Depth of field and Circle of confusion.

[Grandpa Ron]

I came across a comment on the web that indicated the film plane and/or the film holder could warp enough to cause the image be out of focus on the film. I suppose this is true, but dealing with my 1960's vintage 4x5 holders, it seems any significant warping would cause light leaks.

This brought to mind the depth of field (DOF) and the circle of confusion (COC).

DOF is defined by the distance at any given aperture, that is acceptably sharp, there is a lot of discussion on this. The COC is the point of focus by the lens on the film plain. It also has a region, albeit very small, of acceptable focus.

So the question is, "Does stopping down to increase the depth of field, also increase the region of acceptable sharpness for the circle of confusion?

I simply do not have enough experience with my view camera to figure this one out.

 

[compur]

"When you set the lens aperture, you increase or decrease the size of the Circle of Confusion resulting in a larger or smaller area in front of and behind the focal plane being in focus."

Understanding the Circle of Confusion (BEST guide)

 

[Ysarex]

I came across a comment on the web that indicated the film plane and/or the film holder could warp enough to cause the image be out of focus on the film. I suppose this is true, but dealing with my 1960's vintage 4x5 holders, it seems any significant warping would cause light leaks.

This brought to mind the depth of field (DOF) and the circle of confusion (COC).

DOF is defined by the distance at any given aperture, that is acceptably sharp, there is a lot of discussion on this. The COC is the point of focus by the lens on the film plain. It also has a region, albeit very small, of acceptable focus.

So the question is, "Does stopping down to increase the depth of field, also increase the region of acceptable sharpness for the circle of confusion?

I simply do not have enough experience with my view camera to figure this one out.

Given your opening comments about the film holders and film plane I think what you're asking about is depth of focus as opposed to depth of field. Try this: Depth of focus - Wikipedia

 

[StephenBatey]

The circle of confusion has nothing to do with focus or depth of field per se. It's the diameter of the largest circle that the eye will be fooled into accepting as a point when viewing from a normal distance.

If you have a series of points at different distances (and we'll assume a perfect lens, and diffraction doesn't exist) then one and only one - the one in focus - will be representing by a point in the image. The others will be represented by progressively larger curcles as they go progressively out of focus. At some point (no pun intended) these "points" will become larger than the circle of confusion, and no longer appear as points. As you stop down, the depth of focus increases, and the circles produced by out of focus points become smaller. The circle of confusion is unchanged.

The size of the circle of confusion is arbitrary. It depends on the degree of enlargement and assumed viewing distance.

 

[Original katomi]

Stopping down will increase DOF but there is a limit after which the image degrades
Most lenses will have a sweet spot
I have an app from photographers friend that I can set the format f stop and a number of settings and it will work out all sorts of info like DOF cof near limit and so on

 

[Grandpa Ron]

Okay, more terminology to master. You have to love this stuff.

So based in the information above, the depth of field on the image side of the lens increased with the smaller aperture and the depth of focus on the film plane side of the lens also increases with a smaller aperture.

My concerns about film plane shift of my decades old film holders is somewhat relieved by the fact that most of my equally old lenses have a max shutter speed of 1/100 or 1/200 second so I am often using smaller apertures and I usually print 5x7 prints.

Larger format are rather forgiving.

Thanks

 

[StephenBatey]

The easiest way to grasp it is with a diagram. Light rays from infinity are focused by a lens one focal length behind the lens (that's the simplest way of putting it, without going into optics). If you draw the lines out, you'll easily see that the wider the aperture, the larger the angle the lines make as they meet at a focus. This translates to a very small distance before and behind that plane before the point at the intersection becomes a sizeable disc.

Now bring the parallel lines close together by cutting the distance, which is what stopping the lens down does. You can now go further out from the focal plane because the angle of intersection is less.

One other point. Depth of field increases as focal length decreases, but depth of focus works the other way round. Film position matters more with short focal length lenses than long ones. Hence film flatness is more important at the short end of focal lengths.