Today’s Question: I have noticed that the histogram I can display on my digital SLR with live view enabled does not always match the histogram for the actual photo I capture. In some cases the live view histogram shows no clipping, and then there is clipping in the captured photo. Why don’t the histograms match?
Tim’s Quick Answer: The exposure preview and histogram available with live view is an estimation of what the final exposure will look like, but it can’t be expected to be completely accurate because the live view histogram preview is not based on the exact photographic exposure you will produce when you capture an image.
More Detail: This question was actually posed during my current photo workshop in the remarkable Palouse region of eastern Washington State. The example I provided to clarify the difference between the preview histogram and the final histogram related to exposure time.
If you consider that you might have your camera configured for a 30-second exposure, I think it is easier to understand that you can’t expect the image preview or histogram on the live view display to be completely accurate. After all, you can see the live view preview and histogram instantly, while the photo will require thirty seconds to actually capture. So the camera in this case would need to apply amplification to the signal being gathered by the sensor to enable the live view display, in order to simulate what the actual exposure will look like.
In some ways this is similar to the fact that the live view display will show you what the scene looks like with a wide-open lens aperture, even if you’ve set the aperture to be stopped down by several stops. You would need to press the depth of field preview button to see the actual depth of field you can expect, and even then the exposure is once again a simulation enabled by amplification of the signal being received by the image sensor for the live view display.
Having said all that, the histogram based on a simulated exposure for the live view display will generally be reasonably accurate. However, especially in situations where the light is very tricky, such as with high contrast for a sunset scene, you can expect some variability between the histogram you see in the live view display and the final result for the actual capture.