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Relationship of focus throw to focus length?


cnick6

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So I have been noticing that my Summicron-M 75mm has a very long focus throw which means I have to focus very precisely in order to get that perfect IQ. Then it sort of hit me that there must be some relationship between the FL and the amount of focusing required for each lens.  

 

Do the shorter FL lenses such as the 28mm offer a simpler focus throw (less turn of the focus ring) to acquire that high IQ -- even compared to the 35mm lenses?  If so, is this why most Leica photographers stick with the wider FL lenses?  (I'm not suggesting there has been any official study done on this fact.  I just have noticed Leica professionals best pictures seem to always be with the primary 28/35/50mm lenses.)

 

 

Thanks,

-Nick

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Leica could, if they wished, make the throw shorter however 75mm is close to the precision tolerance of the rangefinder. Having a long throw allows them to best coordinate the RF with the subject. I rather doubt you would like a shorter throw, especially at relatively (f/5.6 or wider) apertures.

Edited by pico
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I often like longer throws, which generally offer more range to dial in precise focus.  With practice, most M users can get close to proper focus before even lifting the camera to the eye, leaving only fine tuning adjustments at time of shot.  

 

It helps, too, to have a smooth focus action; the sticky focus on my 50 Summilux ASPH made fine-focusing problematic until it was fixed by DAG (who replaced the grease with the old Leitz grease stock...better than new).

 

Jeff

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I have Canon and Yashica rangefinders, both have a very short throw. That is fine if you want to focus quickly to grab a shot, but if you find yourself doing a lot of that you should probably be using an SLR anyway. I much prefer the long throw of the Leica lenses, it makes critical focusing much easier.

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[...] there must be some relationship between the FL and the amount of focusing required for each lens.  [...]

 

Or between focus throw and DoF perhaps. 75/2 and 75/2.5 have shorter focus throws than 75/1.4 for instance. And 90/4 macro has a shorter focus throw than 90/2 apo. But it is also a matter of vintage, earlier lenses having longer focus throws generally. 21/3.4 vs 21/3.4 asph, 50/2.8 v1 vs 50/2.8 v2, 90/4 collapsible vs 90/4 macro, etc.

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I often like longer throws, which generally offer more range to dial in precise focus.  With practice, most M users can get close to proper focus before even lifting the camera to the eye, leaving only fine tuning adjustments at time of shot.  

 

Thanks Jeff.  Yes, I definitely prefer the longer throw for that extra precision.  I just learned this past weekend those 'extra precise' adjustments are super-critical in getting the maximum IQ.  I'm a newbie, so I'll get better at it.  This is just a new world for me!  Albeit, an exciting one. ;)

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Or between focus throw and DoF perhaps. 75/2 and 75/2.5 have shorter focus throws than 75/1.4 for instance. And 90/4 macro has a shorter focus throw than 90/2 apo. But it is also a matter of vintage, earlier lenses having longer focus throws generally. 21/3.4 vs 21/3.4 asph, 50/2.8 v1 vs 50/2.8 v2, 90/4 collapsible vs 90/4 macro, etc.

 

Good point lct.  I think you're right about the DoF in that it also plays a key role in the focusing.  I have my fancy Leica book at home which discusses the Leica lens history, etc.  I'll need to re-read it and see if this subject is mentioned.

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Leica could, if they wished, make the throw shorter however 75mm is close to the precision tolerance of the rangefinder. Having a long throw allows them to best coordinate the RF with the subject. I rather doubt you would like a shorter throw, especially at relatively (f/5.6 or wider) apertures.

 

Yeah I get your point pico but I sure like shorter throws on my binoculars!  ;)

 

PS.  My Leica Ultravid HD Plus 8x42 has a long-ish throw too.  Perhaps Leica just likes the longer throw design.  My other binocular models have a much shorter throw and it's great for quick focusing.  

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If you search the forum (APO-Summicron-M 75 ASPH), you will find a discussion some years ago when a very helpful person tracked down the relative focus throws of what could be "problematic" lenses.  The starting point for the discussion, if I recall, was that the APO-Summicron-M 75 ASPH was particularly difficult to focus accurately (I sold mine for that reason).

 

Anyway, the comparison showed that the throw on the 75 Summicron was shorter than potentially more difficult lenses like the 75 Summilux, the Noctilux and the AA Summicron 90.  I always found my 75 Summicron terrible to focus because it was hit and miss, whereas the Noct and the 75 Summilux (with their shallower depth of field wide open) easier because of the longer focus throw.  As for the 50 Summilux ASPH, when mine was new it had the stiff step when the FLE was engaged (at least, that was the explanation I remember being given).  With use, it focusses freely and is actually my smoothest focussing lens.  I guess use is the key!

 

Cheers

John

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 As for the 50 Summilux ASPH, when mine was new it had the stiff step when the FLE was engaged (at least, that was the explanation I remember being given).  With use, it focusses freely and is actually my smoothest focussing lens.  I guess use is the key!

 

 

 

Or better grease (post #3).  DAG debunked the FLE explanation, and proved Leica NJ Service wrong when they said that mine couldn't be improved.

 

Jeff

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... DAG debunked the FLE explanation ...

 

 

 

How so?

 

Mine shows a definite and consistent increase in pressure at a very distinct point.  That said, no grease, no servicing, and mine is smooth as butter, but with a two stage feel to it.  I think I'll stick with the FLE explanation ...

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Just a matter of QC if you ask me. My 50/1.4 asph was stiff and using it did nothing to fix it. Was returned two or three times to Wetzlar and after several months there it is now butter smooth with no two-stage feeling at all. While my 75/2 has always been smooth but has some two-stage feeling at 2 metres more or less.

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So I have been noticing that my Summicron-M 75mm has a very long focus throw which means I have to focus very precisely in order to get that perfect IQ. Then it sort of hit me that there must be some relationship between the FL and the amount of focusing required for each lens.  

 

Do the shorter FL lenses such as the 28mm offer a simpler focus throw (less turn of the focus ring) to acquire that high IQ -- even compared to the 35mm lenses?  If so, is this why most Leica photographers stick with the wider FL lenses?  (I'm not suggesting there has been any official study done on this fact.  I just have noticed Leica professionals best pictures seem to always be with the primary 28/35/50mm lenses.)

 

 

Thanks,

-Nick

 

Working through your questions in reverse order:

 

Most Leica photographers stick with wider lenses - if true - because a rangefinder focus system has a fixed precision, which is very competent for a 50mm lens, vast overkill for wide angles that have massive depth of field, but starts to be very picky and barely precise enough for longer focal lengths that have much more limited DoF. You'll notice Leica does not even offer lenses longer than 135mm - they know the limitation of the RF system. Back in the 1960's when pro SLRs first became available, most pros immediately switched to using the SLRs for anything longer than 85mm, even if they kept their lighter, smaller Leica RFs for the 21-50mm range.

 

Thus you find Larry Burrows in Vietnam, with a couple of Leicas for 35/50 and perhaps 28mm - and a couple of Nikon SLRs for 105 and 200mm.  https://s-media-cache-ak0.pinimg.com/236x/7c/45/22/7c45224fde91096c3d46cba50f439436.jpg

 

It has tended to be that way ever since. The Leica RFs, especially once the lower-magnification "M2" finder with 35mm viewing became the standard, have been favored for the shorter, less focus-critical focal lengths. (There are chicken and egg effects as well - the relative silence and compactness of the Ms make them more "friendly" working in close with a wide angle, while who notices the size and noise of an SLR from 20 feet away with a 200mm?)

 

As to "focus throw" - this depends on exactly what you mean. Focus throw usually means the amount, in degrees, that you have to turn the focus ring on a lens to get from infinity down to, say 1 meter/3 feet. I.E. the spacing of the distance numbers around the focus ring. Although I can see where it also makes sense to use the term to describe the actual linear lens movement in and out.

 

In the second case - yes, there is a direct proportional relationship between the focal length, and the amount you must move the lens in and out to change focus. You need to move a 35mm lens only a few mm to cover the same focus range that takes several cms/inches of movement with a 75 or 135 or 400mm.

 

In the first case, the relationship is the same - IF you simply move the lens in and out like a trombone (Leica f/6.8 Telyt telephotos 400/560/800mm) or rack it in and out with a rack-and-pinion (most view cameras and most 6x6 TLRs).

 

But if you use a ring to move the lens in and out, then the focus throw of the ring can be non-proportional to focal length, because there is a "gearing system" - the focusing helical (a threaded tube) that converts rotary motion into linear motion.

 

The pitch of the focus helical threads can be steep (a lot of in-and-out movement per degree of turn) or shallow (a lot of degrees of turn required for a given amount of movement). So it's perfectly easy to design a 35 and a 75 lens that have the exact same "throw" (in terms of the focus ring rotation) - just make the threads shallow in the 35mm design, and steeper in the 75mm design. The Summarit 35 and 75 ring throws are almost identical at about 90° to get from infinity to 3 feet/0.9m.

 

The kicker is usually weight - how much lens mass you have to move. A shallower, longer focus throw requires less finger pressure/effort (but over a longer distance and time) to move a heavy lens, than a steep, rapid throw. So the massive 75mm f/1.4 uses a much longer ring throw than the much-lighter 75 Summarit, so that weaker fingers can move it at all. My 80mm f/2.8 Hasselblad lens uses 270° of throw to get from infinity to 3 feet, compared to the Summarit 75mm f/2.4 (about 90°) - for virtually the same focal length and aperture.

 

Now, as to the 75 Summicron. First, it is the tightest-framing M rangefinder lens ever made, except for the 90 macro (which uses an extension tube). Therefore it needs more linear movement to get from infinity down to its minimum focus of 0.7 meters. It gets really long extended to focus at 0.7 meters. Additionally, as others have mentioned - it has a floating element to change the optical formula slightly as it focuses closer for improved close-up performance. The front sections of the glass are moved forward more than the back elements.

 

As to focus precision and focusing ring throw - yes, a longer throw tends to improve vernier focus precision, simply because a given finger movement of the ring produces a smaller change in focus. It's like trying to stop a car precisely on a white line painted on the pavement - for best precision, would you rather speed towards the line at 60mph and then slam on the brakes, or creep up to the line at 10mph?

Edited by adan
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Thanks Adan for the detailed, technical explanation.  And yes, by using "focus throw" I was referring to the number of degrees that focusing is possible.  My binocular terminology intermixing with photography again.  

 

For some reason, I was thinking in terms of a normal lens system (Sony, Nikon, etc.) and as soon as you bought up the threading that jarred my memory and I then realized we're talking about "Leica" lenses...  

 

Also, I forgot to check last night, but I do remember a section in one of my Leica books about the limitations of the RF with respect to focal length.  This might be the reason I posed this question.

 

BTW, I did search older posts regarding the 75AA's past issues but my like-new model seems to be a good one.  I'm not having any issues with focusing.  I think it just comes down the user getting it right the first time! ;)

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