# DOF - OK, I'm confused!

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• 04-25-2011, 08:18 PM
tkamiya
Thank you Ralph...

Working on combining formulas for df and dr into dF=f(u,f,c,n)
Looks like a lot is going to cancel each other....
• 04-26-2011, 01:35 AM
RalphLambrecht
I like to see your results, but you'll find that DoF always depends on those four variables in one way or another, unless some kind of assumption was made. For example, some people combine f and N to the true aperture dimension or use the magnification factor instead of u and f. But none of that makes DoF equations easier or more practical, in my opinion. Still, like to see your results.
• 04-26-2011, 08:47 PM
tkamiya
I've been at this for two nights now and results aren't good. I got it down to:

2cnuf^2(u-f)
---------------------------
{f^2-cn(u-f)}{f^2+cn(u-f)}

I see a common term "cn(u-f)" but can't do anything from here...

So much for wanting a simple explanation!
• 04-26-2011, 09:30 PM
2F/2F
IMO, it is best not to think about AOV as if it has a direct affect on D of F; it does not directly affect D of F. Think of D of F in terms of magnification and f stop, and it is more simple to understand. Anything that changes magnification will change D of F. Once you understand this, you can think of AOV in terms of how it affects magnification, and it will make more sense.

This means same format, same f stop, same location, different lens, and you will have different D of F. Longer lenses shot from the same distance as shorter lenses make things larger in the viewfinder, meaning more magnification. Therefore, less D of F.

Also, same AOV, same f stop, but different formats, and the larger format will have less D of F because it has higher magnification (image is larger on the film simply because the format is larger, even though the spatial relationships are the same with the equivalent FL on the smaller format).

And, of course, same format, same lens, move camera closer. Subject gets bigger, meaning more magnification. Thus, less D of F.

At a given f stop, you can figure out whether you will be getting more of less D of F by making any change if you simply reason out how that change will affect magnification.

And you can definitely get shallow D of F with subminiature cameras such as 110 format or APS-C/H. You just get more D of F at a given f stop and identical composition than you do with 35mm or larger formats.

The cameras that are hard to get shallow D of F with are the ones that have even tinier sensors or film frames than APS-C.
• 04-26-2011, 09:33 PM
tkamiya
AOV?
• 04-26-2011, 09:39 PM
lxdude
Angle of view.
• 04-26-2011, 09:41 PM
tkamiya
oh...
• 04-27-2011, 03:14 AM
RalphLambrecht
Quote:

Originally Posted by tkamiya
I've been at this for two nights now and results aren't good. I got it down to:

2cnuf^2(u-f)
---------------------------
{f^2-cn(u-f)}{f^2+cn(u-f)}

I see a common term "cn(u-f)" but can't do anything from here...

So much for wanting a simple explanation!

In my opinion, thinking about DoF in terms of magnification is not any simpler than thinking about it in terms of distance and focal length, but in case the subject magnification is already known or calculated, the equation to determine the depth of field (dF) simplifies to (see attached), where ‘c’ is the circle of confusion, ‘N’ is the lens aperture in f/stops, and ‘m’ is the subject magnification. This equation is adequately accurate for subject magnifications larger than 0.1, which means it can be used for close-up but not landscape photography.
• 04-27-2011, 07:05 AM
markbarendt
You are correct.

For a moment and think about it one piece at a time.

Assume you have a camera where you can change anything you please. For large format cameras this is actually the norm.

If you have a given subject say a person and you want them to be a certain size in the frame, your camera is on a tripod and all you change is the format what happens?

The subject doesn't fit the same way anymore.

To fix that you can;

Move the camera. Getting closer while using the same lens and aperture reduces our DOF and vice versa.

Change the lens. Here longer lenses have shorter DOF at any given aperture and vice versa.

Once you fix the issue above then you adjust aperture.
• 04-27-2011, 02:45 PM
rjbuzzclick
Quote:

Originally Posted by RalphLambrecht
Simply speaking: you are correct in all points.

Hope this helps.

Wow, a Fiesta! I had one of those in the mid 1980's!

(Sorry...way, way, way, OT...)
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