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Thread: Under/Over Exposure Canon 7D

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    Macro and Flora Moderator Jonathan Ashton's Avatar
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    Default Under/Over Exposure Canon 7D

    I have been taking quite a few shots of BIF in the back garden and I have tended to go for high ISO to obtain a fast shutter speed. I think I am right in the assumption that a correctly exposed image set at ISO 800 will give less noise than a similar shot taken at ISO 400 at -1.0EV. I wonder if anyone actually knows this to be true or if this is a somewhat subjective issue and the noise generated will be more apparent in some images than others. I am thinking for example noise becomes more apparent on darker plumage and where there is subject movement or where the area is out of focus.
    I have just been looking at an image taken by someone of a lemur in a tree under low light and the photographer used ISO 400 at -1.0 and I am impressed by the low level of noise and I am wondering if I should reconsider my assumptive self imposed guidelines? It may be that under exposure by up to say -1.0 EV does work but -2.0 for example would be way out.
    I will be interested to read any comments or views on this matter.

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    Super Moderator Daniel Cadieux's Avatar
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    That depends Jonathan. An image can be underexposed and noisy at +2.0, and another properly exposed (or exposed to the right) and noiseless at -1.0, regardless of ISO (excpet for the extreme high ISOs...which 800 is not). Any image that is grossly underexposed in the darks look very grainy on the 7D, with blacks almost unrecoverable without lots of careful attention. Images should be looked at individually, rather than lumped together by set values. Use whatever settings will get you the proper results. Having said that, I am very satisfied with the IQ on this camera...

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    Roger will know the answer Jonathan! In the meantime, the particular example you provided of two images one at 0ev 800ISO and one at -1ev at 400ISO should give the same amount of noise. I think this is because for these two scenarios you are letting the same amount of light impinge upon the sensor. Anyway, it's relatively easy to test this empirically.

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    Macro and Flora Moderator Jonathan Ashton's Avatar
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    Thanks Dan and John - I have already PM'd Roger to ask him if he would mind having a look see when he has a mo. It is an interesting point to ponder and I can see the logic in your replies. Dan the question is all about ISO related noise, I appreciate a high ISO image, even if over exposed my be noisy but my point is related to using the same scene, one image being underexposed by 1 stop at 400 ISO and one correctly exposed at 800 ISO. So the effective ISO is 800 in either instance. Now having said that I appreciate John's point relating to the same amount of light impinging upon the sensor, but if that is the case why do we need ISO 800, 1600 etc when all would need to do is dial in an under exposure? I think the 100, 200 400 800 etc are there for a good reason, but what do I know?

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    Robert Amoruso
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    As an owner of the Canon 7D, this is my impression.

    Assuming a white bird against a white BG: ISO 800 correctly exposed (which would be about a +2 EC) would be better than ISO 400 exposed at the metered value (which would be -2 stops less then required to be properly exposed. I am talking about modifications to the exposure when in an automated exposure mode. You would want to place white values high in the histogram where they belong. Brighter values = more digital date = less noise.

    The key here is getting the correct exposure for the key tones of the image. In the example about my exposure is for white on white and at ISO 800 I expect some luminance noise but no color noise as there are no dark areas

    If I was photographing a predominately dark subject against a blue sky, I would push mid-tones to the right on the histogram and correct the dynamic range in post-processing. If this was a BIF, and I want it sharp I will adjust ISO and shutter speed to make that possible. In this circumstance I would expect some luminance noise in the sky and perhaps some color noise in my dark (black) subject but both would be correctable. If the subject was blurred due to inadequate shutter speed, it is the delete key and I have gained nothing.

    I feel you need to approach this by deciding upon what you want to accomplish, and making the adjustments to the camera settings to make that possible. If I need a high ISO to get the shutter speed you need, so be it. Better to deal with noise in PP then get a blurry image that is a sure delete.

    With the 7D I find ISO 400 very usable with the need to correct luminance noise in blue skies. ISO 800 still good with more correction needed for luminance noise. If I have no large dark areas, color noise is not an issue at ISO 800.

    However, as soon as light intensity allows it, I reduce ISO. My favorite to use with the camera is ISO 200.

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    Jonathan,

    Your example is simple. Both exposure times are the same, so John is correct, the noise in the mid and highlights would be the same because the noise is dominated by photon statistics. However, in the deep shadows, the noise from the electronics will come into play and there the ISO 400 images will look worse. Also, there is more of a chance that fixed pattern noise (which is pretty minimal on the 7D) might also bee seen more in the ISO 400 images. One reason to expose at ISO 400 and -1 stop is you want to maintain the greater dynamic range and headroom, and are not concerned with noise in the shadows. If that is not the case, then I would say ISO 800 would be better. So each has an advantage.

    Roger

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    Macro and Flora Moderator Jonathan Ashton's Avatar
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    Thank you very much indeed gentleman I appreciate your taking time to reply. I have a better understanding over what is going on now and I can now make a more informed decision over which option to chose. In short if my background is not too dark and the bird has mainly mid tones and light tones and few dark tones then ISO 400 -1.0 would be a better option. If I sense there will be significant/important areas that are dark then ISO 800 would be a better option.
    Baring this in mind I wonder just how smart the auto ISO option is when using Tv mode or perhaps even manual mode. I suppose the camera would be governed by purely by the overall exposure, it would not of course be smart enough to know which was of most importance to me i.e. the dark or light.

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    I'm not sure it's related to the issues discussed in this topic, in outdoor condition with good equal lighting between the background and the subject, I find my 7D correctly exposes at about +2/3 to +1.0 EV. On the other hand, my Rebel XSi correctly exposes at about -1/3 to -2/3 for the same kind of lighting condition. I'm beginning think that not all the sensors and electronics of the camera behave the same way, and one must find the sweet spot before shooting for real. These days I shoot manual exposure to fit the proper exposure of each camera and may use Av or Tv mode with the appropriate exposure compensation when needed. I would love to hear others' view as well.

    Raymond

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    I think you are right Raymond. Each camera could have a different exposure sensor and each may calculate exposure in a different way. The 1DIV allows you to micro-adjust exposure. I would be interested to know what you mean by "expose correctly". Some take this to mean that the image looks great on the rear LCD with a good looking histogram more or less in the middle. Devotees of "exposing to the right" would consider this type of image under-exposed. As cameras do not expose to the right out of the box, some form of exposure compensation will always be needed, and usually +EV.

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    Quote Originally Posted by John Chardine View Post
    I think you are right Raymond. Each camera could have a different exposure sensor and each may calculate exposure in a different way. The 1DIV allows you to micro-adjust exposure. I would be interested to know what you mean by "expose correctly". Some take this to mean that the image looks great on the rear LCD with a good looking histogram more or less in the middle. Devotees of "exposing to the right" would consider this type of image under-exposed. As cameras do not expose to the right out of the box, some form of exposure compensation will always be needed, and usually +EV.
    Thanks! My way of thinking of exposing correctly is the brightness and exposure matches to what I see with my eyes. It's interesting that the 1DIV has the exposure micro-adjustment, and now I'm wondering if 7D has that too even though I doubt it. I was actually thinking of the exposure microadjustment many months ago.

    Cheers,
    Raymond

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    Macro and Flora Moderator Jonathan Ashton's Avatar
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    Raymond I use Av most of the time and try to get a quick histogram check, usually I use evaluative metering and I intuitively over or underexpose and check the histogram when I can. If I am doing BIF shots abs the background is likely to change between sky and trees etc then I would use manual mode, usually taking a meter reading off something like green grass. got to say most of the time the meter reading does not appear to be far out.

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