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03-22-2009, 07:51 AM   #1
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DOF for 6 x 7 on Crop Sensor

I recently began shooting some 67 lenses on my K20d. One of the things which strikes me most about this setup, is depth of field seems shallower than normal--compared with regular lenses on my K20.

I should state I never shot 6 x 7 before this experiment. So I have no prior knowledge upon which to compare my current observations. This brought me to post my first thread in this section of the forum. I have come here for knowledge. Here are my questions.

Is it possible shallow DOF is inherent in 6 x 7 lens design? Also, is it possible that DOF is shallower on 6 x 7 systems in general? If so, would using 6 x 7 lenses on my K20d crop sensor make the DOF even shallower, or would it be about the same?

I am just curious if this is a real or perceived phenomenon. So far I really like the few images I have shot with 6 x 7 on the K20. I'm sure I will learn more as I shoot with the 67 lenses, but was hoping to get a headstart by picking the 67 brains in this forum first. Thanks.

03-22-2009, 11:51 AM   #2
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A 300mm lens would give shallower DOF that a noraml 50 or 40mm lens.

Try the DA300, FA300(35mm) FA300(645) M300(67) on your KxxD and amazingly they all give the same DOF. If there is a way of mounting my 300 Sironar onto my K20D it will give the same DOF at the same f setting.

It is just law of physics (Light and optics)
03-22-2009, 12:12 PM   #3
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One of the most difficult transitions for me converting from 35mm to 6x7 about ten years ago was the loss of DOF. 6x7 has approximately half the DOF of 35mm and then less than that vs the crop factor sensors. So an f.4 6x7 lens has about the same DOF as an f.2.8 lens on 35mm and nearly the same DOF as an f.2 lens on the K20D sensor.

Part of the flip side is that 6x7 has less risk of diffraction than 35mm and consequently even less risk than the crop sensor cameras. So you can use much smaller apertures to gain back DOF. Where f.16 is a perfectly usable f stop for DOF on 35mm, f.16 tends to show lots of diffraction effect on the crop sensors. With 6x7 it's common to shoot at f.22 without too much diffraction effect.

I have no personal knowledge of how that translates into 6x7 (or 67 or 67II) lenses on digi bodies...never tried it. I'd try stopping down severely with those 6x7 lenses on your K20D. Hopefully, you'll get the DOF through stopping down without the diffraction problem that you might expect on the small sensor.

I guess the question is: do you get the characteristics of the small sensor or the characteristics of the medium format lens? Or is it a blending of the characteristics...too bad it doesn't seem to produce the DOF of the small sensor and the lack of diffraction of the MF lens!

Have fun!
03-22-2009, 05:05 PM   #4
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Here is an example

Here is a link to a shot with the 67 (later model, close focus) 200mm f4 on the K20--last post in this thread, currently: I believe this is f8--does this tell you guys anything which you can relay to me? Thanks


https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/49428-medium-f...-sensor-7.html


Last edited by Jewelltrail; 03-22-2009 at 07:37 PM. Reason: correct link
03-24-2009, 07:45 AM   #5
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In terms of DOF, 200mm and f8 is 200mm and f8 whether the lens is for 35mm, 6x7 or APS-C systems.
03-30-2009, 09:55 AM   #6
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This isn't the same issue as the 200mm is 200mm regardless of crop factor argument.

Shoot cropped digi, 35mm and medium format for one full week in the field and you will have no doubt whatsoever that the smaller the format the greater the apparent depth of field. The diffraction differences have been well documented in past threads as well--though many photogs don't recognize diffraction effect as easily as most can identify depth of field.

Note that it isn't uncommon to shoot large format--4x5" or 8x10" at f.32 or even f. 64 which isn't possible in smaller formats even on lenses with such apertures--(long teles and macros). Larger format equals less risk of diffraction effect (or at least less noticeable diffraction effect). Almost all large format cameras also include movements like tilt, swing, shift etc to manipulate depth of field and to make up for the format's inherent difficulties with getting enough depth of field.

I'm not mathmatically inclined and so can't discuss the formulas involved to prove it. But like jeweltrail, I noticed the reduced depth of field shooting through medium format lenses immediately. I was shooting on medium format film however and find it interesting that the characteristics of the lenses seem to hold true for jeweltrail even when shooting on a smaller format.
03-31-2009, 01:14 PM   #7
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QuoteOriginally posted by brkl Quote
In terms of DOF, 200mm and f8 is 200mm and f8 whether the lens is for 35mm, 6x7 or APS-C systems.
Are you saying 200mm absolute focal length? Even that produces different depths of field depending on the sensor/film size.

For a 200mm lens at a fixed aperture, the actual "circle of confusion" size (ie, blurriness of out-of-focus areas) at the sensor/film is the same no matter what format is used (APS-C, 6X7, etc). But the APS-C image needs to be enlarged a lot more produce an image of a given size than the 6X7 shot does. This means that for a smaller sensor those "circles of confusion" are larger in the final print and this means less depth of field.

This is why a Depth Of Field Calculator requires that you supply a sensor size as well as a focal length.
04-01-2009, 03:12 AM   #8
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QuoteOriginally posted by Jewelltrail Quote
Is it possible shallow DOF is inherent in 6 x 7 lens design? Also, is it possible that DOF is shallower on 6 x 7 systems in general? If so, would using 6 x 7 lenses on my K20d crop sensor make the DOF even shallower, or would it be about the same?
No, yes (when using a 6x7 camera), and no (would be the same). What you observed is pure imagination.

QuoteOriginally posted by Ron Boggs Quote
Shoot cropped digi, 35mm and medium format for one full week in the field and you will have no doubt whatsoever that the smaller the format the greater the apparent depth of field. [...]
I'm not mathmatically inclined and so can't discuss the formulas involved to prove it.
QuoteOriginally posted by Sean Nelson Quote
This is why a Depth Of Field Calculator requires that you supply a sensor size as well as a focal length.
DoF calculators easily obfuscate a simple fact. Which is why I keep repeating the following statement from time to time:
Depth of Field depends on two variables only (whatever be the recording medium or size):
- Field of View (in degrees)
- Physical diameter of the lens aperture (in mm)
Of course, as soon as you know the sensor size, you can compute circle of confusion, focal length, f-stop and all of a sudden, things look more complicated than they are You may google an earlier post of mine here for the mathematical side of things.

Now, because medium format tends to use larger lenses (larger focal length and aperture in mm), DoF tends to be shallower.

But the effect is smaller than one would normally think because typical f-stop values tend to decrease with the larger systems as well.

Here is a comparison of DoF for 35mm full frame and 645 medium format taking available glass into account:

https://www.pentaxforums.com/forums/531863-post7.html


Last edited by falconeye; 04-01-2009 at 03:17 AM.
04-01-2009, 07:22 AM   #9
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I built a spreadsheet tool last week specifically because of discussions like this one and the confusion on the subject. It has all the sensor/film size and circle of confusion data built in and you just feed it focal lengths, apertures and distance to subject and compare multiple formats side-by-side. You can download it from this thread: https://www.pentaxforums.com/forums/general-pentax-photography/55197-camera-lens-math-easy.html
04-01-2009, 08:12 AM   #10
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My simplified version--sorry no charts!

Using a "standard" focal length, take a picture at f22 with a K10d and the same picture with your 67II at f22--tell me you can't see the difference in diffraction effect. If you can't see it EASILY, you aren't very worried about critical focus.

Now take a wide angle shot with each system at "only" f11 but with a close foreground element--3 to 5 feet away--as well as infinity background...distant mountains or similar...If you can't see the big difference in depth of field between the two, then I can't help you any further.

Look, I carry hyperfocal distance charts with me in each individual lens bag. I'm no newbie, but I'm also only interested in hands-on results. I couldn't care less about theories and math. So perhaps something in the lens design of the lenses I'm shooting makes a difference in my "seat of the pants" analysis. For example, diffraction effect varies based on the distance of the diaphram blades from the film/sensor plane. The farther the blades are separated from the film/sensor the greater the risk of diffraction (John Shaw). Perhaps my FA* lenses have blades farther out the lens barrel than what would be considered "normal" and my 67 stuff has closer blades than a theoretical "normal"? As with much in life, there can be alternative explanations. I'm just stating what I've observed--sober, by the way.

Take a few images and get back to me. The differences are huge and very noticeable when checking critical focus. But if you've never shot a variety of formats, there's no way you can actually understand. Sorry!
04-01-2009, 09:57 AM   #11
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QuoteOriginally posted by falconeye Quote
Depth of Field depends on two variables only (whatever be the recording medium or size):
- Field of View (in degrees)
- Physical diameter of the lens aperture (in mm)
...which is why a 200mm lens has different depths of field when used on different sized sensors - it's field of view changes.
04-01-2009, 06:41 PM   #12
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QuoteOriginally posted by Ron Boggs Quote
I couldn't care less about theories and math.
[...]
But if you've never shot a variety of formats, there's no way you can actually understand. Sorry!
Is this an answer to me?
If so, I consider it to be inpolite and rude.

And I did shoot on a variety of formats and still I cannot disagree more.

"Understanding" means understanding, not having seen something. Having seen something is "Observing". You observe w/o understanding.

Understanding requires a certain level of theoretization allowing you to predict things before you actually try it out. Which, in the case of optics, involves some rules which are shortest when written in the language of math.

So, I am sorry, but you cannot be more wrong. It was people which were able to truely understand which gave you the toy named camera you have been gifted to use now.

To answer your question: Look up a text-book on lens design or classical optics. Read it. See that both, circle of confusion and f-stop value are determined by the optical focal length and the optical entrance pupil. And so is DoF. The entrance pupil is a virtual image of the aperture stop and already incorporates any variation an optical design may have.

Therefore, an optical design cannot change how DoF depends on the f-stop. As I said, only Field of View and the diameter of the entrance pupil do.

This answer is final. I only will respond to you again if your tone gets a lot more polite.


Addition: I actually assume you have missed the trivial part of all answers given so far. You write:
Using a "standard" focal length, take a picture at f22 with a K10d and the same picture with your 67II at f22--tell me you can't see the difference in diffraction effect.
Well, standard lens means to keep constant the Field of View. Of course, then the aperture with 67II at f/22 is much larger than the aperture with a K10D. Because the aperture is f/22 (as it actually says in bold letters). And f of a standard lens for a 67II is much larger than for a K10D. Divide by 22 and you'll understand. Maybe.

QuoteOriginally posted by Sean Nelson Quote
...which is why a 200mm lens has different depths of field when used on different sized sensors - it's field of view changes.
Yes, or if you crop into an image (a major source of confusion ...).

Things only appear to be complicated before they are understood.

Of course, a 200mm lens designed for 67II and designed for K10D still have identical DoF when used at the same body and using the same f-stop.

Last edited by falconeye; 04-01-2009 at 06:55 PM.
04-01-2009, 07:27 PM   #13
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QuoteOriginally posted by Ron Boggs Quote
I noticed the reduced depth of field shooting through medium format lenses immediately. I was shooting on medium format film however and find it interesting that the characteristics of the lenses seem to hold true for jeweltrail even when shooting on a smaller format.
It's certainly true that shooting in medium format at the same field of view and f/number will give you shallower depth of field than on APS-C. That's because the medium format camera requires a longer focal length to get the same field of view, and the longer focal length means the f/number you've chosen translates into a larger physical aperture. The larger the physical aperture, the shallower the depth of field (for the same field of view).

But that doesn't apply to using the different lenses of the same focal length on an APS-C camera, even if one was designed for medium format. If the lenses are the same focal length, then at a given f/number they have the same physical aperture diameter, and since they're being used on the same sensor the result is that they'll both have the same field of view and the same depth of field.


QuoteOriginally posted by Ron Boggs Quote
For example, diffraction effect varies based on the distance of the diaphram blades from the film/sensor plane. The farther the blades are separated from the film/sensor the greater the risk of diffraction (John Shaw). Perhaps my FA* lenses have blades farther out the lens barrel than what would be considered "normal" and my 67 stuff has closer blades than a theoretical "normal"?
An interesting idea, and I certainly wouldn't discount it from having an effect on the picture. But diffraction would apply equally across the entire image, not just those parts of the subject at a specific distance from the lens. Therefore what you'd see is a difference in overall sharpness, not a difference in depth of field.
04-02-2009, 09:17 AM   #14
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QuoteQuote:
Sean Nelson: ...which is why a 200mm lens has different depths of field when used on different sized sensors - it's field of view changes.
Thanks, this is a short and telling statement from which I can gather a lot of facts and understanding.

Also, thank you to everyone for this excellent and enriching discussion. I have not seen a lot of discussion on DOF at the forum, although crop factor is common place. I learned a lot from this discussion, from each of you, and this is what makes this forum such a great place.
04-03-2009, 04:42 PM   #15
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QuoteOriginally posted by Sean Nelson Quote
This means that for a smaller sensor those "circles of confusion" are larger in the final print and this means less depth of field.
DOF is measured in centimeters. How many centimeters deep the in-focus area of your photo is. If you take a photo and crop half of it away, the "circles of confusion" become larger, but the DOF remains exactly the same.

Say you're shooting an MF picture and an APS-C picture from a fixed distance with the same lens. If you need f11 in MF to get your entire subject in focus, you need f11 to get that same subject from the same distance in focus with the APS-C camera. You can't get away with using a larger aperture on the APS-C camera -- your subject won't be entirely in focus anymore.
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