The depth of field markers on the lens barrel

[Finnkare]

Advertisement (gone after registration)

Does anyone know, what is the magnification rate Leica has in mind, when marking the DOF scales on the lens barrels? As you know, the size of a circle of confusion is absolute, not relative, so it doesn't scale along with the magnification.

[SJP]

The DoF scale is based on a 31 micron circle of confusion, which is (was) considered the right value for a 35 mm film, printed onto a "standard size print", viewed at a "standard distance". The "standard distance" is usually taken to be equal to the diagonal of the print. I hope this helps.

 

The markings are basically according to the age-old standard that was adopted in the 1930s or so.

[Advertisement]

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

[jaapv]

Yes it was based on thick and grainy film of the period and 6x9 cm (!) prints.

That means it is hopelessly obsolete by now and on a sensor I would advise narrowing it by two stops, depending on the magnification desired. Personally I regard DOF as non-existent for focussing purposes. The DOF scale is very practical for IR photography though, as the marking 2 stops down (i.e. 5.6 for a 2.8 lens) is usually about the correct focus index for scale focussing.

[lct]

Does anyone know, what is the magnification rate Leica has in mind, when marking the DOF scales on the lens barrels? As you know, the size of a circle of confusion is absolute, not relative, so it doesn't scale along with the magnification.

The size of the circle of confusion (CoC) depends on the format of the camera. If one uses half frame (24x18mm) for instance the CoC size is half that of full frame format (36x24mm). Then if one chooses 0.03mm as CoC value for full frame, the half frame's will be necessarily 0.03:2 = 0.015mm.

Discussions about the actual value of full frame CoC size (more or less 0.03mm) do not change the simple fact that the half frame CoC value will always be 2 times smaller than the full frame one. Then if one prefers 0.02mm as FF CoC value for some reason, the half frame's will be 0.01mm.

Easy to check with good DoF & CoC calculators:

Online Depth of Field Calculator

Circles of Confusion for Digital Cameras

Edited August 11, 2009 by lct

[lct]

....If one uses half frame (24x18mm)....

I meant 17x13mm sorry.

[lars_bergquist]

It is true that calculations of d.o.f. are based on assumptions about final enlargement. Jaap is right, the people at Leitz (may it have been MaxBerek?) assumed at the end of the 1920's, when lenses got their d.o.f. scales, that people would be satisfied with an enlargement ratio of about 3x. That would make a 72 x 108mm print. This had nothing to do with the viewing distance. All objects -- prints, books, whatever -- that are small enough to be seen comfortably within a visual field of about 15--20 degrees are viewed at the eye's close focusing limit, 250--300mm, i.e. we try to maximise the perception of detail. The sizes of print in books and newspapers are chosen with this criterion in mind.

 

This means that when you look at a 6x9 print, you may well wonder if the neg will stand an enlargement to a larger size -- because that larger size, maybe 12x18cm, would be viewed at the same 'reading distance' as the 6x9. Our attempts to adjust the viewing distance to approximately equal the picture diagonal do not begin until that diagonal is greater than our close viewing limit, i.e about the size of a 18x24cm, 8x10" or A4 print.

 

Only gradually did the Leitz assumption of a circle of confusion of 1/30th of a millimeter in the neg become the universal industrial standard. Zeiss did originally calculate with 1/1000 of the focal length, because they assumed that everything would be printed contact! I have worked with cameras with scales based on an assumption of 1/10th (a Mamiya Press) and 1/20th (early Retina). Now why do not manufacturers change over to a more realistic assumption? They do not dare! Indicated d.o.f. would shrink drastically, and with long lenses, it would essentially disappear. So it is thought that the public would believe the lenses had suddenly deteriorated. The assumption is that a sizable segment of the customer base are photographic idiots. This is probably true.

 

In practice, I halve the indicated d.o.f. just as Jaap does, meaning that my personal circle of confusion is 1/60mm. D.o.f. or 'zone focusing' does not enter my thinking except with wide angle lenses, and not always then. And I am conscious of the fact that even this is not enough for a print with the critical diagonal of 25--30cm. Fortunately we humans tend to accept pictures where at least >something< about the main subject is acceptably sharp! And of course, there are other things to a successful picture than sharpness -- "the fetish of boring photographers".

 

The old man from the Age of Contact Printing

[lct]

Advertisement (gone after registration)

...my personal circle of confusion is 1/60mm...

Interesting indeed as this CoC value (0.017mm) is close to those usually associated to 4/3 (0.015mm) and APS-C (0.020mm) cameras.

A far as i'm concerned the DoF markings of Leica lenses work fine with FF bodies.

With APS-C i use the nearest faster f stop i.e. f/5.6 when i choose f/8 for metering by example.

[Finnkare]

And of course, there are other things to a successful picture than sharpness -- "the fetish of boring photographers".

 

Of course there are, but it's one of the most important ones, imho. And normally the "sharpness fetishists" want some kind of absolute sharpness, they rarely talk about dof.

[lars_bergquist]

The only thing I know about 'absolute sharpness' is that it does not exist.

 

At a viewing distance of 30cm, most of us can indeed see the difference in a print between a c.o.c. of 1/10mm and one somewhat smaller, say 1/15. Does that mean that the print with the smaller c.o.c. is 'absolutely sharp'? Well, what about a 1/20mm print? I presume that there is a lower limit set by the packing density of the retinal cells in the fovea, but there will always be individual differences. And basically, the standard of a print c.o.c. of 1/10 (from which the negative-standard was calculated, from an assumption of 3x enlargement) does only set the limit of what people tend to see as acceptably sharp. This is natural as this standard is used to define the >lower limit< of sharpness, at the near and far limits of the d.o.f. 'zone'. It must of necessity be sharper at some plane inside that zone -- no matter if we are able to see it or not. Does it really matter?

 

I have never claimed that sharpness is unimportant, just that content does always trump it. How many complaints have you heard about the sharpness, or lack of it, in Cartier-Bresson's pictures? Mostly however, sharpness is a useful >potential< in lenses, sensors and printing. What you actually do with it is up to yourself. --Just a few days ago I was accused of unduly overrating sharpness, when I pointed out that modern Leica lenses are tecnically superior to those of bygone times!

 

The old man of a certain age