What is reach?

Started Sep 30, 2013 | Discussions thread
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JimPearce Veteran Member • Posts: 9,188
What is reach?

I'm certainly not capable of answering this question, but I do have a couple of thoughts. One is that if we have a target file size we can define reach by:

reach factor = crop factor * sqrt(cameraMP/targetMP)

But all files are not equal. So I wondered if using the dxomark low-light ISO criteria at ISO 400 might be a reasonable approach. The criteria are:

"An SNR value of 30dB means excellent image quality. Thus low-light ISO is the highest ISO setting for a camera that allows it to achieve an SNR of 30dB while keeping a good dynamic range of 9 EVs and a color depth of 18bits."

Looking at Nikon's D800 and D7100 it was immediately clear that the 30 dB SNR ratio would be decisive. I calculated a minimum file size of 4 MP for the D800 and 5.4 MP for the D7100. This gave me reach factors of 3x for the D800 3.23x for the D7100. I think the 8% spread is more realistic than the 25% advantage that a more naïve approach might attribute to the D7100. In fact it conforms to my experience that a D800 with a 600 f4 has more reach than a D7100 with a 500 f4.

But the arbitrariness bothers me. Any thoughts from the wizard crew?

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 JimPearce's gear list:JimPearce's gear list
Nikon D7100 Nikon D500
Canon EOS 30D Nikon D7100 Nikon D800
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