11-18-2016, 08:59 PM
Quote:Your post is confusing. You say that to have no focus breathing it needs to shorten focal length (correct), but then you say that is unacceptable to you because it changes perspective.
If you talk about perspective as it is talked about in art, that actually changes when FOV changes.
If you talk about perspective as in point of view, it will not change, either with focus breathing or shortening of focal length.
As for your calculations, they still do not make sense (well, the outcome you typed). Version 1 of the Nikkor 70-200mm f2.8 VR has a narrowing FOV towards MFD. Your post just above seems to assert that that changing FOV will mean a longer focal length.
Also, for a single lens calculation you still need to know the front nodal point to judge the subject distance.
And yes, to have no focus breathing, one has to take care of that in the lens design. That is why specialized film lenses which have low to no focus breathing are very expensive.
About that Nikkor 70-200mm f2.8 VR:
1.5m MFD, max. mag. 0.16x
Taking a 1.3m object distance (assuming a single 200mm lens), gives a focal length of 179mm.
Sorry if it was confusing (and sorry for the late reply, been busy again with other stuff).
I actually used your definition of focus breathing, and worked from there. Personally, although correction for focus breathing in your sense is what videographers appear to like, I prefer lenses to behave according to general optic laws, IOW, that one gets a larger magnification when focused closer, and for the FL to stay the same over the entire focusing range, IOW, preferably no IF either, as that generally is effectuated by a zooming type group of elements, decreasing FL when focusing closer.
I would expect lenses which keep the FoV the same to be very expensive, as it requires quite an optical and mechanical tour de force. However, it does require for the FL to change. For me, focus breathing is for the FL not to stay the same over the entire focus range, rather than the FoV to stay the same, as a changing FoV is a given with primary lenses which focus by increasing the distance between film or sensor and the lens itself. That has my preference.
With many lenses, especiall zoom lenses, I think it is cheaper to have them change focal length with closer focus, rather than make sure the FL stays the same, as only a few elements have to move in the former case, whereas all emlements have to move in the latter, in principle anyway (it could be done differently too, as in moving the FoV creating groups rather than the inital image forming group(s)). Even more expensive would indeed be lenses which retain FoV consistently over the entire focusing range, as that at least triples the complexity, optically and mechanically.
I reckon that many lenses which are ceated for videography these days, specifically when they are based on existing photography lenses, they do nothing but offering T-stops instead of or in addition to f-stops, and possibly a smooth, linear way of focusing. These two alone make a video-ready lens 3 or 4 times as expensive as a lens meant for stills photography. I shudder to think what a properly created videolens which would keep the exact same FoV at all times when focusing would cost.
Anyway, I prefer a lens created for stills photography, which keeps its FL when focusing, at least to a large degree. The EF-S 60 macro is such a lens (goes from 60 mm to 50 mm from infinity to 1:1), as are most primes (those which do not have IF). The 60 macro does have (part) IF but is not too bad. It is one of the reasons I never liked the Canon 100 USM macro, or the 100L macro. Both have significant FL lens and get too close to the subject at close focusing distances, for my taste anyway.
I can therefore understand that someone objects to loss of expected magnification with lenses which do have lower FLs at closer focus.
In addition, there is an unexpected perspective change: when FL changes, not only does it appear as if one is further away from the subject, but the change of FL also has a different effect. The proportions in distance of a subject appear to change. It is like using a wideangle in close-up vs a tele in close-up, to use an extreme example.
Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....