Canon’s new 600mm and 800mm lenses

Started 3 months ago | Discussions thread
bclaff Forum Pro • Posts: 10,477
Re: DoF of Canon’s new 600mm and 800mm lenses

alanr0 wrote:

bclaff wrote:

alanr0 wrote:

Old Greenlander wrote:

Chris in UK

tell me how you get a shallow DOF with those lenses...

How shallow do you want?

At 10 m distance, assuming 0.03 mm Circle of Confusion, DOFmaster tells us that depth of field extends from 9.95 m to 10.05 m for the 800 mm lens at f/11.

For a portrait, it will be tricky to get nose and ears in focus at the same time, if the subject is facing you.

At 6 m minimum focus distance, total depth of field is probably less than 40 mm for the 800 mm lens (depending on the pupil magnification and the working f-number), with a similar value for the 600 mm f/11 at its 4.5 m minimum focus distance.

Predicted total DoF at f/11 nominal aperture is 34 mm at maximum magnification 0.14.

Hyperfocal distance is 1.9 km.

Rather than DOFmaster did you try the formulas you cite and a pupil magnification less than unity, like 0.6 ?

The 34 mm total depth of field value at minimum focus comes from equations 10 and 11 in the post I linked.

The usual formula for working f-number Nw = (1+m/p) N, at pupil magnification p=0.6, would increase the effective f-number by a factor (1 + 0.14 / 0.6) = 1.23, to f/13.6.

At minimum focus, image magnification is 0.14 for both lenses, so total depth of field would increase to 2 Nw CoC / m² = 42 mm.

However the usual formula assumes that the focal length is fixed, and the lens moves as a single unit to adjust focus.

The diagram you linked, and the associated patent text, both suggest that focusing is achieved by moving the central lens group of three. The aperture is shown as part of the third lens group, closest to the image plane. If the location of this third lens group and the system stop are both fixed, then NA and working f-number will not change with focus, and my estimate of 34 mm total depth of field applies.

I agree that the diaphragm to image plane distance is unchanged with focusing but that doesn't mean pupil magnification is unity at infinity and pupil magnification will certainly change as you focus closer (as will the focal length).

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

Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

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