Histogram Question

thebigsky

Sorry if the answer is obvious, but if the read out of a histogram in no way touches the right hand edge of the graph does this mean that no part of the picture is over exposed and if so is it therefore good practice to increase exposure until it does?

pathfinder

Most cameras do not display the histogram in each RGB channel, so yes, it is possible to blow out one channel and still have the standard histogram report that there are no blown highlights on the camera's LCD. Photoshop WILL display the three RGB channels in its histogram display and you can see the blown pixels when setting thresholds for white and black points.

The Pro level cameras - 1 series in Canon, at least, do offer the display of each of the channels in RGB in the histogram. I think the better Nikons do also, not sure as I shoot white lenses.

You can see this frequently when shooting highly saturated pinks, reds, and purples in flowers. It usually is not too big of a problem, but it is worthwhile to remember if you are shooting a highly saturated color, like a purple iris. Just dial back the exposure a little more if needed. Shooting in RAW helps avoid this since you cab dial back the exposure in RAW conversion.

You are correct that you do want to tend to keep the histogram to the right, WHILE avoiding blowing the the histogram all the way to the right. Most of the data in an image is in the grey levels farthest to the right on the histogram. If you do not get your histogram to the right, you may be underexposing your image and increasing the color noise in the image as a result. That is why the phrase "Expose to the right!"

thebigsky

Thanks Pathfinder, excellent info as always, I think my 5D does display each RGB channel in the histogram. I shall endeavour to expose to the right in future.

gtc

also
beware of spikes hitting the top...

Richard

gtc wrote:

beware of spikes hitting the top...

What do these spikes really mean? I never could figure that out.

HeldDown

That would mean an excess of "volume" or data in that wavelength. Don't know exactly how to apply it though.

Richard

HeldDown wrote:

Don't know exactly how to apply it though.

Right, that is really what I don't get. I always assumed that it was really just a problem in the algorithm that scales the histogram, not a sign that there was anything wrong with the exposure--as long as the spikes were not at the extremes of the X axis. scratch

colourbox

I don't think the spikes are that much of a concern. I think the real problem is when tones are bunched up against the ends, suggesting clipped shadows or highlights. But mostly I go by the clipping display. On my XT and P&S, the image flashes in places where the histogram says highlights are clipping.

Even then, I've learned to not be too alarmed if small or nonessential areas are clipping. We don't know how a camera histogram is tuned. It could be tuned sRGB if it assumes you are shooting sRGB JPEG, and therefore a display indicating clipping may not necessarily be clipping in the camera's much larger raw color space. I need to double-check but I'm pretty sure there are times when the camera tells me it's clipping, but later in Adobe Camera Raw it shows there is still room to move in that file. (Another reason I don't shoot JPEG which would clamp the extremes too soon.) You can also see how the clipping level changes when you change the color space in Camera Raw.

In the end you probably have to run some tests and figure out how much headroom you actually do or don't have when your camera histogram says it's clipping. This seems much like the film+development exposure range tests Ansel Adams recommended running in Appendix 1 of his book The Negative, to determine the actual ISO of your film versus what it says on the box. I don't know if I'll ever run tests as precise as his, but someday I will probably try and figure out what my camera assumes is the highlight clipping limit vs the actual upper limit of the raw file.

The point is that you don't really know what the histogram means in terms of clipping until you've compared what you got in your JPEG or raw file versus what the histogram said you were going to get.