90mm Apo Summicron and Leica M9

[120]

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...depth-of-focus does not depend on image format...

 

Depth of focus depends on the acceptable blur circle, which depends on format.

 

 

...You are missing that depth-of-focus is not depth-of-field.

 

Was not that long ago that "depth of focus" in a textbook meant depth of field; within the memory of some of the forum members.

[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?

 

...

hmm, now I'm confused...how does the depth of focus depend on focal length? I'm not sure what you are trying to show. The next to the last sentence is unclear, and the expression for depth of focus you started with is approximate. Can you make this clearer?...

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[120]

Hi:

 

I have a chance to buy a 2002 model 90mm Apo lens for my Leica M9 and I wanted to check to see if it's more trouble than not. I've looked through several threads on this topic on LUF and at the end I'm still unclear. Often I've found that one's biases tend to influence how one interprets the threads - in this case, I really want to buy the lens :-)

 

If the camera is reasonably well calibrated, what are the chances of a used lens bought online to have focusing issues - I seem to see more posts about this particular lens than other lenses (it could be that I'm just noticing these more). I'm checking with the seller, but not sure if they are able to verify or not.

 

Thanks in advance.

 

Anil

I have not had any problems with this lens.

[120]

...

o.k. I can prove the first Wiki-formula, and then t = 2CNv/f = 2CNu/(u-f) to get t as an increasing fcn of f; but what in the heck are you doing, and why do e.g. Stroebel and others take such pains to point out the depth of focus is independent of f ?

[01af]

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.

Well, in fact it does ... albeit at smaller magnifications the sensitivity of depth-of-focus to variantions of f is small, so when simplifying the formula to cut out f, the error will be negligible for small magnifications or at long subject distances. By the way, depth-of-focus also depends on the pupil magnification, so even the 'full' formula in the current Wikipedia article is a simplification.

 

 

... depth-of-focus does not depend on image format.

Depth of focus depends on the acceptable blur circle, which depends on format.

Ugh. Sure. So my first statement was right, and my 'correction' thereof was wrong.

 

 

Was not that long ago that 'depth of focus' in a textbook meant depth of field; within the memory of some of the forum members.

Depth-of-field and depth-of-focus have been confused with each other for a long time—in some textbooks and in the memories of some readers.

 

 

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?

The next-to-the-last sentence is unclear [...]. Can you make this clearer?

Be f1 < f2. Then, for a given subject distance shorter than infinity, v1/f1 < v2/f2.

 

That's why depth-of-focus is not independent of f.

 

 

... and why do e. g. Stroebel and others take such pains to point out the depth of focus is independent of f?

No idea. Maybe they're falsely taking a simplified formula for the real thing?

[120]

Well, in fact it does ...

 

I agree, see my last post. Where did you find this fact and where do you find the fancy version of the depth of focus formula? I seem to remember you got a fancy version of the depth of FIELD formula from a website (P. van Walree). That one seems to be his own creation.

 

Thanks

[120]

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...

Depth-of-field and depth-of-focus have been confused with each other for a long time—in some textbooks and in the memories of some readers.

 

...

 

No I mean the terms had different meanings; someone did an article on this. Supposedly depth of field is a nineteenth century notion (Dallmeyers, Taylor, etc.) and depth of focus is a twentieth century notion. Depth of focus or depth of definition was the original term for depth of field. The housecleaning did not begin until the 1950's.

 

...

Be f1 < f2. Then, for a given subject distance shorter than infinity, v1/f1 < v2/f2.

 

That's why depth-of-focus is not independent of f.

 

...

o.k. that is a first and last step but it's not clear (to me at least) how you are using the extension and filling in the details.

[algrove]

I bought my APO 90/2 ASPH and love it. Some of my best and most memorable images have been taken with it. I stopped using my 90/2 #11136 after getting the APO 90/2 even though I had it CLA'd and 6 bit coded by Leica NJ.

[120]

@Olaf

 

From Stroebel, View Camera Technique...

 

(He gives the depth of focus = 2NC approximation but here N is the effective f-number.)

 

"Four significant relationships involving depth of focus follow:

1. Depth of focus increases as a lens is stopped down. This is a direct proportion, as indicated in the preceding formula, so that stopping down from f/11 to f/22 doubles the depth of focus just as it doubles the depth of field. [sic]

2. Depth of focus increases as object distance decreases. Athough the marked f-number does not change as the subject to camera distance becomes smaller, the effective f-number becomes larger, and this value must be used in the preceding formula for short object distances.

3. Depth of focus is not affected by focal length. At the same f-number, all focal length lenses subtend the same angular cone of light. Note the term focal length does not appear in the formula.

4. Depth of focus increases as the film size increases. The viewing distance and therefore the acceptable circle of confusion increase with negative size."

 

In a diagram that appears elsewhere in a couple of places (and I did a borrow from), the legend is "Depth of focus t is independent of f provided that lenses are at the same value of N = f/D." No other commentary, but N is relative aperture.

 

So (I guess) no. 3 and the diagram only consider the case of infinity.

 

It does not look like focal length has much practical effect.

[01af]

Be f1 < f2. Then, for a given subj. distance shorter than infinity, v1/f1 < v2/f2.

 

That's why depth-of-focus is not independent of f.

O.k. that is a first and last step but it's not clear (to me at least) how you are using the extension and filling in the details.

To fill in the details, assume the two focal lengths being focused at said given subject distance.

 

 

... and why do e. g. Stroebel and others take such pains to point out the depth-of-focus is independent of f?

Maybe they're falsely taking a simplified formula for the real thing?

... or they were discussing depth-of-focus for a given magnification rather than for a given subject distance. In the former case, it is independent of f indeed; in the latter case, not.

 

EDIT: Uh. It's the 'simplified formula' thing.

 

 

So (I guess) no. 3 and the diagram only consider the case of infinity.

Exactly.

 

 

It does not look like focal length has much practical effect.

Not at, uh, 'regular' (i. e. non-macro) subject distances. Still, not much is more than nothing, at least in principle.

Edited April 29, 2013 by 01af

[120]

I bought my APO 90/2 ASPH and love it. Some of my best and most memorable images have been taken with it. I stopped using my 90/2 #11136 after getting the APO 90/2 even though I had it CLA'd and 6 bit coded by Leica NJ.

 

A couple of people here liked their last-version pre-asph. better for close up, and sold the apo. E. P. has a comment about the apo. close up.

[120]

Thank you Olaf for an interesting thread...

[fried.egg]

I have this lens bought new last year, using with my M9. I love the lens but this is the only lens in my bag that I cannot focus correctly, even with an 1.4 mag. It is a bit front focus and I have to manually compensate by turning the focusing ring after focus. I have no problem focusing other thin DOF lens such as 75 lux and 50 nocti and 135 telyt.

 

Despite the front focusing issue, I am using the lens a lot and love the color and resolution it gave it. The draw is unique compared to 75 lux. They both have their own drawing.

 

FRieD.eGG

 

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