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

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• 01-02-2013, 09:11 PM
pinholer
For my black and white prints I just make a test strip where I am changing the times by a quarter stop per strip. For example 10, 12, 14, 17, and 20 seconds. This gives me a better spacing between exposures than a 5, 10, 15, 20 second strip.

For color I have a spread sheet printed out using the formula T2 = T1 (H2/H1)^1.94 where H1 is the original height from the center of the lens to the baseboard, H2 is the new height measured the same way, T1 is the exposure for H1, and T2 is the new exposure. I am using Omega enlargers and found that an exponent of 1.94 works better than the exponent of 2 required by the inverse square law. I adjust the aperture so that my exposures stay in the 10 to 20 second range.

Prior to this, again for color, I calibrated an EM10 exposure monitor so that it read directly in seconds and filed out the edge of my negative carrier so that I could always take a reading off the blank film between frames. Both systems have worked well for me.
• 01-02-2013, 09:23 PM
tkamiya
I've struggled with this for a while now.

What I have right now is both mathematical and seat-of-the-pants method combined. When I go from 8x10 to 11x14 to 16x20, simply doubling exposure each step will get me close. Then, I'll often have to bump up contrast by 1/4 to 1/2 grade each time to get each print to look similar. While contrast shouldn't change when the size change, how we perceive the contrast to be change depending on size. When I print larger, I print larger because I want the print to look larger. I don't necessary back up farther from the print to look at larger print looking the same size as before. With that in mind, I usually tweak contrast as well. I also need to change my dodge and burn schedule if I'm going larger by say 2 stops worth. Large patch of dark shadow look far more oppressive at 16x20 than did so at 8x10.

So I basically gave up on math to give me the answers. It will get me close and sometimes close enough. Then my gut and eye tells me what to change and by how much.
• 01-02-2013, 10:45 PM
Bob-D659
Try this ruler, print it and tape to a yardstick, not much math required.

http://www.darkroomautomation.com/su...stopsruler.pdf
• 01-03-2013, 11:03 AM
Nicholas Lindan
There is a long interminable sticky thread on this subject at the top of the forum.

My view is that the need to increase contrast as print size goes up is largely perceptual. There isn't much physical effect with modern enlargers and lenses when combined with a blacked-out (or redded-out) enlarger alcove.

AA had a terrible time getting contrast on his mural prints. They were 5x7's - but measured in feet. The reason does seem to have been stray light. His darkroom was painted white and the enlarger was homemade, with an unknown amount of light leakage. And the lens wasn't multicoated.

If physical effects are contributing to contrast loss then:

Stray light from the enlarger can be dismissed (unless yours leaks like a sieve). The greatest stray light source is lite bouncing off the paper. The second greatest contribution is stray light from the lens - peer up at the lens and you can see quite a bit of illumination that shouldn't be there. To see the enlarger proper's contribution you can make an experiment: Turn on the enlarger and look at the illumination on the walls, then cover the lens and look at the contribution of light leaking from the enlarger; it's not night and day, exactly, but the enlarger's contribution to total stray light is tiny in comparison.

A lens hood on the enlarger might not be a bad idea. A multicoated lens will also reduce stray light.

If the aperture is opened to compensate for larger print size then printing time stays the same and the fog from the enlarger and the lens doesn't change.

The greatest contribution to lowered contrast, fog from light bouncing off the paper, will increase with print size: this was probably AA's greatest contribution to reduced contrast. The total amount of light from the lens is constant with print size, assuming you are using time to compensate. A certain percentage of the lens output bounces from the paper, then to the walls and back on to the print. So the total bounced light is independent of print size. However, as print size increases the exposure time increases and thus the effect of this bounced light increases. Using the aperature to compensate won't help here: if you open the aperture then the amount of bounced light increases (more total light).

Painting the walls and ceiling around the enlarger matte bright red (or black) is the only way to mitigate the effect of bounced-light fogging. It also reduces any effects from stray lens and enlarger light.

Reciprocity failure increases contrast. It's the same with paper as it is with film, but rather than the shadows going empty it's the highlights.

Using an EM-10 requires that you use the lens aperture to compensate for print size. If you are using an EM-10 then it is a good idea to make your work prints at a small aperture, so that the lens is at optimum when opened up to make the final print. The EM-10 was designed for use with Ilfochrome printing, where changing the exposure time would shift the color balance. There is another company (cough) that makes a meter better suited to this purpose, this meter has 1/100th of a stop of resolution and can meter any contrast changes accruing to the larger print size.

My experience is that small work prints are fine if they are no more than 1 size smaller: 11x14 for a 16x20, say. But that is still reducing paper wastage by half.
• 01-03-2013, 11:15 AM
Nicholas Lindan
Quote:

Originally Posted by ic-racer
WTF is a 'sweat spot'...

• 01-03-2013, 03:11 PM
Bob-D659
Only one comment to Mr. Lindan's post, red walls are bad idea if you print RA4 paper, I found that a brown shiny bookshelf unit is bad too. :( Go for black, paint or cloth. :)
• 01-03-2013, 03:47 PM
Nicholas Lindan
Quote:

Originally Posted by Bob-D659
... red walls are bad idea if you print RA4 paper ...

I don't know that I hold with that newfangled color stuff.

Yeah, red (or OC orange, even brighter) only work for black and white. I once used a darkroom with black-painted enlarger cubicles. Couldn't see a thing. I
would have wanted a dedicated Thomas safelight in each cubby. No, that wouldn't work, the ceiling was black.

I knew an outfit that painted their darkrooms blue. Now that has to be the worst color for darkroom walls.
• 01-04-2013, 08:44 AM
mr rusty
Quote:

Since I got an Analyser Pro, I don't putz around with that stuff anymore. I just take two readings then push the "Print" button.