a focus question

[jjjjuin]

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if i set apeture f 11, the diagram on the lens shown 0.5m ~ infinity.

 

does that mean the image from 0.5m to infinity will be focused even though I didn't turn the ring?

 

THANKS.

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[lars_bergquist]

if i set apeture f 11, the diagram on the lens shown 0.5m ~ infinity.

 

does that mean the image from 0.5m to infinity will be focused even though I didn't turn the ring?

 

THANKS.

You don't say which lens. There is lots of depth of field, but if you keep the lens set to infinity, half of it will fall beyond infinity, where you cannot use it, or even find it.

 

Let me use my own Summilux 50mm as an example. Lens at infinity, d.o.f. from c.9m to infinity+ (if we may believe the scale, BUT see below).

 

The infinity mark set against right f:11 marking: d.o.f. from infinity to 5m, all right.

 

Here come the bad news. The image is not equally sharp between 5m and infinity. In the example above, it is sharpest at 10m (the point on the focusing scale midway between infinity and 5m) and sharpness does fall off gradually on both sides. If you take a photo with the above setting, and make even a moderate size print, things at 10m will be sharp but those at 5m and at infinity will be visibly unsharp. Not good.

 

How come? We must judge what in the print is sharp enough. It is often felt that if a point, or a theoretically dimensionless line, in the subject is rendered no more than 0.1mm wide in the print, then people will accept this as sharp enough -- though they can indeed se the difference between that and, say, 0.8mm. Now sometime in the 1920's, when the depth of field tables were computed, people actually believed that no greater enlargement would ever be required than 3x! So the largest acceptable fuzz on the film was deemed to be 0.1 / 3 = 0.033mm. And the d.o.f. scales are true -- for this degree of enlargement.

 

Silly? In those days the competition was contact (i.e. 1:1) prints from 6x9cm roll film. Three times 24x36mm makes 7.2 x 10.8cm. Fine! We have beaten roll film folders! But then some crazy people started making 30x40cm prints or even larger, and the 0.033mm standard was rendered useless -- and with it, the d.o.f. scales. But Leitz did not dare change the tables and the scales. People would say: "Hey, the new lenses have less depth of field than the old ones! Leica lenses are going to the dogs!"

 

For a very long time now, knowledgeable photogs have been using numerically halved f-values. So, if the lens is set to f:11, read d.o.f. between the 5.6 marks. If f:8, use 4. If 5.6, use 2.8. This gives an acceptable unsharpness (the technical term is 'circle of confusion') that is just half as large. That allows a 6x enlargement, or a whopping 14x22cm! Hooray!

 

This means that when you zone focus, the 'zone' is way narrower than you think. With a 50mm lens, it is simply meaningless. With a wide angle lens, be very sceptical. My advice is to forget about zone focusing etc. and point focus on the most important part of the subject. Do by all means stop down a bit, because that will give you a margin of safety. But do not go farther than f:8, because a rigorously stopped down lens loses definition because of increasing diffraction.

 

The old man from the Age of Box Cameras