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01-07-2023, 10:28 PM   #1
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Curious about Pentax 6x7/67 lenses focus breathing behaviour (esp. 45mm)

I'm looking into a Pentax 6x7/67 system and as somebody who is very picky about composition including angle of view, I'm curious about the focus breathing behaviour in particular the 45mm lens. At close focus, is the magnification greater or less than infinity, and by how much?

I'm also curious about the other lenses, so if you have any and can comment that would be great! Having that info in the same place would be useful!

Thanks everyone!

01-08-2023, 07:44 AM - 1 Like   #2
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All lenses that have a helicoid that moves the entire set of lens elements back and forth relative to the film plane will have focus breathing. At infinity focus, that type of lens is closer to the film plane (and farther from the foreground, midground, and background elements) so it has widest FoV (the one typically specified in the lens design). At close focus, that type of lens is farther from the film plane (and closer from the foreground, midground, and background elements) so it has a slightly narrower angle of view.

To the best of my knowledge, the vast majority of vintage photographic primes focus the lens by moving the whole assembly back and forth meaning they will definitely have focus breathing. I've never seen the SMC Pentax 67 / SMC Pentax-6x7 45mm F4 Reviews - 67 Wide-Angle Primes - Pentax Lens Reviews & Lens Database but would guess it, too, moves the whole lens assembly and would have focus breathing.

Designing a lens that avoids focus breathing is not easy and only the makers of lenses dedicated to cinematography would take the trouble to make such a lens. Such a lens typically requires a fixed front group that defines the front nodal point relative to the film plane and then a moving back group (or groups) that adjust the focus.

If you need maintain a specific angle of view regardless of the focus setting, you actually need a lens that slightly changes focal length as it focuses. You could use a zoom lens and add your own markings to it for adjusting the zoom slightly to compensate for the focus setting.

The other way to create images that all have the identical angle of view regardless of the focus setting is to crop the infinity-focus images in post to match for angle of view the closer-focused images.

Last edited by photoptimist; 01-08-2023 at 10:33 AM. Reason: typos
01-08-2023, 09:51 AM   #3
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I'm think of my personal experience taking pictures with my P67 and I'm not sure how focus breathing affects the composition of any of my shots. Are you planning on doing focus stacking or something with these lenses on a digital camera? The 45mm on a Pentax 67 is similar to a 21mm on small format. You can get some distortion on the edges pointing it up/down on a medium format camera.
01-08-2023, 03:58 PM   #4
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That is helpful and well written, thank you! Seems like it shouldn't be too hard to calculate different angles of view at various focus distances based on infinity.

---------- Post added 01-08-23 at 04:02 PM ----------

QuoteOriginally posted by tuco Quote
I'm think of my personal experience taking pictures with my P67 and I'm not sure how focus breathing affects the composition of any of my shots. Are you planning on doing focus stacking or something with these lenses on a digital camera? The 45mm on a Pentax 67 is similar to a 21mm on small format. You can get some distortion on the edges pointing it up/down on a medium format camera.
Good to know it isn't noticeable! I'm not focus stacking; I'm more concerned about pre-visualisation and repeatability, and consistency between shots that may be viewed side by side.

01-08-2023, 04:42 PM   #5
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QuoteOriginally posted by dstnss Quote
...I'm more concerned about pre-visualisation...
Okay. I take it you'll be using the folding hood (aka waist level finder) or the ridged version of it? The prism does not give you a 100% view, FYI.
01-08-2023, 05:05 PM - 1 Like   #6
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QuoteOriginally posted by tuco Quote
Okay. I take it you'll be using the folding hood (aka waist level finder) or the ridged version of it? The prism does not give you a 100% view, FYI.
Yes definitely, thanks for checking. Don't mind saving that weight either
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