Focus Finder paranoia

Hi everyone!
Just bought a paterson focus finder and now I'm the middle of a new paranoia.

From what I've understood, the focus finder use its mirror to show you the grain, and for
some "magic" reason (related to the distance of the mirror to the pedestal) the focusing on the mirror is the same as on the paper.
This means that if I put a glass just over the paper to keep it flat, and put the focus finder over it, the focusing is not perfect, right?

right. the finder goes on the paper.

Not paranoia.
The thicker the glass, the greater the error.
Some people do use a sheet of paper on the easel or baseboard when focusing anyway, though it's a bit of a debate whether this is necessary (because of depth of focus).
But put a 5mm thick sheet of glass in and you could easily see problems.

As with so many questions like this, your best bet is to try it and see.

If you can't see a difference between a print made with and without glass, you don't have a problem.

Simple answer, if you want to use a focus finder with that setup you would focus the negative on the baseboard without the paper and the glass plate. Then place the paper and the glass on the baseboard, expose and you will be fine.

To go into more detail, in common enlargement situations (common apertures, focal lengths and magnification ratios) the thickness of the paper will not make a substantial difference for practical purposes, as the depth of focus on the base board (usually >5mm) covers it. So, it does not make a difference if you focus with a sheet of paper under the focus finder or not. The glass plate, however, might make a difference (depending on its thickness and the other factors mentioned above), so putting the focus finder on top of the glass could give you trouble, especially if some other errors creep in. Before you start, you should check by trial and error that your new focus finder works and is adjusted properly (For now, I simply assume you know how to adjust and use it correctly, if not, please ask). Also, remember that the depth of field on the side of the negative is very, very small. So it pays off to make sure that your enlarger is well aligned and the film is kept as flat as possible.

You just focus on the baseboard. No scrap piece of paper necessary.


Steve.

I have two focus finders, one is a Patterson 'Major' which is great for enlagements larger than 10x12 and a very high quality one sold in UK under the name of 'Peak'. Both are absolutly accurate even on paper. However, what I have done is to attach (glue), a used piece of printing paper to the base of each finder and then focus the negative on the masking frame without the new paper being in place. This can save physical marks appearing on the developed print which may be caused by the focus finder.

For all practical purposes, even without the paper attached to the finder, when the enlarging lens is closed down a couple of stops, the depth of field will cover any minor errors.

I do not remember where I saw this, but someone from Peak said that when designing the finder, they already took the paper thikness. Basically, the paper is not necessary (and I did that also, until I read that).

I print on so called double weight fiber paper, which is pretty standard. I have some different sized sheets from rejected prints in my darkroom and use those to set up the easel and to focus with the focus finder. I think in general the cheaper focus finders work well in the middle of the image, and the expensive focus finders also work well closer to the corners. Given that getting sharp grain in the corners is more critical I always try to focus nearer to the corners. Obviously there are parts in an image that make seeing the grain more difficult (darker parts in particular).

The Peaks are probably the better ones. I also have that tall patterson but struggle with it and always go back to the Peaks

We had a long thread on this a few years ago. I contacted some manufacturers and the replies basically said don't bother with the paper.

Barry Thornton did some experiments which are detailed in one of his books. He focused and made a print with his easel on a 1/2" thick piece of board. Then without changing the focus, he made a print without the board (1/2" lower) and with a second board (1/2" higher). He couldn't tell any difference in sharpness - and he was a sharpness freak.

Master printer Gene Nocon also states not to bother with paper under the focusing aid in his book, Darkroom Printing.


Steve.

There is another reason for not using glass to lay over the paper, flaws in the glass will be reproduced on the print, also there may be a slight loss of contrast due to light scattering from the faces of the glass. Paper only needs to be held down at the edges, the middle stays flat, this is how enlarging easels work, I recommend them.

tedr1 said:

There is another reason for not using glass to lay over the paper, flaws in the glass will be reproduced on the print, also there may be a slight loss of contrast due to light scattering from the faces of the glass. Paper only needs to be held down at the edges, the middle stays flat, this is how enlarging easels work, I recommend them.

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+1 I also think it would be near impossible to keep the glass sheet clean.

For added paranoia, you can worry about:

1. If you focus on the paper or easel, then add glass over it, you will shift the plane of focus down by a small amount, equal to (n-1) * thickness. The value n is the index of refraction of the glass (about 1.5). For a 5mm thick glass, the focus will shift by 2.5mm.

2. You will get ghost reflections from the top surface of the glass. This will happen because the undeveloped paper is white, with strong diffuse reflection. The top glass surface will reflect about 4% of this light back to the paper. This will be most noticeable at a sharp edge between a black area and white area. The black will "bleed" into the white area on the print.

There is no easy remedy for 1. If you use glass, you should focus with the grain in place. you could do this by:

a. Using your unaided eye.
b. Using an optical device that aids your eye (hand magnifier?) (long-throw microscope?) (short-throw theodolite?)
c. Trimming the bottom of your grain focuser by the correct amount to compensate for the thickness of your glass, which would be 0.5 * (n-1) * thickness.

To remedy 2. you could invest in a cover glass with antireflection coating, like how our lenses are coated.

Both of these effects are small. You will have to decide if they are bad enough to worry about.

I agree with tedr1's comment. I use a grain focuser and an enlarging easel, and use the backside of a rejected print for cropping and focusing. RC papers will lie very flat. FB papers may curl at the edges, but not enough to cause any noticeable problem. Old, dried-out FB papers will have excessive curl, but they are likely to have other problems as well.

I use a sheet of scrap print paper to focus with; the grain magnifier goes right on top of that. I know the depth of focus will cover small errors such as a paper thickness. However, my easels are yellow and I prefer focusing on the brighter white paper surface.

Glass isn't a good idea for the reasons mentioned above unless you get really good quality glass and keep it as thin as possible. Even then, focus without it.

One thing to mention: all the Peak and the Paterson grain magnifiers I own have an adjustment for individual eyesight. Make sure you position the eyepiece so you can focus sharply on the reticle before using your magnifier to focus prints. Not doing this can make focusing more difficult and less accurate.

Best,

Doremus

Doremus Scudder said:

One thing to mention: all the Peak and the Paterson grain magnifiers I own have an adjustment for individual eyesight. Make sure you position the eyepiece so you can focus sharply on the reticle before using your magnifier to focus prints. Not doing this can make focusing more difficult and less accurate.

Best,

Doremus

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+1