f/ stops

[400dabuser]

I am so confused about these numbers.

A magazine article was explaining a feature about sports photography, in particular, skiing.

First and foremost was to get the correct exposure for the snow, otherwise it would look grey. To which he first used f/16 to get the correct exposure, using the manual mode (assuming he meant the exposure, however he got in the middle with the Tv set accordingly)

Then the magazine article, now this is what confuses me, said to the Av down four stops to f/8

I tried it on my camera, moving it to f/8, I had to set it not four times to get it to f/8, but six? Or is that four stops???

Btw, there isn't snow, which is a shame, seeing that it came out a month after our snow fall

[johnz]

1.0 1.4 2 2.8 4 5.6 8 11 16 22 32

That's how Full stops go. Stopping down from 16 to 8, and saying that it's four stops is just wrong. That is two full stops, not four. Perhaps the writer does not quite understand f/stops and is talking about his cameras steps - those might be 1/stop, 1/2 stop or 1/3 stop. You camera probably is moving 1/3 stop at a time, that way you have 6 clicks from f/16 to f/8.

[400dabuser]

Wow, very confusing now, lol

[tzalman]

The camera wants to make the snow medium grey because it doesn't know that it is being pointed at snow and it doesn't know that snow is white. So you override what the camera wants by increasing exposure by 2 stops (six clicks). You can do this in M or by using the EC button in Av, Tv or P. Try it on a white sheet of paper, wall or any thing else that is white, but get close enough so the white object fills most of the viewfinder.

[rooeey]

I dug this oldy up ....I have read it a few times now and each time it gets clearer..


"F," I learned, could actually mean "fraction". The "4"
in "f4" actually stood for 1/4. The diameter of the
aperature at f4 would be 1/4th the focal length of the
lens. A 50mm lens set to f4 would have an aperature
diameter of 12.5mm (50mm X 1/4). A 1000 lens set at F22
would have an aperature diameter of 45mm (1000mm X 1/22).
So with this information, you could derive the formula:
F-Stop = A/FL; where A is the diameter of the aperature, FL
is the focal length of the lens, and F-stop is expressed in
it's fraction form.
So we now know what the numbers mean, but why do we use
THOSE particular numbers? Before we can answer that, we
have to go back in time and get a little history.

When the first lenses where used in cameras, a 50mm lens
was actally 50mm long, a 1000mm lens was actually 1000mm
long. Lenses where basically nothing more than a convex
lens at the end of a tube. A 50 mm lens would be a convex
lens at the end of a 50mm tube, and so on.... The above
formulas would be directly applicable to these types of
lenses. But as photography became more sophisticated,
photographers tired of lugging around huge long lenses.
Techniques were developed to use multiple elements in a
lens to make the effective focal length of a lens much
longer than its real lengh (example: a 1000mm lens could
now be made 250mm long). The above formulas cannot be
directly used on modern lenses to determine exact aperature
diameter, but they can be used to express the ratios
between different aperatures.

Ok, with that out of the way, let's get on to the
"Inversed Squared Law" (ISL). Part of the ISL states that
the area of a circle is directly proportional to the change
of the diameter squared. Therefore, if we multiply or
divide the diameter of an aperature by 1.4, the area of the
aperature would be twice as big or half as big as it was
before (1.4 squared equals 2).
From here it is easy to see that if the area of the
aperature is twice or half of what it was before, it would
let in twice or half as much light in; which would equal 1
stop of exposure either way.

*So to put it in a nutshell: if you multiply or divide
you aperature diameter by 1.4, you get 1 stop more or less
in exposure.*

Now, why do we use the f-stops we do? For ease of
computation, let's start out with a non-descript lens with
a focal length of FL. Let's give the largest aperature
that lens could have a diameter of FL. We could use our
formula: F-stop = Aperture Diameter/Focal Length to get:
F-stop = FL/FL, which would equal 1 (or 1/1). So the
maximum aperature for this lens would be f1.
To find the aperature that would give us one stop less
exposure than f1, we would divide the aperature by 1.4;
this would be 1 divided by 1.4 which is 1/1.4; so the next
aperature for this lens would be f1.4.
To find the aperature that would give us one stop less
exposure than f1.4, we would divide the aperature by 1.4;
this would be 1/1.4 divided by 1.4, which would equal 1/2.
So the next aperature for this lens would be f2.
So far our f-stops on this lens are 1, 1.4, and 2. If
you continued this process, you would find that the
following stops would be 2.8, 4, 5.6, 8, 11, 16, 22. The
reason that lenses don't have a maximum aperature of f1 is
that it is nearly impossible to have a maximum aperature
diameter that is equal to the focal length of the lens.

[400dabuser]

The camera wants to make the snow medium grey because it doesn't know that it is being pointed at snow and it doesn't know that snow is white. So you override what the camera wants by increasing exposure by 2 stops (six clicks). You can do this in M or by using the EC button in Av, Tv or P. Try it on a white sheet of paper, wall or any thing else that is white, but get close enough so the white object fills most of the viewfinder.


Sheez, keep forgetting to do that all the time:(

[Jim G]

Google the tedious explanation of the f-stop. That ought to help.

[400dabuser]

I did, plenty of strange explanations out there

[tzalman]

I forgot to explain why we increase the exposure of white things by two stops and not four or one or sixteen. The camera has a certain range of tones it can portray, called the Dynamic Range, and for jpg shots with a 400D it is about nine stops in all. However, that range is not distributed evenly. If we take that medium grey that the light meter wants to create as our zero point, the camera can render about six levels (stops) darker than medium until it gets to a point where the noise interferes with the image too much, but only about two and a half levels brighter than medium. Any higher than that we just get blank, detailless white - what is called "clipped" or "blown". So by adding two stops of exposure we brighten the white object just about as far as it is safe to go and still get details in the white surface.

[johnz]

I am not a fan of sunny sixteen, or other rules. They just don't work for me as well as metering the light and setting exposure manually.

But shouldn't the sunny16 rule give you a CORRECT exposure under sunlight? So if your shooting outside on a sunny winter day, you should not need to compesate the exposure to get white snow to appear white in the photos. If you do, than what is the rule good for anyway..
I understood the sunny16 rule to be like this, on a clear sunny day, correct exposure = ISO 100, 1/100 @f/16, and if you want to make changes you have to calculate it from there to get the same exposure like ISO 200 1/200 @ f/16, or ISO 100, 1/200 @ f/11.

[tzalman]

I am not a fan of sunny sixteen, or other rules. They just don't work for me as well as metering the light and setting exposure manually.

But shouldn't the sunny16 rule give you a CORRECT exposure under sunlight? So if your shooting outside on a sunny winter day, you should not need to compesate the exposure to get white snow to appear white in the photos. If you do, than what is the rule good for anyway..
I understood the sunny16 rule to be like this, on a clear sunny day, correct exposure = ISO 100, 1/100 @f/16, and if you want to make changes you have to calculate it from there to get the same exposure like ISO 200 1/200 @ f/16, or ISO 100, 1/200 @ f/11.
Yeh sure, Sunny 16 is great if the sky is clear and your subject is facing the sun and it is midday, but what do you do if you are a newby who is afraid of M, you are shooting a white flower under a tree and it is an hour before sunset? Light meters are handy things, if you use them right.

[johnz]

Yeh sure, Sunny 16 is great if the sky is clear and your subject is facing the sun and it is midday, but what do you do if you are a newby who is afraid of M, you are shooting a white flower under a tree and it is an hour before sunset? Light meters are handy things, if you use them right.

Well, if you're a newbie ( i know You are not :) ) - you should study how exposure works and learn to use the meters in your great camera and skip these silly rules :)

[Blue Deuce]

skip these silly rules :)

f/8 and be there.

[johnz]

f/8 and be there.

Yeah.. how creative... :rolleyes:

[Grimes]

f/8 and be there.

Haha, "old school" checking in I see....

Well yes, googling f-stop helps first, there is a large amount of info out there. Just remember that it helps to keep in mind that the f-stop of a lens is a ratio. When you set a lens at f/8, the diameter of the aperture you are creating will fit into the focal length of the lens 8 times. f/2 will fit the aperture twice into the focal length. This reasoning is why f/16 is a smaller hole than f/8.

The fact that it is a ratio also explains why we can compare f-stops between lenses. If we used just the "diameter of the aperture", it would be wildy different between different lenses (for example say a 400mm telephoto and 30mm prime). F-stops are a ratio, thus allowing us to compare apertures among different lenses.

[20droger]

It's really much simpler than some of the explanations would have you believe.

The f/number is the apparent aperture of a lens, expressed as a diameter. The same apparent aperture allows the same amount of light to fall upon the film/sensor regardless of the focal length of the lens. That is, f/8 is f/8, regardless of whether the lens is 50mm or 300mm.

The physical aperture of a lens for a given apparent aperture is a function of the focal length of the lens. That is where the "f" in the f/number comes into play: the "f" is the focal length of the lens. For an apparent aperture of f/8, a 50mm lens has a physical aperture of 50mm ÷ 8 = 6.25mm, while a 300mm lens has a physical aperture of 300mm ÷ 8 = 37.5mm.

However, the physical aperture of a lens is irrelevant. Only its apparent aperture matters.

The f/number is a fraction, where "f" is the numerator and the numerical value is the denominator. That is why the f/number should always be written with a "slash": as "f/8" and never as "f8".

As with any fraction, making the denominator larger makes the fraction smaller. 1/8 of a pie is smaller than 1/4 of a pie. Likewise, f/8 is smaller than f/4, and lets in less light.

A "stop" is a doubling or halving of the light received. With shutter speeds or ISO values, this is a simple and straightforward doubling or halving of the numerical value: 1/60 second is one half of 1/30 second, a decrease of one stop. Very simple.

Not so with aperture! The series of whole stops in apertures are: f/1.0, f/1.4, f/2.0, f/2.4, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22, f/32, f/45, f/64, f/91, etc. They have a ratio of approximately 1.4 between each stop, not 2.

Why? Because apertures are areas, but f/numbers are diameters. An area is a function of the square of the diameter.

For an easy to understand example, consider a square 1 inch on a side. let us say that this square has a "diameter" of 1 inch (f/1). Such a square has an area of 1 inch × 1 inch = 1 square inch. If we were to double the "diameter" of the square to 2 inches (f/2), the area would quadruple, not double: 2 inches × 2 inches = 4 square inches.

Therefore, to double the area of our square, we must increase it's "diameter" by the square root of 2, which is approximately 1.4 (1.41421356...): 1.4142 inches × 1.4142 inches = 1.99996164 square inches.

This works exactly the same for circles.

So, to change an aperture by one stop, to halve or double the amount of light passing through the lens, the f/number (diameter) must be multiplied or divided by the square root of two, or approximately 1.4. This results in the current f/number sequence of f/1.0, f/1.4, f/2.0, f/2.4, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22, f/32, f/45, f/64, f/91, etc.

Yes, there are rounding errors in the sequence. That is because it was created by photographers, not mathematicians. The values are historical, and cannot be "corrected" now.

I hope this helps.

Roger

[Blue Deuce]

Yeah.. how creative... :rolleyes:

Haha, "old school" checking in I see....

Obviously the OP hasn't come to fully understand aperture in relation to ISO and shutter speed. f/8 and be there will serve well under most conditions for most people ensuring proper exposure and dof. My comment was sort of in jest but is a standard practice used by photo journalists for decades.

I have probably forgotten more then you know about being creative having to learn on a fully manual body, lens and years spent in a darkroom ;).

[Persephone]

Is that what's suggested? Set to f/8? I'm relatively new to photography and all I really know is that I shoot wide open during nighttime sports and events...but for sunny outdoor sports I've usually left it in P mode and let the camera decide what to do. I think for the tennis game on Wed. I set it to M at f/6.3 and 1/800...does it matter, how much DOF I have, then?

[johnz]

Obviously the OP hasn't come to fully understand aperture in relation to ISO and shutter speed. f/8 and be there will serve well under most conditions for most people ensuring proper exposure and dof. My comment was sort of in jest but is a standard practice used by photo journalists for decades.

I have probably forgotten more then you know about being creative having to learn on a fully manual body, lens and years spent in a darkroom ;).

I just meant to say, that learning the relation between these three settings and learning how to use your camera meter is probably easier and more useful than understanding the mathematics behind the term f/stop. I have never needed to explain how fstop numbers are calculated, but i have explained quite a few times what they mean for the light that gets to your sensor - and that is the thing that matters, for me atleast :) For me - they are just numbers that define my DOF and the amount of light that gets inside my camera :)

I am sure you haven't forgotten anything! I just meant that sticking at f/8 - if you don't understand what will happen at f/1.8 might not be the best solution in learning how to use your camera properly - if someone does not want to learn ISO, shutter speed and aperture, better stick with P IMO:)

[johnz]

Is that what's suggested? Set to f/8? I'm relatively new to photography and all I really know is that I shoot wide open during nighttime sports and events...but for sunny outdoor sports I've usually left it in P mode and let the camera decide what to do. I think for the tennis game on Wed. I set it to M at f/6.3 and 1/800...does it matter, how much DOF I have, then?

I would not stick with any 1 value, you have to choose your f-stop according to the light, as you probably know already. When shooting in dark spaces, you have to shoot wide open - or use a flash. But lets stick with the sunny day scenario. In those conditions your f-stop has to be selected according to the depth of field and shutter speed that you desire. Do you need to freeze motion, or do you want to imply motion in your image. If your stuck at f/8 under sunlight, it's impossible to imply motion because your shutter speed would be 1/400 @ ISO 100 ( sunny16). But if your shooting tennis with a telephoto of 300mm, than you might want even faster shutter speeds and might want to use f/5.6 and 1/800.

It all depends on what your trying to do, but i find that my telephoto lens that is f/5.6 @ 400mm is too slow even on sunnydays, well not under direct sunlight, but in shadows your shutter speed gets dragged to somewhere like 1/30 and that is too slow. Those are the days when i wish i would have f/4 telezoom and FF body so that i could use ISO 800 with little noise..

This all started with the suggestion that you have to overexpose white targets to get the right exposure and underexpose black targets to do the same, this was because camera assumes that it's metering gray and its actually metering something else. Remember that camera is "color blind" - atleast when metering is concerned!

We are already little off topic, but i hope your learning a little from this :)

[20droger]

The guideline "use f/8 and be there" has more to do with maximum lens sharpness and DoF problems than it does with exposure. I personally would not have brought it up in an exposure discussion.

Exposure consists of three parameters: aperture, shutter speed, and ISO (sensitivity). All three interact in a simple and straightforward manner.

Each of these parameters has both a primary effect upon exposure and a secondary effect not directly related to exposure.

As regards exposure, aperture determines the amount of light that reaches the film/sensor per instant, shutter speed determines the length of time the film/sensor receives light, and ISO determines the sensitivity of the film/sensor to the light it receives. Manipulating these three parameter determines the exposure. That's it in a nutshell.

Secondary effects are somewhat more complicated:

Aperture affects the depth of field: the greater the aperture, the shallower the depth of field. The use of a large aperture produces a shallow depth of field and allows the photographer to produce a beautifully blurred background while keeping the subject in crisp focus. A small aperture causes a large depth of field and brings the background more into focus, resulting in a "snapshot" look.

Shutter speed affects motion blur: the longer the shutter speed, the greater the tendency for the image to blur. This is not necessarily bad. A long shutter speed will blur moving water into a soft gauze-like stream, while a short shutter speed will show every drop.

ISO affects image grain (film) or noise (digital). The higher the ISO, the grainier/noisier the image. As a general rule, ISO should be kept as low as is practical for the situation. With film, this means selecting the appropriate film for the shoot. With digital, this means keeping the setting appropriate for the shoot; using an ISO setting that would represent the proper film speed were film being used.

And yes, there are countless exceptions to every guideline, these included. But one should master the "rules" first, so that one would know when it is appropriate to break them.

[Grimes]

Obviously the OP hasn't come to fully understand aperture in relation to ISO and shutter speed. f/8 and be there will serve well under most conditions for most people ensuring proper exposure and dof. My comment was sort of in jest but is a standard practice used by photo journalists for decades.

I have probably forgotten more then you know about being creative having to learn on a fully manual body, lens and years spent in a darkroom ;).


....my comment was meant in jest as well. No need to get feathers ruffled!

[20droger]

As far as the Sunny-16 Rule is concerned, the rule is: an average front-lit subject on a clear sunny day will be properly exposed at f/16 when the shutter speed is set to the reciprocal of the ISO.

Therefore, a front-lit person in direct sunlight would be properly exposed at f/16 at 1/100 second at ISO 100. For side lighting, add one stop. For back lighting, add two stops.

These settings should not be used as an absolute, but as a starting point for the aperture and shutter speed desired. For the kids at play, try f/8 at 1/400 second at ISO 100. And so forth.

The Sunny-16 rule assumes twelve different scenes, with corresponding adjustments:

Bright — A sunny day on a bright background, such as sand or snow. — Subtract 1 stop.

Sunny — A typical bright sunny day. Shadows are distinct and sharp. This is the basis of the "Sunny 6" rule.

Hazy — A bright hazy day, where the sun shines through haze or thin high clouds, Shadows are distinct but soft. — Add 1 stop.

Cloudy — A bright cloudy day, where the sun is basically shining but is currently behind a bright white cloud. Shadows are very soft. — Add two stops.

Overcast — A typical overcast day. Alternatively, open shade with no direct sunlight. There are no shadows. — Add three stops.

Heavy — A heavily overcast day. A typical rainy day or overcast winter day. — Add four stops.

Deep — A very heavily overcast day. Alternatively, deep shade and found in deep woods on a sunny day. — Add five stops.

Storm — A dark overcast day, as just before a heavy thunderstorm. Alternatively, dusk on a sunny day. — Add six stops.

Office — A brightly lit office or store. — Add seven stops.

Stage — A well lit stage or an indoor sports arena, such as a basketball court or a hockey rink. — Add eight stops.

Home — A well lit home. This is typical kitchen or reading area brightness. Alternatively, right under a bright streetlight. — Add nine stops.

Soft — A softly lit home, such as a TV room or bedroom. Alternatively, a typical city street at night. — Add ten stops.

The plans for a do-it-yourself Sunny-16 Slide Rule is available free of charge for the asking. Just PM me your email address.

[400dabuser]

It's really much simpler than some of the explanations would have you believe.

The f/number is the apparent aperture of a lens, expressed as a diameter. The same apparent aperture allows the same amount of light to fall upon the film/sensor regardless of the focal length of the lens. That is, f/8 is f/8, regardless of whether the lens is 50mm or 300mm.

The physical aperture of a lens for a given apparent aperture is a function of the focal length of the lens. That is where the "f" in the f/number comes into play: the "f" is the focal length of the lens. For an apparent aperture of f/8, a 50mm lens has a physical aperture of 50mm ÷ 8 = 6.25mm, while a 300mm lens has a physical aperture of 300mm ÷ 8 = 37.5mm.

However, the physical aperture of a lens is irrelevant. Only its apparent aperture matters.

The f/number is a fraction, where "f" is the numerator and the numerical value is the denominator. That is why the f/number should always be written with a "slash": as "f/8" and never as "f8".

As with any fraction, making the denominator larger makes the fraction smaller. 1/8 of a pie is smaller than 1/4 of a pie. Likewise, f/8 is smaller than f/4, and lets in less light.

A "stop" is a doubling or halving of the light received. With shutter speeds or ISO values, this is a simple and straightforward doubling or halving of the numerical value: 1/60 second is one half of 1/30 second, a decrease of one stop. Very simple.

Not so with aperture! The series of whole stops in apertures are: f/1.0, f/1.4, f/2.0, f/2.4, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22, f/32, f/45, f/64, f/91, etc. They have a ratio of approximately 1.4 between each stop, not 2.

Why? Because apertures are areas, but f/numbers are diameters. An area is a function of the square of the diameter.

For an easy to understand example, consider a square 1 inch on a side. let us say that this square has a "diameter" of 1 inch (f/1). Such a square has an area of 1 inch × 1 inch = 1 square inch. If we were to double the "diameter" of the square to 2 inches (f/2), the area would quadruple, not double: 2 inches × 2 inches = 4 square inches.

Therefore, to double the area of our square, we must increase it's "diameter" by the square root of 2, which is approximately 1.4 (1.41421356...): 1.4142 inches × 1.4142 inches = 1.99996164 square inches.

This works exactly the same for circles.

So, to change an aperture by one stop, to halve or double the amount of light passing through the lens, the f/number (diameter) must be multiplied or divided by the square root of two, or approximately 1.4. This results in the current f/number sequence of f/1.0, f/1.4, f/2.0, f/2.4, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22, f/32, f/45, f/64, f/91, etc.

Yes, there are rounding errors in the sequence. That is because it was created by photographers, not mathematicians. The values are historical, and cannot be "corrected" now.

I hope this helps.

Roger


I will give it a read, tomorrow, haha, but thanks

[Blue Deuce]

....my comment was meant in jest as well. No need to get feathers ruffled!

I know it was and I will stay on topic.

[Bosscat]

The camera wants to make the snow medium grey because it doesn't know that it is being pointed at snow and it doesn't know that snow is white. So you override what the camera wants by increasing exposure by 2 stops (six clicks). You can do this in M or by using the EC button in Av, Tv or P. Try it on a white sheet of paper, wall or any thing else that is white, but get close enough so the white object fills most of the viewfinder.

I'm sure glad I didn't follow this advice when I took this shot. Or wasted time with a handheld meter either.

[johnz]

I'm sure glad I didn't follow this advice when I took this shot. Or wasted time with a handheld meter either.

Did someone suggest using a handheld meter under these circumstances? I can't recall that.

That photo sure is nice, but if did shoot that in Av, TV, or P and took many shots, i am guessing your exposure varies quite a lot between the shots. Well, according to your sig, you seem to have some kind of a mission against M, so i will leave let it be :) The Tv and Av modes do great in most of the shots, but you cant avoid using EC under those modes, your camera is not That smart.

[400dabuser]

Originally Posted by tzalman View Post
The camera wants to make the snow medium grey because it doesn't know that it is being pointed at snow and it doesn't know that snow is white. So you override what the camera wants by increasing exposure by 2 stops (six clicks). You can do this in M or by using the EC button in Av, Tv or P. Try it on a white sheet of paper, wall or any thing else that is white, but get close enough so the white object fills most of the viewfinder.


You have limited control in these modes, because the sensor will in Av mode, just correct the best timing for the shutter, to achieve the "correct" exposure, in M mode, you have full creative control. In P mode, that is what I really would semi-Auto mode, the timing and aperture is really based on what you set the Ev and ISO values. The writer/pro photographer, who was teaching an amateur/readers on how to get the correct exposure for the snow

[PhotosGuy]

This shows how the subject can affect the exposure & why manual keeps me worry free:
Post #47 (http://photography-on-the.net/forum/showpost.php?p=5191658&postcount=47)
Click the "Thread: (thread title)" link at the top-right if you'd like more info on the subject.

[krb]

Then the magazine article, now this is what confuses me, said to the Av down four stops to f/8

I tried it on my camera, moving it to f/8, I had to set it not four times to get it to f/8, but six? Or is that four stops???
Others have covered the theory but I didn't see anybody answer this specifically. By default, one click of the adjustment wheel is 1/3 of a stop. Some cameras allow you to change this to 1/2 stop per click. Moving from f/8 to f/16 is 2 stops, which means either 6 clicks at 1/3 stop per click or 4 clicks at 1/2 stop per click.

[400dabuser]

Others have covered the theory but I didn't see anybody answer this specifically. By default, one click of the adjustment wheel is 1/3 of a stop. Some cameras allow you to change this to 1/2 stop per click. Moving from f/8 to f/16 is 2 stops, which means either 6 clicks at 1/3 stop per click or 4 clicks at 1/2 stop per click.

Does the number of stops per clicks or vice versa depend on the camera???

[krb]

Does the number of stops per clicks or vice versa depend on the camera???
1/3 stop per click is the old standard and what I think most people are used to. I know that on the 40D there is a menu option to select which one you prefer and 1/3 is the default.

[20droger]

Others have covered the theory but I didn't see anybody answer this specifically. By default, one click of the adjustment wheel is 1/3 of a stop. Some cameras allow you to change this to 1/2 stop per click. Moving from f/8 to f/16 is 2 stops, which means either 6 clicks at 1/3 stop per click or 4 clicks at 1/2 stop per click.
This was covered in posts #2 and #4.

[20droger]

Does the number of stops per clicks or vice versa depend on the camera???
Yes, it does. In some cameras, this is fixed at whole, half, or third stops. In other cameras, this is settable. There is no "standard."

In the 40D, the choice is either third stops or half stops, controlled by Custom Function 1 (CFn-1), and is covered on page 154 of the manual.

In the 400D/Digital Rebel XTi, the choice is also either third stops or half stops, controlled by CFn-6, and is covered of page 105 of the manual.

[400dabuser]

Yes, it does. In some cameras, this is fixed at whole, half, or third stops. In other cameras, this is settable. There is no "standard."

In the 40D, the choice is either third stops or half stops, controlled by Custom Function 1 (CFn-1), and is covered on page 154 of the manual.

In the 400D/Digital Rebel XTi, the choice is also either third stops or half stops, controlled by CFn-6, and is covered of page 105 of the manual.


OK, thanks for that

[tzalman]

1/3 stop per click is the old standard and what I think most people are used to. I know that on the 40D there is a menu option to select which one you prefer and 1/3 is the default.
You don't go back very far. :) The old standard was one stop per click. Actually, if you want to go back a lot further than any forum member can remember, there were no aperture clicks at all, just exchangeable metal plates with different sized holes.

[krb]

You don't go back very far. :) The old standard was one stop per click. Actually, if you want to go back a lot further than any forum member can remember, there were no aperture clicks at all, just exchangeable metal plates with different sized holes.
One click of the aperture ring per stop, but the shutter speed and ASA were in third of a stop increments and most people I know think of stops as being broken into thirds.

[PhotosGuy]

and most people I know think of stops as being broken into thirds. Kids! :D

[rdenney]

Kids! :D

Boy, you got that right.

When Ansel Adams was a young lad, did his eyes glaze over when considering the arithmetic behind the F-stop? What about Imogen Cunningham? Paul Strand? Alfred Stieglitz? Dorothea Lange? Margaret Bourke-White? Etc., etc. Uh, no. They did not believe, as many "kids" apparently do today, that being artistic requires an abhorrence of fairly simple geometry and arithmetic.

"F/8 and be there" is a statement that where you point the camera and when you push the button are at least as important as how you twiddle the exposure dials on the camera. But that doesn't mean we can get away with not caring about that exposure. I once asked my music teacher: "Which is more important, technique or musicality?" His answer: "Yes."

An early poster stated that lenses were their length--a 50mm lens was 50mm long. That is not correct. A 50mm lens is always just this: The optical center of the lens is 50mm away from the film/sensor when focused at infinity. The optical center of the lens may be in the middle, in front of, or behind the glass itself based on the design, and that was true in the olds days, too.

Long after plates with holes in them, we had aperture controls that involved moving a pointer smoothly from one marking to the next, like focusing a lens. Having detents (clicks) was not common, and the Copal shutters/diaphragms used on modern view camera lenses still don't have detents on the aperture dial. They do have detents for shutter speeds, but that is for mechanical reasons. Detents on the aperture control was an innovation mostly related to small, portable cameras of modern design.

F-stops and shutter speeds regulate amount of light reaching the sensor, and ISO sets the sensitivity of the sensor. If you think of light as a flow of water, the F-stop determines how much you open the faucet, and the shutter speed determines how long the faucet is on. The objective is to fill a glass of a certain size (the size of the glass is determined by the ISO) exactly. Too much water, and the highlights will blow out. Too little water, and the shadows will lose tonal separation, even with post-processing.

The F-stop series is related to the inverse-square law (which I learned no later than junior high school). When you double the distance light must shine, the light that reaches the surface now has to cover four times as much area and is therefore one-fourth as bright. As Roger mentioned, the area of the aperture is what dictates the flow of light through it, and that area is related to the square of the diameter. So, when you cut the diameter of the hole in half, the area is only one-fourth as much. Since we define a stop to be a doubling or a halving of the light, reducing it to one-fourth as much is two stops (we are halving it twice). Thus, when I go from an aperture of f/4 to f/8, I'm moving two stops.

To double the area, we are increasing the diameter by the square root of 2, which is 1.4. (1.4 times 1.4 = 2.) So, when we go from one stop to the next, we double the area in order to double the flow of light, and the diameter of the aperture increases by a factor of 1.4. Thus, f/4 to f/5.6 is one stop, because 4 times 1.4 equals 5.6.

(This all works the same no matter how big the image frame happens to be or how long the lens happens to be.)

Because it's related to the inverse square law, there's lots that we can do with this series of numbers. For example, if you have two lamps lighting your subject, and you want the dark side of the face to be 1/2 as bright as the bright side, move the lamp lighting the dark side 1.4 times farther than the lamp on the bright side. If the bright-side lamp is 8 feet away, move the other lamp to 11 feet away (f/8 to f/11 is one stop, which means that 8 time 1.4 is 11).

To fill a glass of a given size, when you increase the flow, you must decrease the time the faucet is on. If you increase the flow by double, then the time the faucet is on must decreased by half. That is a reciprocal arrangement, and you can move up and down the scale for whatever reason and still keep the exposure the same. Thus, in terms of exposure, f/1.4 at 1/125 is the same as f/2 at 1/60, which is the same as f/2.8 at 1/30, f/4 at 1/15, f/5.6 at 1/8, f/8 at 1/4, and so on.

Now, each of those aperture and shutter speed settings will have other effects (secondary effects, as Roger calls them), and learning how to choose those is one of the principal expressions of technique by a photographer. If you let the camera do it all the time, you are letting Canon software programmers express your technique for you. Where's the fun in that?

Rick "who lets the camera make the choices pretty often, but only under skilled supervision" Denney

[tzalman]

One click of the aperture ring per stop, but the shutter speed and ASA were in third of a stop increments and most people I know think of stops as being broken into thirds.
Like I said, there are some real oldtimers around here, guys who remember mechanical shutters. When the curtain speed was determined by spring tension and the slit width by clockwork escapements, it was an engineeriing marvel to get speeds that were roughly half or twice the adjacent settings. 1/3 accuracy was beyond our dreams until electronic shutters appeared in the mid '70s.