Grain focusing by moving enlarger up and down

[Nicholas Lindan]

I threw out an El Nikor 50mm f/2.8 because it was soft on the edges. Now I see similar on the APO Rodagon (not N) 50mm f/2.8 wide open...

I went through the same - then finally followed Bob Solomon's advice and got a glass carrier and a Zig-Align. Then I found the lenses performed identically at f5.6. I sold the APO. I rarely use the glass carrier - the dust and occasional fingerprint smear is a PITA.

YMMV - as has been pointed out, sample variation between 'identical' lenses can be just as large as variation between brands and types.

 

[Bill Burk]

I’ve abandoned the “crank up and down enlarger” method in favor of the dial indicator.

I stop down first, because this jig “sees” a focus shift.

This also has me reconsidering the whole “paper under the grain focuser” too. Because I can see that too.

But I know that we still have a 5/8 inch play. So in the end it is not important to have the paper. But I am investigating how much it matters. I think it might be in the ballpark of one mark on the dial.

I am really surprised nobody’s done this before, that can’t be the case because I think it is really nice.

 

[ic-racer]

I’ve abandoned the “crank up and down enlarger” method in favor of the dial indicator.

I stop down first, because this jig “sees” a focus shift.

This also has me reconsidering the whole “paper under the grain focuser” too. Because I can see that too.

But I know that we still have a 5/8 inch play. So in the end it is not important to have the paper. But I am investigating how much it matters. I think it might be in the ballpark of one mark on the dial.

I am really surprised nobody’s done this before, that can’t be the case because I think it is really nice.

The thread is a little convoluted; I'm not fully following. Can you give an update of exactly what you are doing with the dial indicator?

 

[Bill Burk]

It’s attached to the lens board with a c-clamp. The design of the indicator gives a wide range of reach that can be locked into position. So with a 35mm lens the best position is a u-turn and touch up to part of the negative stage.

It can be switched to spring the feeler up or down and I have it springing up.

After initial focus I position it in the middle of its +/- 0.015 range and then focus the lens now (instead of the enlarger).

Now I can see the range of lens focus with precision and find the middle. I like this more than moving the whole enlarger up and down.

And I am learning as I go, that things aren’t exactly what I expected but I can believe what I see.

 

[ic-racer]

Is the middle correct? I don't know. In the view camera formula at the object side, the DOF is 2/3 behind and 1/3 in front at the hyperfocal distance, whereas at the image (film) side it is 50/50. On an enlarger, we can compute the answer using geometry and the diagram below, but what have you found with your actual measurements?

Full text in "Ins and Outs of Focus" Harold M. Merklinger 1990

 

[Bill Burk]

Worth considering third of way through may be the best focus.

 

[MattKing]

Worth considering third of way through may be the best focus.

Is this consistent with our earlier discussion about whether the changes are linear?

 

[Bill Burk]

Yes, the hyper focal distance is well known and makes sense there is some of that at play here.

So I am looking forward to suggestions, but will take it for granted there is a 1/3 slash 2/3 breakdown and I will try to get it in the right direction.

 

[ic-racer]

I can do the math if you can give us the parameters of the above diagram. Actually, without doing math, it can also just be plotted on graph paper.
Object distance (lens to paper) = ?
Subject distance (lens to negative) = ?
Focal length = ?

 

[Bill Burk]

Thanks ic-racer.

Focal length 50mm.
Negative to paper 22 3/16 inch.
I haven’t measured the lens position - It’s whatever it takes.

 

[ic-racer]

Bill's 22 3/16 inch lens-to-baseboard distance is about 564mm
Circle of confusion size for the math = 0.15mm

 

[ic-racer]

So, plugging practically used numbers to the equation is important in this case. The angle that forms the DoF limits is so narrow, the model shows the focal point in the MIDDLE of the Depth of Field, not 1/3-2/3 like the view-camera model in the earlier post. The math is the same, but the magnitude of the numbers is very different and yields different results!

 

[ic-racer]

I did not zoom in on the left side of the model (the negative side), but as Bill has already shown us, and as one can imagine, the Depth of Field is very very small.

 

[ic-racer]

Ok, in case you would like to see it, this is the DoF at the negative with 10x enlargement and F8 50mm lens. The DoF is about 1/10 of a millimeter.

 

[Bill Burk]

Wow that’s beautiful!

At the negative it looks like 1/3 and 2/3

 

[ic-racer]

So that answers two questions:

When focusing with the enlarger head, where is the focal point located between the extremes? Near the middle.

Does a thickness of paper matter under a grain focuser? No. At f8 with 10X enlargement, the depth of field at the baseboard is sixty times greater than the thickness of Double Weight paper.

 

[ic-racer]

Wow that’s beautiful!

At the negative it looks like 1/3 and 2/3

That is just from the lines being so hard to draw that close together with the software. Already in the view camera the DoF is very close 50/50 at the negative. In the enlarger in my model it is even more exact at the middle.


REMEMBER REMEMBER, the lines in the model are not the light rays from the lens to the baseboard. This is just a geometric model to calculate the angles that form the DoF.

So, as the two divergent lines have a smaller angle from the pivot at the corner of the box, and as they intersect the "negative plane" at more of a 90 degree angle, the focal point is more right in the center.

The reason for the 1/3-2/3 DoF in the scene when using a camera is the shown mathematically when the two lines diverge a lot form the pivot at the corner of the box and they intersect the subject plane at a very shallow angle.

 

[ic-racer]

This was a real surprise to me as all a long I thought the diagram would look like that in post #55 above.