1

In After Effects' 3D camera settings there's a nifty one called "Focus Distance". Creating Motion Graphics with After Effects by Chris Meyer and Trish Meyer says:

Aperture’s not-so-evil twin is F-stop: the ratio between Focus Distance and Aperture.

While Wikipedia on f-number says:

The f-number N is given by: N = f/D where f is the focal length, and D is the diameter of the entrance pupil (effective aperture)

In After Effects, "Focus Distance" and "Focal Length" are separate settings; adjusting one does not affect the other. This does allow the possibility to "pull focus" with "Focus Distance" without actually changing "Focal Length" and thus keep the framing completely intact (i.e. no zooming whatsoever). But I still wonder what this "Focus Distance" corresponds to in real-life cameras, because most tutorials I Googled on "rack focus/focus pulling" involves actively changing focal length, which results in some (albeit subtle) zooming in or out.

3

To very much over-simplify… [full explanations in Wikipedia links]

Focus distance == distance from lens to the plane in sharpest focus.
Focal length == amount of 'zoom' of a lens, how close or far your subject appears, measured in mm.
F-stop [or in cinema T-stop] == the width of the 'hole' (aperture) that allows light into the camera body. [F-stop is measured in physical distance across the gap/focal length, T-stop is measured in absolute light transmission & is much more difficult, & consequently expensive, to achieve a constant value.]
Aperture (& to an extent focal length) determines your depth of field - the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image

On a traditional stills camera lens, changing focus distance will slightly affect focal length*, even on a non-zoom lens. This is generally known as 'breathing'. The same effect is seen if you change the focal length on a zoom lens, the focus distance changes.

Cinema lenses are what's known as parfocal - which makes them stunningly expensive (≅ £40,000 vs £1,000) - but means that changing focus distance does not allow the lens to 'breathe' or change its focal length at all. Neither do they change the absolute amount of light transmission (the aforementioned T-stop). A regular 'stills' camera lens will always change focal length, albeit slightly, if you change focus distance. Most stills lenses will also change aperture (F-stop) as you zoom.

Purely in software, where there is no real light transmission, of course, parfocal qualities are easy to achieve.

I found this B&H blog post - How to Use Cinema Zoom Lenses - which covers this in more detail without getting too technical. They've also used to opportunity to try sell you some 'cheaper' parfocal lenses for a mere 20 grand, but they're a bit slow compared to the 'good stuff', eg this ARRI 40-250mm T2.6
More for my savage amusement than any real relevance - selections of Angeniuex & ARRI lenses at a mere $100,000

*Strictly speaking, on a stills lens, changing any one aspect will change its relationship to all the others, to greater or lesser degree.

5
  • 40k does not seem like a lot of money for a specialized industrial tool. Thats about the cost of most of our industrial machinery. I mean the crew to operate a movie shoot easily a few thousand a day.
    – joojaa
    Oct 13 '20 at 12:03
  • It's not, I suppose, when the rest of the camera body, picture transmission system & dolly it sits on will be another 250k or so ;) People mostly rent all these things, though. Only the 'biggies' like Warner & Disney tend to own them all in-house. I'm in the middle of shooting on a big Warner at the moment (week off, so I'm on here instead ;) - gods know what the on-set equipment cost would be on any given day… astronomical.numbers.
    – Tetsujin
    Oct 13 '20 at 12:07
  • Yeah well a million isnt all that much money really, its just a lot to a single person but if you think about the salary of a 20 persons... I mean i use equipment worth in excess of a million every day
    – joojaa
    Oct 13 '20 at 12:10
  • Sure - when you've got a year from principal photography to release, that's about a million a day… let me just check my lottery ticket again, just in case ;)
    – Tetsujin
    Oct 13 '20 at 12:13
  • What is even a billion nowadays? Won't even be enough for a space program or a secret nuke proof headquarters inside a volcano. 😭
    – Wolff
    Oct 13 '20 at 15:25
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Often these two things go hand in hand with aperture. If you close the aperture (set a larger f-value), you get a greater depth of field. At the same time you will also get a slower shutter speed. Always aperture makes beautifully blurred backgrounds and helps make sharpness and depth of light. You want the foreground to be in focus as well as the mountains in the background. You also want the water to be smooth. This will require a long shutter speed to smooth out the fine ripples on the surface. To increase the shutter speed, you can attach a neutral density filter to the lens. If a filter isn't enough, you can also close the aperture to f/22 or a narrower value that your lens supports

1
  • Hi Welcome to GDSE and thanks for contributing, however this doesn't really answer the question. The OP is asking what focal distance is, and what that equates to in a camera in the real world. Check the other answer given here, which does in fact answer the question.
    – Billy Kerr
    Jul 16 at 12:28

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