Circle of Confusion query

[peter brooks]

I'm looking at hyperfocal distances for a wide angle lens on 10x8 at a fairly small aperture. A 'normal' CoC for 10x8 is (I think) 0.2, whereas a CoC for 5x7 is 0.15.

Would I get a better result (in real life rather than just theoretically) using the smaller CoC of 0.15 (or even smaller)? Would it even be noticeable in contact prints?

By my calculations the difference between a 0.2 and 0.15 CoC would move the hyperfocal distance only about 3ft further away from the film plane.

(I think I understand a little of what the Circle of Confusion is about - but seldom has a technical term been so aptly named )

[Jac@stafford.net]

A rule of thumb suggests that you close the aperture (larger numeric ƒ) to gain more DOF. Calculating via CoC just confuses practical expectations. CoC relates more to degree of enlargement which is nil in contact printing. Some people become concerned with diffraction with small numeric apertures, but for contact prints it is not likely critical for any aperture on the lens scale.

[peter brooks]

Thanks for the reply. Yes, I understand the relationship between aperture and DOF. The application is a box camera so it will have a fixed focal length (I have done this with 5x7 and am now going to build a 10x8, using f32 for the calculations). Looking at the hyperfocal distance gives me a range of 'usefullness' (so to speak). As the CoC is part of the formula for calculating the hyperfocal distance I was wondering what practical effect (if any) reducing the CoC would have on the final image.

[Pere Casals]

The application is a box camera so it will have a fixed focal length

If I understand well, you have to decide what hyperfocal settings you are to use, the problem is that when using fixed hyperfocal you are to have distant subjets in the worst case...

You can use this app:



...to understand how the system will behave across subject distance, to decide what f/ and what focus plane distance you want.

[peter brooks]

I was replying to Jac@stafford.net and in the meantime two more replies... Maybe my reply makes more sense? Sorry if I wasn't clear in the original query...

[Jim Jones]

The CoC is a personal choice. It should be chosen after considering how the final image will be presented, not on what others might recommend. I suspect many photographers would never notice the difference between CoCs of 0.2 and 0.15, but a few certainly will. Remember, a WA pinhole 8x10 camera may have a pinhole of 0.5mm and a somewhat slightly smaller effective CoC, and still produce photos that many consider pleasing.

[Drew Wiley]

I ignore the whole topic. The only thing that counts is how you want the print to look. Unsharpness as well as sharpness is a tool of composition. That's what so wonderful about a big groundglass. And if you need more critical information, a basic loupe will tell you far more than all the hyperfocal formulas in the world.

[peter brooks]

Hmm. On reflection my reply makes no sense - reducing the CoC in the calculation would have absolutely no effect on the final image.

What it does is moves the hyperfocal distance (slightly) further away (which should be a sharper point) and reducing the band of 'acceptable' sharpness. So is the CoC acting like a kind of resolution tolerance?

[Jac@stafford.net]

So is the CoC acting like a kind of resolution tolerance?

Kinda. For 10x8" contact prints use a CoC of 0.2 mm which approximates the resolving power of the human eye under normal viewing conditions. Simplifies things, no?

[peter brooks]

Kinda. For 10x8" contact prints use a CoC of 0.2 mm which approximates the resolving power of the human eye under normal viewing conditions. Simplifies things, no?

Yeah, that makes sense, thanks. I'll stop considering the CoC now

(side note: the box camera is me convincing myself that I need some 10x8 holders etc... before long I'll convince myself that I need a 'proper' 10x8 camera... You know how it is... )