Topic : Photographic
Article 35 
20 August 2007
Digital Bird Photography - Expose for data
When photographing a bird you are trying to record the image that you see. The human eye has the amazing ability to adjust and let us see detail in the brightest highlights and darkest shadows in a scene which we could equate to about 10 stops. By comparison, a digital camera can only capture about 6 stops, but in most cases captures closer to 5 stops.
Graphic representation of of the 5 stops captured by Digital cameras
Most digital cameras capture 12bits per pixel (often advertised as 16bits), but this is actually only 12bits in a 16bit colour space (newer Canon models can now capture 14bits). This means that we can capture 4096 distinct levels of brightness per channel. You would think therefore that if we divided this up by the 5 stops we could capture about 850 (4096/5) levels of brightness per stop. This is however not the case, as solid state camera sensors CCD and CMOS record data in a linear fashion. This means that the data is not distributed equally among the 5 stops, so the brightest stop holds half of the total number of brightness levels (up to 2048). The second brightest stop holds half that (1024), and the third half that again (512) and so on, 256 in the fourth and 128 in the darkest.
Two photos showing "normal exposure" on the left and "exposure for data" right
Knowing this, and photographing in RAW mode (which is what you should be doing anyway after reading my earlier article!) we should try and take advantage of this knowledge and capture as much information as possible. In other words, we want to capture as much data or as many levels of brightness as possible with as little noise as possible.
To do this we need to expose as bright as possible without over-exposing any highlights. This means you should try to expose till you just don’t have any flashing warnings on the LCD, and the histogram on your display should extend to the right hand side without clipping the right hand edge. If you over-expose so clipping occurs, all clipped information is lost. You should expose to touch the right hand margin, and by doing this you have taken advantage of retaining the maximum amount of data in these brightest stops. If you have done this and the photo looks too bright you can tone it down in Camera Raw, or in whatever raw converter you use to bring the image back to the level it should be. This ‘extra’ data also allows for safer editing in Photoshop when using curve adjustments and sharpening, lessening the chance of posterization and excessive noise in the shadow areas.
When time allows, take comparative photos, edit to get optimum exposure and compare results it can be worth the extra effort.