Easier to focus 75mm cron or 90mm apo

[stump4545]

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I have both the 75mm cron apo and the 90mm apo.

 

Anyone else here find it easier to hit focus with the 75mm over the 90 apo?

 

That extra 15mm in focal length for me at least really makes it hard to hit exact focus.

[uhoh7]

I doubt anyone would disagree, though does the 75 cron have shorter throw?

 

Anyway those are two incredible lenses I wish I had LOL

 

Instead I use 75 Lux and 90 Summarit

[IkarusJohn]

I have both the 75mm cron apo and the 90mm apo.

 

Anyone else here find it easier to hit focus with the 75mm over the 90 apo?

 

That extra 15mm in focal length for me at least really makes it hard to hit exact focus.

 

Curiously, I found the reverse.

[lct]

At f/2, any 90mm lens is at the limit of the RF accuracy on 0.72x and 0.68x full frame bodies (same as a 75 at f/1.4) so when comparing well calibrated lenses, the 90 should me more tricky to focus at full aperture than the 75 at f/2 normally. I mean with a rangefinder of course as there is no problem with EVF's obviously. Now in practice i have no problem with either lens on my M240 but focussing needs indeed some concentration with both lenses and perhaps a bit more with the 90. 

[lawman]

Leica 1.4 x magnifier really helps with both of these lenses.

[CheshireCat]

I had a 90 APO and exchanged it for a 75 APO which is easier (actually, less difficult) to focus.

Apart from that, the 75 is a better lens technically.

[Herr Barnack]

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I have the 90mm Summicron APO and am finding that nowadays I am missing focus at f/2; if I stop down to f/4 and shoot from a tripod the problem resolves itself.  This was not a problem 12 years ago when I bought this lens.  Ah, the joys of old coothood!   

 

Oddly enough, I can nail the focuse at f/1.0 with my Noctilux so I am wondering if the problem is the 90/2 APO rather than my eyesight.

 

I have been thinking about getting the 1.4x magnifier.  I'm beginning to think even more about it now.

[lct]

There is less DoF at 90/2 than 50/1 which could explain that.

[adan]

lct - didn't you used to have a chart graphing focal lengths against RF mag against DoF/aperture, that showed the limits of a lens/aperture against the various rangefinders? May have been one of the other DoF/optical experts here. RF mag on the y axis, and apertures along the x axis, and upsloped colored curves for each focal length showing what each lens/aperture required for good focus consistency.

 

IIRC, 75 @ f/2 was just below the .72x finder limit - 90 @ f/2 nudged above it slightly, 135 APO @ f/3.4 "required" .85x or higher. etc. (Although, as with you, I find practical reality to be more tolerant than theory).

 

However, as cheshirecat notes, the 90 APO is not quite as sharp @ f/2 as the 75, especially in the close range, where the 90 has no FLE correction - even if focused perfectly.

 

All I know is that I get a larger number of sharp pictures from the 75 than the 90 - consistently.

[otho]

"All I know is that I get a larger number of sharp pictures from the 75 than the 90 - consistently."

I strongly agree with above statement for M system, but AA 75 or AA 90 paired with Sony A7x (in my case - A7R II) they doesn't matter for me: both lenses are equally stunning sharp pieces of glass.

[lct]

We had interesting discussions with charts in the thread below but i'm no expert in those matters at all. Bottom line DoF is slightly shallower out of 90/2 than 75/2 lenses expectedly but others factors come into play in actual hit rates like the weight of the lens or the length of focus throw that are not taken into account by DoF formulas. In my modest experience RF and/or lens calibration is the main culprit as far as rangefinders are concerned when results are not as good as expected. The best way to check it is to compare with and w/o EVF for those having live view.

http://www.l-camera-forum.com/topic/233759-75mm-apo-depth-of-field/

[jaapv]

I have both the 75mm cron apo and the 90mm apo.

 

Anyone else here find it easier to hit focus with the 75mm over the 90 apo?

 

That extra 15mm in focal length for me at least really makes it hard to hit exact focus.

0,03 mm DOF @ 3m is 8cm for 90, 12 cm for 75. You are focusing both lenses exactly the same, but you are seeing your  mistakes better on the 90 Having said that I always found the 90 AA one of the most difficult lenses to hit focus with, for the reasons mentioned in LCT's post.

[adan]

Actually I just found this ancient graph (pre-1954, since it compares the Leica IIIc and Contax II RFs), which more or less equates to the colored chart I saw on our forum once upon a time.

 

http://static.photo.net/attachments/bboard/00E/00EUiN-26943584.bmp

 

Y-axis at left is the rangefinder effective baseline (basislänge) - the actual distance between the centers of the RF windows (68.5 mm for the Leica M RF) multiplied by the finder magnification (e.g. 0.68x for a digital M)

 

X-axis lists focal lengths

 

Curved datum lines represent apertures (see top right for labels)

 

Find your focal length along the bottom, draw a vertical line to hit the curved line for the aperture you'll be using, draw a line to the left to read off how long an effective baselength you'd need.

 

Obviously a certain DoF (what counts as "correctly focused") is assumed, but I don't see it listed, unless it is in the German text (that "z = 0,03mm" looks suspiciously familiar). What looks correctly focused in a 5x7 print may not be what looks sharp at 100% pixels or a large print.

 

But (per this chart) for 75mm f/2 you need a 35mm effective base length

 

For 90mm f/2, you need a 40mm effective base length

 

The 135 f/3.4 "requires" 55mm EBL - or 0.8x magnification

 

Digital M baselength = 68.5mm actual base x 0.68 viewfinder = 46.58mm effective. So either a 75 or 90 @ f/2 should be focusable to whatever standard of precision this chart assumes. A 135 (in theory) needs to be stopped down to f/4.5 (surprise, surprise, Leica's screw-mount 135 Hektors were - f/4.5!) - or a higher magnification than the digital bodies provide.

 

Other values:

 

75 f/1.4 requires right at 46mm or so

 

50 f/1.0 requires about 28mm EBL, f/0.95 maybe 1mm more (if that).

[CheshireCat]

0,03 mm DOF @ 3m is 8cm for 90, 12 cm for 75. You are focusing both lenses exactly the same, but you are seeing your  mistakes better on the 90

 

However, to be fair, for the same portrait framing, you need to be a little closer with the 75.

 

Also, non-apochromatic lenses will somewhat hide focusing errors, as fewer spectrum frequencies will be in "correct focus". Though I must say that I do not consider the 90 APO-Summicron an apochromatic lens, and its lack of FLE further decreases near focus performance.

[bencoyote]

We had interesting discussions with charts in the thread below but i'm no expert in those matters at all. Bottom line DoF is slightly shallower out of 90/2 than 75/2 lenses expectedly but others factors come into play in actual hit rates like the weight of the lens or the length of focus throw that are not taken into account by DoF formulas. In my modest experience RF and/or lens calibration is the main culprit as far as rangefinders are concerned when results are not as good as expected. The best way to check it is to compare with and w/o EVF for those having live view.

http://www.l-camera-forum.com/topic/233759-75mm-apo-depth-of-field/

 

Heh I had a little trouble when I first got the 90 AA but once I put in some concerted time learning to focus it quickly and accurately, I haven't found it that hard. I had to fail really miserably at one event, to clearly show me that nailing focus on a 90mm f/2 was substantially harder than a 50mm at f/1.4 and that I needed to practice. Most of this was shot with the 90 AA at F/2. https://goo.gl/photos/i4ACFQj1v5uhAHxM6

 

Shooting that event inspired me to add an addendum to Capa's old quote: "If your photos aren't good enough, you aren't close enough." To which I add, "except you don't want to be too close when people are swinging flaming objects like swords and whips. It is OK to use a telephoto lens." ;-)

[lct]

[...] Digital M baselength = 68.5mm (69.25) actual base x 0.68 viewfinder = 46.58mm (47.09) effective. So either a 75 or 90 @ f/2 should be focusable to whatever standard of precision this chart assumes. A 135 (in theory) needs to be stopped down to f/4.5 (f/4). [...]

 

My results (in red) are very close to yours. They are based upon the formula b = e*f^2/k*z where b is the critical RF base length, e the visual acuity (0.0003), f the focal length, k the aperture and z the circle of confusion (0.03 for FF, 0.022 for APS-H).  BTW i've retrieved an old chart of mine below. Not sure if it is accurate though. 

 

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[bencoyote]

 

Obviously a certain DoF (what counts as "correctly focused") is assumed, but I don't see it listed, unless it is in the German text (that "z = 0,03mm" looks suspiciously familiar). What looks correctly focused in a 5x7 print may not be what looks sharp at 100% pixels or a large print.

 

One part that I never quite understood about all the math like this is the circle of confusion and the relationship to a print and a viewing distance. That seems to make some sense in the case of a print from film but it does not seem to make sense to me in the case of digital. Why isn't the standard based upon actual pixel dimensions on the sensor?

 

For example: "out of focus" means something objective like: a beam of light small enough to illuminate just one pixel when properly focused ends illuminating adjacent pixels. Or if that results in something too narrow, reintroduce the circle of confusion and say something like "out of focus" represents a case where a beam of light illuminates a circle of pixels only X in diameter.

[lct]

We would have different CoC values depending upon sensor resolution then. Usual CoC values are a mere translation of the 5 line pairs per millimeter that human sight can perceive at an average viewing distance of 25 or 30cm if i recall correctly. They are not related with film or digital per se and are not made for billboards nor for pixel peeping obviously. Other CoC values can be chosen for more demanding purposes if need be but they would have consequences on RF accuracy necessarily.

[adan]

Ben - Your proposed standards would work - if all pictures were viewed at 100% pixels. But since they are not, except by pixel-peepers, the old concept that DoF is tied to final image size still applies, digital or film; images viewed on a screen or as a print.

 

Remember what DoF is - the range of distances outside the correct focus plane, within which objects thus out-of-focus will appear sharp in the final image.

 

Take the image below, sized as a "4x6" print - what is the DoF? I.E. what appears to be sharp? Fingers, tip of nose, eyes, eyebrows, mouth, side of hand? Can you say for certain whether I nailed focus on the eyes, or got the bridge or the tip of the nose instead?

 

Whatever parts of the lady's face appear to be sharp, fall within the DoF - for this image size. You can even estimate it in distance between the nearest and farthest things apparently sharp. 2 inches? 4 inches? 6 inches?

 

Will your standard of blur or no blur outside a single pixel help you at all?

 

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Same for this landscape (coincidentally with a 75) - Does the DoF cover the distance from the mine office to the distant hills? If it all appears sharp, then it does.

 

 

In other words, you don't get access to my dng files, or Ansel Adams' negatives, to peruse with a loupe. What you get is the final image, whether it be a 16x20 print, or a 7x9 book reproduction, or a 800 x 600 web image. And what looks sharp in those, is within the DoF.

 

Poke around "objectively" in one of AA's negs with a 100x magnifier, and you may find that a rock in the corner isn't quite in focus, even though it appears so in all the prints. Or the mine office or the mountains in my color shot may be oof blown up to 100% pixels. But it just isn't useful. Publish your "discovery", and people will go and look at the prints in their final size or the web reproduction in its final size, and say "I don't see anything out of focus!" - End of story.

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Hallo,

Look here: Leica M

[asiafish]

At f/2, any 90mm lens is at the limit of the RF accuracy on 0.72x and 0.68x full frame bodies (same as a 75 at f/1.4) so when comparing well calibrated lenses, the 90 should me more tricky to focus at full aperture than the 75 at f/2 normally. I mean with a rangefinder of course as there is no problem with EVF's obviously. Now in practice i have no problem with either lens on my M240 but focussing needs indeed some concentration with both lenses and perhaps a bit more with the 90. 

 

 

 

Not APO, but I find the 90 Pre ASPH Cron quite easy to focus on my M-E and CCD Monochrom as well.