Canon’s new 600mm and 800mm lenses

Started 3 months ago | Discussions thread
AiryDiscus Senior Member • Posts: 2,138
Re: Canon’s new 600mm and 800mm lenses

John Sheehy wrote:

Old Greenlander wrote:

Chris in UK

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

Shallow DOF depends on how larger the pupil looks as an angle, from the subject's perspective. It has no direct relationship to f-ratios.

Depth of field and depth of focus are two sides of the same coin; object and image space.  They are related by the longitudinal magnification.

F-ratios do not appear visibly in images. Pupils do.


pupils do not appear in focused images. The focused image is the convolution of some truth with the magnitude squared of the fourier transform of a pupil.

Your way of thinking is irreconcilable with lenses like these, which have both pupils at infinity and of undefined size:

On the other hand, these lenses have finite NA/F#, so nothing is lost the way that has "no direct relationship."

Like so many other similar situations in this big world, people have a tendency to confuse casuation and correlation, ignoring confounding variables.

Yes, if you vary the f-ratio, all else being equal, the higher f-ratios give the deepest DOF. That does NOT mean that DOF is a direct characteristic of f-ratio.

Perhaps I used to be confused myself at one time, but it is still hard for me to fathom why people ascribe so much absolute meaning to things like f-ratios, ISOs, and exposure, when in fact, they are removed from direct relevance by a number of other factors. This may not affect the landscape photographer much in a way that causes miscalculations, but for people using long lenses, or who always crave for longer lenses, the pupil and distance paradigm tends to be more direct, as you do more object- or subject-relevant calculations, and less sensor-frame-relevant ones.

What is real in a photograph (as far as the underlying analog image is concerned), is exposure time, perspective, and effective pupil size. What we are left with is whether or not the size of the sensor is too narrow to host our composition, and whether there are enough pixels to resolve our composition to our satisfaction.

f-ratios are for completing exposure triangles, if that floats your boat, or for AF systems, where f-ratios of open lenses can affect AF ability, because of low light flux, but also because of optical geometry,

that "geometry" argument, without commenting on its legitimacy, would lead you to support an NA/F# based formalism, no?

despite sufficient light quantity. Low (not high!) f-ratios can run into problems with photosite/microlens optics, losing more of the light from the outer regions of the exit pupil, but this generally only occurs at f-ratios less than 2.8.

The microlenses on most sensors these days are on the order of F/1.5.  Canon puts NA 0.8 lightguides on their cine cameras.

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