Why are AP67 lenses so big?

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Having this camera, it is of no big importance importance to me, but just because I4m curious: why are the lenses for this camera so big? I have owned a Fuji pro 6x4,5 wide, and I recently got hold of a Rolleicord 6x6. Both camera4s, giving some attention, produce very sharp and contrasty pictures. The lenses however, are 10 times as small as similar AP lenses. Why? Is more glass per definition better? It is not because AP lenses are much faster, or because the image cirkel is much larger. Even if they are sharper (which I know they are) does the size do this?

-- Peter Gooijer (rockrose@gironet.nl), October 03, 2000

Answers

Peter: In Physics there is a proven reality that when two lenses of different diameters but equal correction are compared, the one of larger diameter will have better resolution. So, you can do one of two things to improve resolution according to this law. You can either increase the frequency of the energy source(light, etc.) or you can increase the diameter of the optic. The electron microscope uses the former method but the photographer can't use electrons and must use light. So, larger diameter lenses do make a difference. But the amount of aberration correction by the designer is a far more important consideration than the diameter of a lens. BTW, very fast lenses are difficult to make sharp due to their use of off axis rays. Astigmatism and coma become difficult to correct. I doubt very much that Pentax considers lens diameter/size as a performance feature, but does consider it for focal ratios. SR

-- Steve Rasmussen (srasmuss@flash.net), October 04, 2000.

Bill, I'm not sure why Pentax does this on the 165LS but please note that the 165LS and 200mm Pentax are of the same design. They are called Ernostars and originated in Germany in 1924. It is very popular for this focal length, even in 35mm format. The 90mm LS uses a modified Double Gauss so I would think the 165LS could use it also, just like the 165 f/2.8. Pentax seems to like the performance of the Ernostar design. Why the 200 and 165LS don't use the full diameter of the barrel, I can't say. SR

-- Steve Rasmussen (srasmuss@flash.net), October 04, 2000.

The reasons for your observation are twofold. First, as you already suspected, the image circle determines the lens size. The 6x7 diagonal exceeds the 6x6 diagonal by 1cm. Second, you are comparing lenses for SLR (Pentax), RF (Fuji) and TLR (Rollei) cameras. Lenses for RF and TLR cameras frequently extend into the camera body which is not possible for SLR cameras because of the mirror. Contrary to your opinion, I find that lens size is usually (i.e. in the 35-200mm range) an advantage of the P67 system in comparison to its SLR competitors.

-- Joachim Inkmann (Joachim.Inkmann@uni-konstanz.de), October 04, 2000.

Don4t get me wrong, I4m not unhappy with the lens size. I just wondered what the main advantages of a big lens are. I know view camera lenses don4t have to be small (the camera is big itself, so why bother making minute lenses), and probably it is easier to avoid abberations and other faults in big lenses. In 35mm slr, however, often the slower lenses (1,2/50 versus 2,0/50) are better than the very fast ones, and considerably smaller. Only if you need the big aperture, you4re willing to pay the extra $$. Pentax itself showed with the very flat 40mm (pancake) that a small lens can perform well.

-- Peter Gooijer (rockrose@freeler.nl), October 04, 2000.

Peter,

Joachim was right. The biggest reason for the lens size is having to project the image circle with the mirror box clearance in the body. This does not exist in RF or TLR cameras where the rear, correcting element is only millimeters from the film plane. LF lenses get HUGE when large image circles are required at long focal lengths (read large rear element to film plane distances), and that includes fairly slow lenses. Front element sizes (in mm) should consistently be focal length (in mm) divided by maximum aperture, that's due to physical realities.

-- Patrick Drennon (sierraengineering@worldnet.att.net), October 04, 2000.



What seems odd to me is that the Mamiya 7 incorporates a small mount projection even though it's a rangefinder. I assume there must be limit of distance from focus plane to rear element below which it becomes difficult to design a sharp lens for a given image circle. It'd be a lot more attractive system if it were more compact.

Anyway, it's pretty damn amazing that Pentax could design a 6X7 165mm/ f2.8 lens whose front element fits inside a 67mm filter ring. Why, then, did they drop down to f4 when designing the 165mm LS, even though they used a 77mm filter ring?

-- Bill Baker (wab@well.com), October 04, 2000.


Hi Steve and Bill,

the 165LS has a speed of 4 instead of 2.8, becourse of the size of the leaf shutter and its maximum shutterspeed to cover,I guess, so it has noting to with the front element.

Kind regards Gert Jan

-- Gert Jan Bollen (gert.jan.bollen@philips.com), October 05, 2000.


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