omd:ettr

Shoot RAW and Expose to The Right (ETTR) for optimal image quality

introduction

  • normal camera metering will aim to expose scenes “correctly” for your in-camera jpeg images
  • however, this exposure will generally result in most of your scene being in the left 3/4 of your histogram which means you miss out on using at least 75% of the data bits available in the file, and thus potentially miss out on tonal range, and you get more image noise in the dark areas
  • mirrorless camera systems with Live View and a accurate highlight shadow warning system and a wide dynamic range such as the Olympus OM-D cameras are ideal cameras with which to use the ETTR technique to meter with.
  • Expose-To-The-Right (ETTR) technique aims to over-expose your image just enough that non-specular highlights do not get blown out in the RAW file:
    • this results in a fairly useless jpeg image
    • BUT in RAW processing, you reduce the exposure back to where it looks best for your aesthetic needs and you potentially gain a cleaner looking image with more tonality
    • the benefits of this technique are greatest when you do not need to increase ISO to achieve the over-exposure

why ETTR helps in shooting RAW

the two brightest stops render 75% of all the shades the camera is capable of producing

bit depth of files

  • a jpeg file is 8 bit - this means each of the 3 color channels can only store 2^8 or 256 possible values for each pixel
    • image manipulation of 8 bit files is likely to lead to artefacts such as posterization
  • a RAW file is generally 12-bit - this means each of the 3 color channels can only store 2^12 or 4096 possible values for each pixel and thus allow for much more tonality and is much more forgiving with subsequent image manipulation
    • 14-bit RAW allows 16,383 levels per color channel and perhaps more shadow detail
  • so this is the reason you should shoot in RAW mode if you want the best quality shots
  • BUT, there is more - the files are structured so that 50% of the data range will be devoted to the brightest 1 stop, 25% to the next brightest 1 stop, and so on down the brightness scale
  • this means is you expose as per your camera, you are probably going to miss out on the brightest stop and perhaps even the 2nd brightest stop, meaning your data in your image will be only using less than 25-50% pof the possible data range!
  • exposing to the right aims to resolve this - as long as your bright non-specular highlights do not get blown out by too much over-exposure

tonal range available at each stop of brightness:

file type brightest stop 2nd brightest stop 3rd brightest 4th 5th 6th 7th
14bit RAW 8192 levels 4096 levels 2048 levels 1024 512 256 128
12bit RAW 2048 levels 1024 levels 512 levels 256 128 64 32
8bit jpeg with gamma 69 levels 50 levels 37 levels 27 20

Imagine then doing a low key portrait:

  • with “correct” metering, your jpeg may have none of the brightest stop used (apart from sclera of eyes), while your subject's facial skin tones are compressed into just 50 possible values - had you shot RAW, these skin tones could have had 20x more tonality with 1024 levels, and had you shot ETTR, they could have had even more than that much.
  • just as importantly, the darker regions which may have had 20 levels per stop in a jpeg, with ETTR could have over 200 levels and thus much less image noise.

ETTR technique with the OM-D cameras

option 1: use the highlight blinkies

  • use the highlight-shadow “blinkies” in the Live View so that there are no red blown highlights in non-specular areas of your image

option 2: spot meter highlights

  • aim centre spot at the brightest subject in which you want details and not be blown out
  • adjust exposure so that the camera meters that subject to a value which on your testing will not be blown out when you process the RAW file:
    • Olympus would traditionally use +2.3 for a white subject (and this is what is used for their Hi Spot mode)
    • however, you should be able to use +3.0 or a touch more for ETTR BUT on most OM-D cameras, the exposure indicator stops at +3.0, so for this technique, you must use Hi Spot and aim for meter reading of +0.7 or so (+3.0 or so minus +2.3 for the Hi)
      • optionally, you can verify the best exposure with the highlight blinkies as in option 1

option 3: normal metering and jpeg settings with bracketed over-exposures

  • there are times when one wants to use normal jpeg settings
  • you can still achieve ETTR but now you will have to bracket your exposures and choose the one with no blown highlights in RAW processing software
  • the OM-D camera meter will generally meter a scene at around 1 stop under-exposed for ETTR, thus, you could do 2-3 brackets such as +0.3, +1.0 and maybe +1.7 or +2.0
omd/ettr.txt · Last modified: 2014/10/30 23:54 by gary1