Grain Focuser Not Focused

[Silverpixels5]

For some reason my grain focuser isn't giving me a totally focused image. When I foucs w/o it by eye the image is sharper than when I focus on the grain with the focuser. Any ideas as to whats going on here? I use a Peak focuser if that makes any difference. Thanks!

 

[Tom Stanworth]

I think this is a can of worms! Sorry to keep mentioning Barry Thornton, but he looked into this in 'edge of darkness' finding that out of about half a dozen finders he owned, all gave different indications of focus.

I also have the same problem. When mine (paterson) is calibrated as per instructions for my eye, it is very slightly 'out' compared to what I can see on the baseboard (need a big print to be able to do this well). I then calibrated my finder so that it gave sharp grain when I could see a sharp image on the baseboard (having carefully chosen a neg that made this evaluation easy by eye). The two then continued to agree (it and my eye). However, the little hair line inside, which should be crisp to show that it is set up for your eye, was fuzzy as hell, showing that it should be miles out.

The end result? When set up to agree with what my strained eye could see or calibrated as per instructions, focus points were different. Print sharpness was identical. Weird and I never got to the bottom of it. I moved on.

Can of worms!

Tom

 

[eric]

This reminds me of an email thread on the pure-silver list a while ago. I should look at the archives or ask Jim Brick cause he mentioned, focusers do not focus whats on the easel, but what is leaving the lens or something like that. As an example, put something a few mm on the easel and use the grain focuser. It'll be focused with or without it. As I recall, it was a heated thread about a week long. Arguments went back and forth as "thickness of paper, ect, etc. But at the end, man, I need to find the archives, I was persuaded that the focus was up top, not the bottom, hence the mirror on the grain focuser.

Okay, I will look up the thread but if anyone else on the pure-silver list, you probably remember it as well. Pure-silver list left tundraware but I"m sure the archives are somewhere.

 

[JackRosa]

Ouch! This is a tough one. My guess would be that something is wrong with the grain focuser, although I know Peak to be top quality (I own one myself). The distance from the lens to the mirror is very important. The angle the light hits the mirror is also important. Are you using the focuser close to the centerline of the lens, where the light rays are vertical (down).

 

[dancqu]

Ouch! This is a tough one. My guess would be that
something is wrong with the grain focuser, ... The distance
from the lens to the mirror is very important.

The image seen through the eyepiece must be that of
the image at the surface of the paper. Lifting the focuser
a bit may help. Lowering is another matter.

Good optics are required if the color of light used for focus
and the color of light used for exposure are to coincide.

I usually use the most powerfull reading glasses the
law allows. Dan

 

[mikewhi]

Yes, this seems like a simple thing but can probably get hairy pretty fast. I also use a Peak model and I don't have any problems.

It's probable that you're focusing a small area of the image and then standing back and looking at the image overall and noticing that it isn't in focus. One variable you should look is the overall alignment of your 3 planes in your enlarmenent setup. It could be possible that your grain focuser is indeed focusing that small patch correctly but that your planes are out of alignment somewhat and that you can get a better overall focus by eye. As a sanity check, check out the alignment of your enlarger negative, lens and easel planes.

Are you focusing with the focuser sitting on a piece of printing paper, which will more exactly be the image position projected at the time of exposure? Some people say this makes a difference. I do it out of habit but can't prove that it makes a difference in reality.

I assume your Peak is setup and focused on the black line correctly? If any part of it that would affect the distance that the light travels to the eyepiece is out of wack, this would cause it to focus incorrectly.

Are you focusing wide open then stopping down and noticing a focus shift? Could be your lens is much sharper stopped down a few stops. I ask because it is natural to focus wide open to get a bright image and then to stop down and watch the image sharpen up. You may get a focus shift as a side-affect of stopping down or you may just be noticing a sharper image stopped down as a natural result of the lens projecting a sharper image when stopped down a few stops. If you are focusing wide-open, try focusing at the stop that you will expose at or vice versa.

Did you ever drop your focuser or anything that would move the mirror or lens elements?

Just brainstorming here. Hope some ideas help. You have a good focuser there, so it would be the last thing that I'd blame for this unless you have good reason to suspect that it's broken.

Good luck.

-Mike

 

[Bob Carnie]

Ok this may be a stupid response but ...
are you using a glass carrier????

I have a large assortment of enlargers and grain focusers, from peaks down to those little blue devices with a hair line in the middle.

with a sheet of paper matching the desired material under the magnifyer adjusted to my eye I have not had any problems.
Without a glass carrier your negative may be heating up as you focus and drawing in the middle towards the light bulb>
then as you turn off the enlarger to get your paper and place in the easel and then find the timer and ready to expose> the negative cools down and flattens out again , this will cause you no end of problems ..

I strongly advise trying glass carrier.

 

[gainer]

For some reason my grain focuser isn't giving me a totally focused image. When I foucs w/o it by eye the image is sharper than when I focus on the grain with the focuser. Any ideas as to whats going on here? I use a Peak focuser if that makes any difference. Thanks!

Are you using a blue focusing filter? If so, don't.

I did a series of experiments some years ago in which I got the probable erors of focusing with a grain focuser with three different filters and none. There was a focus shift on the average in one direction with blue, the other direction with red, and none with white or green light. The cause was the chromatic aberration of the eye. Also, the standasr error of focus was greater for both red and blue focusing filters than it was for green or no filter. You can be as scrupulous as you want about the 1/2 mm or so of paper thickness, but the random errors of focus swamped that out. These results were in Darkroom and Creative Camera Techniques, now Photo Techniques.

No matter what printing filter you use, the focusing should be done by the light that the eye sees best, which is green or white.

The effects of chromatic aberration of the eye are well know to astronomers, where they show up as apparent chromatic aberration of the telescope.

 

[Silverpixels5]

as a matter of fact, my focuser does have a blue filter. Not so sure how to use it w/o it though. The filter is part of the eyepiece, which does come off, but then where do I get a clear piece from? Do they sell those for Peak focusers? Thanks for the info though!

Are you using a blue focusing filter? If so, don't.

I did a series of experiments some years ago in which I got the probable erors of focusing with a grain focuser with three different filters and none. There was a focus shift on the average in one direction with blue, the other direction with red, and none with white or green light. The cause was the chromatic aberration of the eye. Also, the standasr error of focus was greater for both red and blue focusing filters than it was for green or no filter. You can be as scrupulous as you want about the 1/2 mm or so of paper thickness, but the random errors of focus swamped that out. These results were in Darkroom and Creative Camera Techniques, now Photo Techniques.

No matter what printing filter you use, the focusing should be done by the light that the eye sees best, which is green or white.

The effects of chromatic aberration of the eye are well know to astronomers, where they show up as apparent chromatic aberration of the telescope.

 

[Silverpixels5]

Well it would seem that it was the blue filter that was causing my problems. I figured out that the filter was an attachment on my eyepiece and not actually part of it. So I took it off and focused....and of course the image was both sharp on the paper and in the focuser. I put the filter back on and it looked slightly out of focus. Damned blue filter! Thanks for the help!

 

[dancqu]

No matter what printing filter you use, the focusing should be
done by the light that the eye sees best, which is green or white.

If it is knowen that the enlarger's optics are very well color
corrected that is correct. If in doubt focus by the light the
paper sees best. That is the reason for the blue filter.

VC papers see both blue and green and that complicates
matters. I avoid that complication by using graded. Dan

 

[Tom Stanworth]

Actually BT said that an often recommended blue fiilter made print sharpness worse......

 

[Thilo Schmid]

With a grain focuser like the Peak, you are focusing on a so-called airy image. Since your eyes do focus automatically, you need a reference point where the paper plane is. Otherwise you will always see a sharp image. This reference point is critical (not the mirror). The reference rectangle of your Peak may be out of alignment or your eyes are not or no longer able to focus this distance.

 

[dancqu]

Well it would seem that it was the blue
filter that was causing my problems. ... I took it off and
focused....and of course the image was both sharp on
the paper and in the focuser. ... Damned blue filter!

"Damned blue filter" Peak and Omega and likely some
other manufacturers may be throwing us some curves.

I think though it may be bad news. Your enlarging lens
may not be well color corrected. Your blue filter may be
doing it's job and Peak may not be throwing you a curve.
Your blue filter is showing you that your blue focus is not
the same as your white focus.

For a lens which is not well color corrected and when using
VC papers I'd suggest a blue-green filter and for graded
papers, a blue filter. Dan

 

[Ed Sukach]

Your enlarging lens
may not be well color corrected. Your blue filter may be
doing it's job and Peak may not be throwing you a curve.
Your blue filter is showing you that your blue focus is not
the same as your white focus.
For a lens which is not well color corrected and when using
VC papers I'd suggest a blue-green filter and for graded
papers, a blue filter. Dan

That is not what "color correction" is all about. It is properly referred to as "Chromatic Abberation". Its effect is color "fringing" where one side of an image edge will appear red (to me it looks orange) and the other side will appear blue.... and this is usually most prominent at the extremites of the field.

The blue filter is there to remove the longer wavelengths of red light, and it effect will be to reduce or remove the "red part" of this fringing - it also replicates more closely the sensitivity of photographic paper ... at least as far as graded paper is concerned.

Possibly, it can best be understood in terms of "curvature of field", where the focal plane of the red part of the spectrum curves differently from the focal plane of the blue. They usually - I have never seen otherwise - coincide at the optical axis of the lens ... I have never seen a lens with two completely distinct focal planes.

 

[Ed Sukach]

With a grain focuser like the Peak, you are focusing on a so-called airy image. Since your eyes do focus automatically, you need a reference point where the paper plane is. Otherwise you will always see a sharp image. This reference point is critical (not the mirror). The reference rectangle of your Peak may be out of alignment or your eyes are not or no longer able to focus this distance.

The lens focuses in space. The rays do not require a medium to intercept them.
A "grain focuser" works on an "aerial" image ... As an example, one uses a microscope on an optical bench to examine that aerial image. The focal plane is rather easily determined with that microscope. The naked eye works the same way, focusing on an aerial image, when we look through a telescope or microscope.

However, there are limits to the system. If the eye is not in the mathematically correct position, the image will not be in focus.... The placement of the mirror in a grain focuser affects the optical path and the ray trace... so it IS critical.

 

[Ronald Moravec]

The blue filter was for graded paper that is sensitive to blue light, not the blue/green of VC paper. If your lens focused green, red and blue in different places, you could compensate with the blue filter.

I never used mine and never had trouble.

 

[fschifano]

If you are even slightly far sighted, what generally happens when you start needing reading glasses, anything awash in blue light will give you the devil's own trouble. The blasted Germans who designed the lighting for my VW Passat's instrument panel should be spanked! They lit it up in a very attractive blue color, but seeing the darn thing at night hurts my eyes.

 

[chad wyatt]

Peak grain focuser

Speaking only to practice, not theory, I have had the same focusing trouble with my Peak Model #3. My old Thomas Scoponet is set correcetly; I placed a spacer beneath my Model 3, so that it agrees with the Scoponet. Sharpness accomplished. I use a cold light source.

 

[Claire Senft]

Eye problem

The problem alluded to by Mr. Gainer is not in reference to the color correction of the focussing aid or the enlarging lens etc. The problem originates in the human eye not on these optical devices. Human eyesight is very poor in focussing via blue light. not to good in red either. No filter is best, a green filter is much better than the other primaries. Focus first and then insert whatever filtration is desired for making the photo into the light path.

 

[edz]

This reminds me of an email thread on the pure-silver list a while ago. I should look at the archives or ask Jim Brick cause he mentioned, focusers do not focus whats on the easel,
.
.
Okay, I will look up the thread but if anyone else on the pure-silver list, you probably remember it as well. Pure-silver list left tundraware but I"m sure the archives are somewhere.

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