So-called focus breathing is very common and not unique to internal focusing lenses,
which are preferred not least because they accumulate less dust.
https://photographylife.com/focus-breathing
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So-called focus breathing is very common and not unique to internal focusing lenses,
That would be true if the lens retains its infinity focal length and reaches 1:2 entirely by extension. But in general not true.bbobby wrote: ↑Sat Jun 25, 2022 3:14 pmMany older macro lenses are going only to 1:2 and need extension tube to reach 1:1. For example Nikon PK-13 is 27.5mm and if I remember correctly it is for 55/2.8. 25.7 is half of 55mm - so my guess here is that for a lens to go from 1:2 to 1:1 it needs 50% extension. Is that always true?
No, because this Sigma only reaches 1:3 without extension. (Limiting case: suppose it did not focus at all, then the extension needed to reach 1:1 would be equal to its focal length, not focal length/2.)Example: to get 1:1 this Sigma will need 110 mm extension - meaning its focal length is 220 mm and voila - I got the 2nd confirmation.
No, many lenses that obviously extend also shorten their focal length. Any lens that reaches 1:1 and auto-focuses will probably shorten its FL to focus closer, because lenses that focus entirely by extension behave very perversely around 1:1. If you slog through the math, you will find that exactly at 1:1, changing the extension on a lens with constant FL will change its magnification but not its focus! Near but not exactly 1:1 it is a little better, but still difficult to handle. Users of bellows quickly discover that around 1:1 they need to set the bellows for magnification, then focus by changing distance to subject.My 2nd question is about shortening the effective focal length going close to the target - is this happen only with internal focusing lens?
A few hours ago I posted a new FAQ: How to find focal length and principal planes for arbitrary lens? Perhaps that will help.Or I can be completely wrong and this would not work like that at all... and in this case better to know it beforehand and not waste time in pointless comparisons ;-)