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Why lensmakers' sweet spots in focal lengths??
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PostPosted: Sat Mar 27, 2010 7:52 am    Post subject: Why lensmakers' sweet spots in focal lengths?? Reply with quote

On another forum, a question arose: why are old 135s so common and 85s so rare? I generalized that: why did (35mm) lensmakers produces many interchangeable lenses in certain clusters of focal lengths, and not in others? Thus we have many 35s (and the odd 37), many 50-55s (and the odd 58), a fair number between 85-105 (many macros), many 135s, a few (expensive) 180s, many 200s, etc? And since 43mm is the 'normal' (frame diagonal) focal length for 35mm film, why so few there?

I charted focal lengths from 16mm up, and found that if each longer lens was about 25%-35% longer than its predecessor, I got a sequence of: 16, 21, 28, 35/37, 50, 65, 85, 105, 135, 180, 240, 300, 400, 500, 650. I own manual lenses in each of those focal lengths except 65 and 650 (with a 100 rather than 105). Notably missing from that sequence are 24, 43, 58, and 200, all of which I own (except the 'normal' 43).

I am curious about the gaps, and about why 180s and 240s are rare (and 180s usually costly). I'm especially curious about 43s. I know that many 35mm consumer rangefinders and P&S cams had fixed lenses in the 40-45 range, like my Yashica GSM's 45/1.7. One explanation I heard was that, before computers, lens design was tedious, so lensmakers stuck to a limited set of focal lengths. Yet these rangefinder 45s, and the 65s and 75s and 80s on many consumer MF cams, shows that they couldn't have been THAT hard to design and produce. Many MF cams have 'normal' lenses; why not 35mm SLRs? I don't buy the difficulty argument.

I've read that MF frames have so much area that it's feasible to crop a great deal and still have a usable, printable image, and that 50mm was popular because it essentially "pre-cropped" the image -- a 24x36mm frame had too little real estate to waste any. Yet in the heyday of 6x9cm folders, many snapshots were contact-printed, with no cropping; and many 35mm prints ARE closely cropped. So I don't buy the real estate / precropping argument either.

I have a suspicion that the SLR mirror may be a factor. Interchangeable 35/37 and shorter lenses contain retrofocus rear elements to push them far enough from the film/sensor plane that they don't impede a swinging mirror. Do the optical constraints of lens design preclude that for 43/45s, so that they just won't fit on, say, a M42 SLR? Does 43mm have the sheer bad luck to nearly unbuildable in M42? Is it a matter of optics, economics, tradition, or what?

And that brings up a related question: why so many fast 50s? I own 50/1.4, 50/1.8, 55/1.7, 55/2, 58/2. And a 24/2 and a couple of 85/2s. None of these was terribly expensive; some were dirt cheap. I've had a couple rangefinders with fixed f/1.7 lenses. Why so many CHEAP very fast lenses in just these focal lengths, and not others? Why these optical "sweet spots"?

I can't quite afford a copy of Kingslake's A History of the Photographic Lens and I'm not finding other references online. I'd appreciate if anyone here with knowledge of the history of lens design could enlighten me. Thanks so much.


PostPosted: Sat Mar 27, 2010 9:07 am    Post subject: Reply with quote

If I remember right, the first 'normal' lenses on SLR cameras were 55-58mm because anything wider than that would be more complicated to design without the rear of the lens hitting the mirror. Fast normal lenses were cheapest to produce and most versatile for the user. Large aperture was more important in the film era. ~50mm lenses were kit lenses of the time, that explains the cheap price.

Last edited by Riku on Sat Mar 27, 2010 9:20 am; edited 1 time in total


PostPosted: Sat Mar 27, 2010 9:19 am    Post subject: Reply with quote

Slightly longer lenses (55-58mm) were easier to design with faster apertures. Increase the focal lenght (ie 85mm and more), to keep the same aperture you have to increase the amount of glass, metal, and it became difficult to get a sharp lens. In the other way, decreasing the focal lenght of a SLR lens means you have to use a retrofocus design, which can't give very fast lenses (we know *some* 24/1.4 but no 20/2 sold in wide series) Wink An other thing, is that we human easily are used to fixed conditions, by example, for focal lenght, and their rendering Wink


PostPosted: Sat Mar 27, 2010 12:17 pm    Post subject: Reply with quote

Very interesting thread Smile

Many of these aspects ar related to construction of the lenses. For range-finder and viewfinder camerase there is a lot of freedom for lens designs. 40-50mm lenses were more common, that on SLR cameras.

SLR cameras have mirror box, which prevents to use lenses, which have protruding rear elemens. This is typical for lenses using "standard" construction + short focal lenght. E.g. 50mm Sonnars, 35mm biogons etc.

There are a few early 40mm SLR lenses - they wasn't very popular, because these early designs were quite compromised. Tessar 40/4.5 was sharp, but too slow and protruding rear element prevented usage on many SLR cameras. Meyer Helioplan was more compatible, but slow and less sharp than the Tessar. Steinheil 40mm Cassomething was triplet based, so not as sharp... etc..

That's why retrofocal lens design become so popular - it allowed to make wide leneses without protruding elements. The negative side of retrofocal lenses are CA, distorsion and large size of fast models - because of that they were never used as "standard" lenses.

When better types of glass allowed construction of non-retrofocal 40-45mm lenses, it become quite popular - Pentax 40/2.8, Pentax Limited 40/2.8, Pentax Limited 43/1.9, Voigtlander Ultron 40/2.0...

But another problem appeared - current SLR cameras are mostly based on APS-C sized sensors, so 40mm isn't as interesting, as it could be on full-frame camera (allowing quite wide angle of view without compromises of retrofocal designs).

As for 180mm lenses - many of them are f/2.8 => very fast for this focal length. Fast lenses are always expensive. According to my experience, good copy of 180/2.8 Sonnar is a bit sharper and has slightly less CA, than good copy of 200/2.8 Sonnar. Maybe this is the reason of their popularity.


PostPosted: Sat Mar 27, 2010 4:43 pm    Post subject: Reply with quote

180mm and 240mm (actually mostly 250mm) were very common focal lengths pre-1970 for 35mm, and they were not particularly expensive when not made in large apertures.

The Exakta in the 1950's (the first major camera market for SLR lenses) had low-cost 180's and 240/250's available, but there were very few 200's in that mount until the early 1960's, and the 300's were the more expensive large aperture types until then also.

The usual third party Japanese makers all had 180 f/3.5 and 250 f/4.5 in their lines until the early 1970's. I have plenty of these.

I suspect that the shift came in the 1960's as a desire to parallel Nikons product line instead of the Exaktas. Nikon had an expensive 180, a cheap 200, and no 250.

The 135 may be a holdover from the rangefinder days, as 135mm is about the longest practical FL for a rangefinder, besides the fact that 135mm was already a standard FL for the typical German 9x12cm plate camera. It was easy to make a decent f/3.5-4.5 135mm for a rangefinder, just grab your old plate camera lens, shrink it a bit. and put it in a focusing mount. Voila, a low cost "extra" resellers can add to sweeten an SLR camera deal.


PostPosted: Sat Mar 27, 2010 6:18 pm    Post subject: Reply with quote

Consumer preferences must have played a part. Longer lenses are more difficult to use. Maybe 135 is the best compromise between magnification and necessary shutter speeds for the normal, hand-holding user in shooting in daylight.


PostPosted: Sun Mar 28, 2010 12:42 am    Post subject: Reply with quote

I think there is some marketing involved here also...many lenses marked "50mm" are actually anywhere from 47-53mm. My Distagon 25 was probably really closer to 27mm...I'm sure other more knowledgeable forum members can bring up other examples. The same goes for f-stop values I would imagine. If you were a lens/camera maker, and your design for a new lens was really a 53/1.5, I would think you would market it as a 50/1.4.

That being said, I really want a 75-85mm Sonnar for EOS mount, that I can actually afford...the Tak is far too collectible to be reasonably priced. Maybe I could sell my child! Wink