Lens/DOF advice

[E Thomson]

Hi folks,

This is a question from a relative newbie. I purchased a Graflex 4x5 a few years ago to eventually produce work in the photogravure process. After working with it some I got seriously sidetracked by the consumer DSLRs from Nikon. I now have a project which brings me back to 4x5. I would like to produce a series of portraits with the Graflex, shooting quite tight to include only neck and partial shoulders. This will be done in the studio. The lens that came with the Graflex Speed was the Ektar 127mm/4.7. This nearly puts me in my subject's lap, so I am in the market for a longer lens.

I have settled on the 203/210mm range as a reasonable compromise between closeness to subject (still pretty close at perhaps 40") and the bellows extension of the Graflex (which I calculate will be more or less used up; correct me if this seems wrong).

Here is my question. I am looking at several lenses at auction and I see f6.8 and f7.7 on some of these longer standard lenses. With my limited experience on the 4x5 I'm having trouble understanding if this will mean I will not be able to shoot at very thin DOF if I want to. I want to try working with thin DOF with the portraits, perhaps limiting sharpness to, say, two inches. With other subjects perhaps thinner.

I would appreciate if someone would help me figure out what DOF effects I will see with, say, a 203mm/7.7 wide open, in terms of the 35mm lenses and format which I am more familiar with.

Thanks very much.

[Ron Marshall]

On 35mm a 57mm lens will give about the same perspective as a 203mm on 4x5.

Assuming that you shoot both lenses at the same distance, and you use the same circle or confusion for both formats, I put some numbers into a DOF calculator.

57mm at 2 meters and f1.4 gives 10 centimeters DOF.

203mm at 2 meters and f7.7 gives 4 centimeters DOF.

This gives you a rough idea of how DOF differs between these two formats. Of course with different enlargements COC becomes a consideration, but this gives you a rough idea.

Here is the link:

http://bobatkins.com/photography/technical/dofcalc.html#calc

[E Thomson]

Ron, thanks for your succinct answer and for the DOF calculator. That would seem plenty thin enough for my purposes.

[Alan Davenport]

You probably won't be using the same circle of confusion for 4x5 as for 35mm. Because 4x5 film requires much less enlargement to reach a given print size, it tolerates a much larger CoC than does 35mm. Typical CoC values are 0.03mm for 35mm film, and 0.1mm for 4x5 film.

Plugging those values into f/calc:

35mm film; 57mm lens, f/1.4, CoC = 0.03mm: 10cm DOF
4x5 film, 203mm lens, f/7.7, CoC = 0.1mm: 12cm DOF

[E Thomson]

Hi Alan,

Thanks for your response. What subject distance were you using? My subject is about 3.5 feet or figure one meter (Ron used two meters above).

I went back to Ron's calculator and the assumption there for 4x5 COC is 150 microns or .15mm (consistent with your assumption). Result; DOF = ~2" at 3.5' subject distance.

I checked this against the calculator here (could have done this in the first place but I guess I wanted to talk it through):

http://www.dofmaster.com/dofjs.html

That calculator assumes your .1mm COC for 4x5. Result = 1.4" (subject at 42", f8).

Were you calculating for a further subject?

Thanks,

Eric

[Leonard Evens]

At the distances you are talking about, provided you arrange that the size of the image stays more or less constant, the depth of filed is essentially independent of focal length. The rule for the depth of field in front or in back of the point of focus is

(f-number times circle of confusion divided by square of magnification) times (one plus magnification)

(The factor on the right is the bellows extension factor. If the magnification is small, you can ignore it.)

The magnification is the ratio of subject size to image size. Suppose the head to neck distance is 300 mm, and it fills up 100 mm on the film in portrait mode. (That would mean the head plus neck would essentially fill the frame.) The magnification would be 100/300 = 1/3. The circle of confusion is dependent on what standard for sharpness you choose. A common value for 4 x 5 is 0.1 mm, but some people prefer something smaller. The bellows extension factor would be 1 + 1/3 = 4/3 ~ 1.33. Using the above rule, you get the following results in mm.

At f/7.7 7.7 x 0.1/(1/3)^2(1/33) ~ 9.24 mm

That would be the DOF on either side of the point of focus, so the total DOF would be twice that or about 18.5 mm or about 0.73 inches.

If you know the DOF at one f-stop, you can obtain it at another f-stop by multiplying by the ratio of the f-numbers. So
So suppose you wanted the depth of field at f/16. You would multiply the above numbers by 16/7.7 ~ 2.08 to get plus or minus 19.2 mm, The total DOF would be about 38.4 mm or 1.54 inches.

I suspect that in your case the magnification is not quite as large since you probably don't have the distance from the top of the head to just below the neck almost filling the frame. That would increase the depth of field a bit, A very rough estimate would be obtained by multiplying by the ratio of your magnification to mine of 1/3. For example, if you only filled 60 mm vertically, your magnification would be 1/2 and the ratio would be 3/2, so the DOF would be roughly 1.5 larger. (But this ignores the contribution from the bellows extension factor, so it is not quite right. If you want more exact answers, you need to use the complete formula.)

The above rule still applies in 35 mm photography provided the focal length is not too short. But the magnification would be much less, typically about one eighth of what would be appropriate for 4 x 5. Indeed it would be so small that you could ignore the bellows extension factor in the formula. At the same time the proper coc would also be about one eighth. The combined effect is that the DOF at any f-number would be a little less than about 8 times as large. (Because of the bellows extension factors.) You would have to change the f-number by a division factor of 8 to get the same result. So to get the same result as you would with f/7.7 for 4 x 5, you would need to use f-number about 7.7/8 = 0.965 ~ 1.0, i.e, you would use an f/1 lens.. Looked at another way. If you did 35 mm at f/2, you would get the result very roughly the same as f/16 with 4 x 5. (If you want more exact answers, you will just have to use the complete formula.)

[E Thomson]

Thank you Leonard for a very generous reply. I think the general look of f/1 on miniature film (24x36) should be plenty thin for my shooting, to say the least. Thanks for all the replies, this is a wonderful resource.

[Alan Davenport]

Eric,
I was using 2m as the subject distance.

I just reran the 4x5 calc in f/calc:


203mm lens, f/7.7, 2 meter distance, 0.1mm CoC

Near DOF 1.935027 m
Far DOF 2.069488 m
Total Depth: 0.134461 m

I must've forgotten to reset the CoC the first time; f/calc defaults to a slightly smaller value for 4x5. (BTW, setting the stop to f/8 in f/calc produces exactly the same result as the dofmaster link.)

Leonard's post also points out that there is more than one way to skin this cat.

It's worth mentioning, that depth of field is ultimately a very subjective thing. What looks good to me, may not be good enough for you.