Saturday, 22 February 2014

Exposure Exposed


A big thanks to those who have been in touch, especially those who pledged to help with the project.  
Special gratitude here to Phil Jeffrey for sharing some of his expert photographic knowledge.  

Phil wrote re my description of overexposure

"...probably describes something closer to an optimally exposed image (ETTR) because definitionally contrast goes down when you start truly saturating images (in the pixel counter way i.e. clipping).  You fall afoul of using saturation in two ways (colour saturation as a good thing, generally, but you also use as pixel over-saturation in a technical way).  
Your description would also rate a correctly-exposed Blackbird (Euro or Icterid) on a snow field as being pathologically over-exposed, and a Snow Bunting on a black sand beach as being pathologically under-exposed.  What you need to say is that the exposure needs to be adjusted to maximise the tonal range of the subject, while minimising the number of burnt out (white) or deep shadow (black) pixels in that subject.  Even then that's technically too bright (blackbird) or too dull (snow bunting) an exposure but it's closer to what you want to achieve.  Given that the image is likely to be JPEG that's the situation to put one in the strongest position to post-process control for exposure and colour.  A technically correct exposure male (Euro) Blackbird probably conveys less information than it might, anyway."
As stated in the introduction to this blog, this is not a photographic tutorial or a technical exercise but I really do appreciate expert intervention like this.  If it looks like I am going off on a tangent in a technical way please feel free to advise as Phil has here.  

Below I have taken various exposures of a test image and laid them alongside each other to illustrate the impacts of over-exposure and under-exposure on detail and colour.  Under-exposure tends to be less of a problem.  We have a greater chance of reversing the effects of under-exposure and drawing detail and colours out of our image.  With over-exposure the effects tend to be more damaging.  I know some photographers intentionally under-expose their images to try and avoid BLOWN HIGHLIGHTS, i.e. patches of severe over-exposure in their images.

I have illustrated the relative impacts of moderate and major under-exposure and over-exposure on this particular test image.  I have rated the relative impacts on detail and colour on the right as they appear to my eye.  Others may have a different take.

I am beginning to think that I should probably alter the scoring in the quality tool for under-exposure relative to over-exposure.  I should probably negatively score over-exposure more severely than under-exposure.  One for Rev. 1.2 I guess.  Again your input here would be greatly welcome.

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