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# Thread: Does paper thickness change the critical focus?

1. Originally Posted by ic-racer
'dv' = millimeters of focal depth on the baseboard.

'dv' = 2.56mm
IC - who uses f/16? Wouldn't f/8 be more realistic?

2. I'm sure this could be sorted out really easily. Does anyone have the instructions which came with their focusers?

The problem is we have two respected printers, Fred Picker and Gene Nocon mentioned in this thread who's suggestions are complete opposites.

One says use paper, the other says don't. I know the majority do and I don't. It is human nature to defend the way you do something as being right even if you have no proof so a bit of guidance from the focuser manufacturers would be really useful.

Steve.

3. Originally Posted by leicam5
I thought that the distance neg. <—> lens and the distance lens <—> paper are 'reversed proportional' ( is this the right expression in English?) to each-other. So, when one of the two distances is wrong the other one is equaly wrong, or am I wrong?

Considering the enlarger as a camera, I came to these assumptions:

For 1:1 reproduction, I agree with your 'reversed proportional' theory. i.e. distances and depth of field/depth of focus being equal.

When focused at greater distances, a huge change in subject distance translates to a small change in lens to film distance. e.g. a change from infinity focus to ten feet may result in a lens position change of about 1" (thinking about a view camera here).

My thinking then is that if you go in the opposite direction, e.g. the image on the film (or paper on the enlarger) is larger than the subject, then the reverse should be true. i.e. a small depth of field at the subject (negative) translates to a larger depth of focus at the paper.

Steve.

4. Originally Posted by Steve Smith
I'm sure this could be sorted out really easily. Does anyone have the instructions which came with their focusers?

The problem is we have two respected printers, Fred Picker and Gene Nocon mentioned in this thread who's suggestions are complete opposites.

One says use paper, the other says don't. I know the majority do and I don't. It is human nature to defend the way you do something as being right even if you have no proof so a bit of guidance from the focuser manufacturers would be really useful.

Steve.
Steve

But we have proof. The mathematics of it tell us that it does not matter if you use paper or not. Picker and Nocon, both give good advise, because both approaches work.

BTW, I checked my Peak instructions. It assumes no paper!

5. Originally Posted by RalphLambrecht
BTW, I checked my Peak instructions. It assumes no paper!
So what are you going to do in future?

Steve.

6. [QUOTE=RalphLambrecht;789553Picker and Nocon, both give good advise, because both approaches work.[/QUOTE]

Yet they both suggest that the other's method is wrong! I think we need to stop worrying about it and do whatever works for us personally.

Steve.

7. Originally Posted by Steve Smith
So what are you going to do in future?

Steve.
Same as now. Use a scrap piece of paper for easy focus (not because f the thickness) and leave the grain focuser paperless.

8. Originally Posted by Steve Smith
Yet they both suggest that the other's method is wrong! I think we need to stop worrying about it and do whatever works for us personally.

Steve.
Exactly, that's what I keep saying. Using a piece of paper is theoretically more precise, but in practise, the error is so small that it disappears in the 'noise' of the focusing system.

9. Originally Posted by Kirk Keyes
IC - who uses f/16? Wouldn't f/8 be more realistic?
Remember, that particular example was for an 8x10 negative, so that is an 300mm lens. f/16 and f/11 are the best apertures based on the MTF curves for that lens. I didn't want to get in to details, but that lens is not 'flat' at 1:1, so I use f/16. I have measured the focus spread for that lens at 1:1 from edge to center and it is curved to the extent of 2.5mm at the baseboard. So it is no coincidence that I use f/16 for my 1:1 prints with that lens. Again, no surprises here as the manufacturer does not list 1:1 in the recommended magnification range of that lens.

A little OT, but to continue with these thoughts. If I were in a "Resolution Contest" with someone doing contact prints of 8x10, then my personal 0.15mm CoC may not be good enough. So a process lens (with a flat field) and good MFT curve at f5.6 may be needed. Again this is if the prints are going to being closely scrutinized side-by-side.

Now, that first example I gave (9x enlargerment) was more typical of a 35mm enlargement. I used f2.8 in that example, but, as you suggest, using a more realistic f8 would increase the focus spread way beyond 2mm, further discounting the 'paper thickness' myth.

10. One thing I would like to summarize is that when enlarging, understanding depth of field is just as important as when taking pictures. Every enlarging system suffers from the following things to some extent:

1) Lack of parallelism
2) Lack of field flatness due to the lens
3) Curved or wavy negatives
4) concave, convex or wavy paper easels

The question of "how much to you need to stop down" to correct those things can be answered, to some extent, by the Peak magnifier. The main problem, is that things can look a little fuzzy under the Peak, and still be OK in a print viewed from a distance.

Therefore, determining the acceptable F-Number from the focusing equation takes into account a personal "acceptable circle of confusion" for viewing a print from a distance, and can take some of the guesswork out of interpreting the image under the Peak.

I have considered writing up an article on focusing the enlarger but,
a) Jeff Conrad came up with the math that substantiates that equation,
b) Jeff already has two good articles on focusing the view camera on the LF site,
c) the math in those articles is difficult to follow,
d) if you can follow the math for the view camera model, you don't need a separate article for the enlarger (its just a large format macro camera) and
e) if you can't follow the math in the view camera focusing articles, you probably won't be able to follow it in an enlarger focusing article

However, if there were interest I could try to come up with some kind of article like "The Basics of Optimum F-number Selection for Projection Printing Using Modular Transfer Function Criteria." Problem with that title is that no one will understand the title. If I called it "How to Focus the Enlarger" no one will read it

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