Understanding Exposure
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Old 07-23-2007   #1
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Default Understanding Exposure

I wasn't sure exactly where to put this thread, and after considering many of them, I figured this was the best place to post.

I've heard it and read it many times. Exposure control with digital cameras is more crucial than it was with film. I also know that getting it right in the camera saves a ton of time in post processing as well as providing a better image. What I have also learned is that proper exposure does not seem to be an absolute, a steadfast scientific recipe.

Proper exposure seems, at best, an approximation of what would be the best exposure. Many elements can throw off exposure...fool the in camera system.

Therefore, to get the best exposure, the photographer needs to take control over the camera and use manual settings. I do this the majority of the time, and many times I am disappointed with the outcome.

I have read "Understanding Exposure" by Bryan Petersen, and "Exposure and Lighting for Digital Photographers Only" by Michael Meadhra and Charlotte K. Lowrie, but I still have some questions.

For outdoor portraits...it appears to me that spot metering is the best way to meter for an outdoor portrait. You have a family against a lush green background. You meter for their faces and shoot. Simple solution? Seems like it. But what part of the face do you meter? The shadow side, the highlight side?

Or...do you throw in an 18% grey card in front of the subject, meter it, and set your exposure based on the reading from the 18% grey card? This will then allow all of the higlights and shadows in the faces to be exposed correctly, not losing details in either?

Or I have heard some phtographers say they meter the brightest part of the face and the darkest part of the face and average the readings. I don't know what that means. Forgive me for being so thick.

Zone system...Michael Meadhra gave a comprehensive explanation of Ansel Adams zone system, but for the life of me, I just don't get it. Perhaps I need to go to a landscape and just practice what the book says and then it will all make sense.

According to Meadrha, "Select an area of the scene and mentally place it's brightness on the zone scale..." "Using a spot meter, read the exposure for the selected area and note the exposure settings recommended..." "Mentally adjust the metered exposure by the number of steps between the selected zone and middle gray..."

So...for example...I meter a highlight in the clouds...find it to be in Zone zone 8. It meters f/11, 1/500 at ISO 200. Zone 8 is three steps away from Zone V, which should be 18% gray. The setting for camera based on this reading would be to adjust my settings 3 stops lower?

I meter another spot, considered to be in Zone 2. When metering it, and if I have correctly assigned the zone, then it should be three stops away from the midtones. Determining the proper exposure for midtones, I would raise the reading three stops to Zone V. If I have properly placed my highlights and shadows in the correct zones, then both readings, after adjusting them, would be approximately the same.

Am I understanding this correctly?

I know a few folks are probably frustrated with me because I can't seem to wrap my mind completely around this exposure thing. I am just trying to take my understanding to a higher level. In the past, when I was on the run, I shot in aperture priority, metered the higlights on the face, and shot away. When time allowed, I used manual mode, metered the face, and shot away.

I am now wanting to improve the quality of my images by understanding the exposure system to a greater degree. Am I foolish for wanting a greater understanding? I don't know.

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Old 07-23-2007   #2
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Default Re: Understanhding Exposure

Quote:
Zone system...Michael Meadhra gave a comprehensive explanation of Ansel Adams zone system, but for the life of me, I just don't get it.
Have you read Ansel himself? Chapter 4 of The Negative, by Ansel Adams, is titled "The Zone System". I think I'll make it a goal to get to that chapter today.

Quote:
Or I have heard some phtographers say they meter the brightest part of the face and the darkest part of the face and average the readings. I don't know what that means. Forgive me for being so thick.
It's just like averaging numbers. Take the different numbers, add them up, divide by how ever many numbers there were, only we're doing this with meter readings. Also, I find it easier to use the difference between the readings, and move equidistantly from either reading to one in between.

Let's say you've chosen an aperture of f/4 for your desired DOF, and you are going to meter in this method. Choose spot metering, and meter the brightest or darkest part of the face. Let's say you meter the darkest part of the face, and it's 1/250 second at the chosen aperture of f/4. Now, meter the lightest part of the face- let's say it is 1/400 second. The difference is 2/3 stop, so dividing that by two we move 1/3 stop from each reading and choose 1/320 second.

Quote:
I know a few folks are probably frustrated with me because I can't seem to wrap my mind completely around this exposure thing. I am just trying to take my understanding to a higher level.
Nope, they aren't, or at least shouldn't, be frustrated with you. You should be commended for your incredible dedication. =)

Quote:
Am I foolish for wanting a greater understanding? I don't know.
Of course not! I've dedicated part of this summer to redoing all the fundamentals, so I've picked up the same Petersen book, plus the first two of the three-part series by Ansel Adams. I already finished The Camera (minus some of the view camera adjustment stuff) and am about to dive deeper into The Negative; The Print was out of stock but I'll be reading that eventually.
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Old 07-23-2007   #3
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Default Re: Understanhding Exposure

Thanks Michael. I appreciate your explanation. I was feeling that averaging stops was the same as averaging numbers, I just couldn't find anyone to finish the sentence! You have finished the sentence for at least three authors who I have read. Blessings to you!

It also makes me feel good to know you too are reviewing the fundamentals.
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Old 07-23-2007   #4
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Default Re: Understanhding Exposure

Glad I could provide some help. The explanation made less sense than I had hoped it would when I reread it, LOL

By the way, going back to the original post...

Quote:
Many elements can throw off exposure...fool the in camera system.
...this is doubly true with the advent of all the different kinds of complex metering systems being used today.

Center-weighted, and spot still assume "18% grey" (won't get into the whole "the in-camera meter value is really closer to 12%" thing), regardless of what they're pointed at; this is why you can center-weight off of your hand under the light, open up one stop, and be reasonably close to the "correct" exposure, if not dead on it. These are relatively easy to fool, but they are fooled consistently and simply, which means it is easy to understand and correct the exposure based on readings taken with these systems.

Matrix metering (dubbed "Evaluative" by Canon, and also called "multi-pattern"), however, tends to perform other things outside of the usual "assume it's '18% grey'" routine used by the other meters. I half-jokingly refer to the things done as "voodoo". Basically, the meter uses a ton of sensing points (current Nikon cameras use 1005-pixel and 420-pixel meters) to survey the scene and take an average reading- sort of like an average meter that reads the entire frame. This sounds simple enough, right? Except, it doesn't stop there; this where the voodoo comes in. The meter compares the scene to other scenes- stored as exposure data programmed into the meter circuits- and tries to guess "what" you are shooting before making an adjustment; if the camera thinks you're shooting a snow scene, it'll adjust the exposure differently than if it thinks you're shooting a person in front of a bush, etc.. More recently, the some cameras also take color into account, and this means another adjustment based on whether the camera sees red, green, and so on. The reason I call this voodoo is because you really only have a general idea of what the camera is doing to arrive at the final exposure value displayed, and while it is very often "correct" the times it is wrong you don't always know why/how the camera got to where it did.

I really did not quite grasp exposure metering and how to compensate effectively until I stopped using matrix metering and switched to center-weighted and spot. I want to play with matrix metering some more, but each model Nikon I have behaves slightly differently from the others, which means one basically has to profile his or her camera's matrix meter in relation to the camera's tone curve.
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Old 07-23-2007   #5
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Default Re: Understanhding Exposure

Mmmmm...Kelly, I don't recommend the classic zone system for digital camera users. The visualisation of the placement of zones depended completely on how the film was developed, and what film was chosen in the first place.
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Old 07-23-2007   #6
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Default Re: Understanhding Exposure

Michael, I have heard this process of evaluating the scene and the camera judging the scene based on a data bank before. I explained that to a friend and felt like I was making something up.

The shocking thing to me is that true, accurate exposure is a subjective, mysterious thing. Almost like a quantum particle. Hard to put your finger on.
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Old 07-23-2007   #7
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Default Re: Understanhding Exposure

Bob...one of the reasons I was thinking about the zone system was because when I first came to the Camel and would bump around the Landscape area, many people would comment on how Cedric was able to properly expose so many different elements of his pictures at the same time. This made me think that I needed to know more about the zone system. Because of those early reads, I have always felt that I am missing something in my understanding of proper exposure.
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Old 07-23-2007   #8
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Default Re: Understanhding Exposure

Bob...so in metering a landscape...what is your method?
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Old 07-23-2007   #9
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Default Re: Understanhding Exposure

OOOPPPPPSSSS More, (I pushed the wrong button)

Digital cameras have different exposure response issues from film...and they are many depending upon the kind of sensor.....and how the manufacturer has "tweaked" it for his particular camera.
Fortunately, none of this is really a big issue. Forget the classic zone system. I taught Adam's Zone System for College of New Caledonia, and also Island Mountain School of Art...I only mention that to establish a credential in the subject....if you don't name names lots of people will doubt your statements. OK. Generally your cameras exposure system will work adequately on most subjects automatically.......the main thing is to be on the lookout for over-exposure, as it is almost irretrievable.....and also ensure that you aren't asking the camera to capture a dynamic range which is simply beyond its ability.
My advice is to take a preliminary exposure.....LOOK at it....THINK about it....and adjust up or down as needed. That's going to take care of 90% of everything you do.
The other ten percent is usually handled by you using additional lighting, or HDR.
Now wer'e up to about 98% of everything youll ever shoot.
And that last 2% is going to be interesting for you.
But....honestly......the classic Zone System is NOT the tool in digital exposure.....Bob
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Old 07-23-2007   #10
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Default Re: Understanhding Exposure

Great advice by Bob. Usually, with landscapes, if you have time to shoot it once, you can shoot it twice. Hence, with digital, you can go ahead and fire off a test shot like a polaroid in most cases, and spend some time chimping.

I usually center-weight (using a 6mm circle set using custom functions, not the default 8mm in my cameras) the different areas, average them, and shoot. Usually I weigh a little extra towards the sky to keep some blue (if it's completely blown it's white... not especially desirable). After the first shot, I chimp, adjust, and shoot again until I have it.


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