90mm Apo Summicron and Leica M9

[01af]

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Sorry, Olaf. [...] What was your point?

My point originally was to agree with UliWer in one of his points (focusing throw) and disagree with another (depth-of-focus), which both add up to the Apo-Summicron-M 90 mm Asph being easier to focus than the Apo-Summicron-M 75 mm Asph, even though the former's focal length is longer so it has less depth-of-field.

[albireo_double]

I've had (and sold) two of them - splendid lens, no focusing problems, great character. Since my return to the M9 stable earlier this year, I've been using the Elmarit 90/2.8 which is also a great lens; more compact and lightweight, super sharp, a bit less "character" than the Cron Apo. Apart from the weight and size, there is nothing not to like about the Cron Apo 90.

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[Prosophos]

The 90mm APO Summicron is marvelous. I've had difficulty taking mine off my camera since I acquired it recently:

 

Peter.

P r o s o p h o s | Photographing Life's Little Moments

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Peter.

P r o s o p h o s | Photographing Life's Little Moments

[marcpruiz]

The 90mm APO Summicron is marvelous. I've had difficulty taking mine off my camera since I acquired it recently:

 

Peter.

P r o s o p h o s | Photographing Life's Little Moments

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Peter.

P r o s o p h o s | Photographing Life's Little Moments

 

Very beautiful pictures, overcoat the third!

[Anil Kalagatla]

Interesting discussion on this thread so far - definitely convinced me that the 90mm apo is worth buying. Unfortunately for me I was not able to convert the purchase, but thanks to this thread, I will not hesitate the next time I see a good deal on the 90mm APO :-)

 

Somebody was mentioning the disadvantages of short focus throw for accurate focusing - I think that's a great point. I think a lens like the 50mm Summilux-ASPH which has a longer throw than the 50mm cron probably needs this when wide open - if you need precise focusing. With a lens like the 50mm lux, you really need the precise focus to take advantage of the amazing "3D-pop" at 1.4. However, my understanding is that the shorter focus throw of the summicron lenses (along with the focus tab) is really useful when you you do rapid/scale focusing at higher apertures - one is able to very quickly focus by feel and achieve reasonable focus (but not precise focus). To me this is still theory, since I haven't had enough experience to be able to focus by feel and position of the focus tab. But I'm hoping to gain this experience :-)

 

Anil

[Anil Kalagatla]

BTW, great pictures Peter! Especially love the third one - wonderful feeling comes through when I see the picture.

 

Anil

[luigi bertolotti]

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For the life of me, I cannot understand those who say they prefer today's Leica M lenses' ludicrously short focusing throws.

 

I agree... short throws MAY be quickier to get focus, and this can be an advantage for experienced photogs in action pics.... but about precision the old longer throws are definitely better... the short throw of my Summarit 75 is a feature that I have never appreciated

[luigi bertolotti]

My point originally was to agree with UliWer in one of his points (focusing throw) and disagree with another (depth-of-focus), which both add up to the Apo-Summicron-M 90 mm Asph being easier to focus than the Apo-Summicron-M 75 mm Asph, even though the former's focal length is longer so it has less depth-of-field.

 

DOF (Depth - Of - FOCUS ) ... is indeed smaller on a 90 than a 75, if one wants to take the same FIELD : at least this is the answer of the DOF calculator at f2 :

 

2 meters 75 = 8cm / 2,4 meters (same fov) 90 = 6cm

3 meters 75 = 19cm / 3,6 meters (same fov) 90 = 13 cm

4 meters 75 = 34cm / 4,8 meters (same fov) 90 = 23cm

 

and so on...

Edited April 25, 2013 by luigi bertolotti

[01af]

... short throws MAY be quicker to get focus ...

No, they're not. More finnicky, hence more finicking.

 

In fact, with a little practice, older lenses with particularly long focusing throws can be focused fairly accurately even without looking through the viewfinder, just by feeling the position of the focusing tab with your fingers. You see the shot and set focus while lifting the camera to the eye. Impossible with modern lenses

 

 

DOF (Depth - Of - FOCUS) ...

"DOF" stands for depth-of-field. Depth-of-focus is a different thing.

 

 

... is indeed smaller on a 90 than a 75 ...

In comparison to to 75 mm lens, depth-of-field is narrower but depth-of-focus is greater for a 90 mm lens.

Edited April 25, 2013 by 01af

[Anil Kalagatla]

 

"DOF" stands for depth-of-field. Depth-of-focus is a different thing.

 

Hi 01af: I'm curious what "Depth-of-focus" is. I know what dept of field is, but have not heard of depth of focus before. Can you provide a brief description?

 

Thanks

Anil

[01af]

I'm curious what "depth-of-focus" is. I know what dept of field is, but have not heard of depth of focus before. Can you provide a brief description?

Let me google that for you

[Anil Kalagatla]

Let me google that for you

 

Cool, thanks - good to learn a new concept. So, from what I can tell, depth-of-focus refers to the "tolerance" for the image plane within which you can still attain focus (i.e., an image plane equivalent for depth-of-field which is for the object plane).

 

Is this really such a big driver of focus for something like the M9 which (I'd imagine) has a pretty flat image plane? I think this was a concern for film cameras (due to buckling and misalignment and such). This also seems to be a much more important factor for microscopy and telescopy where the eye is at the image plane and there's a lot of variation in human eyes :-)

 

One thing I'm still confused about: according to the formula in the Wikipedia article, depth of focus ("t") is proportional to v/f where v is the image distance and f is the focal length seeming to imply that it's inversely proportional to focal length. Or does the corresponding image distance ("v") increase disproportionately to compensate? I.e., is v/f larger for longer focal lengths?

 

Finally, I guess it's still not super clear to me how this relates to ease of focusing (sorry for being dense). I understand the longer throw helping (since you have more room to more precisely rotate the lens and hence better chance of aligning the image in the rangefinder). But how does a larger depth of focus help (assuming the image plan is flat)? Sorry if this is a well known subject (could not find satisfactory answers via googling; already tried :-)).

 

Again, I'm fairly new to all this technical stuff of cameras and optics, but very interested in learning. Also, we're probably way off topic at this point as well - so sorry about hijacking my own thread.

 

Anil

[01af]

Is this really such a big driver of focus ...?

No-one said it was "a really big driver". It is one (small) factor among many.

 

 

... and telescopy where the eye is at the image plane ...

When looking through a telescope, the eye is not "at the image plane".

 

 

One thing I'm still confused about: according to the formula in the Wikipedia article, depth of focus ("t") is proportional to v/f where v is the image distance and f is the focal length seeming to imply that it's inversely proportional to focal length.

It seems to imply that indeed, but doesn't due to the nature of the image distance v. Because v/f can be re-written as (f + e)/f, with a non-negative e which is the extension which is zero at infinity and increases for shorter subject distances. At a given subject distance, e will increase in a super-proportional way for greater f, so even when f is in the denominator, v/f will increase along with f at a given subject distance (except at infinity focus where v/f will always be unity, regardless of f). After all, at a given distance, the greater focal length will give you a greater magnification, right?

 

Now (f + e)/f, in turn, can be re-written as (f/f + e/f), or (1 + m) where m = e/f = v/f - 1 is the magnification (which at infinity focus will always be zero). Note how depth-of-focus increases with magnification—much unlike depth-of-field which decreases with magnification.

 

 

Finally, I guess it's still not super-clear to me how this relates to ease of focusing ...

More depth-of-focus means more tolerance for focusing errors on the image side of things.

Edited April 26, 2013 by 01af

[Anil Kalagatla]

Cool, thanks for the clarifications.

 

Anil

[jto555]

No-one said it was "a really big driver". It is one (small) factor among many.

 

 

 

When looking through a telescope, the eye is not "at the image plane".

 

 

 

It seems to imply that indeed, but doesn't due to the nature of the image distance v. Because v/f can be re-written as (f + e)/f, with a non-negative e which is the extension which is zero at infinity and increases for shorter subject distances. At a given subject distance, e will increase in a super-proportional way for greater f, so even when f is in the denominator, v/f will increase along with f at a given subject distance (except at infinity focus where v/f will always be unity, regardless of f). After all, at a given distance, the greater focal length will give you a greater magnification, right?

 

Now (f + e)/f, in turn, can be re-written as (f/f + e/f), or (1 + m) where m = e/f = v/f - 1 is the magnification (which at infinity focus will always be zero). Note how depth-of-focus increases with magnification—much unlike depth-of-field which decreases with magnification.

 

 

 

More depth-of-focus means more tolerance for focusing errors on the image side of things.

 

 

And in English we say...

[lct]

OK free translation: Olaf dislikes short focus throw lenses.

[IkarusJohn]

OK free translation: Olaf dislikes short focus throw lenses.

 

I agree with him on that, but came to the conclusion the rest was more esoteric than I needed to know. Both my 75 Summilux and 90 Summicron are easy to get reliably sharp images. I couldn't reliably focus the now sold 75 Summicron.

 

Lines of maths don't help my photography. Depth of field v deep the of focus - more than I need to know. But your advice is always helpful, thank you, Olaf. It's usually better to give a more complete answer than less than is needed ...

 

Cheers

John

[01af]

Both my 75 Summilux and 90 Summicron are easy to get reliably sharp images.

Same here.

 

 

I couldn't reliably focus the now sold 75 Summicron.

Same here (umm, except my Apo-Summicron-M 75 mm Asph isn't sold yet but will be in the near future).

[lct]

No problem with my 75/2 on the M8.2 so i don't expect any difficulty on the M240 but i've not received my sample yet so wait and see.

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[120]

...

It seems to imply that indeed, but doesn't due to the nature of the image distance v. Because v/f can be re-written as (f + e)/f, with a non-negative e which is the extension which is zero at infinity and increases for shorter subject distances. At a given subject distance, e will increase in a super-proportional way for greater f, so even when f is in the denominator, v/f will increase along with f at a given subject distance (except at infinity focus where v/f will always be unity, regardless of f). After all, at a given distance, the greater focal length will give you a greater magnification, right?

 

Now (f + e)/f, in turn, can be re-written as (f/f + e/f), or (1 + m) where m = e/f = v/f - 1 is the magnification (which at infinity focus will always be zero). Note how depth-of-focus increases with magnification—much unlike depth-of-field which decreases with magnification.

 

 

 

 

Or just draw the diagram and you can see immediately if the f-number stays the same, the depth of focus does not depend on the focal length.

 

No notions of image distance, magnification, extension, or "super proportionality" needed.