Canon’s new 600mm and 800mm lenses

Started 3 months ago | Discussions thread
alanr0 Senior Member • Posts: 2,326
Re: DoF of Canon’s new 600mm and 800mm lenses

bclaff wrote:

bclaff wrote:


Too bad I can't model this lens; I'll look for something similar.


This lens also focuses without affecting the diaphragm to image plane distance.

At infinity focus

At close focus

At infinity focal length length is 776mm and P'D/PD = 33.21mm/137.10mm=0.24
At close focus focal length is 541mm and P'D/PD = 31.23mm/138.45mm=0.23

Since we compute DOF on-axis we should simply use the on-axis pupil magnifications of 0.24 and 0.19 respectively. (The difference is due to curvature of the pupils).

Not so much of a change in pupil magnification as I expected, but a change.
Substantial change in focal length.

Thanks Bill.

Note that with this lens fully open (f/5.66), at close focus it is the front element, rather than S1 which forms the system stop.

Stopped down to f/6.3, S1 becomes the limiting aperture throughout the focus range, and the exit pupil diameter P'D is 29.6 mm at both focus settings.

With the working f-number fixed, there is a small dependence of near and far focus distances on pupil magnification, due to the image-space asymmetry. However, the total depth of field (far-near distance) is independent of pupil magnification. See my equations 18, 19 here, or in more detail here.

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Alan Robinson

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