Does paper thickness change the critical focus?

[Steve Smith]

I think it's worth finding out from the manufacturers if they suggest using a piece of paper under them or not.
I bought mine secondhand from ebay and don't have any instructions but to those of you with instructions, please have a look to see if they recommend using paper under them or even suggest not doing that and post your findings here.


Steve.

[Steve Smith]

I think that, particularly while printing larger format negs., the DOF of a reasonably stopped down (LF-) printing lens is not that 'deep', not to mention the circle of diffusion...

I have no actual proof from testing but I think that the negative to lens distance is much more critical than the lens to paper distance.


Steve.

[Philippe-Georges]

I have no actual proof from testing but I think that the negative to lens distance is much more critical than the lens to paper distance.


Steve.

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?

Philippe

[Steve Smith]

I'm not sure. The enlarger is really a macro camera with the subject (the film) much closer to the lens than the film (in this case, the paper!).

If it was focussing an image at infinity onto the paper then a change in the subject distance of a few hundred feet (think mountains in the distance) would be negligable.

In the case of a macro camera, there is a very narrow depth of field and this translates to the position of the film in an enlarger. Obviously this is ideal for an enlarger as the film is very narrow too as long as it is flat but it does have to be in the right place.

Doesn't really answer the question though!


Steve.

[ic-racer]

I do a lot of 1:1 projection printing of 8x10 negatives.
This is how the numbers work out. (but you can easily check 'dv' for yourself by moving the column up and down)

N = 20/(1+M) * square root of 'dv'

N = Aperture number
20 = constant (circle of confusion 0.15mm)
M = magnification
'dv' = millimeters of focal depth on the baseboard.

So, for me a 1:1 enlargement works out like this:
f16 = 20/2 x square root 'dv'
f16 = 10 x square root 'dv'
'dv' = 2.56mm

Again, enough focal depth for any paper out there.

This is a real-life situation. If you substitute other hypothetical numbers (ie smaller f-number and smaller acceptable CoC, then you may approach a number small enough that paper height comes in to play)

Now, if you do reductions, then the story is different.

One of these days I'm going to try to compare 8x10 to 4x5 reductions to 4x5 to 4x5 contact prints. I predict paper thickness my need to be considered, but when I do it, I'll post my empiric results.

[RalphLambrecht]

Something else worth considering is the possibility that the grain focuser manufacturer has already accounted for the paper thickness and using a scrap piece may put the focuser too high.

Let's hope they don't.

[RalphLambrecht]

Then I have the paper there for focusing which can't hurt.

It won't hurt, but the human focus error is likely to be more than the paper thickness anyway. Depth of field is several mm, apiece of paper is much less.

[Steve Smith]

Let's hope they don't.

Conversly, I hope they do!


Steve.

[RalphLambrecht]

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?

Philippe

That's my understanding too.

[RalphLambrecht]

Conversly, I hope they do!


Steve.

No, you don't!

Photographic paper is quite different in thickness, and what thickness do you want them to anticipate? Also, many printers like to have a sheet of paper on the easel to see and focus on. If the grain focuser assumes paper, it would double up on the paper thickness correction. What a mess!

You are better off without the paper assumption, which is likely to be wrong anyway, and add YOUR paper where needed. However, in practise, the paper thickness is only a minute part of the depth of field, and hence, this all makes no difference whatsoever.