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
1

bclaff wrote:

alanr0 wrote:

bclaff wrote:

bclaff wrote:

...

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

Alan,

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

<snip>

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.

Thanks for pointing that out. It was actually R9 (L14) that was limiting it.
I adjusted the diameters to resolve it. This makes sense because I would expect R14/R15 to be the "choke" point.
This patent, like most, does not have effective diameters so I normally just try to match the patent figure. But patent figures also often depart from a perfect 1:1 aspect ratio so this adjustment is not that unusual.

I get 33.21mm as the exit pupil at both infinity and closest focus so I wonder where 29.6mm came from.

Is that after opening up R9? My 29.6 mm is at f/6.3, not fully open.

Revised close focus

With the working f-number fixed,

Not quite. If you toggle between the two scenarios while zoomed in close to the focal point you'll see the angle changes slightly.

Is that after opening up R9? My 29.6 mm is at f/6.3, not fully open.

I see the change in angle with focus distance fully open. I did not see it with the lens stopped down.

Taking a closer look, at f/6.35 your reported P'D changes from 29.57 mm to 29.62 mm, which produces a barely perceptible change in the marginal ray angle on my screen. If this is not a rounding error, I do not understand why it occurs if the lens elements between the physical stop and the image plane are fixed.

there is a small dependence of near and far focus distances on pupil magnification, due to the image-space asymmetry.

I don't think it's small.

However, the total depth of field (far-near distance) is independent of pupil magnification.

I don't think so.

See my equations 18, 19 here, or in more detail here.

I'm curious, what do you get for near and far focus for a circle of confusion of 0.29mm at closest focus?
(I have numbers from the ray trace and I'm curious to see if there is agreement.)

From your Optical Bench for the Nikon 800 mm f/5.6, at f/6.35 (re-setting exact aperture is tricky) and closest focus:

Focal length f = 540.91 mm

Magnification 6.50:1 m = 0.154

At working f-number Nw = 6.35; CoC = 0.29 mm, I calculate:

  • Exit pupil diameter p A = 29.3 mm
  • Ufar = 4132 mm; Unear = 3977 mm; 2 Δu = 155 mm.

Closer to the Canon aperture, using working aperture f/11, smaller circle of confusion diameter CoC = 0.029 mm, but the same magnification as above m = 0.154

  • Exit pupil diameter p A = 16.9 mm
  • Ufar = 4066.8 mm; Unear = 4053.3 mm; Total DoF = 2 Δu = 26.9 mm.

Calculated using my equation 18, with similar total DoF predictions from my equation 19.  Object distances (Ufar, Unear) are measured from first principal surface, so add 1236 mm for working distance from first element, and 1746 mm for distance to image plane.

You are quite careful in your work and usually correct so I wonder why we're not lined up on this. Perhaps there's a misunderstanding.

Even with your large CoC of 0.29 mm (did you mean 0.029 mm?) and an aperture of f/11, the image-space depth of field is (± CoC Nw) = ±3.2 mm. Exit pupil distance to image plane is 187 mm, so there is only ±1.7% asymmetry in the effective aperture at near and far image points. The first order contribution to object-space DoF cancel, so the impact on the total depth of field is negligible.

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Alan Robinson

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