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Thread started 16 Jun 2007 (Saturday) 16:09
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Understanding your camera’s built-in metering system

 
TMR ­ Design
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Jun 16, 2007 16:09 |  #1

I see a lot of posts and questions from people that are getting poor exposures. In many cases the user tries to point the finger at the camera or lens somehow being defective. Most of the time this is not the case and it’s just an issue of understanding how your camera is metering and the differences between the metering modes.

Some users will settle into one metering mode and learn to work with it, which is fine. Some don’t even realize that different metering modes will yield different results and they just rely on evaluative metering to do the job. The reality is that all too often the camera does not make the correct decisions about exposure because of the metering mode used and lack of understanding how it’s working.

The meter is easily fooled by high contrast areas and an uneven proportion of highlight to shadow. When there is an even distribution of highlight and shadow the meter doesn’t get fooled and it’s able to correctly calculate the correct exposure (or close to it).

You can manually fool the meter by metering off a particular area of the scene, using exposure lock and then recomposing and shooting with that exposure.

You can also use exposure compensation to override the meter and adjust exposure for a particular area or to prevent a blown out sky or shadows that are in black. That can take some practice and experience to know how far to adjust and in what direction, plus or minus.

As a means of just showing a very basic example of how this works I’ve attached 3 images that were taken using evaluative metering. They were shot in Aperture Priority (Av) mode at f/8. I intentionally used a scene that had a mixture of highlight and shadow with significant contrast.

The first image was shot as you would normally take a photograph. I’ve composed the shot as I want, metered and took the photo. In the second shot I pointed the camera up so the entire center of the frame was on the sky, metered and locked exposure, recomposed and shot. Finally, the last shot was taken by pointing the camera down to the wooden deck, metering and locking exposure, recomposing and shooting. No exposure compensation was applied to any of the images and you can see how the metering and exposure are affected.

The camera determined that for the first exposure 1/100s was the appropriate shutter speed and for the most part did a good job, giving me good exposure on the sky, the trees in the middle and the wooden deck. In the second image that was exposed for the sky, I got a beautiful rich blue sky but the trees and wooden deck are slightly on the dark side. The camera determined that 1/200s (1 full stop less than standard exposure) was correct for the sky to be properly exposed. There is nothing wrong with that exposure and some might even like it more because of the saturated colors, and even though the trees and deck are dark there is no real loss of detail and it looks good. The third image, on the other hand, although exposed properly for the wooden deck, is now overexposing the sky and trees to the point where it is no longer pleasing or correct. For that shot, it determined that 1/60s (2/3 stop more than standard exposure) was correct to properly expose the wooden deck.

This is where the concept of creative exposure meets correct exposure. If you wanted to take control of the exposure and get the best of what the first and second exposure offer then you could dial in some negative EC, probably -1/3 or -2/3 stop, but not going down as far as the second image, which is 1 full stop down. Knowing this helps to figure out when to use exposure compensation and gives you a feel for how much just by being observant of the camera's 'suggested' exposures when it sees different amounts of shadow, highlight and contrast.


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Jun 16, 2007 17:01 |  #2

Here is another example of how the camera's metering system is evaluating the scene and exposing for the average of the scene. In this case the trees and dark brown dirt area are properly exposed but as a result the sky is once again blown out. This is very common.

This time I adjusted using exposure compensation and dialed in -2/3 stop to bring the sky back down to where it should be and to get some of the blue back. Now there's some contrast between the sky and the white clouds on the right side of the sky. The trees, grass and dirt are a bit more saturated and none of the shadow areas are lost or black. Much better exposure but it required that I override the camera's exposure to give me not only the desired exposure but it's also much closer to what I saw that day, whereas the first image is not at all what I saw.


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RichNY
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Jun 16, 2007 17:34 |  #3

Robert- You've done a good job of establishing that one can not trust the camera to properly meter an image. In your second set of photos you state that the 2'nd picture is an accurate image of what you saw, and the first image is how the camera metered it.

I think the most important lesson of this post has yet to be made. When looking at image number 2 (What you saw) what was it that you saw that made it obvious to use a -2/3 EC?

In essence, how does one visually judge whether there is -1/3EC, -2/3EC, etc. more darks in the image than lights? It's much easier to judge how much +EC to use with snow covered scenes.


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Jun 16, 2007 17:57 as a reply to  @ RichNY's post |  #4

Hi Rich,

In all honestly I have learned how much EC to apply though experience. I can almost always count on the fact that a scene with some or little sky is going to be blown out when the rest of the scene is darker and I find that a blown out sky usually needs -2/3 to 1 stop to compensate. The amount I apply will depend on what else is in the scene. If I have very dark areas then I have to use caution when applying negative EC because those dark areas might go to black or lose detail. Sometimes there is a careful balance and experience is what lets me intuitively adjust. I don't think there is any generic way to tell someone how much EC to apply because there are too many variables that contribute to overall exposure.


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Jun 16, 2007 18:25 |  #5

Good examples and explanations. They will help those that are inclined to search and read before posting their question, and those of us that point people to threads that explain solutions to their problems. But for the rest.......:)

Mark


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Jun 16, 2007 18:58 |  #6

Here are two shots of one of my favorite rides. It was morning and the shadows were pretty deep. I underexposed the first by one stop and got an exposure very much like what I saw. The second, also underexposed by one stop, should have been +1 stop instead.

Poor meter, it doesn't know the difference. :rolleyes:

-js


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Jun 16, 2007 19:36 |  #7

Brian Petersen's book - "Understanding Exposure" is a great book to help learn this subject. I have enjoyed it immensely!


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Jun 16, 2007 19:40 |  #8

number six wrote in post #3388985 (external link)
The second, also underexposed by one stop, should have been +1 stop instead.

That's questionable. Granted, the entire foreground is underexposed but look at the top of the scene. If that is -1 EC and the trees are that bright with a blown out sky, can you imagine what it would look like if you were at +1 EC. That sky would be 2 full stops brighter and probably pure white at that point. The trees in the background would also be so blown out. This is a good candidate for either using a Grad ND filter or shooting 2 images, one correctly exposed for the foregraound and one for the background and then layering them in Photoshop to combine the exposures.

In reality there might not be a happy compromise to get a correct exposure for that entire shot. I would be more inclined to revert to the standard exposure or -1/3 EC on that one to bring back some of the detail in the foreground without completely destroying the background.


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Jun 16, 2007 19:54 |  #9

lcfr-nl437 wrote in post #3389096 (external link)
Brian Petersen's book - "Understanding Exposure" is a great book to help learn this subject. I have enjoyed it immensely!

Hi Harry,

Understanding Exposure is a great book and Bryan Peterson certainly does a great job of explaining exposure. Unfortunately he never ties it in with metering modes and never shows anything other than perfect exposures. This is the kind of subject that users benefit from seeing incorrect exposures and understanding why and what happened to make it an incorrect or bad exposure as well as seeing the correct exposure.


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Jun 16, 2007 20:06 as a reply to  @ TMR Design's post |  #10

Thanks, nice examples and great write up.


Is it fair to say that a circular polarizer would "handle" these adjustments "for you"?


(I'm all for knowing the basics, I'm just sayin' ;) )



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Keith ­ R
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Jun 16, 2007 20:12 |  #11

TMR Design wrote in post #3389108 (external link)
That's questionable. Granted, the entire foreground is underexposed but look at the top of the scene. If that is -1 EC and the trees are that bright with a blown out sky, can you imagine what it would look like if you were at +1 EC. That sky would be 2 full stops brighter and probably pure white at that point. The trees in the background would also be so blown out.

Good call - that scene is a metering nightmare with no obvious in-camera solution.

That said, I wonder what spot (or partial) metering would have done?

Metering on (say) the distant trees to the right of the telegraph pole - they look fairly "mid toned" - could get a reasonable tonal compromise.




  
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Jun 16, 2007 20:18 as a reply to  @ Keith R's post |  #12

Hi js,

Just to show you what your second image would look like with +1 EC, here it is.


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Jun 16, 2007 20:19 |  #13

TMR Design wrote in post #3389108 (external link)
That's questionable. Granted, the entire foreground is underexposed but look at the top of the scene.
(snip)
In reality there might not be a happy compromise to get a correct exposure for that entire shot. I would be more inclined to revert to the standard exposure or -1/3 EC on that one to bring back some of the detail in the foreground without completely destroying the background.

More than one way to skin a cat. A little PP improves things substantially.

(Please ignore the jpeg degradation - I modified and resaved the file which was already compressed about max for good quality on the web.)

In many cases like this I don't have time to compose carefully and examine histograms, since I'm sitting on the bike with my helmet on. :lol:

-js


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Jun 16, 2007 20:22 as a reply to  @ TMR Design's post |  #14

Hello Keith,

Yeah, that's a tough one. Unfortunately it wouldn't matter which metering mode he was using. You might nail correct exposure for part of the scene but that can't change the fact that it requires a very wide dynamic range that just can't be reproduced accurately.


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Jun 16, 2007 20:34 |  #15

danpass wrote in post #3389204 (external link)
Thanks, nice examples and great write up.

Is it fair to say that a circular polarizer would "handle" these adjustments "for you"?

Hello Dan,

Without getting into the specifics of what Circular Polarizing filters are used for and how they work the answer to your question is NO. A CPL filter will cause the whole shot to lose about 1 full stop, so in a way it's not doing any more for you than dialing in -1 EC. Yes, it will help with those saturated colors but it can't bring blue back into a blown out sky and it certainly won't help getting detail back into an area that has gone into black.


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