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:

alanr0 wrote:

bclaff wrote:

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.

First, my apologies. I did not open up one of the diameters quite enough and have modified the optical prescription again. The change is minimal.

However, after doing so the Optical Bench shows an exit pupil diameter increase wide open from 33.21mm to 33.24mm. See angle comment below.

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

While it's true that nothing changes behind the diaphragm the changes in front of it change the angle that the marginal rays pass the diaphragm and ultimately the angle at which they arrive at the image plane.

But doesn't a change in angle at the edge of the diaphragm mean mean that the marginal ray intercepts the axis at a different location?

If so, then we can interpret this difference as a change in spherical aberration with focus.  Sorry - I should have thought of that.  Unfortunately, my current DoF calculations ignore aberrations.

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:

The Optical Bench Aperture Stopdown slider works in 1/3 stop steps so 1 click is f/6.35 at Infinity and 6 clicks is f/11.32 at Infinity.

OK - got it.  I need to click within the circle of the slider, but not at the centre to get a single 1/3 stop step.  A click in the centre does nothing, and a click outside the circle jumps multiple steps.

And yes, anticipating your question below. I carelessly omitted a zero and did intend 0.029mm COC.

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.

Why 2Δu ? Isn't Total DOF = uFar - uNear = 13.5mm ?

In the equations I referenced,  Δu is difference between geometric focus and either near or far point, so total DoF is twice this.

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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