why is my focusing off?

[jordanstarr]

It happened just now and I can't fix it. I focus my print, run it through and it comes out not-in-focus. I readjust, act quickly and it's still out of focus. So, I adjust the tension knob on my 4x5 beseler and still the same problem. What I found out is that when I look through the grain focuser and hit the expose button, it starts out of focus and comes into focus gradually over 15 seconds. Any thoughts on solving this besides waiting with a shhet under the lens 20 srconds before exposure?

[Dear]

I tried something like it once.
No matter what I did, I just couldn't get it crisp after the enlarger.
Turned out that I had exposed the whole thing through the back of the paper!
It was matte paper so it was difficult to see which side the emulsion was on.
But that doesn't sound like what you're dealing with, since you say it gradually get's to focus?

[bdial]

Is this a color head with a fan?

If the lens stage isn't moving and the head assembly isn't moving, you shouldn't see any change. is it possible that your eye is adapting with the grain focuser, and the image is out of focus the whole time?

What happens without the grain focuser, can you see the same change when you switch on the enlarger?

[paul ron]

Negative buckling from the heat?

What size neg are you printing?

Try a glass neg carrier.

.

[jeffreyg]

With some grain focusers you can focus its eyepiece. Mine has an etched square on the mirror and the the eye piece can be focused first. Also I put a piece of print paper (previously processed) in the easel with the grain focuser on top of it. If the negative is popping, wait the 20 seconds before focusing.

http://www.jeffreyglasser.com/

[bobwysiwyg]

Negative buckling from the heat?

My thought as well.

[Ian Grant]

Sometimes the head on enlargers slip, they usually have a counterweight or spring system. This happens with Durst's and some other makes because the pressure pads wear, these lock the head assembly and it's very simple adjustments to cure the problem. Ironiocally the head creeps up the column.

Ian

[Ian C]

The problem is almost certainly the negative warming from the heat contained in the light passing through it. That makes the part of the negative exposed to the light expand as it warms. The rest of the negative is clamped by the metal carrier and stays cool and doesn’t expand. The expanded film must go somewhere, and it does.

It warps getting a pregnant belly upward and out of the shallow depth of field projecting a fuzzy image. At the same time, other parts of the negative that stayed within the depth of field project a sharp image.
To keep the negative cool the enlarger must be equipped with a heat-absorbing glass filter. That’s a special type of glass that contains an iron compound that absorbs the excess heat from the light passing through it. The HA glass actually gets quite warm in the process. Regular glass is worthless in this function, because it is transparent to heat.

The HA filter is placed below the lamp before passing to the condensers, although some older machines had the HA filter placed between the two condenser lenses. The glass sheet negative carriers are also used to keep negatives flat during projection. They’re expensive and introduce 4 additional dust-catching surfaces.

Diffusion enlargers, like most dichroic color and VC enlargers, have the filter placed between the lamp and the mixing chamber. Omega dichroic enlargers place the filter on the side of the mixing chamber. Dichroic filtered diffusion enlargers tend to run relatively cool.

Even so, for really long projections, such as must be the case when making large prints, require glass carriers to keep the negative flat, because such long projections allow the heat to build sufficiently to pop the negative out of the plane of focus.

Sometimes the center is sharp, but one or more of the edges is fuzzy. This can happen if the lamp is on long enough to pop the negative. The center is bellied up and brought to focus. But the outer regions are below the center and defocused.

I’ve found that by adjusting the focus so that the grain is sharp through the grain magnifier the moment that I snap on the lamp and watching until the negative begins to defocus due to heating, I can establish the time limit during which the image stays sharp.

Suppose that a negative that is initially in focus will stay sharp for at least 7 seconds. By dividing the exposure into segments no longer than the safe limit, I can make much longer exposures as needed to make large prints. I do this by making shorter exposures with, say 10-second cool-down periods in between.

For example, if my test strips indicate 34 seconds, then I can make 5 exposures of 6.8 seconds. For 23 seconds 4(5.8seconds) = 23.2 seconds. The extra 0.2 seconds is meaninglessly small.

Pre-heating the negative is a poor solution. If you focus the center of the image, then the edges are still held at or close to the original plane and cannot move upward due to heating. Focusing a “popped” negative gives a sharp center that tapers into fuzziness as you move out radially.

[pentaxuser]

IanC The way the OP describes his problem it sounds as if the heat warping actually brings the neg into focus - admittedly the exact opposite of what you'd expect.

Beat me.

pentaxuser

[Bob Carnie]

Jordan If you are not using glass carriers this will be a constant PIA.
If you are using glass carriers then something needs fixing on your setup.

It happened just now and I can't fix it. I focus my print, run it through and it comes out not-in-focus. I readjust, act quickly and it's still out of focus. So, I adjust the tension knob on my 4x5 beseler and still the same problem. What I found out is that when I look through the grain focuser and hit the expose button, it starts out of focus and comes into focus gradually over 15 seconds. Any thoughts on solving this besides waiting with a shhet under the lens 20 srconds before exposure?