I have read that a 50mm f2 lens does not have to be a retrofocus design on a SLR as there is plenty of room between the film plane and rear element of the lens for the mirror box. With a 50mm f1.4 it does need to be a retro focus design (just, depends on camera make there is some variation in this distance) and with f1.2 definitely because the rear element is closer to the film plane than the mirror box will allow.
My question can you work out when a lens on a 35mm rf needs to be a retrofocus design because of its focal length and/or its aperature e.g. 25mm f1.4, 28mm f1.4, 21mm f2, 12mm f3.5...?
I think there must be an error in what you read. It would be a rare 50mm lens that would need to be a retrofocus, or 35mm either, on a 35mm slr.
[The sonnar needs the additional elements for correcting the errors coming with the bigger aperture and they interfere with the mirror.
While there is no direct link aperture - mirror clearance there is an indirect one.
As for the symmetrical lenses (gauss lenses): they need additional elements as well. A 50mm/1,4 has typically 7 elememts, a 50mm/2 has 5 or 6. The lens block gets physically longer and has to be moved slightly more outside for mirror clearance./QUOTE] This is what I was getting at.
OK you could make a simple 3 element non retrofocus f1.4 50mm - but the the opticla errors would make it a grim lens to use (compared to a more complicated design). so if some one wants to manufacture a high quality say f1.2 50mm lens - due to the length of the optical train, when would the 15mm rear lens /fim plane distane become a concern for the lens designer. I thick the 50mm f1.2 would not cause them concern but a 24mm f1.4 etc. OK with modern computing they can try diferrent optical lay outs which would have a slight effect on the rear lens element so it won't be a hard and fast rule.
TomThanks for that, so a 35 f1.2 does not have to be a retrofocus design - this is what I am trying to get at. People say its a lump of a lens ...but its not as big or heavy as the ZF F2 35mm (but I love the handling of this lens).My 35mm f1.2 CV Nokton is my fastest M mount lens, it is an Aspheric non-retrofocus wide angle design. The front-most lens element diameter is about 52mm. The rear-most lens element diameter is about 29mm.
Thanks ath[The sonnar needs the additional elements for correcting the errors coming with the bigger aperture and they interfere with the mirror.
While there is no direct link aperture - mirror clearance there is an indirect one.
As for the symmetrical lenses (gauss lenses): they need additional elements as well. A 50mm/1,4 has typically 7 elememts, a 50mm/2 has 5 or 6. The lens block gets physically longer and has to be moved slightly more outside for mirror clearance./QUOTE] This is what I was getting at.
OK you could make a simple 3 element non retrofocus f1.4 50mm - but the the opticla errors would make it a grim lens to use (compared to a more complicated design). so if some one wants to manufacture a high quality say f1.2 50mm lens - due to the length of the optical train, when would the 15mm rear lens /fim plane distane become a concern for the lens designer. I thick the 50mm f1.2 would not cause them concern but a 24mm f1.4 etc. OK with modern computing they can try diferrent optical lay outs which would have a slight effect on the rear lens element so it won't be a hard and fast rule.
Tom Thanks for that, so a 35 f1.2 does not have to be a retrofocus design - this is what I am trying to get at. People say its a lump of a lens ...but its not as big or heavy as the ZF F2 35mm (but I love the handling of this lens).
You are welcome! Of course, the Zeiss ZF F2 35mm is not an M mount lens (it's an SLR lens and IT IS a retrofocus design).
The Zeiss Biogon T* 2/35 ZM is an M mount lens and IS NOT a retrofocus design.
I also have the ZM 50mm f2 Planar and the ZM 50mm f1.5 Sonnar.
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