...from Erwin's Lens Compendium, which my lady bought me yesterday.

Which of the following lenses requires the MOST focusing accuracy wide-open (longest baselength rangefinder)?

50 f/1 Noctilux 75 f/1.4 Summilux 90 f/2 Summicron

(if you have erwin's book and already know, feel free to answer - just don't give aweay the source.)

-- Andy Piper (apidens@denver.infi.net), April 28, 2002

Answers

I have Erwin's book but not in front of me. What I do have are DOF tables in the Leica M Compendium (Eastland). The book has at least one glaring error and a lot of subjectivity, but the tables are said to be taken directly from Leica. Accordingly, the DOF of both the Noctilux @ f/1 and 90mm @ f/2 and 1m (their closest focusing distance) is 0.02m; for the 75 Lux it's 0.013m, making it the most demanding of rangefinder accuracy, but not by much. What is most interesting is according to the same tables, the DOF of a 135mm lens @ f/2.8 and 1.5m (it's closest focusing distance) is the same as the Noctilux and 90 Summicron (0.02m)yet Leica did not deem it necessary to attach goggles to either of the latter.

-- Jay (infinitydt@aol.com), April 28, 2002.

The 75/1.4 would have the smallest DOF of the three (for a given subject distance), so it should need the most accurate rangefinder.

-- Mike Dixon (mike@mikedixonphotography.com), April 28, 2002.

My recollection (from OSTERLOH's) book is that DOF is inversely related to the size of the aperture and to the square of the focal length, That would make the DOF at the same distance (eg. 1 m), the smallest for the 75/1.4 Summilux wide open. Moreover, the 75 Lux focusses to 0.75 m while the 50 Nocti and the 90 Cron each focus to 1 m, which is even more reason that the 75 Lux requires the most focussing accuracy. The latter factor would make this a trick question, since DOF very strongly decreases with decreasing subject distance.

ANSWER: 75/1.4 Slux.

-- Eliot (erosen@lij.edu), April 29, 2002.

Puzzled - my 75 lux's closest focus is just a bit below the last mark of 0.9m (older version...?) - and my 135mm/2.8 did come goggled...?!

-- Lutz Konermann (lutz@konermann.net), April 29, 2002.

The 75/1.4 Slux is officially spec'd as closest focussing distance of 0.75 m. There were early versions with detachable plastic shade (the same as the 50/1 Nocti). It is possible that there are early versions of this lens with longer close focussing distances. After all, Leica reduced the close focussing distance of the 50/1.4 Slux from 1 m to 0.7 m, so they could have done the same with the 75/1.4,

-- Eliot (erosen@lij.edu), April 29, 2002.

"The 75/1.4 would have the smallest DOF of the three (for a given subject distance), so it should need the most accurate rangefinder. "

This isn't right. Depth of field varies with the square of the focal length but varies with aperture only in simple proporion - therefore, going from a 75 to a 90 gives (75/90) squared (= 0.7) and going from f2 to f1.4 gives 1.4/2 (=0.7). So, for a given distance, the 2 lenses have identical depths of field. Of course, the 75 focusses more closely than the 90, so that at minimum focus distance, you can get yourself in more trouble than you could with the 90.

-- Steve Jones (stephenjjones@btopenworld.com), April 29, 2002.

But Steve. The "aperture" is actually related to the [inverse] square of the aperture number, not directly to the [inverse] of the aperture number. An aperature is an area (cm2) not a linear dimension. So isn't what I stated correct? Is not the DOF of the 75/1.4 the smallest?

Eliot

-- Eliot (erosen@lij.edu), April 29, 2002.

Eliot, The short answer is "no", as far as I know.

-- Steve Jones (stephenjjones@btopenworld.com), April 29, 2002.

In a fit of math geekness, I went over to the lens tutorial section of photo.net, played with the equations for DOF, and came up with the following formulas for the relative depth of field of the three lenses mentioned at maximum aperture when they are focused to the same distance. c = circle of confusion, and x is a variable I introduced for the magnification (M) of the 50mm lens (with M = 1.5x for the 75, and M = 1.8x for the 90). After crunching and cancelling terms, here are the simplified formulas.

50mm at f1 DOF = c * 1/x

75mm at f1.4 DOF = c * 0.93/x

90mm at f2 DOF = c * 1.11/x

-- Mike Dixon (mike@mikedixonphotography.com), April 29, 2002.

Sorry - I was posting this when we were so rudely interrupted on Monday:

Interesting....

Actually, what Erwin lists is not DOF per se but the effective base length needed for accurate focusing (with the whole range of lenses)

A 'fair use" sample

90 f/2 - 63.1mm (needs M3 .92x finder)

75 f/1.4 - 52.3mm (needs .85x finder)

50 f/1 - 32.5mm (gets by with .58x finder)

He also gives a number of about 70mm EBL for the 135 @ 3.4, which means even an M3 is questionable for focusing a 135 without the magnifier.

In other words, the 90 f/2 needs twice the focusing accuracy of the 50 f/1. This kind of makes sense in that if you put a 2x converter on the Nocti, you would get a 100 f/2 - projecting the underlying same image but magnified (and cropped) 2x - which would make the inherent circles of confusion twice as big.

At any rate HIS figures seem to indicate that focal length trumps aperture - longer lenses are more difficult to focus, ALMOST regardless of aperture.

His numbers show that the 90 even at f/2.8 still requires more EBL than the Nocti at f/1.

But there it is....

-- Andy Piper (apidens@denver.infi.net), May 02, 2002.

What I was taught in Optical Engineering is that DOF depends on the Aperture; and the distance that the lens is focused upon... Thus if all three lenses are focused at say same distance; then one only has to compare the apertures used...... >Thus the Noctilux @f1 has an aperture of 50/1= 50mm >the Summilux @f1.4 has an aperture of 75/1.4= 53.6mm >the 90mm Summicron @f2 has an aperture of 90/2= 45 mm..

If one photographs an 3D object at 10 ft with all 3 lenses wide open; the LARGEST aperture lens will have the smallest DOF; which is the 75mm f1.4 Summilux....

-- Kelly Flanigan (zorki3c@netscape.net), May 04, 2002.