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03-24-2010, 06:01 PM   #16
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QuoteOriginally posted by Lowell Goudge Quote
What you are suggesting is that every maker make lenses in 1mm focal length increments so you have infinite selection.
You obviously didn't read what I wrote. I made no suggestions at all. I asked WHY there are clusters of common focal lengths of interchangeable 35/FF lenses. We see MANY at 35mm, 50-55mm, 85-105mm, and 135mm, and FEW around 43mm (the true normal for 35/FF) or 70mm or 120mm. A little further out, 200mm's are common and cheap, while 180's are rare and expensive. I just asked WHY this is so? Tradition, economics, optics, or what?

EDIT: I'm not totally ignorant of patterns. I put together a chart of focal lengths, and their percentage of increase from the previous (shorter) FL. It's instructive:

FL -- PCT
--- -- -----
12 --
16 -- 1.33
21 -- 1.31
28 -- 1.33
37 -- 1.32
50 -- 1.35
65 -- 1.30
85 -- 1.31
105 - 1.24
135 - 1.29
180 - 1.33
240 - 1.33
300 - 1.25
400 - 1.33
500 - 1.25
650 - 1.30
850 - 1.31

An obvious pattern -- each FL is (roughly) about 1/3 longer than the previous. But notice the anomalies. No 24's or 35's or 90's or 200's. The 'normal' 43 is missing. The unknown 65 and rare 37 and 180 and 240 are there. There's no straight progression of focal lengths. I just wonder WHY? Yeah, I understand that 200 and 300 and 400 and 500 are nice round numbers. I'm just curious why some focal lengths were produced, and others weren't.

EDIT2: I ain't done with this yet! I did another simple calculation, starting at 43mm (the diagonal of a 35mm frame and thus what in larger formats would be called 'normal') and ran up and down the line from there. Each smaller FL is 1/3 smaller, each larger is 1/3 larger, and the sun pours down like honey. Here's the sequence: 10, 14, 18, 24, 32, 43, 57, 76, 102, 136, 181, 242, 322, 430, 573. We could fudge a little to get a few more familiar numbers: 10, 14, 18, 24, 32, 43, 58, 75, 100, 135, 180, 240, 320, 425, 575. Ah, lens numerology! I'd better stop now.


Last edited by RioRico; 03-24-2010 at 07:14 PM.
03-25-2010, 08:08 AM   #17
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QuoteOriginally posted by sjwoo Quote
Why is it that the 85mm primes I see are so much more expensive on eBay than the 135mm?
If you come across the SMC Pentax-A* 135mm F1.8 on eBay, you will usually find it the other way round. The last one I saw sold, went for $1976.00 in March this year.

But it is truely a great piece of glass.
03-25-2010, 08:28 AM   #18
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QuoteOriginally posted by RioRico Quote
You obviously didn't read what I wrote. I made no suggestions at all. I asked WHY there are clusters of common focal lengths of interchangeable 35/FF lenses. We see MANY at 35mm, 50-55mm, 85-105mm, and 135mm, and FEW around 43mm (the true normal for 35/FF) or 70mm or 120mm. A little further out, 200mm's are common and cheap, while 180's are rare and expensive. I just asked WHY this is so? Tradition, economics, optics, or what?
You've been given a few answers which you've rejected out of hand.
Why don't you stop bugging us until you've figured it out, and then get back to us with the one true answer.
03-25-2010, 09:08 AM   #19
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This is all David Letterman's fault for making that joke about Palin's daughter.

03-25-2010, 10:23 AM   #20
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QuoteOriginally posted by Wheatfield Quote
You've been given a few answers which you've rejected out of hand.
I only reject answers that don't match observable facts. Lenses of many different focal lengths have been designed and produced, so they're not impossible; yet lenses in a few focal lengths predominated in film days, with some glaring holes. Maybe it is tradition. Like I said, I'll have to look for documentation of the history of optical design. Now calm down. Have some chamomile tea. Think pleasant thoughts. Don't think about Palin's daughter.
03-25-2010, 11:06 AM   #21
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QuoteOriginally posted by RioRico Quote
I only reject answers that don't match observable facts. Lenses of many different focal lengths have been designed and produced, so they're not impossible; yet lenses in a few focal lengths predominated in film days, with some glaring holes. Maybe it is tradition. Like I said, I'll have to look for documentation of the history of optical design. Now calm down. Have some chamomile tea. Think pleasant thoughts. Don't think about Palin's daughter.

I am wondering which explanation didn't match the observable facts though.
Oh yes, perhaps it was designing lenses on graph paper making manufacturers reticent about designing lenses in every concievable focal length.
Pity you don't see the obvious.
Don't stick anything too sharp into your eye while looking for those glaring holes.
Or perhaps you already have, given your ability to see the obvious.
03-25-2010, 02:48 PM   #22
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QuoteOriginally posted by Wheatfield Quote
I am wondering which explanation didn't match the observable facts though.
Oh yes, perhaps it was designing lenses on graph paper making manufacturers reticent about designing lenses in every concievable focal length.
Pity you don't see the obvious.
Don't stick anything too sharp into your eye while looking for those glaring holes.
Or perhaps you already have, given your ability to see the obvious.
I'll speak of lenses from the pre-computer era.
I'll say this very very slowly.

Yes, lens design (by hand) is very tedious work.
Yes, lens production is a non-trivial procedure.
And yet, lenses of many focal lengths were designed and produced.
That's why I don't buy the "it's too difficult" argument.

Lensmakers tend to cluster their production at specific focal lengths.
Those clustered focal lengths don't bear any fixed relationship.
Some lenses are produced outside those clusters, but relatively few.
Some lensmakers produce more than a few lenses outside those clusters.
They apparently didn't find the design and production too difficult.
That's why I don't buy the "it's too difficult" argument.

Trying to reason through this with inductive logic is pointless.
Dismissing the question with "you don't see the obvious" is pointless.
I am still curious; that's why I'm looking at the history of optics design.
Perhaps I'll find designers talking of why they chose certain options.
This is not worth arguing about. Bye now.
03-25-2010, 03:38 PM   #23
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QuoteOriginally posted by RioRico Quote
An obvious pattern -- each FL is (roughly) about 1/3 longer than the previous. But notice the anomalies. No 24's or 35's or 90's or 200's. The 'normal' 43 is missing.
You're saying Pentax didn't make a 40mm or 43mm?


Last edited by tuco; 03-25-2010 at 03:48 PM.
03-25-2010, 05:17 PM   #24
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QuoteOriginally posted by tuco Quote
You're saying Pentax didn't make a 40mm or 43mm?
No, I'm saying that few lensmakers produced interchangeable FF 40s or 43s -- I certainly don't see them on eBay. 43mm ISN'T in the pattern or lineup of common FL's of interchangeable FF lenses. And that's odd, because it's 'normal' for 35mm film. ("Normal' means the diagonal of the film or sensor frame.) And medium-format cams commonly have near-'normal' lenses.

35mm film rangefinder or viewfinder/P&S cameras often had fixed, non-removable lenses in the 40-45 range, so designing them apparently wasn't that difficult. And since many were used to shoot Kodachrome and other transparency films with little leeway for sloppiness, they had to be sharp. My Yashica GSN rangefinder wears a 45/1.7 lens that's nice and crisp.

I'll do more research, but I suspect that for lenses in the 40-45mm range, the mirror is the culprit. In a non-SLR (rangefinder, viewfinder) the lens rear elements can project a bit into the space behind the mount. There's less room available in a SLR because of the swinging mirror. Wide-angle lenses have an inverted teleconverter element (I forget the technical name) in back, to make them 'look' shorter and keep them away from that mirror. Maybe the optics of 40-45mm just doesn't allow that kind of trick. And maybe I'm totally wrong about that. More research, more research...

Which still doesn't explain the lack (or scarcity) of lenses around 65-70mm. That's the major hole in the lineup of 35/FF lenses. Why? The quest continues.
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