What's a stop?

[DocFrankenstein]

I keep hearing it over and over... What's the relationship of the amounts of stops and shutter speeds?

And how can you calculate aperture? What is the definition of aperture from the physics POV?

[RikWriter]

If I might make a reccomendation, when I first purchased my Rebel, I found the books "The Complete Idiot's Guide to Photography Like a Pro" very helpful. Gives you all the basics.

[DocFrankenstein]

:lol: I was hoping there are some articles on the net explaining it...

[Longwatcher]

Lets see if I remember things right from memory.

A full stop is a setting which allows in twice as much (or half as much) light as the previous stop.

The values involved change based on focal length and most modern lenses have the setting built into them for easy reference and convience. A larger aperture will have a smaller number, which is what always caused me confusion. If I remember though it is because the number represents the fraction of light it lets in. Thus an aperture of 1.0 will let in 100% of the light that is available (or at least the equivilant). Where as say an aperture of 2.8 is letting in 1/2.8 of the available light.

I believe the 10D/300D does overall 1/3 stops for the balance of shutter and aperture (1D and 1Ds do 1/5 stops I think). ISO is usually in full stops.

Now lets see if I got it right from memory with out checking my facts.

[Pekka]

Check this out:
http://www.silverlight.co.uk/tutorials/compose_expose/exposure.html

[RikWriter]

:lol: I was hoping there are some articles on the net explaining it...

There might be, but I find it very hard to sit down and read a very long technical explanation on my monitor. I need to sit back with it in book form.

[robertwgross]

Lets see if I remember things right from memory.

A full stop is a setting which allows in twice as much (or half as much) light as the previous stop.

The values involved change based on focal length and most modern lenses have the setting built into them for easy reference and convience. A larger aperture will have a smaller number, which is what always caused me confusion. If I remember though it is because the number represents the fraction of light it lets in. Thus an aperture of 1.0 will let in 100% of the light that is available (or at least the equivilant). Where as say an aperture of 2.8 is letting in 1/2.8 of the available light.

I believe the 10D/300D does overall 1/3 stops for the balance of shutter and aperture (1D and 1Ds do 1/5 stops I think). ISO is usually in full stops.

Now lets see if I got it right from memory with out checking my facts.

Longwatcher, I think you have the right idea, but your numbers are off a little. If we start from an aperture of f/1.0, then the next f-stop down is f/1.4. Each stop lower (higher number) is 1.4 times the first one. So, you see a progression like 1.0, 1.4, 2.0, 2.8, 4.0 etc.

Why 1.4? Well, it turns out to be the square root of 2. Each stop lets in half of the light of the earlier stop. So, in theory, if you saw the progression of apertures 1.0, 1.4, 2.0, 2.8, 4.0, and if you keep sensitivity constant, then you ought to see shutter speeds progressing by halves and doubles to keep the exposure value constant. So, from a starting shutter of 1/100, if the aperture allowed in half of the light (down one stop), then the shutter must stay open twice as long, which would be 1/50. If the aperture allowed in two stops less, then the shutter must change by two speed stops, which would be from 1/100 to 1/50 to 1/25.

---Bob Gross---

[robertwgross]

In all urban areas, they have community colleges, and I suspect that one near you has a basic photography class. In general, they teach about f stops and shutter speeds in the very first class session. In some cities, there is an adult education class that does it. Virtually every basic photography textbook teaches the concepts.

---Bob Gross---

[cmM]

A stop and an f stop are NOT the same.

Aperture is measured in F stops... aperture reffers to the area of the opening of the lens (I think).

Stops measure brightness. As in... a 2 stop Neutral Density filter. That has nothing to do with aperture.


There are plenty of articles on the web about this. Look at "ALl the best links" sticky thread. A lot of good stuff there.

[robertwgross]

Stops measure brightness. As in... a 2 stop Neutral Density filter. That has nothing to do with aperture.


Think about it.

It can have everything to do with aperture. If you have an exposure solution in your camera, and then you add a 2-stop neutral density filter, you will need to either open the aperture by two stops to adjust, or else slow the shutter two stops to adjust.

If it is a 2-stop filter, then it has reduced the light intensity to one fourth. That gets fixed by opening the aperture to let four times the light through, which is opening it two stops.

---Bob Gross---

[cmM]

That's true. There's a relationship between them. Just like the one between aperture and shutter speed.

But as far as terms go, when you are reffering to "stops" you are not talking about aperture.

That has nothing to do with aperture.
I phrased it wrong.... sorry :oops: It does have a lot to do with aperture. It just isn't talking about aperture.

[Aylwin]

I simply love these informative threads! Great stuff, Bob! :)

I think I understand the aperture part but I want to clarify the shutter speed part. So, increasing the shutter speed 2 stops means adjusting it till it's 4 times the original setting (eg. 1/50 --> 1/200)?

Another thing, I've noticed it's possible to set shutter speeds to 1/2 or 1/3. But 1/3 is better because aperture is in increments of 1/3 stops also. Is that correct? If yes, then is the 1/2 setting appicable? Why is it even an option?

As for f-stop vs. stop, 1 f-stop refers directly to 1 aperture stop while stop simply refers to half (or double, as the case may be) the amount of light. Am I understanding all this correctly?

[ron chappel]

All light measurements in photography are done in stops.

Stops (which get's the name from a historical gadget) are either a doubling or halving of the amount of light. Light is measured this way in photography because there is such a huge range of it! Percentages or other ways of measuring would be just hopeless.


Film ISO,aperture,shutter speed ,filter factors and exposure compensation are all totally interchangable.If you double one of them you just have to halve one of the others to have the exact same amount of light
Most of these are easy to understand as the numbers are a dead givaway-they are twice as big/small as the number above/below.
The exception to this is aperture which has a strange geometric patern to it.
These are calculated be dividing the focal length by the aperture size.The best way to remember them is simply to memorize the numbers.
The reason these weird aperture numbers are used is because they tell a whole lot about depth of field,depth of focus,hyperfocus,etc and are completely interchangeable from one lens to another.
i.e if you have a 100mm lens focused at 10 meters and a 50mm lens focused at 5 meters then the depth of feild will be identical if the aperture is the same!

Once you put all this together with the knowledge of how many stops of light a film/sensor can see,photography becomes a whole lot simpler!!! :D

Oh...and by the way.Several posters above have implied that f1 is the largest aperture a lens can have.This is way wrong !


Because f stop is simply focal length divided by aperture then ANY Fstop is theoretically possible
There have been quite a few sub f1 lenses made.The f0.7 Zeiss/NASA lens used by stanley kubrick to film parts of the movie 'barry lyndon' is the most well known but there have been others.

Such large aperture lenses tend to be huge in diameter due to the large front elements required to give enough light to the huge aperture

There are some good online tutorials but i havent had to look at them for awhile so i'll have to find them again
I'll get back once i've found some usefull ones :)

[HTP]

stop = appeture

[robertwgross]

I think I understand the aperture part but I want to clarify the shutter speed part. So, increasing the shutter speed 2 stops means adjusting it till it's 4 times the original setting (eg. 1/50 --> 1/200)?

Increasing the shutter speed 2 stops FASTER means setting it 4 times FASTER than the original.


Another thing, I've noticed it's possible to set shutter speeds to 1/2 or 1/3. But 1/3 is better because aperture is in increments of 1/3 stops also. Is that correct? If yes, then is the 1/2 setting appicable? Why is it even an option?

A 1/3 stop is just finer measurement than 1/2 stop. In the old, old days, metering any closer than 1 whole stop was a challenge. Then for many years, photographers have learned to think finer, like in 1/2 stop increments. Now some like to go finer yet, so they use 1/3 stop increments. There are some light meters that go to 1/10 stop increments.

Find me somebody who says "Let's see, light is about 1/100 at f/6.7. No, f/6.8." For typical shooting either 1/3 or 1/2 will get you good results. If you are getting into really critical bracketing, it will make a difference.


As for f-stop vs. stop, 1 f-stop refers directly to 1 aperture stop while stop simply refers to half (or double, as the case may be) the amount of light. Am I understanding all this correctly?

F-stops refer to aperture. Speed stops refer to shutter. One tends to run one way while the other runs the other way. Just call them stops.

I'm sure this will make sense, if you think about it. If you change shutter from 1/100 second to 1/50 second, you have doubled the light. Right? The numbers always run in halves and doubles, or a very close approximation of that.

Now, if you change aperture from f/5.6 to f/4, you have doubled the light through the lens. Right? (That is one stop)

The only tricky thing there is that the aperture number stops do not look like halves and doubles. This will be tough for liberal arts majors, but if you can learn to think in multiples of 1.4 (square root of two), then it works easy. So, the apertures do not run like f/1, f/2, f/4. Instead, they run like f/1, f/1.4, f/2, f/2.8.

Once you hang around with your camera long enough, you can rattle off the whole f/stops like that, and then you can learn the half-stops in between. Maybe even the 1/3 stops. No, let the camera do most of the work. It is designed to do that quickly, easily, and it is relatively foolproof. You need to be able to "know" what it is doing and not be deceived. Pay no attention to the man behind the curtain. He probably shoots a Nikon, anyhow.

---Bob Gross---

[cmM]

You guys are saying that stops always reffer to aperture. From what I read, that is not the case. I'm not an experienced photographer, but thought I had understood these concepts, or at least most of them

Here's what one article on the web says....

I hear stops referred to a lot. Are these always f/stops?

No. A source of confusion is that "stops", as in f/stops, has become something of a handy shorthand for other doubling/halving relationships when referring to exposure. Thus, when someone says they "stopped down", they probably did change the aperture from, say, f/8 to f/11. However, if someone says they wish they had a stop more light, they mean they wish they had twice as much. If they say they got some ASA 400 film which is two stops faster than their Sensia II, it means it is four times as sensitive and you can infer that the Sensia was ASA 100 (from 400, 200 would be one stop, one halving, and 100 would be the second stop, the second halving). Even experienced photographers get confused sometimes; I had one guy tell me he "pulled his film 6 stops, from ASA 100 to ASA 6". Well, that's not six stops, it's four. Here, count along: 100 to 50 is one, 50 to 25 is two, 25 to 12 is three, 12 to 6 is four.

Note that stops always refer to exposure things. You would not say a 100mm lens is a "stop longer" than a 50mm because it was twice as long! You would say it was twice as long, or just that it's a 100mm.



http://www.uscoles.com/fstop.htm - the whole thing.

[CyberDyneSystems]

stop = appeture

... er,.. you might want to actually read some of this thread there pal :wink:

[CyberDyneSystems]

You guys are saying that stops always reffer to aperture. .

Actually,. so far I think only HTP did that.. and that would be incorrect :)

[robertwgross]

You guys are saying that stops always reffer to aperture.

No, that was not what we said.

F-stops always refer to aperture.

Shutter speed stops always refer to shutter speed.

I suppose that ISO stops always refer to ISO sensitivity also, but you don't hear that much.

F-stops always refer to halving and doubling of light through the lens.

Shutter speed stops always refer to halving and doubling of time the shutter is open.

---Bob Gross---

[CyberDyneSystems]

ISO stops are starting to be discussed more often with the digital aspect of things.. there are even auto ISO settings in some digitals.. and Apeture/Shutter priority settings that only adjust the ISO to acheive proper exposure.. ISO bracketing etc...

The ISO stop is here now.. we just haven't gotten used to it :)

With film of course.. the adjustment was made by changing film... (or by "pushing" ... but not everyone knew about that.)

[robertwgross]

The ISO stop is here now.. we just haven't gotten used to it :)


Yes, it just does not come spinning off my lips yet.



With film of course.. the adjustment was made by changing film... (or by "pushing" ... but not everyone knew about that.)

Yes, I could move from Ektachrome 100 speed to 200 speed to 400 speed. But those ISO stops came one roll at a time. Ahh, ISO 64! That always threw my math off.

---Bob Gross---

[Aylwin]

Very good link, Chris. Thanks. But yeah, I already have better understanding now of stops, etc. thanks to this thread.

there are even auto ISO settings in some digitals..
Yep, even on the 10D. If I'm not mistaken, in full auto with the flash on the ISO is automatically set to 400. Actually, I'm not even sure if you can set ISO in any of the full auto modes.

One thing with ISO settings though, I haven't really noticed that I could double the shutter speed when changing from ISO 100 to 200. Or maybe I just haven't been paying attention.

By the way, Bob:
Increasing the shutter speed 2 stops FASTER means setting it 4 times FASTER than the original.
I thought that's what I said. Isn't "increasing... speed" the same as "faster"? :wink:

[robertwgross]

By the way, Bob:
Increasing the shutter speed 2 stops FASTER means setting it 4 times FASTER than the original.
I thought that's what I said. Isn't "increasing... speed" the same as "faster"? :wink:

I think I know what you meant. This is just a terminology trap, and we should not let it confuse us.

Going from 1/100 sec to 1/200 sec. Is that increasing or decreasing?

By one way of thinking, it is speeding up, or increasing. But by another way of thinking, it is reducing the light that gets to the sensor. You can turn the shutter speed scale around and look at it from whichever direction you want.

Going from f/1.4 to f/2. Is that increasing or decreasing?

By one way of thinking it is closing down the lens, which means less light gets to the sensor. By another way of thinking, the aperture number is increasing. Turn the scale around to whichever direction you want.

The big tendency, of course, is that for any one exposure solution, you should be able to increase the light as a result of the shutter, and that will cause you to decrease the light as a result of the aperture. All of these inverse relationships have to be burned into your brain if you are going to do this stuff.

What makes it triply complicated is that the digital metering display on a lot of cameras will show "100" when it means a shutter of "1/100". Then it might show " 8 " when it means "f/8". That is another inverse (or perverse) thing that throws the newbies off. That has to be burned into your brain also so that it is instinctive as to what the smart camera is really trying to tell you.

Damn. Where are the old match-needle meters that we had thirty years ago (back when I was only a tiny child, of course). Film speed was 25 then.

---Bob Gross---

[HTP]

stop = appeture

... er,.. you might want to actually read some of this thread there pal :wink:
perhaps u should look it up before u open ur arse to talk

[Aylwin]

Very good clarification! I understand your points. It turns out I did know what I was talking about after all. :)

Going from 1/100 sec to 1/200 sec. Is that increasing or decreasing?
Without qualifying, it depends. In terms of speed, increasing. In terms of time, decreasing. That is, the length of time the shutter is open.

Going from f/1.4 to f/2. Is that increasing or decreasing?

This used to confuse me. Especially that term "stop down". I was never really sure what people meant. In fact, I'm still not 100% sure on it. I hope this is still on topic because we are talking about stops, but...

Does "stop down" refer exclusively to aperture? Couldn't it also mean that the photographer changed the speed from 1/100 to 1/200. Or, perhaps changed from ISO 400 to 200? And if we are talking about aperture, what if I "stopped down" from f/1.8 to f/2.0? Do I say, "I stopped down 1/3 (or whatever)"?

Am I just getting confused again?

[Jesper]

Stopping down and the f/stop number just means setting the aperture. It doesn't have anything to do with shutter speed in itself. Ofcourse, if you talk about the exposure, you're talking about the combination of aperture setting (f/stop) and shutter speed.

Here's an online explanation:
A Tedious Explanation of the f/stop (http://www.uscoles.com/fstop.htm)

And now for some mathematics!

The f/stop number is the ratio between the focal length of the lens and the diameter of the aperture. So, if you have for example a lens with a focal length of 50 mm, and the diameter of the aperture is 50 mm, you have an f/1.0 lens.

The area of the aperture hole determines how much light enters the lens. If you make the area of the hole twice as small, you are making the diameter smaller by a factor of the square root of 2 (1.4142...). This is because the area of a circle is pi (3.14159...) times the radius squared (and the diameter is 2 times the radius).

You are letting in half the light if you make the area half the size, which means you are making the diameter smaller with a factor of 1.4. So for the 50 mm lens, if you make the diameter 35.4 mm you are setting it to f/1.4.

One stop down = 1.4 (1/2 of the light compared to f/1.0)
Two stops down = 1.4 x 1.4 = 2.0 (1/4 light compared to f/1.0)
Three stops down = 2.0 x 1.4 = 2.8 (1/8 light compared to f/1.0)
Four stops down = 2.8 x 1.4 = 4.0 (1/16 light compared to f/1.0)
etc...

Note the factor 1/2, 1/4, 1/8... To get the same exposure (the same amount of light for photo), you need to make your shutter speed 2x, 4x or 8x longer than the shutter speed you'd use at f/1.0.

[DocFrankenstein]

WOW!

No wonder I can't take pictures indoors with my standard lens!

I am usually at the telephoto end of the 18-55 lens, so the aperture is 5.6... Had I used the 50 mm f/1.4 I'd be able to get... 5 stops... 32 times more light... :shock: :shock: :shock:

Would that make my shutter time 32 times faster???? :shock:

Or did I mess up my calculations? :shock:

[hmhm]

Or did I mess up my calculations?

The progression of f-stops would be 1.4, 2.0, 2.8, 4.0, 5.6, so the difference between 1.4 and 5.6 is _4_ stops, not 5, which means 16 times as much light at 1.4 as at 5.6. So if you don't change the ISO, then you'll see a shutter speed at f/5.6 that is 16 times longer than the shutter speed at f/1.4 .
-harry

[hmhm]

Does "stop down" refer exclusively to aperture?

As it is typically used, the phrase "stop down" specifically means "switch to a smaller aperture". Note that smaller apertures are the ones with larger f-numbers (e.g. if I change from f/5.6 to f/8 then I have stopped down 1 stop).

For some reason, nobody ever seems to say "stop up". I guess they usually say "open up".
-harry

[Aylwin]

I always understood "open up". Maybe because it's more visual. Aperture is pretty much the only thing you can "open up". However, "stop down" never stuck with me as a direct opposite to "open up".

So, "stop down" is generally understood as referring to aperture. Specifically, it refers to selecting an f/stop value that is 1 stop down from your current reference (as in Harry's example).

Now, back to my previous questions. If my current aperture setting is f/1.8 and I stop down, what aperture setting do I end up with? My calculator says f/2.52. Do I simply choose the closest one to that? If I decide to take the photo at f/2.0, how do I describe what I just did? Would that still qualify as "stopping down"?

[robertwgross]

If my current aperture setting is f/1.8 and I stop down, what aperture setting do I end up with? My calculator says f/2.52. Do I simply choose the closest one to that?

Yes. That is pretty close to f/2.8.

If I decide to take the photo at f/2.0, how do I describe what I just did? Would that still qualify as "stopping down"?

Yes, you stopped down just a little from f/1.8 to f/2.0, but that isn't very much at all.

Suggestion: Let's not split hairs any finer than this right now. Run around and shoot with one-third stops or one-half stops or whatever you want. Once that is all mastered, then come back sometime, buy a couple of beers, and have a discussion about one-tenth of stops.

---Bob Gross---

[Aylwin]

Suggestion: Let's not split hairs any finer than this right now. Run around and shoot with one-third stops or one-half stops or whatever you want. Once that is all mastered, then come back sometime, buy a couple of beers, and have a discussion about one-tenth of stops.
HAHAHA! Good suggestion! :D

I tend to get carried away. Actually, I know enough to be able to take shots. And I have a basic idea of things like the inverse proportional relationship between aperture and shutter speed. I guess what I'm really after is the terminology and more detailed theory. Basically, I want to learn the language of photography. I want to understand what you guys are talking about and to be able to communicate correctly as well.

[Pekka]

stop = appeture

... er,.. you might want to actually read some of this thread there pal :wink:
perhaps u should look it up before u open ur arse to talk

C'mon please leave that "dpreview attitude" out of this forum NOW.

Stop is not same as aperture. It is a term used to communicate exposure relations. Sometimes it is equal to aperture up/down, sometimes it is equal to doubling/halving of amount of light.

[robertwgross]

Actually, I know enough to be able to take shots. And I have a basic idea of things like the inverse proportional relationship between aperture and shutter speed.

That is a good start. In contrast, one friend of mine has a Canon film Rebel, and she breaks into a sweat if somebody suggests that she move the mode off "Green Box."

---Bob Gross---

[DocFrankenstein]

You mean full auto mode?

:shock:

[Aylwin]

Oh is that what the green box means? I usually just choose the appropriate picture/icon. ;)

[awagner]

http://www.fredparker.com/ultexp1.htm


try this link I have found it pretty usefull