I find the most difficulty is when I am making a large enlargement and
can't reach the controls, it would be nice if I had about four more
inches of finger length but then my knuckles would surely drag on the
ground, since my arms are long but my body is short >:-D

As of right now I am in process of moving to a new house so my
darkroom will be more user friendly, and that means making some
device for focusing remotely from the base board,....I have heard
bicycle chains work well.

 What you need is a pair of strong reading glasses.
And I'm not pulling anybody's leg. I've a very down to
Earth view of focusing.
 I focus with the lens wide open; DOF is least. With both
eyes open and reading glasses on I adjust the focus. If the
image looks sharp on the "focusing" paper at a very close range,
and then the lens is stopped down, the resulting print will be
sharp at any viewing distance. That includes viewing the
print with strong reading glasses.
 I use my grain focuser for checking the grain. I could use
a better. That Jobo Hocus Focus sounds interesting.          Dan

    Yep. That's why I love the Durst. No moving parts.

Regards,

  John S. Douglas, Photographer -  http://www.puresilver.org
             Please remove the "_" when replying via email

I find there is a focus shift that occurs, especially when filtration is
present.  The phenomenon was described in an in-depth article by Patrick Gainer,
"Hazards of the Grain Focuser", Photo Techniques Magazine, Jan-Feb 1997, p. 41.
 I myself find that some refinement is needed once the filtration is added and
the lens stopped down.

Francis A. Miniter

Hey Nicholas, I've been thinking of getting one of these. Do you have
any brand suggestions?  Do you find the 30x magnification to be optimal?

Regards,
Peter De Smidt
www.desmidt.net

<pre>
Hi John!

The easy answer is that the top glass is important when you need it.

Frequently, 35mm and roll film are curved with the emulsion toward the
inside of the curve. If exposed to heat for some time, the curve will
increase due to emulsion dehydration. At large magnifications (16X),
the total depth of field at the carrier for a good APO lens used at
f/5.6 will be less than 0.1 mm. Maintaining that sort of tolerance
over the entire frame usually requires a two sheet glass carrier,
though one sheet above the negative is also common. The single sheet
works to counteract the curvature if the lower frame forces the
negative's edges against the glass (some frames are "loose").

Sheet film is different in that it ends to sag in a glassless carrier,
and of course, the required tolerances differ. The sag can be checked
simply by placing a straight-edge from corner to corner across the
film supported by the lower half of the carrier. Glass under the film
solves that problem. However, the film border should be pressed into
contact with the glass. This morning I checked some old 4x5 negs, and
though they appear flat when resting on the glass, a corner or part of
the border often had a space under it. A glassless top frame may
correct the problem, but don't assume that is the case - check it -
and check for heat induced focus shifts (leave the head on and check
focus accuracy over time via the parallax method without touching
anything). Dropping a second sheet of glass onto the film solves
planarity problems.

Years ago there was a detailed discussion of APO / non-APO lens
sharpness for 4x5. A glass carrier allows one to get very sharp corner
images from APO lenses at f/11. I think Barry Sherman confirmed my old
tests, and he could see repeatable differences even between 4x5 APO
lenses when the system is optimized (flat film and aligned enlarger).
I also remember Bob Solomon at a camera expo ordering a customer to
get a glass carrier for his enlarger before Bob would sell him an APO
Rodogon for 4x5 - good advice. The glass carrier also helps less
aspiring lenses, and I've seen marked improvements in B&W corner
sharpness ("microcontrast") from Tessar designs, though they need to
be stopped down more (f/22) to get the off-axis abbe rations under
control.

Another aspect is enlarger alignment, which also must be within the
tolerance for film flatness. I've pasted in a couple of tables to
illustrate where and when one is likely to run into problems, or
shall we say, where there's room for improvement in acceptable work.

The simple macro formulas:

Depth of field at the negative carrier:

DOF = 2 * C * f * (M + 1)/(M * M)

where D is the total depth of field (two-way), c is the circle of
confusion permitted on the paper, f is the marked lens f/stop and M is
the magnification. Usually DOF at the carrier is a relatively small
distance, and decreases with magnification.

Depth of focus at the easel:

D = 2 * C * f * (M + 1)

where D is the total depth, c is the circle of confusion
permitted on the paper, f is the marked lens f/stop and M is
the magnification.

Depth of field at the negative carrier (mm)

COC = 0.25 mm
                                f/stop
        2.8     4       5.6     8       11      16      22
Magn.
2       0.525   0.750   1.050   1.500   2.063   3.000   4.125
4       0.219   0.313   0.438   0.625   0.859   1.250   1.719
6       0.136   0.194   0.272   0.389   0.535   0.778   1.069
8       0.098   0.141   0.197   0.281   0.387   0.563   0.773
10      0.077   0.110   0.154   0.220   0.303   0.440   0.605
12      0.063   0.090   0.126   0.181   0.248   0.361   0.497
14      0.054   0.077   0.107   0.153   0.210   0.306   0.421
16      0.046   0.066   0.093   0.133   0.183   0.266   0.365
18      0.041   0.059   0.082   0.117   0.161   0.235   0.323
20      0.037   0.053   0.074   0.105   0.144   0.210   0.289

COC = 0.125
                                f/stop
        2.8     4       5.6     8       11      16      22
Magn.
2       0.263   0.375   0.525   0.750   1.031   1.500   2.063
4       0.109   0.156   0.219   0.313   0.430   0.625   0.859
6       0.068   0.097   0.136   0.194   0.267   0.389   0.535
8       0.049   0.070   0.098   0.141   0.193   0.281   0.387
10      0.039   0.055   0.077   0.110   0.151   0.220   0.303
12      0.032   0.045   0.063   0.090   0.124   0.181   0.248
14      0.027   0.038   0.054   0.077   0.105   0.153   0.210
16      0.023   0.033   0.046   0.066   0.091   0.133   0.183
18      0.021   0.029   0.041   0.059   0.081   0.117   0.161
20      0.018   0.026   0.037   0.053   0.072   0.105   0.144

Depth of focus at the easel (mm)

COC = 0.25
                                f/stop
        2.8     4       5.6     8       11      16      22
Magn.
2       2.1     3.0     4.2     6.0     8.3     12.0    16.5
4       3.5     5.0     7.0     10.0    13.8    20.0    27.5
6       4.9     7.0     9.8     14.0    19.3    28.0    38.5
8       6.3     9.0     12.6    18.0    24.8    36.0    49.5
10      7.7     11.0    15.4    22.0    30.3    44.0    60.5
12      9.1     13.0    18.2    26.0    35.8    52.0    71.5
14      10.5    15.0    21.0    30.0    41.3    60.0    82.5
16      11.9    17.0    23.8    34.0    46.8    68.0    93.5
18      13.3    19.0    26.6    38.0    52.3    76.0    104.5
20      14.7    21.0    29.4    42.0    57.8    84.0    115.5

COC = 0.125
                                f/stop
        2.8     4       5.6     8       11      16      22
Magn.
2       1.1     1.5     2.1     3.0     4.1     6.0     8.3
4       1.8     2.5     3.5     5.0     6.9     10.0    13.8
6       2.5     3.5     4.9     7.0     9.6     14.0    19.3
8       3.2     4.5     6.3     9.0     12.4    18.0    24.8
10      3.9     5.5     7.7     11.0    15.1    22.0    30.3
12      4.6     6.5     9.1     13.0    17.9    26.0    35.8
14      5.3     7.5     10.5    15.0    20.6    30.0    41.3
16      6.0     8.5     11.9    17.0    23.4    34.0    46.8
18      6.7     9.5     13.3    19.0    26.1    38.0    52.3
20      7.4     10.5    14.7    21.0    28.9    42.0    57.8

Note that paper thickness and other concerns at the easel
plane are not very important, since one can stop down at low
magnifications without noticeable losses to diffraction. Some
of the values in the table are not practical (large magnification
and small aperture) since the effective f-stop is in a range
where diffraction will degrade performance.

(effective f/stop = marked f/stop * (M + 1)}

Now to get back to your question, for a critical 4X print from 5x7
exposed at f/16, you are permitted a total of 0.625 mm of slop in
focusing which includes film flatness and alignment of the lens axis
perpendicular to the film plane. My woodworking skills aren't that
bad, but I'd take a hard look at the birch carrier and its planarity.
Consider using 1/8" aluminum with studs to align two sheets of good
glass dropped on top.
</pre>