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11-19-2009, 06:32 AM   #1
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DOF in Using a Manual Focus Lens on a DSLR

I have been using Pentax film gear for years, and am thinking of buying a K-7 (or prehaps a K-x). I prefer focusing manually, and thus I would intend to use K-mount lenses (A-series) and a few screwmount lenses on a DSLR. Thus, would the DOF on the lens scale work in the same manner as it would with a film SLR? For example, with a 35mm focal length, all will be in foucs from about 8 or 10 feet and beyond. This 35mm lens will convert to about a 52mm lens when used with digital. However, will I still get my "all in foucs" at the same 8 or 10 feet if I use the lens on a K-7 or K-x?

11-19-2009, 07:53 AM   #2
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Actually, upon further investigation, there would be no chance I would buy a K-x, as there is no battery grip available for it. The K-7 remains a possibility, but I would need to know what type of DOF behavior I could expect with manual lenses.

I am somewhat surprised that no one has chimed in with an immediate answer, as there are many knowledgeable users here (and, of course, many, many Pentax fans). Does everyone just use autofocus these days?...and focus only on the subject? An image can be greatly enhanced with a simple understanding of DOF in regard to a particular lens.
11-19-2009, 08:12 AM   #3
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QuoteOriginally posted by zx-m Quote
Actually, upon further investigation, there would be no chance I would buy a K-x, as there is no battery grip available for it. The K-7 remains a possibility, but I would need to know what type of DOF behavior I could expect with manual lenses.

I am somewhat surprised that no one has chimed in with an immediate answer, as there are many knowledgeable users here (and, of course, many, many Pentax fans). Does everyone just use autofocus these days?...and focus only on the subject? An image can be greatly enhanced with a simple understanding of DOF in regard to a particular lens.
The reason no one chimed in may be that it is a surprisingly difficult question to answer precisely. Here is a precise answer I think:

If you use the same lens and stand at the same distance from the subject and print such that the size of the subject on the print is the same, then yes, the DOF marks on the lens are correct in both cases. This is because the print from the small sensor camera will be equivalent to a crop of the print from the larger frame camera (ie. make a print from the large frame camera and trim it with scissors in the ratio of sensor sizes.....this has no effect on DOF.)

BUT people rarely do this. Rather, they make the same sized print; this requires that the image from the smaller sensor camera be enlarged more than from the full frame sensor (3:2 for APS-C). The extra enlargement messes with the interpretation of the DOF marks on the lens.

Dave in Iowa
11-19-2009, 08:55 AM   #4
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Thanks, Dave...you are right: this is rather complicated, much more than I had imagined. I suppose this is why I have ignored digital for so long.

I thought the "full frame" (3:2 or 2:3) designation of cameras such as the K-7 and D300 would make for easy, accurate printing. I suppose I have a rough understanding of what you are trying to explain, but I do not fully understand the concept. I am trying to determine if a 2:3 print (say a 4"x6") will offer the same DOF as it would with an SLR. It seems to me that if things work in the manner in which I understand them (which could be incorrect), then I would be getting something like a 50% increase in depth-of-field in using a manual focus lens on a DSLR--that is, the DOF of a 50mm focal length would be more like that of a 35mm focal length.

11-19-2009, 08:55 AM   #5
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Well, I had a look to this problem with a DOF calculator, and it seems that on APS-C, you will lose 2 stops of DOF...

I mean that on an APS-C body, your lens at f/4 will have the DOF it has at f/2.8 on an FF camera (at least, that's what the maths tell us)...

Quite counter-intuitive, no?
In fact, it's logical, in a way : APS-C enlarges the central portion of a FF frame (for the same output size), so any slightly out of focus element considered as in focus on FF will be more easily seen on this APS-C crop/enlargement.
11-19-2009, 08:57 AM   #6
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I have read many posts on this subject, and the consensus seems to be that in practice, you should set the aperture to one stop slower than the scale f stop you are registering. So if you use the f8 scale on the lens, you should set the aperture to one stop slower ie f11
11-19-2009, 09:07 AM   #7
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Thanks for the question, by the way... I'd never have thought DOF would behave like this!

So, the Three rules of FF to APS are :
- for Equivalent FOV, divide focal length by 1.5 (use a 35mm for a 50mm equivalent)
- for Equivalent DOF for a given Equivalent FOV, open by 1.5 stops (a 35/2 is equivalent to a 50/3.5)
- for a given lens, DOF is two stops less than on FF (a 35@f/16 has the same DOF than f/8 on FF)
11-19-2009, 09:33 AM   #8
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Thanks...

dlacouture: in your first post, you mention f/2.8 and f/4...this would be only one stop (not two)...f/16 to f/8 would be two stops.

After considering all of this, I may remain in the world of film...

11-19-2009, 09:51 AM   #9
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I would also add that in order to take digital shots in a more normal manner--i.e. without awkward calculations and approximations--one would have to buy a pro camera--such as a $5,000 Nikon D3 (with its FX sensor, as oppsed to a DX sensor).
11-19-2009, 09:57 AM   #10
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oh geez this is so simple

acceptable circle of confusion goes down by a factor of 1.5 when you crop to APS-C
stopping down by one stop reduces the diameter of the blur disks by 1.4

So you lose approximately "one stop" of DOF; i.e. use the film-lens marks for f/5.6 for APS-C pictures taken at f/8

(Assuming same print size from both cameras.)
11-19-2009, 10:02 AM   #11
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troyz: in the manner in which you state it, it is, in concept, simple. Remembering this in the field, especially when shooting a moving object and/or when things have to be done quickly, is another story.
11-19-2009, 10:40 AM   #12
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How about shooting with a digital-specific lens--such as the DA 40/2.8? Would this also need the one-stop reduction for accuracy, or would its DOF markings be correct?
11-19-2009, 11:12 AM   #13
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QuoteOriginally posted by zx-m Quote
Thanks, Dave...you are right: this is rather complicated, much more than I had imagined. I suppose this is why I have ignored digital for so long.

I thought the "full frame" (3:2 or 2:3) designation of cameras such as the K-7 and D300 would make for easy, accurate printing. I suppose I have a rough understanding of what you are trying to explain, but I do not fully understand the concept. I am trying to determine if a 2:3 print (say a 4"x6") will offer the same DOF as it would with an SLR. It seems to me that if things work in the manner in which I understand them (which could be incorrect), then I would be getting something like a 50% increase in depth-of-field in using a manual focus lens on a DSLR--that is, the DOF of a 50mm focal length would be more like that of a 35mm focal length.
I'm sorry, by "3:2" I meant it is necessary to enlarge the APS-C print by a ratio of 3/2= 1.5X to match the print size from a full-frame camera.

Stated another way, when focused on a particular subject at a particular distance the same lens throws the same image on the sensor plane for both cameras, so at that level the DOF is the same.

However the smaller APS-C sensor only captures a central portion of what the full frame's sensor captures.

To make the same sized print from both cameras you've got two choices:
(1) set the enlarger so the size of the subject (eg. a face) on the easel is the same. In this case the print from the full frame camera will be larger, it so must be trimmed with scissors to make the two prints the same size. The two resulting prints will be identical with identical DOFs.

(2) grab a piece of 8x10" paper (for example) and enlarge what was captured by the sensors just enough to fill the paper. In this case the APS-C sensor's image will have to be enlarged 1.5x that of the full frame sensor's image (the face on the ASP-C print will be 1.5 times bigger.) This has the effect of altering the effective DOF.

I hope this helps clarify what's going on rather than adding to the confusion.

The rules given most places about DOF, sensor size, etc, assume you'll follow the second of the above alternatives with the result being DOF is increased by the crop factor (1.5X in this case).

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11-19-2009, 11:34 AM   #14
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QuoteOriginally posted by zx-m Quote
How about shooting with a digital-specific lens--such as the DA 40/2.8? Would this also need the one-stop reduction for accuracy, or would its DOF markings be correct?

I don't think you need to apply a correction for a DA lens on a digital camera. In general, the DOF scale should be calibrated for the format for which the lens is intended. I know the medium format lenses are calibrated differently than the 35mm film lenses.

The only DA lens with a DOF scale that I own is a DA21. The scale seems to be calibrated correctly, but then again, I'm rarely DOF-limited by a DA21!

Someone with a DA40 should check for sure.
11-19-2009, 12:10 PM   #15
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Everybody really makes this much too complicated.

The DOF of any lens at a specific focal length and f stop will always be the same. The DOF scales will always be as accurate as they always were.

The lens does not know or for that matter care what sort of camera it is attached to. It will project the same image onto a digital sensor, film, ground glass, or a blank sheet of paper.

The only exception I know of is the focus screens on DSLR's. They use a few optical tricks that have the effect of making fast lenses look like they have the DOF of f2.8 or so.
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