Exposure math?

epetti

Anyone know what the math is for the exposure controls found in most photo finishing software -- everything from iPhoto to Photoshop has a little exposure dial that mimics changing the exposure of the image, but what is it actually doing? Is it some combination of a brightness and a gamma? If anyone knows or could point me to somewhere that would say I would appreciate it.

Ernie

William M Porter

epetti wrote:

Anyone know what the math is for the exposure controls found in most photo finishing software -- everything from iPhoto to Photoshop has a little exposure dial that mimics changing the exposure of the image, but what is it actually doing? Is it some combination of a brightness and a gamma? If anyone knows or could point me to somewhere that would say I would appreciate it.

Ernie,

I've wondered about this myself but for me it's idle curiosity and I have not had time to pursue it yet. You might have better luck asking over in Adobe's forums, as there are a lot of serious digital-photo-editing propeller heads over there. The wikipedia.org article on "Exposure (photography)" begins by noting that exposure is calculated from the exposure value and the scene luminance. There are links on each of these terms that you can click on for more information. I find wikipedia.org to be a fairly good technical resource for this kind of thing: it's precise but generally pretty accessible.

NOTE: If someone else actually knows the answer to this question - and someone here at SmugMug surely does! - I'd love to hear the answer myself, so please don't be shy.

Will

Tanuki

William M Porter wrote:

Ernie,
You might have better luck asking over in Adobe's forums, as there are a lot of serious digital-photo-editing propeller heads over there.
Will

I've actually been looking for that information in the Adobe Lightroom forums recently, and the best description I found is that Exposure is equivalent to setting the white point in Levels, Blacks is equivalent to setting the black point in Levels, and Brightness is akin to setting the gamma in Levels. However my own experience using these controls while watching the histogram change is that there is a bit more to it than that. Brightness does indeed change the white point to some degree. Too bad that Adobe doesn't just come out and provide the technical explanations for their controls so that we don't have to search Adobe forums to get questionable advice from people that don't really know the answer. Sigh.

Mike

epetti

My research has come up with a little bit but not a lot. In iPhoto the Exposure knob essentially just adjusts the gamma in monitor space, which is also the same as adjusting the grey point in other photo programs. However, if you're working with a RAW file it actually takes the raw data that the sensor received and doing the equivalent of increasing the ISO by actually changing the sensitivity of the calculations applied to the RAW data, the same way the camera would have done for an ISO change.

In Photoshop the Exposure section has three knobs -- Exposure, Offset, and Gamma. These knobs are intended primarily to work with HDR images. All three of these adjustments work in linear space rather than monitor space. The offset is clearly the equivalent of brightness -- just shifting the whole scale. Gamma would then be a gamma change in linear space (which is a very different effect from a gamma in monitor space), but I don't know what exposure is in that case.

Playing around a little bit, doing a gamma on the images in monitor space does appear to be the equivalent of iPhoto's exposure, so that seems to be accurate. But I don't know if that's the same as what Lightroom, Aperture, or others do.

Ernie