[Tig] camera dynamic range quotes are getting silly!

Richard Kirk richard at filmlight.ltd.uk
Thu Feb 17 21:38:41 GMT 2011


>
> Bob Friesenhahn <bfriesen at simple.dallas.tx.us> said....
>
> I don't think that the human eye can view an HDR image so I guess it
> would not be so pretty, or at least it could not be appreciated.  The
> human eye is rather poor with simultaneous contrast.  Contrast ratios
> of 350:1 are still keeping the eye rather satisified.

The inside of the eye is not black. We do not have an anti-reflective  
coating like a cat does. So a fraction of the light entering the eye  
will bounce around and give us flare. Even if your photoreceptors were  
able to measure to enough bits, this would limit our appreciation of  
simultaneous contrast to 100:1.

However, that isn't the whole story. Our eyes scan images, so we may  
be looking in a dark corner of an image, and seeing a local 100:1  
contrast where the average lightness was only 10% of the white point.  
In general, I found you needed about 1400:1 contrast to give a good  
'film-like' image in a grading studio. Of course, it depends on the  
material, but less than 1000:1 was not usually good enough. However,  
it is possible  on some images to trade long-range variation for local  
contrast - so you subtly raise the broad shadows and drop the broad  
highlights, and that can give you a 'super-contrast' effect. However,  
this doesn't work for all subjects: try doing that to text and you  
notice it.

On top of that, it would be nice to have brilliant highlights. I don't  
think you want to have the sun at full brightness in your scene, but  
it is good to have specular highlights. These are usually tiny, and we  
are not good at judging the absolute brightness of tiny highlights as  
they can be only a retinal cell or two across. Film rolls off these  
highlights, and it does make the image look 'flatter' but we are used  
to looking at images like that, and we don't complain. But it is one  
of the things that still separates images from real life. I think HDR  
images look good.

Old CRT video used to permit small excursions all the way to peak  
white provided they were small and they moved. If they were large then  
you drew too much beam current for the CRT HT supply, and if they  
stayed in one place, they could overheat the phosphor. But they could  
be used to add 'sparkle'. No change of getting that though QA these  
days.

Cheers.
Richard Kirk.




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