Clarifying a couple of points from the book "Understanding Exposure"

[Dredd123]

Hi,

I'm reading the book "Understanding Exposure" by Bryan Peterson at the moment and I would just like someone to confirm my understanding of a couple of things I have learned from the book:

1. When photographing snow scenes or beach scenes (basically a scene with a large proportion of highly reflective surface), the camera will get confused by the amount of light being reflected and will tend to UNDER expose the scene. Therefore I should manually override the exposure and dial in settings which the camera light meter thinks are over exposed.

2. When photographing the opposite of 1 above (i.e. a dark scene or a scene filled with a dark object), the camera will get confused and tend to OVER expose the scene. Therefore I should manually override the exposure and dial in settings which the camera light meter thinks are under exposed.

3. What Bryan Peterson calls "Mr Green Jeans (The Sky Brothers Cousin)". He says you can meter off green objects if the scene has a lot of green in it (e.g. a waterfall in a forest), and then expose at -2/3. What does -2/3 mean? Does it mean INCREASE the amount of light allowed to enter the camera, or DECREASE the amount of light allowed to enter the camera?

4. Is increasing exposure by a particular f/ number or shutter speed designated with a "+" sign, and decreasing exposure by a particular f/number or shutter speed designated with a "-" sign?

I would really appreciate if someone could have a look at the above and comment if my understanding is correct.

Many thanks,
Dave.

[TopGear1Ds]

Hi,

I'm reading the book "Understanding Exposure" by Bryan Peterson at the moment and I would just like someone to confirm my understanding of a couple of things I have learned from the book:

1. When photographing snow scenes or beach scenes (basically a scene with a large proportion of highly reflective surface), the camera will get confused by the amount of light being reflected and will tend to UNDER expose the scene. Therefore I should manually override the exposure and dial in settings which the camera light meter thinks are over exposed.


correct


2. When photographing the opposite of 1 above (i.e. a dark scene or a scene filled with a dark object), the camera will get confused and tend to OVER expose the scene. Therefore I should manually override the exposure and dial in settings which the camera light meter thinks are under exposed.

also right

3. What Bryan Peterson calls "Mr Green Jeans (The Sky Brothers Cousin)". He says you can meter off green objects if the scene has a lot of green in it (e.g. a waterfall in a forest), and then expose at -2/3. What does -2/3 mean? Does it mean INCREASE the amount of light allowed to enter the camera, or DECREASE the amount of light allowed to enter the camera?

-2/3 means to decrease the light by 2/3 of a stop


4. Is increasing exposure by a particular f/ number or shutter speed designated with a "+" sign, and decreasing exposure by a particular f/number or shutter speed designated with a "-" sign?


Yup

Seems like you've got the right idea with all of these. feel free to ask any other questions!

[tonylong]

The idea is that when the camera meters a scene (evaluative) or a subject (spot or center) it is programmed to interpret the tone(s) as "medium". With evaluative metering or average metering it tries to average a whole scene; with spot or center it sets an "estimated exposure" to render a chosen part of the scene as "medium".

This is OK in some situations -- if, for example, you are shooting a scene that truly has a balance of tones with no stark highlights or shadows that could mislead the meter, or in a scene such as a shadowy area where you want to brighten things up and any peripheral highlights aren't important to retain.

The problem is that we often want to retain tones that the camera would tend to discard. So, if you are shooting a scene with snow, the camera meter will tend to tone down the snow to "medium" and, in doing that, underexpose the scene, including objects that will end up dark.

So, to get around that, you want to either make sure you are metering a subject that you want to be medium (which may or may not keep the snow acceptably detailed) or meter the snow and then adjust the exposure to "tell the camera" that the snow is bright, close to but not quite pure white, let's say pushing the meter "needle" to +1 2/3, which should retain snow detail but at the same time give enough exposure to other objects so at least you can work with them and they retain shadow detail.

You have a similar "mirror image" situation with dark tones -- the camera meter automatically wants to interpret them as "medium" and so would automatically overexpose the scene as a whole. It's up to you to adjust your settings to properly interpret the tone(s) you are metering: if, for example, you are focusing on a subject with dark clothes, and are metering the clothes, the camera would automatically try to render those clothes as gray, which would overexpose the scene as a whole, but if you adjust your settings to shove the exposure/meter needle down to, say, -1 2/3 (for very dark/black clothes) you would retain a bit of detail in the clothes but at the same time get a well-exposed scene.

So, rather than try to get a set of formulas, try to understand what the camera is automatically doing and compensate for that. There are some handy aids, such as taking a blue sky and setting the camera to interpret that as somewhere around +1, or putting your hand in front of the camera and, if your hand is lit by the same light as your scene, having your camera interpret that as somewhere around +1. You have to decide for yourself in a given scene what the light and the subjects need to be in your image.

So, your question about +/- designations add up to: if you want the camera to interpret a subject as "medium" you leave the meter needle in the middle ("automatic"). If you want the camera to interpret it as "dark", you set a "-" setting, decreasing the exposure either in Manual or using Exposure Compensation. If you are metering a bright subject, set you exposure to a "+" setting.

Hope that helps!

[timbop]

4. Is increasing exposure by a particular f/ number or shutter speed designated with a "+" sign, and decreasing exposure by a particular f/number or shutter speed designated with a "-" sign?

What is hard to wrap your brain around is that the F-number is actually a FRACTION. That is, F 2.8 is actually 1/2.8 and F 4 is 1/4 - so increasing the "F-number" from 2.8 to 4 actually decreases the amount of light entering the camera. What the number refers to is the diameter of the opeining inside of the lens. Without boring you with the math, when you go from a diameter of 1/2.8 to 1/4 you actually HALVE the amount of light getting in. Going from f/2 to f/4 cuts the light to a QUARTER. If you want to know about the math, recall that area = Pi times r squared. The net result is that to cut the light in half you decrease the diameter by a factor of the square root of 2, which is approximated as 1.4. That's why f-stops go 1, 1.4, 2, 2.8, 4, 5.6, etc.

The shutter speed in the viewfinder is also a fraction: 60 means 1/60 sec and 125 means 1/125 sec. So, changing the shutter speed displayed in the viewfinder form 60 to 125 also cuts the amount of light in half.

As for terminology, any time you change the exposure by a factor of 2, it is called changing by "one stop" (or "exposure value"). Changing by a factor of 4 is "2 stops".

[Dredd123]

Thanks all, I think I've got it now.

+ means let more light in
- means let less light in
green must be closer towards black than it is to white, therefore the -2/3 exposure for "Mr Green Jeans".

I need to think in these simple terms so that it becomes second nature. Now I just need to put it into practice.

Really appreciate you guys taking the time to help me out.

All the best,
Dave.

[WaltA]

Dave, your not the only one having trouble "interpreting" Bryan Peterson-speak.

On a thread about exposure from a couple of years back, one of the forum guru's, Bob Lay, pointed us at an unofficial help file he had created to help understand better what Bryan was trying to explain.

It helped me and I thought I'd pass it along to you.

http://www.zaffora.com/W9DMK/BryanPetersonUnofficialHelpFile.pdf

[PhotosGuy]

When you get through the link to "Understanding Understanding Exposure", see if this makes sense to you:
Need an exposure crutch? (http://photography-on-the.net/forum/showthread.php?t=89123)

[gabrioladude]

here is a simple way to remember it that someone gave me:

if the scene is light then you add "lightness" to correct (ie you overexpose)

if the scene is dark then you add "darkness" to correct (ie you underexpose)

[Picture North Carolina]

The idea is that when the camera meters a scene (evaluative) or a subject (spot or center) it is programmed to interpret the tone(s) as "medium". With evaluative metering or average metering it tries to average a whole scene; with spot or center it sets an "estimated exposure" to render a chosen part of the scene as "medium".

Will the histogram of the taken shot be accurate or will that also be "skewed?"

[PhotosGuy]

Will the histogram of the taken shot be accurate or will that also be "skewed?" I'm not sure why you asked that. Are you referring to a live histogram vs. the one showing the results after the shot is taken?

[Picture North Carolina]

I'm not sure why you asked that. Are you referring to a live histogram vs. the one showing the results after the shot is taken?

I guess I should have written "after the shot is taken" instead of "the taken shot." Yes, after. (also, brightness histo, not rgb)

[PhotosGuy]

"after the shot is taken" Maybe I'm still not understanding the question, but I think you're about to have a "D'oH!" moment. Of course the histogram will show the effect of the changed exposure after you take the shot? ;)

[TopGear1Ds]

Will the histogram of the taken shot be accurate or will that also be "skewed?"
The histogram will be accurate in that it is showing whats in the image, but what I think you're asking is "will the histogram bias to the right for a bright scene, and to the left for a dark scene?"

The answer is yes.

ie. don't expect a properly exposed dark scene to be in the middle

same with a properly exposed bright scene (like with snow), it will be heavily biased to the right.

[tonylong]

Will the histogram of the taken shot be accurate or will that also be "skewed?"

The histogram will be accurate in that it is showing whats in the image, but what I think you're asking is "will the histogram bias to the right for a bright scene, and to the left for a dark scene?"

The answer is yes.

ie. don't expect a properly exposed dark scene to be in the middle

same with a properly exposed bright scene (like with snow), it will be heavily biased to the right.

Yesterday, I was doing some wildlife shooting in the late afternoon with quickly changing lighting, and I shot much of the time in Av because I was shooting in a lot of shadows and was content for much of the time keeping my meter centered (giving a bit of a boost to the dim light). I used high ISO speeds to keep my shutter speed up.

There were times, though, when I used Exposure Compensation to tell the camera to interpret a subject as light or dark. For instance, I took some shots of a white goose surrounded by a shadowed background/foreground water, etc. I metered the goose, then adjusted my camera settings to interpret the goose as white, maybe +1 2/3 or 1 1/3 EV (I don't remember exactly which one, I think I changed it for more shadow or to get a slight boost in shutter speed).

After the shot, the histogram showed most of the image data to the left, with a bump to the right, which was a correct exposure -- the white goose came up to the right edge but didn't get blown, wereas the shadowed area, which filled most of the frame, showed up as dark.

If I had just stuck to an average scene setting, the camera might have tried to boost the shadows to medium, and blown the white goose, or it may have metered the goose and rendered it as medium, totally clipping the shadows.

The thing is, whether shooting in manual or a semi-auto mode such as Av/Tv/P, you want to be aware of what your camera is "looking at" and be ready to tell the camera how to interpret it!

[TopGear1Ds]

...The thing is, whether shooting in manual or a semi-auto mode such as Av/Tv/P, you want to be aware of what your camera is "looking at" and be ready to tell the camera how to interpret it!
Right! thats what I was trying to get at.. your camera will always try to stick every scene in the middle. You need to be conscious of your scene, and think for the camera, when you know it will misinterpret what it's looking at.

[Picture North Carolina]

Maybe I'm still not understanding the question, but I think you're about to have a "D'oH!" moment. Of course the histogram will show the effect of the changed exposure after you take the shot? ;)

No, my old friend. I wish I could get Homer to entertain you this time, but I was actually setting up for a comment. :D

Yes, what TopGear and Tony said. To make the comment short, I guess it's just that I've never understood why people shoot in modes or use methodologies where they allow the camera to make decisions for them, and as a result, may have to compensate for wrong assumptions or wrong decisions the camera made. I have always shot fully manual, spot-metering zones, placing the histogram exactly where I want it. As Tony said, I always tell the camera how to and what to do. I figure my brain is bigger than a Canon processor (at least I think it is) and as a result, I don't allow it (the camera brain, that is) to make decisions for me.

[TopGear1Ds]

No, my old friend. I wish I could get Homer to entertain you this time, but I was actually setting up for a comment. :D

ooh, I see what you did there. lol

You could've just said that you begin with, you know :):p

[tonylong]

Heh! I guess Canned Heat wanted to keep the heat canned for a bit:)!

[Picture North Carolina]

You could've just said that you begin with, you know :):p

Yea, you're right. I guess it was just that I was having a hard time fully understanding the thread. Technically I understand, but for some reason (and I am not judging anybody's shooting methods), I just cannot get my mind wrapped around the thought of a camera operator putting a camera into some sort of auto or semi-auto mode, then be forced to twist dials and throw switches to compensate for the mistakes the camera makes when in those modes. (Obviously, I don't shoot much sports or events! ;) )

Edit: and mountains generally don't move so fast that I don't have time to set up a shot. :)

[TopGear1Ds]

...I just cannot get my mind wrapped around the thought of a camera operator putting a camera into some sort of auto or semi-auto mode, then be forced to twist dials and throw switches to compensate for the mistakes the camera makes when in those modes. (Obviously, I don't shoot much sports or events! ;) )

Fair enough, and I definitely agree with you. I only use auto exposure when I'm shooting sports or events with dynamically changing light that I don't have time to react to in M. In any slow or consistent situaton, it's M all the way.


Edit: and mountains generally don't move so fast that I don't have time to set up a shot. :)
:lol:

[sgogula]

all you doubts may be clarified if you read this (http://photography-on-the.net/forum/showthread.php?t=335569&highlight=metering) wonderful thread by Robert.

[PhotosGuy]

all you doubts may be clarified if you read this wonderful thread by Robert. Or you could cut out most of the measurebating stuff & try the link I posted half a day ago? ;) I wish I could get Homer to entertain you this time, but I was actually setting up for a comment. No sweat! And I agree with you 100%.

[tonylong]

OK, yesterday I was out shooting in dim late afternoon light. With the 1D Mk III, I shot in a range from ISO 1600 - ISO 3200, as the light permitted.

On this occasion I shot in Av, because I was shooting birds in rapidly changing light. However, I still had to pay attention -- even though most of my scenes were dim and I was happy with Av rendering them "medium", there were still situaltions where I had to tell the camera "this is not medium".

So, I was shooting a white goose, and when metering the goose I knew that I had to tell the camera "this is white" because I knew the camera would want to expose the goose as gray.

So, I set my exposure compensation to say "this goose is white", and the scene exposed properly. But I played around a bit, and this showed both how to get a correct exposure, plus the fact that a lower exposure can bring out more noise!

Technical note: on top of taking these at high ISOs, I was also using the 100-400 lens with a 1.4x TeleConverter, which is not a combo made in heaven, so the sharpness of the shots is not at issue here:)!

First, my estimated exposure, telling the camera "this goose is maybe 1 2/3 white" at ISO 2000, 1/250 sec:
http://www.pbase.com/tonylong/image/108212965/original.jpg

A close crop of detail:
http://www.pbase.com/tonylong/image/108212964/original.jpg

Then, here is a shot at the automatic "medium" exposure, 1/800 sec:
http://www.pbase.com/tonylong/image/108212967/original.jpg

A close crop shows more noise, despite the same exposure, as well as clipped blacks:
http://www.pbase.com/tonylong/image/108212966/original.jpg

Here was a pretty happy medium, where I took an estimate, then dialed down a bit:
http://www.pbase.com/tonylong/image/108212971/original.jpg

With a closeup that shows some noise, but perfectly manageable:
http://www.pbase.com/tonylong/image/108212970/original.jpg

Finally, I dialed this shot up to the max, just at the limit of overexposing the white, about +1 2/3 EV:
http://www.pbase.com/tonylong/image/108212976/original.jpg

[tonylong]

Dialing down the exposure by a stop gets this:
http://www.pbase.com/tonylong/image/108212974/original.jpg

With this very manageable noise:
http://www.pbase.com/tonylong/image/108212975/original.jpg

[Picture North Carolina]

So, I set my exposure compensation to say "this goose is white",


Bummer! Apparently Canon decided to eliminate this camera setting from my 5D2! I looked in the customs function for it, but all I could find was "That cat is ugly." Bummer! :) ;)

Just kidding, couldn't resist. Good informative post. Oh, also, in your post #24 I noticed that well known little "Creating Thumbnails" Pbase graphic (if still there). Guess this means Pbase is having one of their typical monthly (if not weekly) "problem" sessions?!

[tonylong]

Heh! I see the same ugly PBase malfunction, for some reason.

The "This cat is ugly" Custom Function should work:)!

Edit: The malfunctions were mine, late night stuff, fixed I think!