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C 645 Lens Quality

Mike,

I looked up your quoted statement on Norman's web site. Let's disect his claim. Go to the section just above where the "claim" is made:

"The circle of confusion C at the DOF limit is based on the 0.01 inch = 0.25 mm feature in an 8x10 inch print."

Which, is of course, correct and is simply visual accuity.

"On the film, C (mm) = 0.25/(magnification for an 8x10 print)."

OK...simple arithmatic...

"For a constant angle of view, lens focal length f is proportional to the format size (cropped for an 8x10 inch image) and inversely proportional to the magnification."

Again, no problem here...

"f/C is therefore a constant, independent of the format, about 1600 for a "normal" lens."

Which is focal length over CoC, and of course, that's correct. 50mm / .033 ~= 80mm / .06, all agreed there...

And...then he shows a table, which all seems correct...and then this statement:

"Since f/C is a constant, independent of format, depth of field is constant for constant aperture opening a."

Er, right...so DOF for f11 is the DOF for f11, no matter what the film format, for the same FOV...exactly what I was saying, and the premise/concept DOF/CoC is based on.

And then...:

"And since f-stop N = f /a,"...

We've established f is focal length, and a is aperture...

> "Depth of field is constant when the f-stop is proportional to the > format size, i.e., DOF is the same for a 35mm image taken at f/11, a > 6x7 image at f/22, a 4x5 image at f/45 or an 8x10 image at f/90"

But...he makes no mention of focal length, so I am assuming he means for the same focal length! Well, that means the FOV (Field of View) is different...and that changes the entire meaning of this statement.

35mm format, 50mm lense, f11, 1M DOF = .535 6x6 format, 50mm lense, f22, 1M DOF = .597

OK, so here is the root of the misunderstanding. Of course changing the FOV will give you more/less DOF, BUT, not of the same subject area, only of the OVERALL image, and the images will not be the same, one will contain 2x the subject matter than the other. If you enlarged them to show the same image on the 8x10, then what you'd see is the DOF would be the same.

QED ;-)

Austin
 
A little off topic - but - what what does one multiply the focal length of a 645 format lens to figure out what it equates to in 35mm format focal length? I thought is was just about half - but was recently told this is wrong. (Trying to figure out what a 55-90mm 645 lens equates to in 35mm terms and failing miserably!) Thanks -Lynn
 
Lynn, as near as makes no difference the Contax 45-90mm lens is the same as a 28-56mm lens in 35mm terms, a 55-90mm 645 lens would be a 35-56mm.
 
Hi Lynn,

> A little off topic - but - what what does one multiply the focal > length of a 645 format lens to figure out what it equates to in 35mm > format focal length? I thought is was just about half - but was > recently told this is wrong. (Trying to figure out what a 55-90mm 645 > lens equates to in 35mm terms and failing miserably!) Thanks -Lynn

Easy enough:

focal_length / circle_of_confusion = focal_length / circle_of_confusion

CoC for 35mm = .033mm CoC for 645 = .05mm

55mm / .05mm = x1 / .033mm, so .033 * 55 / .05 = 36.3mm 90mm / .05mm = x2 / .033mm, so .033 * 90 / .05 = 59.4mm

So, a 55-90mm 645 lense equates to about a 36-60mm 35mm lense.

Regards,

Austin
 
Thanks too Austin. Now that I have the formula I should be okay for all lenses. I know you mentioned in a long ago post something that explained "CoC... circle of confusion" however I can't find it. Can you provide a link to this again? -Lynn
 
Hi Austin,

First to address one error in my statements so far, Norman's analysis contains the assumption of a magnification of greater than 1/20th, which is to say the subject must be relatively close (but not too close because of the approximation that relies upon s >> f). It's therefore possible that the relationship you are describing holds for more distant subjects, and since I haven't investigated that I'm not going address it.

Moving on to the errors in your comments:

> "Since f/C is a constant, independent of format, depth of field is constant for constant aperture opening a."

> Er, right...so DOF for f11 is the DOF for f11, no matter what the film format, for the same FOV...exactly what I was saying, and the premise/concept DOF/CoC is based on.

Er, wrong ;o) because the relationship he's describing relates to the physical aperture size 'a', not to the f-stop 'N'. A given opening 'a' is related to the focal length 'f' by the formula N=f/a (as he states) so as you increase the focal length in order to produce the same FoV with a larger format size, the DoF decreases proportionately.

> "Depth of field is constant when the f-stop is proportional to the format size, i.e., DOF is the same for a 35mm image taken at f/11, a 6x7 image at f/22, a 4x5 image at f/45 or an 8x10 image at f/90"

> But...he makes no mention of focal length, so I am assuming he means for the same focal length!

He makes no mention of focal length, but his discussion holds if you assume the same FoV, so why assume otherwise? As you point out, it would be a pointless excercise if you held the focal length constant, as the lens can't "know" how large a film area lies behind it and the only remaining factor affecting DoF is the final print magnification. In a way perhaps this is what you are trying to argue, but you ignore the fact that increasing focal length reduces DoF almost with the inverse square of f at the magnifications we are considering.

The ex&le you have chosen (1m subject distance with a 50mm lens) seems a bit close for the assumptions involved. Try this one:

35mm format, 85mm lens, f/4, 5m subject distance, total DoF is 0.82m.

6x7 format, 170mm lens (giving same FoV), f/4, 5m subject distance, total DoF is 0.44m

The DoF figures were obtained using Bob Atkins' calculator applet: http://bobatkins.com/photography/technical/dofcalc.html

Rudolf Kingslake gives almost exactly the same working as Norman Koren, and uses similar assumptions. He states: "assuming an acceptable circle of confusion c’ in the film, the depth of field of a lens for fairly close objects is proportional to the f-number, and inversely proportional to the square of the focal length, other things being equal" which is exactly what I'm claiming. The reduced print magnification factor cancels out single power of f, and reduces the relationship to approximately an inverse linear one between DoF and focal length in the final print.

Best regards,

-= mike =-

PS. Thanks for the Kingslake page, that's a useful resource for reference.
 
Wow!

Thanks for all of the thoughtful responses. I do trust my eyes, but I was originally educated as a chemist, so I like to square measurements with observations. My assumption was that I am seeing something in Zeiss images that isn't being captured in some of these simple tests.

I have purchased an N1 since my original post. That should do while I prepare my wife for the shocking cost of a 645. The biggest selling point is that, at last, I've found a system that can last a lifetime!
 
Robert,

Jump! Go and get the 645. I have the complete system (except the zoom) with two bodies and havent looked back once. I chose the contax for the lenses. In my work I have been up against work shot on 6x7 or 4x5 and my clients have only praise for the contax 645 images. Plus, basically every digital back Mfgr. has something for the contax so you can grow with this system.
 
I concur 100%=2E When I take my film (Fuji) to my pro lab which services almost all the pros in my city they constantly comment on the quality of m= y images=2E The only piece of equipment I am not really happy with is my TL= 360 flash=2E I am spoiled on my Canon 550EX which I think is far superior=2E = If only I could have a predictable and accurate flash this camera would be perfect=2E I don't even notice the battery consumption anymore!
 
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