# Birding with a 500mm F4 or 600mm F4

Started Feb 1, 2014 | Discussions thread
comparisons, some tricky -- some not
1

Dr Bob wrote:

JimPearce wrote:

It seems to me that the laws of physics must have been suspended.

Jim,

I can't really help with the laws of physics here. In a previous life I was an expert on quantum mechanics (the Dr ..Bob has to come from somewhere!) but optical electronics are not my speciality.

quantum mechanics ! So you must oversee the implications of photon gathering in a light starved situation (photographically I might practically define this as when base ISO is not enough). The light gathering is proportional  to the square of the aperture diameter. And the theoretical diffraction limit for resolution on the object is just proportional to the diameter. So we have this ranking:

600mm f/4 -> 150mm lens diameter

800mm f/5.6 and 400mm f/2.8 --> 141 mm

500mm f/4 --> 125 mm

300mm f/2.8 --> 106mm

(80-)400mm f/5.6 --> 71mm

so its clear which lens is king for light gathering. Though because of almost surely larger lens aberrations, the 600mm presumably passed by the 800mm for resolution on the object. Other considerations, like getting it to the location etc make the over all comparison tricky.

For the camera bodies and photons per pixel there is one practical equivalence

D7100 == D600/610 with TC14

D7100 + TC14 == D600/610 + TC20

if the 7% difference between 1.5 crop factor and 1.414 teleconverter factor is disregarded.

The other differences between the camera bodies on frame rate, buffer size, cropping options have clear winners.

The D7100 is among the clear winners for DSLR with the most pixels on the subject for a given lens (including  TC) upfront.

There are also clear winners for 'per pixel noise' for a given lens etc.

However, contrary to common belief, there is no clear winner in per pixel noise of an image downsized to a common size (for example 1600 pixel along the long side) if all is done right and the same lens aperture diameter and the same field of view was used on capture. (example current DX + 200mm f/2 == FF + 300mm f/2.8 )

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