90mm & 135mm Difficulty in Use

[Guest volker_m]

Advertisement (gone after registration)

Would be interesting to know which formulas and values those figures are based upon.

 

kmin=f^2 / (60*B*V)

 

with B = rangefinder base length, V=viewfinder magnification, f=focal length.

Scholz quotes a textbook "ABC der Optik" as the reference.

[lct]

kmin=f^2 / (60*B*V)

with B = rangefinder base length, V=viewfinder magnification, f=focal length.

Scholz quotes a textbook "ABC der Optik" as the reference.

Thank you volker_m. Do you know what "kmin" means (critical or minimum base length?) and where "60" comes from? Just curious.

[Advertisement]

The Leica Q3 is available for pre-order at B&H Photo Video and Adorama

[Guest volker_m]

Thank you volker_m. Do you know what "kmin" means (critical or minimum base length?) and where "60" comes from? Just curious.

 

Looking at the table and equations, kmin is what he calls the "critical aperture" as shown in the table. There is no explanation for the multiplier 60.

[pgk]

This is from Alfons Scholz' book on the M6 and shows the aperture you mathematically need to be accurate with the rangefinder given the width of basis of the optical triangle involved, which is broader in the M3. So you can see that the 75lux, 90cron and 135Elmar are in fact out of reach when used wide open.

Interesting. My experience, as I said, has been that both the 90/2 and 135/4 have had too low a yield for me. I think that I will accept that, good though it might be, the 75/1.4 might be equally frustrating. Perhaps not such a bad thing to have this 'confirmed' here. Thanks.

[luigi bertolotti]

Would be interesting to know which formulas and values those figures are based upon.

According to the formula b = e*f2/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), those three lenses can be focussed accurately at full aperture on full frame Ms.

 

Sorry... do you mean b = e x f^2 (f x f) ?

(if so, your formula is more convincing than the Scholz's one.... at least in terms of units of measure... unless the obscure "60" is not simply a undimensional factor...)

Edited February 21, 2015 by luigi bertolotti

[lct]

Yes f x f. According to this formula, the critical base length of FF rangefinders is smaller (40mm) than the effective one (47mm) with 75/1.4 lenses, so the latters can be focussed accurately at full aperture. Not the same for the M8 where the critical RF base length (53mm) is significantly longer. Better open at f/2 to get good hit rates then.

[luigi bertolotti]

Advertisement (gone after registration)

Lct, do you know how much was the effective baselength of Barnacks ? (the separate RF had an enlargement I don't know...) ; just curios to verify if they had been fool to make the Summarex 85...

[lct]

58.5mm according to "A Guide to Rangefinder Effective Base Length (EBL) - 35mm" but i have not checked. It this figure is true it is not far from that of the M240 + 1.25x magnifier (58.86mm) and a 85/1.5 lens could fit.

[Guest volker_m]

e the visual acuity (0.0003)

z the circle of confusion (0.03 for FF)

 

Two questions on this:

1. I am not familiar with that e value and the related number. Any references for me to understand this number and the underlying assumptions?

2. Circle of confusion value of 0.03mm was approriate for postcard size prints. Shouldn't we use a much smaller value that gives a better estimate of "sharp" in the digital age, when looking at 100% crops on screen?

 

Bonus question: Shouldn't we include the viewfinder magnification somewhere?

 

For myself, I find the Summicron 90 (after focus calibration) difficult to focus wide open, and the 135/4 (after focus calibration) close to impossible. The 1.25 viewfinder magnifier doesn't really help (me) much.

[lct]

Any references for me to understand this number and the underlying assumptions?

Been using this formula for many years but i don’t recall its cource sorry. Perhaps Josef Stuper (Die photographische Kamera, Lindemanns ed.) but i’m not sure of that at all.

Circle of confusion value of 0.03mm was approriate for postcard size prints. Shouldn't we use a much smaller value that gives a better estimate of "sharp" in the digital age, when looking at 100% crops on screen?

I don't think so. This value is still retained by Leica for the "technical data" of its current lenses.

Shouldn't we include the viewfinder magnification somewhere?

It is included in the effective base length value by definition (mechanical base length x image magnification)

[lct]

For myself, I find the Summicron 90 (after focus calibration) difficult to focus wide open, and the 135/4 (after focus calibration) close to impossible. The 1.25 viewfinder magnifier doesn't really help (me) much.

Did you try a diopter correction lens? I have one on all my M bodies and my first surprise when receiving my new 90/2 apo was how relatively easy it was to focus at full aperture compared to my non calibrated 90/2 pre-apo. I have no problem with my 135/2.8 due to the 1.5x magnification of its goggles but i find f/3.4 a bit tricky on the 135/3.4. No problem for me at f/4 though.

[Guest volker_m]

I don't think so. This value is still retained by Leica for the "technical data" of its current lenses.

 

I agree that this is what Leica uses for the depth of field calculation for historic reasons, but it is far from accurate for today's requirements. We are at 6µm pixel pitch now, so using the old 30µm for acceptable circle of confusion results in blurred pictures when viewed at larger magnifications.

 

It is included in the effective base length value by definition (mechanical base length x image magnification)

 

I see ... thanks!

[Guest volker_m]

Did you try a diopter correction lens?

 

No, I didn't try that. I am wearing glasses and these are up-to-date, so I didn't see value in the diopter correction lens. What I did try was the 1.25 magnifier, but then realized that focus was off for 90mm by a consistent amount. So the lenses went to Leica for adjustment twice (visited CS in person the second time) and now the consistent error is gone, but there is still a lot of random focus error. I've given up on telephoto with the rangefinder.

[lct]

I agree that this is what Leica uses for the depth of field calculation for historic reasons, but it is far from accurate for today's requirements.

I beg to disagree sorry. I'm aware of various parrotings about this on the web but all DoF calculators are still based on the 0.03 value and it works fine for me on FF as well crop bodies. Pay honour to whom honour is due anyway . If i's good for Leica it is good for me. YMMV.

[lct]

No, I didn't try that. I am wearing glasses and these are up-to-date, so I didn't see value in the diopter correction lens

I wear eyeglasses as well but dioper correction lenses work better for me. I would try one if i were you.

[luigi bertolotti]

58.5mm according to "A Guide to Rangefinder Effective Base Length (EBL) - 35mm" but i have not checked. It this figure is true it is not far from that of the M240 + 1.25x magnifier (58.86mm) and a 85/1.5 lens could fit.

 

I take the formula as good : I can personally confirm that M240 with goggled 135 is not difficult to focus at 2,8... and it stays into the computed limit.

[Guest volker_m]

I beg to disagree sorry. I'm aware of various parrotings about this on the web but all DoF calculators are still based on the 0.03 value

 

Parrotings? Sorry, but as an engineer working in instrumentation I'm used to use my math properly. The 30µm in your equation is nothing but an estimate for acceptable sharpness. Those traditional 30µm is just that - a traditional value from very early films days. But of course 30µm is no longer a "safe" measure for sharpness when checking at 100% magnification. The sensor can resolve much better than 30µm - but only when focused accurately.

[lct]

When Leica is convinced by all this i will be convinced as well... provided different formulas, if any, do work for me of course. Do you know any? If not, i will have to rest my case if you don't mind but, as i said above, your mileage may vary .

[otto.f]

Well, if 5 out 6 shots with my 135 are ok, I wonder how Alfons Scholz' formula can be right

[Advertisement]

Hallo,

Look here: Leica M

[Doc Henry]

Volker , I did not need magnifier * for 135 and 90 for the reason mentioned above, that it bothers me to focus,

especially for the 90 not need magnifier but need a mono or tripod !

... question of habit

 

Thanks to click in the top, middle and right icon to enlarge :

Apo 90 Summicron Asph:

L1011173_jpghtdamhon_jpgred700.jpg

Image13statuecath-3lufht___red1200cad.jpg

L1015283orchid___lufgermhtLUG___red700.jpg

L1014302coq leica m8 90aposumasph ___htlug___red1200.jpg

with 90 Macro-Elmar:

L1011987htcplednred1200.jpg

with Apo Telyt 135 Asph:

Image9rosem7apotelyt135kp160lught___red1200cad30.jpg

L1013020-2htcoqavred1200.jpg

L1012999-3htcoqpopies135red1200.jpg

Best

Henry

* I have a Leica magnifier x1,25 but I almost never use

Edited February 23, 2015 by Doc Henry