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Exposure Compensation vs Resetting ISO - Any Differences?

Peter Werner , Mar 28, 2005; 03:41 p.m.

I was wondering if there's any effective difference between changing recommended exposure time by resetting the ISO vs dialing it in on exposure compensation.

I routinely expose slide film at 1/2 stop below what's recommended according to my film's ISO. (This is a pretty standard practice with color transparency film and my tests confirm that this gives me the most accurate gray exposure and color saturation.) If I did this by resetting my ISO from 100 to 80, would there be any difference between that and keeping my ISO at 100 and dialing in 1/2 stop lower exposure compensation? (The latter gives me an effective film speed of 75 rather than 80, but that would change recommended exposure in very few situations I would think.)

(Note that I'm not talking about pulling film here - development time is not altered.)



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Christopher Bibbs , Mar 28, 2005; 03:45 p.m.

The only difference I have with it is when you turn the dial to idiot mode and hand it to a friend to take your picture.

Jim Larson , Mar 28, 2005; 04:02 p.m.

There is a HUGE difference between using exposure compensation and adjusting ISO on a dSLR.

But are you talking about film? In that case, I think there would be no practical difference, since when you tell the camera you are off 1/2 stop on ISO it will change the exposure calcs by 1/2 stop to compensate.

Steve Dunn , Mar 28, 2005; 04:04 p.m.

With film, setting an EI of 80 with 100 film is a 1/3-stop overexposure. It's the same as dialing in +1/3 exposure compensation.

Daniel Taylor , Mar 28, 2005; 04:09 p.m.

> There is a HUGE difference between using exposure compensation and adjusting ISO on a dSLR.

one-sixth f-stop difference is huge?

Jim Simon , Mar 28, 2005; 04:28 p.m.

Adjusting the ISO is exactly what most do in the situation you describe. I think that it has an advantage so that you are free to change the exposure compensation and still be starting at your minus 1/3 or 1/2 exposure. In theory less chance to go wrong in the heat of all the other things you have to do to get a proper exposure.

Derek Hammond , Mar 28, 2005; 05:11 p.m.

Yes there is a marginal difference that won't amount to much even using reversal film. If you are using a DSLR however changing the ISO will change the amount of signal amplification used on the sensor and this will affect the amount of digital noise in the image, while changing exposure compensation will change the shutter speed and/or aperture used.

Mark U , Mar 28, 2005; 05:12 p.m.

As has been pointed out, the ISO scale is marked in 1/3 stop intervals. An advantage of changing ISO is that it applies equally to flash use, whereas otherwise you have to set FEC as well as ambient EC when using flash. As has also been pointed out, with 1/2 stop camera settings for aperture/shutter speed and 1/3 stop ISO settings, this effectively gives you control over exposure to the nearest 1/6th stop when shooting in Manual mode. However, when shooting in Av or Tv, the camera will set exposure at even finer intervals for the other variable. See the articles on "are shutter speeds/apertures stepless on EOS cameras" at eosdoc.com (I can't get to the site at the moment).

Mark U , Mar 29, 2005; 04:58 a.m.

Of course, the maximum "error" in exposure is just half the difference between adjacent bracketed settings (assuming the "correct" exposure lies somewhere in the bracket).

Mike Smith , Mar 29, 2005; 09:51 a.m.

Digital exposure and slide film saturation cannot be compared like for like.

Depends upon the subject image in question, if there are large areas of dark shadow then the recommendation is usually to slightly overexpose the histogram to the right up to the point highlights are not clipped to put more detail into the shadows and then post process by cooling down the image.

Trying to lighten or boost a dark underexposed image runs the risk of increasing digital noise in the dark shadow areas. So routinely underexposing images to increase saturation as per your existing film based technique will not work as well as you think. Canon sensors appear to be more sensitive to dark shadow noise compared to other DSLRs and so are set up to be more "overexposed" lenient ie - expose to the right of the histogram, but this is the increased risk of blown highlights

So if anything you should be routinely overexposing predominantly dark images by half a stop and post processing back, that way you will get better dark area definition.


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