Thursday, 8 January 2009

Interpreting The Histogram

Lots has been written about the Photographic Histogram. Many people still don’t understand what it is and what it does. Here are my notes:

What the histogram is

  1. The histogram is a coarsely scaled and over small 2D representation of the tones. Some cameras have separate RGB histograms.
  2. The histogram has two edges, the left and the right edge of the bounding box
  3. If it looks like a peak has a slope extending past a box edge it means that there is some technical data that has been lost / will be lost regardless of RAW.
  4. RAW has a higher latitude because the contrast curve is not "baked in" and the camera histogram may not show the RAW histogram, it may show the histogram of the embedded JPEG in the RAW. So, you infer that you have half a stop or more of latitude even though the in-camera histogram is showing that there is a truncation of the slope or tail of the peak.
  5. With cameras with LiveView, the histogram is a predictive forecasting tool - it tells you before you press the trigger, what could be captured in tones. Shooting RAW or shooting a bracket of 3 or 5 shots is *NOT* a predictive tool - you have made the shots, you can retire without seeing the shots or you may take the time looking at the shots in camera - either way, you have lost oppurtunity or time. This lost time may be ok for a landscape shot but not useful for a quick assessment of a marching parade or anything in motion.
  6. Some cameras don't have histograms
  7. Some cameras don't have a good EVF or an LCD (e.g. my Kodak P880). This causes me to underestimate the visual quality of the image.
  8. Some cameras have an over beautiful EVF or LCD (e.g. the newer Canon G models and reportedly the Panasonic LX-3, the Nikon D90 DSLRs set....). This can cause people to overestimate the visual quality of the image.
  9. Some cameras don't have auto bracketing e.g. the Nikon D60.

What a histogram IS NOT

  1. The histogram does not measure the visual quality of the shot. It is after all a crude 2D graph of tones.
  2. The histogram does not tell you which subject or part of the subject constitutes the tail or slope of the peaks that is being cut off.
  3. There is no "correct" histogram - the idea of "correct" does not apply because of points a. and b.
  4. The histogram does not tell you how to compose the shot.
  5. The histogram does not tell you what is wrong with the shot. It does not advise you that you are shooting into the sun, it does not tell you that you could enhance detail and micro contrast by changing your position and your lighting angle. It does not tell you that your metering pattern is inappropriate. It does not tell you that you are metering for a 12% grey target when you are facing white snow.
  6. The histogram does not tell you clearly how much to compensate in EV - i.e. how much EV to dial in to move the peak and thus the tail of the hills. You can do some experiments and gain some understanding by dialing in EV and shooting a test subject. And watching the histogram move horizontally. You do that as homework, not on the day.
  7. The histogram does not tell you how much tail to chop off or to force into the bounding box - that's your choice and you have to visualise using whatever method you understand (zone system, experience with the subject etc....). This is a human, visual assessment, certainly the computation and artificial intelligence is getting smarter but the histogram isn't - the histogram is a crude 2D graph.

If a person does not understand how to use the histogram, there is nothing stopping the person from doing a bracket of shots or interpreting visual quality by looking at the LCD screen or the EVF.

The histogram is an informative and useful tool above the:

  1. It's sunny so I use f/16 1/100 @ ISO 100
  2. Oh, the meter says it is EV 17 @ ISO 100
  3. Heck, I'l just dial RAW and shoot a bracket of 5 shots at .3 intervals, one of them should work.

If a person does not want to use the histogram and has some magic recipe that works for them, just do it.

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