enlarging paper under a grain focuser

[radiantdarkroom]

Yes there is visible difference wide open on the enlarger lens when focusing with a sheet of paper as apposed to not having a sheet of paper. I checked this out many years ago. It makes a huge difference when you are blowing up to 40x60 prints from 4x5.

 

[ic-racer]

Since enlargement is basically macrophotography, wouldn't depth of focus make the effect of paper thickness negligible. Does someone want to take on Ctein here?

This is correct and easy to test. It is mathematically and empirically eveident that the degree of enlargement dictates the precision to which the grain focuser needs to be matched to the easel. The MORE the magnification of the enlargement the LESS one has to worry about the thickness of the paper messing up the grain focuser.

Emperic example: When I am enlarging a Minox frame to 16x20 inches I can put a PHONE BOOK under the the grain focuser and the grain is still in focus.

Last night I was doing a 11x14 enlargement of a 4x5 negative and as an experiment, I could put a 7mm MAGAZINE under the grain focuser with no ill effects.

Now as you go the other way and start REDUCING 8x10 negatives to 4x5 prints, the thickness of the paper will be more likely to cause a problem with the grain focuser.

I DO keep a piece of enlarger paper taped to the underside of my grain focuser so it's aluminum base does not SCRATCH my easel.

Everyone can check this for themselves. Just put a 35mm frame in the enlarger with a 50mm lens and crank the head all the way up. Then focus with the grain magnifier. Then just lift the grain magnifier in the air and see how far you can lift it before the image looses focus. It will be a good distance. Then try it stopped down. You may be surprised at how far you can lift it before it goes out of focus.

 

[jstraw]

I hate composing an enlargement on the yellow base of my Saunders easel, So I put in a piece of paper for composing anyway...so necessary or not, it's under my grain focuser.

 

[cao]

This is correct and easy to test. It is mathematically and empirically eveident that the degree of enlargement dictates the precision to which the grain focuser needs to be matched to the easel. The MORE the magnification of the enlargement the LESS one has to worry about the thickness of the paper messing up the grain focuser.

It's fairly easy to visualize this if one considers the cone of light between the exit pupil of the enlargement lens and the plane of sharp focus. The subtended angle gets smaller as the magnification increases (paper is farther away). Hence one needs a greater displacement of the paper for a given circle of confusion. On the other hand, large magnifications also show any misalignment between the negative stage and the lens stage. Hence one should be very reluctant to draw conclusions about focus error unless one uses a laser aligned enlarger and a glass carrier.

 

[dancqu]

Grain focusers, how many diameters of magnification?
Ten, twenty, ...? Say ten and focusing a four diameter
enlarged image. So the focusers image is a forty times
magnified negative. And what is that equal to. A view
of the print from a distance of one half inch?

Rare I use my grain focuser. I use at most a most
powerful pair of reading glasses and keep both
eyes open. Dan

 

[ic-racer]

On the other hand, large magnifications also show any misalignment between the negative stage and the lens stage.

+++
This is so true and frequently overlooked or misunderstood. My routine is to align negative stage to baseboard with the laser, then align lens to baseboard. But the key is to get lens to negative stage perfect. With a laser there is no easy way that I have yet found to align negative stage right to lens without the inbetween step.