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under exposing?

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Thread started 13 Jul 2007 (Friday) 10:35   
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tjmaroney
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Hi,

I've got a 30D, and was wondering if there's any known "issue" with this model typically under exposing by about 1/3 stop? I used to have a 300D, and it appeared to also under expose, but by about the same (sometimes about 2/3 stop). Any info/experiences would be appreciated. Of course, could be that the subjects i'm shooting are predominantly lighter coloured and fooling the internal meter into under exposing a tad.

thanks

Post #1, Jul 13, 2007 10:35:34




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PacAce
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Have you tried shooting a gray card to see if your 30D is really underexposing? It doesn't have to be a perfect 18% gray card as long as it's one solid color. If the histogram shows the spike just to the left of center, the camera meter is working fine.

Post #2, Jul 13, 2007 10:41:42


...Leo

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Gliderparentntn
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Here you go read this thread it's full of some great info...

http://photography-on-the.net ...ht=camera+metering+​system

Post #3, Jul 13, 2007 11:03:31 as a reply to PacAce's post 21 minutes earlier.


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TMR ­ Design
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Hi tjmaroney,

This sounds like it's a case of the camera's internal metering system being fooled. There are many methods to correct this and many are outlined in the thread James recommended.

The metering mode you're using and the complexity of the scene (dynamic range) play a huge factor in achieving correct exposure.

Post #4, Jul 13, 2007 11:09:58 as a reply to Gliderparentntn's post 6 minutes earlier.


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airfrogusmc
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There are tolerances in the manufacturing of all cameras and lenses. They used to be a half stop. Not sure what they are today but if you think about it there would have to be. So you could get two cameras coming of the same production line with a full stop difference between their meters/shutters.

You should always test new equipment. If you know up front how much one way or the other it is or it could be right on you can compensate if need be.

Post #5, Jul 13, 2007 11:13:54 as a reply to Gliderparentntn's post 10 minutes earlier.




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tjmaroney
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thanks everybody, for the very helpful information. i can always count on POTN!

cheers,

Post #6, Jul 13, 2007 11:15:21 as a reply to airfrogusmc's post 1 minute earlier.




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TMR ­ Design
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airfrogusmc wrote in post #3538245external link
There are tolerances in the manufacturing of all cameras and lenses. They used to be a half stop. Not sure what they are today but if you think about it there would have to be. So you could get two cameras coming of the same production line with a full stop difference between their meters/shutters.

You should always test new equipment. If you know up front how much one way or the other it is or it could be right on you can compensate if need be.

Although I do agree with you, 99% of the time there are exposure issues it is due to the lack of undertanding how the camera's meter actually works, and in most cases the results are consistent, with the same consistent results once we do learn about metering, metering modes and how to see an entire scene rather than just the subject we are shooting.

Post #7, Jul 13, 2007 11:25:51


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mnealtx
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PacAce wrote in post #3538064external link
Have you tried shooting a gray card to see if your 30D is really underexposing? It doesn't have to be a perfect 18% gray card as long as it's one solid color. If the histogram shows the spike just to the left of center, the camera meter is working fine.

PacAce - when I was checking out my 30D, pics of a gray card (actually a gray Whibal card) would show the spike left of center, and the junction between the 2nd and 3rd segments of the histogram.

My understanding was that the spike should be dead center in the histogram - is that incorrect?

Post #8, Jul 13, 2007 13:49:47


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davesrose
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mnealtx wrote in post #3539143external link
My understanding was that the spike should be dead center in the histogram - is that incorrect?

Since PacAce said it didn't have to be a perfect 18% grey card.....then no, the spike wouldn't necessarily be at the center or left of center. It would mainly just have to have one spike that is "close enough to the center".

If you google grey card, you'll actually see many debates on whether 18% grey gives true 50% luminance....or if it's closer to 12%. So because no one can agree if 18% is truely 50% luminance or not.....IMO, as long as it's close to center, it's fine. Actually, when I tried typing in 50% luminance in PS's color picker, I got RGB and CMYK values that were slightly darker then "the recommended" values for 18%. So maybe it is true that 18% would be a little left of center.

But getting back to the OP....I totally agree with TMR. "Correct" exposure is a subjective measurement. To get the best exposure that you like, you have to be aware of how your camera's meter modes work. Sure, some cameras might actually have quirks....but they give you manual control and different metering modes for a reason.

Post #9, Jul 13, 2007 14:23:37


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mnealtx
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Ok, I follow what you're saying...and I remember reading something about a Kodak gray needing a half-stop correction, so what I got for my initial gray card shots may have been correct, as I needed a 2/3 correction to 'center up' the spike.

I agree that "correct" exposure is subjective per the photographer.

Post #10, Jul 13, 2007 15:47:12


Mike
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davesrose
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mnealtx wrote in post #3539850external link
I agree that "correct" exposure is subjective per the photographer.

Yeah, that's why I think debates about what is true grey will only intensify with more color spaces that are introduced :D

The way that I see it is that a camera is just trying to reach an average gray with its meter. Now because every grey is slightly different, it's going to be hard to target a grey card to be precisely reading 50%. But you don't really need that, because good exposure actually has a wide varience IMO. You might have to compensate and "underexpose" for the camera trying to reach 50% when much of the scene is in shadow. Likewise, you might have to "overexpose" if much of the scene is light and the camera would want to expose everything to 50% luminence.

Post #11, Jul 13, 2007 15:58:50


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mnealtx
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Assuming you're using evaluative, yes... I've actually gotten better exposures using Peterson's tips on exposure and a sort of Zone approach for where I want to place the tone that I meter (spot/manual on 30D for most things).

Post #12, Jul 13, 2007 16:23:11


Mike
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davesrose
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mnealtx wrote in post #3540097external link
Assuming you're using evaluative, yes... I've actually gotten better exposures using Peterson's tips on exposure and a sort of Zone approach for where I want to place the tone that I meter (spot/manual on 30D for most things).

Well it's true for any meter....it's just that spot has the least amount of area in the viewfinder that the camera is metering. Each meter mode takes in a different amount of the scene, and then decides on the average 50% grey based on that. Spot metering can be the most precise because it's letting you select which area of the shot should be that middle grey.

Post #13, Jul 13, 2007 16:40:13


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PacAce
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mnealtx wrote in post #3539143external link
PacAce - when I was checking out my 30D, pics of a gray card (actually a gray Whibal card) would show the spike left of center, and the junction between the 2nd and 3rd segments of the histogram.

My understanding was that the spike should be dead center in the histogram - is that incorrect?

The way Canon calibrated the meter, it's not supposed to be dead center. I won't go into the details here except to say that it has something to do with the 12% gray that's used as a reference for light meters vs 18% gray. And it doesn't matter what shade of gray the card is because the meter is always going to assume that it's a mid tone gray. AAMOF, even if you use a white card, the meter will center the histogram just left of center.

Post #14, Jul 13, 2007 17:18:56


...Leo

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mnealtx
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PacAce - I appreciate the info - thanks!

Post #15, Jul 13, 2007 17:27:42


Mike
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