Does paper thickness change the critical focus?

Andrey · Apr 23, 2009

I'm just curious what you think in terms of the difference in the circle of confusion and critical focus. If I'm printing my negative and don't use a piece of paper under the grain focuser, would by focus be significantly off? Would it matter at all, considering the negative is not flat, but bent just slightly anyways?

What is your opinion?

BetterSense · Apr 23, 2009

How are you focusing in the first place? If you use a grain focuser, why don't you test it and see if it is within error margins? I personally test on a sheet of paper, but considering that I test at f/3.5 and expose at f/16, I doubt the thickness of the paper is significant. Plus, the paper often "bubbles" up at least a little bit so it's really probably several thicknesses higher than the grain focuser. Still, one less variable.

WGibsonPhotography · Apr 23, 2009

I focus on a sheet of paper from the pack I'm printing on. I usually forget to change the aperture from 2.8 to whatever aperture I'm printing at at least once in the printing session, so I usually have some spare sheets lying around. I usually print at F/8 or F/11, so I dont think the thickness of the paper is going to make a noticable difference.

The fact that I focus on a sheet of paper has nothing to do (for me at least) with the final print being out of focus because of the thickness of the paper. I've been doing that since my first photography class because my instructor said it might make a difference. It's just habbit for me. Better safe than sorry, I guess

BetterSense · Apr 23, 2009

I usually forget to change the aperture from 2.8 to whatever aperture I'm printing at at least once in the printing session, so I usually have some spare sheets lying around.

So I'm not the only one!

Ken N · Apr 23, 2009

I always put my grain focuser on a sample piece of paper to space it upwards a bit. But then again, I always place my paper face down in the developer, wear clean socks in the darkroom and have soft jazz playing on the stereo. Whether any of these things make a hill of beans worth of difference, I doubt very much as long as you aren't exposing with the lens set wide open.

ic-racer · Apr 23, 2009

Andrey said:
I'm just curious what you think in terms of the difference in the circle of confusion and critical focus. If I'm printing my negative and don't use a piece of paper under the grain focuser, would by focus be significantly off? Would it matter at all, considering the negative is not flat, but bent just slightly anyways?

What is your opinion?

I don't have an opinion on the matter, just the optical formula that predicts about a 2mm focus spread at f2.8 at 9x enlargement. This easily encompasses any photographic paper of which I am aware.

Modular transfer function focusing equation (equation #38 in http://www.largeformatphotography.info/articles/DoFinDepth.pdf) :
N_max ~ 20 / (1 + m) sqrt(dv)

N-max = F number
m = magnification
dv = focusing leeway on the baseboard
20 = constant for circle of confusion about 0.15mm on the print

raucousimages · Apr 23, 2009

I did a test once where I focused on the baseboard and then made two prints. One on single weight FB and one on premium weight FB. The premium weight was noticeably thicker than the single weight but both prints were equally sharp to my eye. That said I still focus with with magnifier on a piece of paper I am using at that time. Old habit but it reduces any variables in my printing process. In theory at least a shorter lens high above the baseboard will have a greater DOF then a longer lens close to the baseboard. So a 35mm neg with a 50mm lens making a large image will have less problem with paper thickness than a 4X5 neg with a 150mm lens making a small image. But even at that I doubt it makes a noticeable difference. Maybe I will do a test with a 150mm and a small image just to see what happens.

MattKing · Apr 23, 2009

I have easels that are yellow in colour. It's easier to evaluate the image using the (white) back of a discarded print, so I may as well focus using it too.

Matt

WGibsonPhotography · Apr 23, 2009

BetterSense said:
So I'm not the only one!

good to know i'm not the only one

I have noticed that i'm wasting less and less paper that way, so maybe one day I'll end up having to sacrifice a sheet instead of using a wasted one. Ok, that's not gonna happen. I doubt I'll ever get out of the habit of making careless mistakes :/

Smudger · Apr 23, 2009

I always focus at the aperture I will use to expose the print,on the assumption that any focus shift effect on stopping down will be negated. That said,none of my EL Nikkors show any shift.. Of course,if your focuser is pretty naff you won't have that option.

Anscojohn · Apr 23, 2009

I usually forget to change the aperture from 2.8 to whatever aperture I'm printing at at least once in the printing session,

*****
Yup!

Sirius Glass · Apr 24, 2009

WGibsonPhotography said:
I usually forget to change the aperture from 2.8 to whatever aperture I'm printing at at least once in the printing session, so I usually have some spare sheets lying around.

Been there; done that [many times]!

Steve

jgjbowen · Apr 24, 2009

To quote the late Fred Picker, if you are too lazy to put a piece of the same paper you will use to make the print under your critical focuser, "it probably doesn't matter." That is, if you don't care enough to do it right, why bother...

RalphLambrecht · Apr 25, 2009

Andrey said:
I'm just curious what you think in terms of the difference in the circle of confusion and critical focus. If I'm printing my negative and don't use a piece of paper under the grain focuser, would by focus be significantly off? Would it matter at all, considering the negative is not flat, but bent just slightly anyways?

What is your opinion?

It matters as much as exhaling helps to lose weight.

Bob-D659 · Apr 25, 2009

The easy way to tell is to focus first, then move the head up and down and watch the image in the grain focuser.

Steve Smith · Apr 25, 2009

This question comes up quite regularly. In his book 'Photographic Printing' Gene Nocon regards using a piece of paper under the focuser as a waste of time.

He's a much better printer than I will ever be so I have always followed this advice.

I expect if you made a print with and without the paper, it would be difficult to see a difference.

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.

Steve.

BetterSense · Apr 25, 2009

I always wondered about that bit too.

Personally my easels are grey and yellow, so it's hard to see without white paper there anyway.

Roger Thoms · Apr 25, 2009

I use a piece of photo paper face down, I like the white background to compose the image on. I also mark my borders on the back of the paper so that I know were to adjust the easel blades. That way I get nice even borders. Then I have the paper there for focusing which can't hurt.

Roger

PVia · Apr 26, 2009

I also use a sheet of photo paper face down...lately I've made 8x10 and 11x14 templates in Photoshop with gridlines and bold outlines of various sizes/crops to compose my images on. After composing and setting the easel blades, I replace the template with the photo paper sample to focus. The grid helps make the borders a lot more even than using the numbers on the easel, which always seem to be slightly off even on the more expensive ones.

Philippe-Georges · Apr 26, 2009

I have a black easel (Kostiner) and use a white sheet of silky design paper with a grid drawn on it to compose/frame the image and check the black borders. Then I stuck an extra piece of that paper under the grain-focuser (Omega). So, the thickness of that sheet of paper plus the piece under the focuser equals the thickness of FB paper, more ore less.
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...

About stopping down the aperture for printing, it is the same s*** as while working with a view-camera, after composing/focussing, particularly when age is coming on slowly and one gets easily distracted a little...

Philippe

Steve Smith · Apr 26, 2009

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 · Apr 26, 2009

leicam5 said:
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 · Apr 26, 2009

Steve Smith said:
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 · Apr 26, 2009

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 · Apr 26, 2009

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.

Does paper thickness change the critical focus?

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