Grain Focuser Not Focused

[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]

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]

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.