Canon’s new 600mm and 800mm lenses

Started 3 months ago | Discussions thread
John Sheehy Forum Pro • Posts: 23,791
Re: Canon’s new 600mm and 800mm lenses

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.

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

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, 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.

From the same distance, 400/6.3 or 200/3.2 have a little bit more noise, a little bit more DOF, and a little bit more diffraction than 800/11, in the underlying analog image. 800/11 is photographically equivalent to 400/5.6 or 200/2.8, and one would only get more subject noise, DOF and diffraction with the 800/11 if the narrow angle of view, when combined with the sensor size, forces one to back away from the subject to a longer distance.

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