steve49 wrote:
what can be done w a histogram?
what value does it have?
I see it for each pic but have no idea what it is used for or how it can be useful.
It shows the distribution of tonal values for each channel. Taken together, it will average the channels into a single histogram.
Lots of useful information can be gleaned from a histogram, even if you shoot raw, as long as you consider that the histogram typically reflects the values in the jpeg rendition of the image.
It will show black and white clipping (muddy shadows and blown highlights), but it will also show you if any of the three channels are clipped - when you look at the histogram for the individual channel. If the blue channel is clipped (blown and with out detail), the blue histogram will show one or more flat-topped peaks against the top edge of the histogram. You have several choices here, one of which might be to dial back the exposure a bit to try to bring the channel(s) out of clipping.
It will also show if your image is going to be dark or bright low key/high key - indicated by a large distribution of data on the left and right respectively. A histogram with lots of dark and bright areas will have two main "peaks" on either side.
For those shooters that like to get the most data on the image with the least noise, you can gauge by the histogram how much additional exposure you can add to the scene without blowing out the highlights. Low to medium contrast scenes (where there are gaps between the data on both sides of the histogram) can often benefit from additional exposure, often being able to add 1 to 2 stops before highlights are lost, considerably improving the detail and noise in the shadow areas.
Last, and probably not least, the histogram will quickly let you know if the dynamic range of the scene is beyond the camera's ability to capture it. You will see lots of data across the histogram, and both left and right sides will have data pressed against the borders. Here you can either make a creative decision to save the highlights or the shadows at the expense of the other, or do an HDR capture, which is made easier by being able to see how much under and over exposure you need to capture the whole range of contrast.
The function of the histogram in software is similar to the camera, allowing you to make creative corrections and adjustments based on how data is distributed on the histogram.
I hope this starts to answer your questions.
Here are some links with visuals:
http://digital-photography-school.com/how-to-read-and-use-histograms/http://www.cambridgeincolour.com/tutorials/histograms1.htmhttp://www.techradar.com/us/how-to/photography-video-capture/cameras/histogram-photography-cheat-sheets-for-achieving-perfect-exposure-1320846http://www.youtube.com/watch?v=DXK6ena5Fk8