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C 645 Lens Quality

Thanks Mike! That is very helpful. I have never used MF, so never thought of this before. How does this affect DOF in digital images? (After I read through Normen Koren's page I hopefully can answer this for myself, but....)
 
Hi Steven,

> I have never used MF, so never > thought of this before.

The DOF calculator contradicts what Mike claimed. It showed that for a 50mm lense at f2 on a 35mm camera, vs an 80mm lense at f2 on a 6x6, they both produce the same DOF.

> How does this affect DOF in digital images?

DOF is purely based on imager size/CoC (film is the imager in the case of using film, and the imaging sensor is the imager in the case of digital)...so, you simply need to know the size of the imager, and given it's size, you can calculate the CoC for it.

A full frame sensor, like the N Digital, will have the same CoC as 35mm film, .033mm. A half frame sensor, which is more typical of the "other" cameras (24mm x 18mm), the CoC would be ~.018mm.

So, I'm saying, because it's digital has no bearing on the DOF, it's the same for film or digital if the imaging area is the same.

Regards,

Austin
 
Hi Austin,

Not sure I'm qualified to wade in on this subject, but what you are saying doesn't seem to make common sense to me.

Surely, DOF is a function of focal length and aperture only and independent of image size. If I put a smaller piece of film (or sensor) behind my 50mm lens set at f8 then the DOF (and Coc) will be identical -- the smaller piece of film is essentially just a crop of the full frame.

John
 
Hi John,

> Surely, DOF is a function of focal length and aperture only and > independent of image size.

Correct, but the assumption is the larger the image, the further it's being viewed.

> If I put a smaller piece of film (or > sensor) behind my 50mm lens set at f8 then the DOF (and Coc) will be > identical

No. I believe the part you are missing is that DOF is a visual trick, and only exists because of the way our eyes work. In reality, there is only one very thin plane of focus. Anything else in focus before and after that plane is simply because our eyes can only see so well (visual acuity) (or because we have reached the limit of the recording medium). Remember, you have to enlarge the smaller image more to get the same size image as you get from the larger format. This is why the CoC for larger formats is larger than it is for smaller formats.

It makes no sense that you would want a different film format to give you a different DOF if all the other parameters are the same (image size/viewing distance and visual accuity). Obviously, a 35mm frame has to be enlarged more than a 645 frame to produce an 8x10 image, and that is why the CoC for 35mm is .033mm and for 645 is .050mm.

I would suggest taking a look at this web site:

http://www.ida.liu.se/~marho/dof/dof.html

Regards,

Austin
 
Hi just to add in fact there is no DOF only a plain of sharpness and then various degrees of unsharpness ie a points will become small circles that increase in size as you move away from the plane of focus(known as the circle of confusion) the phenomina refered to as DOF comes from the eyes inability to resolve a very small circle from a point Hence finished print size will have a direct effect on DOF as the more you enlarge the larger the circle will become.
 
Hi Austin,

I know this seems counter-intuitive in some respects, but I spent a good long while poring over the maths (I was a bit rusty in that department so it came down to hauling out some of my old text books) and proved to myself that Norman's workings are correct. I worked from first principles and got the same outcome, and I also plotted a bunch of Excel charts to help convince myself. There is an assumption that the subject distance is much greater than the focal length, so the rule doesn't hold for macro work (I didn't try to find out if any rule of thumb DOES hold there, because I'm not enormously interested in macro work) but the following conclusion from Norman's page is correct I believe:

"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"

If you still have trouble believing this, it's worth sitting down with a big sheet of paper and working through the maths in detail. If you come out with a different result then please let me know, but I wasn't able to find a flaw in Norman's presentation. Also, as Albert points out, the results do agree with practical experience; consider the very thin DoF of large format or the endless DoF of small-sensor digicams.

-= mike =-
 
An intriguing twist to Mike's post is to include camera shake and "graininess" (using Kodak's Portra grain scores)into the calculations when bench-marking medium format against 35mm.

For any given size of print you end up with about one stop of "quality advantage" with 645 versus 35mm. In other words you get about one additional stop with 645 that you can choose to use for more DOF with a smaller aperture, less grain with a slower film, or less camera shake with a faster shutter speed.
 
Hi Mike,

I haven't looked at Norman's work, but I know for a fact what you apparently believe (is being said by Norman) is simply wrong, and that my statement is %100 correct, and is supported by Kingslake, and just about everyone else who has spoken/written about this subject. It's also how every DOF calculator I've seen, and corresponds to every lense DOF marking I've seen.

I have specifically done experiments using both MF and 35mm, shooting at the exact same aperture/distance/FOV, and the DOF is as identical as can be discerned. It's simple arithmetic (as Kingslake shows), and this follows completely with reality

Now, on to this statement:

> "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"

I can't say if that statement is right or wrong. It's missing a LOT of information You can't calculate DOF without knowing subject distance, and focal length. That statement needs more qualifications for it to actually mean anything.

I'll take a look at that web site when I get a chance. My guess is you're somehow misreading/understanding what is being said, or the web site is simply wrong...

Regards,

Austin
 
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