Mar 2016

The purpose of a Log picture profile

The purpose of a logarithmic picture profile is to capture the maximum amount of data at the time of shooting. In doing so we get the most flexible image file for post work. In addition, if the log curve assimilates characteristics of that of film or another digital sensor, it will be easier to mach footage between the two.

A log curve also allows for a sensor to capture more stops of light than the file format natively supports—for example 12 stops of light saved to an 8 bit file.

The true value of a stop of light

As image makers we are used to talking about stops of light. We accept that each new stop of light adds the same about of light that we previously had—it’s doubling. Take what you have, and add just as much, then you’ve got double the amount.

Don’t fall into the trap of thinking that each new stop adds the same amount of light as if a stop was a liter, or a gallon. A fixed amount. It isn’t. Like I said above: it’s always a doubling. Look at this numerical series:

  • 0+1=1…………………… stop 0
  • 1+1=2…………………… stop 1
  • 2+2=4…………………….stop 2
  • 4+4=8…………………….stop 3
  • 8+8=16…………………..stop 4
  • 16+16=32………………..stop 5
  • 32+32=64………………..stop 6
  • 64+64=128………………stop 7
  • 128+128=256……………stop 8

Notice how the first stop was a value increase of 1, but the last stop was a value increase of 128. Notice also that this sequence consists of 8 stops. You’ll recognise the familiar range of 256 values (0-255) as the discreet luminance steps in 8 bit images. When we write 8 bit like so 28, we can see how this represents “a doubling, eight times”. If we would write 10 bit in the same manner we would have 210. A doubling, ten times—which gives us 1024 luminance levels.

How do you fit a 2 foot troll in a 1 foot cave?


If the numerical series above represents light stops and the values are electronic charge levels, we make the following observation: it would be very impractical to just save the data like it’s recorded, because the last stop would contain 128 discreet levels of information, whereas the first stop would only get 1 single level!

Linear

No, instead we use a curve to distribute the light stops evenly. If we recorded 8 stops and have 255 assignable levels we get 255/8 which is just shy of 32 levels per stop.

Log

Logarithmic picture profiles are an essential and powerful tool in today’s professional video cameras. But it has also become a marketing tool for manufacturers when they try to position their cameras as more ‘pro’ than ‘consumer’.
I’m increasingly seeing log profiles that were designed for 10bit space (and 1024 discreet luminance values) being offered in cameras that only record 8bit data. In this case we have to be careful, or at the very least very alert, so that we don’t spread the light too thin. Using too few luminance levels per stop can cause posterisation and banding. But that’s a topic for another day.