Red filter in enlarger throwing off sharpness

Steve Smith

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David A. Goldfarb said:
Paper thickness is not a constant value, so it would be impossible to compensate for it in the design of the grain focuser without adding a separate calibration adjustment for it.
Click to expand...

Just an average would be close enough. Especially when people say it doesn't really matter.

It could only be out by half the difference between the thickest and thinnest papers anyway.


Steve.
 
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ricksplace

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I focus with the lens wide open on a blank sheet of paper, or on the easel. I often adjust until it's fuzzy on both sides of focus, then split the difference, stop the lens down (I usually print at f8-f11) and expose the paper. I have checked it often with a grain focuser, and it's usually pretty close. Stopping the lens down takes care of the fact I might be a little off. If the image is purposely fuzzy, I use the grain focuser. I always focus with the lens wide open with no filters above or below the lens. Both of my enlargers (Beseler 23 and 45) have the red filters removed.
 

David A. Goldfarb

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Sparky said:
BIZARRE. I NEVER heard that before... nor noticed it (which isn't to say that it's not happening)... interesting...
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The issue of focus shift with VC papers is discussed in Ch. 11 of Ctein's Post Exposure, 2nd ed.
 

ic-racer

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Maybe we need a "Stickey Thread" to explain how to focus an enlarger for optimum depth of field and minimum diffraction. This question seems to come up frequently.

The size of Airy disks and circles of confusion can be optimized for any depth of field required as represented in this slight modification of the view camera 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, represented as the distance on the enlarger column between good focus on the highest and lowest portions of the curved or askew negative. (if 'dv' is zero then your negative is not curved or enlarger/baseboard/easel is not askew)
20 = constant for circle of confusion about 0.15mm on the print


To focus your enlarger use the following sequence (based on curved negative in an aligned enlarger). Use different high and low areas if alignment is in question (like far edges of the diagonal).

Focus on the center of the image. Note the distance marker on your enlarger column. Then (without touching the focus knob) move the enlarger head downward to focus on a corner. Make note of the distance between the two points in millimeters ("dv" in the equation above). Then position the head at the middle point between the two positions. You can use the formula to find your optimum f-stop after you figure out your magnification.

The constant of 20 is my own, based on how I look at my prints. You can calculate your own value that may or may not be the same.

***

More on-topic: Why are we focusing with the red light??? And, secondly, if there is a focus shift it is easily determined if the enlarger has a marked column.
 
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David A. Goldfarb

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Re: Can I see a difference in focus with and without the paper under the grain focuser?

RalphLambrecht said:
David

Sorry, but that's simply impossible!
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Depends how closely you look. Just out of curiosity, I set up a macro shot on my Sinar P for an enlargement of about 4-5X, since it's a transparent optical bench with geared movements and scales that I don't have on my enlarger (it has a rapid column), to see how much displacement I could notice by focusing on the rear standard, which would be the equivalent of moving the paper up or down in the enlarger. Observing the groundglass with my nearsighted eyes alone--about the equivalent of using a 2x loupe with normal vision, I'd say the image side depth of focus (which corresponds to what your table calls the "depth of field" for an enlarger) was about 5-6mm; with a 4x loupe, maybe 3-4mm; and with a 6x loupe a bit less than 2mm. A grain focuser is a much more precise instrument than a groundglass and a loupe, since it facilitates focusing on the aerial image and the combined magnification of the enlarging lens and the grain focuser is significantly greater, so I think it shouldn't be so surprising to detect such fine displacements at the image plane with a grain focuser under the enlarger, as I believe I do.
 

RalphLambrecht

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Yes, that's what it means.

The table is calculated with 'standard' values for the circle of confusion. 'Standard' in this case means a minimum viewing angle of 1 arc minute or a near-focus resolution of 7 lp/mm.

There is an argument that our eyes cannot resolve but detect resolution differences up to 20 lp/mm, in which case, we are talking about a minimum viewing angle of 20 arc seconds. But, even if you calculate the table with those values and look at the 16x20-inch print from a 35mm negative at f/4, you are still left with a DoF a 170-times larger than the thickness of a sheet of writing paper.

The more you enlarge, the less you need to worry about the piece of paper under the focusing aid.
 

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Steve Smith said:
I'm not sure either but I would work on the principle of the enlarger being a macro camera with the projected negative as the subject and the paper in place of the film with an image size greater than the subject size.


Steve.
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Yes, it can be done either way. The math stays the same. As David says, it's matter of semantics in the end. I try to remember it as the depth of focus always being at the film or negative plane.
 

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David A. Goldfarb

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RalphLambrecht said:
I try to remember it as the depth of focus always being at the film or negative plane.
Click to expand...

I think this is what photographers usually do, but my impression is that optical engineers refer to "depth of focus" at the image plane (film in a camera, paper in an enlarger) and "depth of field" at the object plane (the photographic subject in a camera, film in an enlarger), just to make matters more confusing.
 

RalphLambrecht

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David A. Goldfarb said:
Paper thickness is not a constant value, so it would be impossible to compensate for it in the design of the grain focuser without adding a separate calibration adjustment for it.
Click to expand...

Exactly. The instructions to my Peak enlarging focuser state:

The bottom surface of the pattern plate and the basement surface of the focuser are adjusted to cause the enlarged image of the negative to correctly focus on the photographic paper.
 

RalphLambrecht

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Steve Smith said:
Just an average would be close enough. Especially when people say it doesn't really matter.

It could only be out by half the difference between the thickest and thinnest papers anyway.


Steve.
Click to expand...

Steve

Here are the contact details for Tohkai. They make the Peak enlarging focusers. Maybe they can tell you, and then you tell us.

Tohkai Sangyo Co. Ltd.
24-2 Yushima,3-chome Bunkyo-Ku
Tokyo, Japan

tel:- 0081 3 3834 5711
web address:- www.peak.co.jp/
e-mail:- ex_div@peak.co.jp
 

RalphLambrecht

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David A. Goldfarb said:
Re: Can I see a difference in focus with and without the paper under the grain focuser?



Depends how closely you look. Just out of curiosity, I set up a macro shot on my Sinar P for an enlargement of about 4-5X...
Click to expand...

This is fun!

Let's calculate, but please tell us the f/stop or we can't.
 

KenS

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Focusing

RalphLambrecht said:
David

Sorry, but that's simply impossible!
Click to expand...

As one who spent a significant number of years printing transmission electron micrographs with a Durst 138S and a point light source... (a 150mm Rodagon wide open as is required for printing with point light source illumination), I always used a 'scrap' piece of paper in the easel. With the Omega focussing device, you are able to achieve sharp focus WITHIN the 'thickness' of the emulsion.... the main reason for my making use of a scrap piece of paper under the focussing device.

Ken
 

Steve Smith

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RalphLambrecht said:
Steve

Here are the contact details for Tohkai. They make the Peak enlarging focusers. Maybe they can tell you, and then you tell us.
Click to expand...

I might do that.

Please don't think I'm being argumentative. I really would like to know the facts rather than opinions. Being represented by just words on a forum loses out on all the body language and tone of voice associated with a proper conversation and sometimes the wrong message goes out.

Another problem we have as humans is that we tend to believe the first version of something we hear from a seemingly knowledgeable source. e.g. I first read about grain focusers in Gene Nocon's book. He said not to use the paper under the focuser so I don't. If I had first read an article by some other authorative figure who said the opposite, I would probably be arguing in the other direction now!


Steve.
 

RalphLambrecht

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Ken
We are talking about making pictorial enlargements at medium apertures, having a depth of field of several mm and even cm. Our negatives aren't even flat enough to worry about the paper thickness at the baseboard, never mind enlarger alignment accuracy.

We are making way too much out of a non issue.
 

RalphLambrecht

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Steve Smith said:
I might do that. ... I really would like to know the facts rather than opinions...
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Then please do it, and let us know, because you won't get a satisfying answer here. Only the designers of the device can put your concerns to rest.

Steve Smith said:
...Being represented by just words on a forum loses out on all the body language and tone of voice associated with a proper conversation and sometimes the wrong message goes out...
Click to expand...

Very true.

Steve Smith said:
..Another problem we have as humans is that we tend to believe the first version of something we hear from a seemingly knowledgeable source...
Click to expand...

Steve
We are very different in this regard.
 

Steve Smith

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RalphLambrecht said:
We are very different in this regard.
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Not really. I was talking generally about human nature. Personally, I will happily change my mind if given the facts.


Steve.
 

RalphLambrecht

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Steve Smith said:
Not really. I was talking generally about human nature. Personally, I will happily change my mind if given the facts.


Steve.
Click to expand...

That's not what I meant. I meant, I never trust what I'm told by an 'authority'. Heck, I don't even trust myself sometimes.

By the way, there are two types of facts; the facts we believe, because we like to, and the facts we just don't want to believe. That's another common human trade in all of us. That's why people play the lottery and believe that they might win, but don't hesitate to jump into a car without getting too worried about an accident. Guess which odds are higher.
 

David A. Goldfarb

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RalphLambrecht said:
This is fun!

Let's calculate, but please tell us the f/stop or we can't.
Click to expand...

75mm/f:4.5 Grandagon-N wide open on 4x5".

A Grandagon-N is an asymmetric lens, but measuring with a ruler, it's not by much. I get an exit pupil of 15mm for an entrance pupil of 16mm. Maybe someone has a spec sheet that's more accurate. In any case, it's closer to a symmetric lens than I would have guessed.

So if I have the formula right--

depth of focus at the image plane=2 * f-number * acceptable CoC (1+Magnification/P)

where P=exit pupil/entrance pupil

So let's say the Magnification is around 4.5 and we go with what I could see with a 6x loupe (depth of focus slightly less than 2mm--call it 1.8mm),

Acceptable CoC for me in that situation is around .034mm then. "Standard" CoC for 4x5" would be around .09mm (your table has 0.89mm, and f/Calc, which I think uses the Zeiss method, has .094011mm)
 

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Dietmar Wolf

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I always have the contact sheet of the film with me when printing a photograph.
I use the back of the contact sheet to focus without red filter and to write the individual print details on its back.
 

RalphLambrecht

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Steve Smith said:
Most of the time that's the only option...
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Not most of the time, but sometimes it's true. However, if we're honest, we have to admit that we do it sometimes, because it's less work than working it out for ourselves. I have witnessed many discussion, which took longer than the simple test, which woud have solved the issue.

Don't forget to report after the call to Tohkai!
 

RalphLambrecht

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David

I used your data provided in post #56 and 69 and did some calculations. The results verify your observations described in post #56.

assumptions:
4x5-inch negative
4x magnification (16x20-inch print)
f/4.5
CoC = 0.030 mm (critical observation)

This calculates to a depth of field at the baseboard of 5.4 mm. You reported 5-6 mm without focusing aid. So, we have a match. You further reported an increase in detection by using a 6x focusing aid to a bit less than 2 mm. Seems reasonable, actually, I would have expected an increase closer to that of the magnification of the focusing aid, so, down to 1 mm, but OK, eyes and optics are never perfect.

This is good info. I agree with your data, and your reported observations sound reasonable.

Now back to the statements, we are trying to address:

David: I can see the difference in the grain focuser between having the paper under the grain focuser and not, and that's enough to convince me... (post #23)

Ralph: Sorry, but that's simply impossible! (post #38)

Double-weight paper has a thickness of roughly 0.25 mm. You were able to detect a 2 mm difference with a 6x focusing aid. The top model from Peak has a magnification of 10x, so, you might be able to detect a 1.2 mm difference with it. I have a 20x focusing aid, with that, let's say 0.6 mm? Of course, we'll lose a bit to the optics again, but let's forget about that for now.

It looks like you are still off from seeing the difference between 'paper' and 'no paper' by a factor of 3x. And this was already assuming critical viewing CoC (0.030), not a huge enlargement (4x), wide open lens (f/4.5) and the highest possible focusing aid available (20x). Changing a few setting to more standard numbers, and you will be off by a factor of 10x.

I must repeat: it's impossible for you to see the difference of a piece of paper under the focusing aid.

But, what's more important, even if you see it under a 10x focusing aid, you'll never see it in the print. Also, your printing paper won't be flat enough to stay within 0.2 mm, and your negative carrier and enlarger alignment won't be good enough to support this ideal condition. So, it looks like we are sweating the small stuff, while the big stuff is eating our lunch.

However, I still think, we are making way too big of a deal of this. If you feel more comfortable to place a piece of paper under the focusing aid, by all means do so. After all, it makes it theoretically perfect. It won't matter, but it won't hurt either.
 

Steve Smith

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I also think we are making too much of it. Use paper if you want, don't if you don't want to but either way we can't say that one way is better than the other.

I doubt that our enlargers can be focused with enough precision for it to make a difference anyway.


Steve.
 
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