How to calculate a new print exposure time for a change in enlarger head height

[Worker 11811]

Why would the contrast change?

I don't know why but it does, sometimes. Smaller prints need less contrast to look the same. Larger prints need more.

When going from, for instance, 5x7 to 8x10, it might not be noticeable. Even if it is, it might not be objectionable but, if going from 4x6 to 11x14, I usually lose at least a grade of contrast. (Reverse if going from large to small.)

Again, it's not always noticeable or objectionable but it does happen often enough that I have to be aware of it and check for it. If not using an Analyser or some kind of meter, all it takes is a test strip to solve this. If using an Analyser or meter, all you need to do is double check but 9 times out of 10, it comes out right because the device makes that adjustment for you in the process of re-metering.

[Ian C]

With respect to post #10:

I find it easier to meter the projection through an empty carrier for maximum brightness to find the difference in stops. I use the aperture wide open for the brightest projection.

I first focus the smaller projection, remove the negative, replace the empty carrier, meter the center of the projection, and record the reading.

Then I replace the negative, resize the image, focus, remove the negative, replace the empty carrier, and meter as before. The difference in readings is Δf. Then the time factor is

2^Δf

The new exposure time is

(2^Δf)*original time

[ic-racer]

Contrast can change by a number of theoretic mechanisms:
1) psychologic impression of the image
2) More fog from a leaky head. A 150 second exposure can fog the paper more than a 5 second exposure if your enlarger head leaks a little light (they all leak some).
3) More fog from stray light bouncing off the paper and coming back on your highlights. 16x20" print (320 square inches of white paper) reflects more stray 'white light' into your darkroom than 4x5" (20 square inches).
4) Slight variations in the paper batch for a different size
5) Potential for less developer activity on 320 square inches of paper vs. 20 square inches
6) If you have to open the lens from its 'sweat spot' it will have more flare.
7) A bigger print requires one to use the far edges of the lens' image circle. The MTF is lower out there and contrast can be lower also.

On the positive side there could be less flare in the enlarger head because the bellows is collapsed more.

[cliveh]

Contrast can change by a number of theoretic mechanisms:
1) psychologic impression of the image
2) More fog from a leaky head. A 150 second exposure can fog the paper more than a 5 second exposure if your enlarger head leaks a little light (they all leak some).
3) More fog from stray light bouncing off the paper and coming back on your highlights. 16x20" print (320 square inches of white paper) reflects more stray 'white light' into your darkroom than 4x5" (20 square inches).
4) Slight variations in the paper batch for a different size
5) Potential for less developer activity on 320 square inches of paper vs. 20 square inches
6) If you have to open the lens from its 'sweat spot' it will have more flare.
7) A bigger print requires one to use the far edges of the lens' image circle. The MTF is lower out there and contrast can be lower also.

On the positive side there could be less flare in the enlarger head because the bellows is collapsed more.

I'm with Steve here, as you are clutching at straws. Contrast shoudn't change.

[nworth]

The inverse square law works, but as you get bigger, some other factors kick in. One is reciprocity. With exposure times greater than a minute, it can be a considerable factor. With really long exposures, reciprocity can affect contrast as well as exposure time. Another is flare, which can lower contrast and generally make things muddy. As you get further away, there are just more things for the light to bounce off of, and there is more chance of stray light and safelight fog affecting the image. Condenser enlargers may work better than diffusion types for big prints.

[David A. Goldfarb]

I'm with ic-racer--contrast often has to be increased as print size increases. You may not notice it from 5x7 to 8x10, but 8x10 to 16x20, it's noticable. Sometimes it could just be because you're changing the way you view the print, and you might need more contrast for the image to be effective from a greater viewing distance. It's fairly common for me at least to print large at a grade higher than I print small for the same image.

To get a ballpark exposure time, figure one stop for every standard print size. You'll notice they follow the f:stop series approximately: 4x5, 5x7, 8x10, 11x14, 16x20, 20x24.

[Steve Smith]

I'm with Steve here, as you are clutching at straws. Contrast shoudn't change.

Whilst I asked why the contrast would change, I didn't actually express an opinion that it wouldn't.


Steve.

[Ken Nadvornick]

Impressions of contrast change as a dependency of print size must be some sort of general visual artifact. I experience the same thing even in the electronic reproductions of the APUG Galleries.

How often have you seen at a thumbnail that looked like it had wonderful tonality, only to click and see the image go flat at its full uploaded size? Or seen a thumbnail that looked like it had way too much contrast, only to click and see a beautifully appropriate full range of tones from white to black?

For some reason if your goal is a full palette of shades of gray, it seems your thumbnails must look too contrasty. And that seems true even with the inevitable compromises inherent in electronic reproduction technologies.

Ken

[ic-racer]

'sweat spot'

WTF is a 'sweat spot'
Thank you spell checker...

[Steve Smith]

Don't use a spell checker. It just reassures you that the wrong words you used were spelled correctly!


Steve.