Lamp WAY too bright - Prints too dark

Gary Grenell · Mar 30, 2005

I am not mathematically minded. I think it is a good idea to put in place the larger lens (even though I've spent a bloody fortune on that APO Rodagon lens). Do you have any idea what the percent increase in exposure time would be going from an 80mm to 100mm enlarging lens (given the same image size on the easel), the same degree of enlargement from the negative?
Gary

L Gebhardt · Mar 30, 2005

My gut feeling is you won't see any difference with a longer lens. You still are focusing the same amount of light coming through the negative into the same area. Think of it as taking a picture. You don't change exposure based on focal length. The only difference is that you are closer to a macro size, but not that close.

Gary Grenell · Mar 30, 2005

But if the longer lens necessitates putting the light source at a further distance from the easel, would that not necessitate a longer exposure time (which is what I'm after)?

photobackpacker · Mar 30, 2005

Gary Grenell said:
I am not mathematically minded. I think it is a good idea to put in place the larger lens (even though I've spent a bloody fortune on that APO Rodagon lens). Do you have any idea what the percent increase in exposure time would be going from an 80mm to 100mm enlarging lens (given the same image size on the easel), the same degree of enlargement from the negative?
Gary

Hi Gary

Boy, I am really rusty here but I will throw it out there to let someone else confirm or correct it.

I believe it will work out this way. Going from an 80mm to a 100mm focal length will give you a 25% increase in distance from the plane of the negative to the easel.

The inverse square law says the your light intensity will decrease by 1/d^2 or one over the square of the change in distance. In that case, believe you will see 1/1.25^2 (1 divided by the square of the increase in distance) which equals 64. This represents a 36% fall-off of light.

If my calculations are correct (that's a big if!) this will not solve your problem and would not justify the $ for a new enlarging lens.

L Gebhardt · Mar 30, 2005

The inverse square law doesn't work in this case because the light is focused. I will try it out tonight, but I would bet a piece of paper on it that it won't change.

Gary Grenell · Mar 30, 2005

Thank you for doing that...and if you have a 135mm lens as well, I'm quite curious.
The issue boils down to this: I have now achieved satisfactory enlarging exposure times with my set-up, having put a ND filter underneath my enlarging lens (B+W glass). Does it make sense to start futzing around with a longer focal length lens, just to deal with "image degradation" that MAY or MAY NOT result from having added glass between the negative and the easel. Frankly, I think I should consider this a "closed case" and move onto the real matter at hand (for all of us?): developing a more artistic eye and being better photograhpers.

L Gebhardt · Mar 30, 2005

I will test a 35mm holder with my 50, 105, 135 and 150 lenses with a meter and one test with paper.

Even if I am wrong, I have read in Post Exposure that is really is best to use the correct lens for the format because of lens distortion. I would think your extra filter would be no worse than this. Ctein also test the effects of stopping down excessively and found it also was noticeable. So most likely you have the best solution for your situation.

You could also start using Forte warmtone paper, which tends to be a bit slower. And it sure is a nice paper.

Bob F. · Mar 30, 2005

The number of times I see the inverse square law invoked with reference to enlargers, I thought they had revised the laws of physics just to spite me... I'll bet a lot more than a piece of paper that the difference will not be more than a percent or two.

Everyone (mostly) happily uses filters on their camera, including bits of plastic: you chose a high quality B+W filter so I would concur with you: problem solved... unless... Durst do an accessory diffuser for their mixing boxes that cuts the light by about a stop over the standard one, perhaps your enlarger makers have a similar option?

Cheers, Bob.

L Gebhardt · Mar 30, 2005

Possibly a stupid thought, but how does light get into the mixing box? On the chromega it is just through a small hole in the side. On my Ilford head it is through a hole on each side (one for each lamp). If you were to cover the hole half with aluminum foil it would block the light and be heat safe. This shouldn't effect the distribution, but I would test it just to be sure.

gainer · Mar 30, 2005

The illumination on the easel depends on the amont of light passing through the iris. The same f-stop with a longer lens will have a larher diameter of iris. On some condenser enlargers, the spacing between lamp and lens is variable independently of the focus. The object is to focus the image of the lamp on the plane of the iris. I haven't tried it, but I think that for the same magnification and the same f-stop, there will not be as much difference in illumination as one might think.

The best place to put a neutral density filter would be between the lamp and the negative carrier, out of focus to both the condenser and the imaging lens. It can be a cheap one, and in fact could be made by exposing and developing a sheet of film.

dancqu · Mar 30, 2005

L Gebhardt said:
... with a longer lens. You still are focusing the same
amount of light coming through the negative into
the same area.

With a longer lens the negative to lens distance is increased.
We all know that with a camera, lens extension is a factor. The
negative is the source of light and that extended distance with
the longer lens is there. The lens is focused on the negative
and on the paper. Lens to paper distance is, of course,
also greater with the longer lens.

Now, with all those greater distances, how does your gut feel?

It all has to do with area and area being a powers of 2
dependent quantity. Think and think and ... about it. Dan

Bob F. · Mar 30, 2005

As the focal length of the lens used is increased, the lens to negative distance for a given magnification also increases. Without doing the mathematics it's pointless making conjecture based on a different model.

As you say, it's about areas. So, for a given light output, over the same area, the intensity reaching the baseboard will be the same (excepting scattering by dust in the air and such like which, unless there is a sandstorm or fog in the darkroom, is unlikely to be a major factor)....

Anyway: I have just tested it with my exposure meter and 50mm and 150mm lenses - no change (0.1EV resolution meter) between them for the same size footprint on the baseboard (no sandstorms today)....

Cheers, Bob.

dancqu · Apr 1, 2005

Bob F. said:
Anyway: I have just tested it with my exposure meter and 50mm
and 150mm lenses - no change (0.1EV resolution meter) between
them for the same size footprint on the baseboard
(no sandstorms today)....
Cheers, Bob.

All in all I think I left myself a little limb. I was a little shy of 100%
convinced of my own argument. I've given some thought recently
to meter calibration using a somewhat point light source. In my
mind I've rays of light going out in all directions.

Good intuition on Mr. Gebhardt's part; and his a view contrary
to at least a few this thread. Dan

Lamp WAY too bright - Prints too dark

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