What is reach?

Started Sep 30, 2013 | Discussions thread
Great Bustard Forum Pro • Posts: 39,704
The way I like to think of reach is...

JimPearce wrote:

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?

....resolution on the subject.  Most people would say that the reach factor is proportional to the ratio of pixel density, per your formula above.  However, I feel this is too simplistic.

For example, let's say we have two lenses of the same focal length on a given sensor where one resolves 2000 lw/ph and the other resolves 4000 lw/ph.  I would argue that the lens that resolves 4000 lw/ph has twice the reach as the lens that resolves 2000 lw/ph, since it could be cropped to half the frame size and maintain the same resolution.

In other words, when speaking of reach, we need to account not just for the pixel count, sensor size, and the focal length of the lens, but how well the lens resolves on the sensor it is used on.

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