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# Thread: Depth of focus (NOT field) calculation query

1. Originally Posted by AgX
My understanding is that depth of focus and depth of field mean the same.

(My photographic encyclopediae and dictionaries confirm this.)
In English depth of field refers to the distance between the nearest and farthest points around the subject that are acceptably sharp.

In English depth of focus refers to the distance between the nearest and farthest points around the film plane that are acceptably sharp.

The two are not at all the same.

FWIW, the depth of focus formula given in S. F. Ray's Applied Photographic Optics, 3d edition, is (approximation for small magnification, p. 221) is 2CN where C is the diameter of the Circle of Confusion and N is the relative aperture, i.e., the f/ number).
In the OP's situation, depth of focus at f/45 with a CoC of 0.2 mm will be 9 mm. I note that Ray recommends a CoC of 0.2 mm for 8x10; this will allow contact printing, is too large for sharp enlargements.

2. and indeed, contact printing (of paper negatives) is exactly the aim!

3. This is just my recent experience building a fixed focus box camera. Yours might be different if you are a more careful builder than I am. I first built a test camera in a cardboard box, like the one you recently built but open on one side. Inside was a movable film plane, with a piece of white paper on it. ( In other words I stuck a piece of cardboard in to be the "back" of the box ) I adjusted the position until the projected image was perfectly in focus, then measured and built my fixed box based on that measurement.

The camera was meant to use only the rear element of a rapid rectilinear lens. In a comedy of errors, I kept forgetting to take the front element off, and ended up over at f295.org asking the same questions you are about depth of focus. I even speculated that my images were blurry because of chemical actinic focus.

It turned out that the focus was just fine at f/45 and f/64.

But here's the part of the story I wanted to get to: after using and being happy with the camera for a while, I tried to make some photos with the lens opened up a little bit more. It was clear that my focus was off just a hair. So I ended up cutting a hole in the back of the camera and inserting a frame identical to what holds the paper negatives, but with some scotch tape stretched across an opening where the paper would have been.

It turned out that the focal length needed to be a bit longer. Shorter would have been easier because I could have added some shims in the back of the box. But I was able to move the lens out the required amount without too much trouble. The adjustment was small, maybe only 1/16th of an inch, but it made a huge difference. It might be my imagination but I think the negatives at f/45 and f/64 are also improved. Now the camera is easy to use and reliable... aim ( with viewing dots ), calculate exposure, and make a picture.

You can see a picture of the camera in this post ( the 1st picture of a camera after "photo details" ) and you can see the tape on the back of the camera where I patched the hole I'd cut to adjust the focus.

I think building with foamcore or matboard is not such a bad way to start... it's easy to fix things with tape and you can learn what works and what doesn't without much expense or time.

4. Also the film plane has to be flat and parallel to the lens mount to maintain even sharpness on the 4 corners.

Here are the final errors on a medium format metal camera I built, measured as shown.
https://app.box.com/s/96ap1stgz0e489vvtl27

TIR of parallel register was about (50 micron), 2 thou inch after a hand lap

Change in register of the aluminum body from minus 10 Celsius to 35 C was calculated to be 65 micron (2.5 thou inch)

Here are some measurements on rollfilm backs for Graflex 4 x5 to get an idea of typical tolerances for well worn components.
For Rollfilm backs, the height of the 4 roller ends with respect to the face was
(in thou inch)
A Singer RH10 206, 197, 202, 202 ( that is within a TIR of about 9 thou inch)
A newer RH10 195, 195, 195, 198
An older RH/8 190, 202, 200, 200

The ground glass, was difficult to measure but I concluded it was around 220 thou inch which puts it about 20 thou inch behind the roller plane of the rollfilm backs.

I expect a fixed focus Large Format camera build would be more of a challenge to get accurate especially if wood is used. The removable shims mentioned by Ned sound like a good idea in the initial design. Shim it longer then home in by removing shims.

5. Ned: Thanks. As you know I'm planning a sliding-box camera for the 213mm lens I also have. When I get bored with the heavy vignette and falloff I'll get from the 150mm on 10x8, I can then take the lens end out and use the box as the back half of the new camera ...

6. Originally Posted by wombat2go
Also the film plane has to be flat and parallel to the lens mount to maintain even sharpness on the 4 corners.

I expect a fixed focus Large Format camera build would be more of a challenge to get accurate especially if wood is used. The removable shims mentioned by Ned sound like a good idea in the initial design. Shim it longer then home in by removing shims.
Yep, but bear in mind that this is really a bit of fun as part of a longer term project, and I won't be expecting deadly accuracy at any stage. As I said in my OP, my craft skills are minimal, and even if I had the sort of kit you clearly have the use of, I'd have no idea where to start!

7. Originally Posted by pdeeh
this is really a bit of fun as part of a longer term project, and I won't be expecting deadly accuracy at any stage.
Yes, I really enjoy building bits and pieces for cameras and I learned a from it too.
If you start with tools the first 2 recommended would be a 300mm stainless rule and a 150 mm dial or vernier caliper of reasonable quality.
The caliper and the vacuum cleaner are the most used items in my little workshop.

8. Originally Posted by pdeeh
wildbill: I explained my reasons for wanting the numbers in the 4th sentence of my OP.

spijker: thanks

bernard_L: Thanks. No, there won't be any macro going on! and yes, I realise the need for DOF, and I do realise the need for minimising error. However, this is a "fun foamcore" project, as I have no workshop, woodworking tools or indeed skills, and thus none of the things of most of the LF community take for granted. I really just want to know how much wiggle room I have, if any. I'm not sure whether your final remark is for me or for spijker - for my part, I simply used a CoC number that seems commonly used for calculating DOF on 10x8 (at Dofmaster.com for instance).
gotcha, somehow I missed that. if you can get gatorboard instead of foamcore, it's much easier to cut accurately and is very rigid. I built a 4x5 fixed camera out of it.

9. That's an amazingly simple equation: d of focus= 2xcxN.nothing to it.

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