Enlarging to Film - Idea for Metering
This is a theoretical proposition that might make enlarging to lith film or sheet film really easy.
I'm interested in the possibility of making enlarged negs from slide film, or enlarged positives from negatives. Or you could make enlarged separation negatives (for tri-color gum for instance, carbon or DCG-dye-transfer) from color negatives by copying those with a slide-copier setup onto 35mm b&w film, with the appropriate filters, and then projecting those.
Let's say you enlarge your image onto a gray card and meter the gray card, or alternatively, take incident readings of the projected light and then tell your meter that you are shooting at f/1. Would this not give you the correct exposure time?
I'd possibly meter the brightest section (thinnest on the film) and add +2.5 stops to that. Or if shooting a pos to a neg, may want to incident the densest section.
Or for neg to pose you may want to spot the densest section and then subtract 2.5 stops.
How would you meter it, though? What aperture would you set on the meter?
I'd possibly meter the brightest section (thinnest on the film) and add +2.5 stops to that.
Subtract 2.5 stops from WHAT? What aperture are you supposed to set on the spot meter when you take the reading? That's the whole question here.
for neg to pose you may want to spot the densest section and then subtract 2.5 stops.
It must be possible to create a direct correlation between the brightness of an enlarger's baseboard image and the exposure given by a common light meter. The question is, if you use a camera meter, whether incident or reflective, to meter the baseboard image thrown by an enlarger, is F/1 the appropriate aperture for calculating exposure? If not, what is?
I'd meter the grey card with no negative in place, then get an incident reading with a negative in place. I would think the two should be fairly close. Then expose the film as you would a test strip, process and see what worked best. Make certain you keep notes, and let us know how it works out.
Stephen Benskin (here on APUG) is probably the man to consult, he just went through the derivation of the equations you need.
From looking at some notes on contacting to TMX you might try setting the meter to the ASA of the film and using the time for f2.0. Meter a grey card with a spot meter with blank film in the enlarger. Bracket at 1/2 stop intervals for 2 stops either side.
Last edited by Nicholas Lindan; 02-16-2011 at 10:51 AM. Click to view previous post history.
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Wouldn't you set the f-stop to what you have your enlarging lens set at? After bracketing you would then know your 'fudge' factor.
*Just in case anyone is not familiar with the term 'fudge factor;' it's the number you add, subtract, multiply or divide into your answer to get the answer you should have had.
Thanks everyone for the input.
I remember a thread a while back where a similar idea was discussed, and f/1 was confirmed as being the right aperture. If you think about it, it makes sense, because technically you're metering the 'film plane', and f/1 is a 1:1 light ratio. That is, it applies no change due to your focal length/aperture, which is nill at the film plane level. In a camera, the light you're metering is outside of the camera and the light has to go through your optical system.
I'm sure the math would be easy to do and to confirm this.
I think the smart thing to do would be to make a test strip in the ballpark determined by this metering method, and that should give a pretty good idea. The main reason I'm asking this is to avoid making expensive test strips on 8x10" T-Max or something!
Thru PM, Ian C provides this, "When film is copied, there is a natural tendency for the copy to have significantly more contrast than the original. Worse, the contrast is distributed somewhat differently on the copy than the original." He goes on to suggest that pre-fogging might be necessary as well. I suppose that I could develop my originals to a lower contrast if it's possible.
All in all, someday I'll give it a shot, and definitely post my results. Can't say it'll be anytime soon however.
Originally Posted by holmburgers
At f1 the film 'sees' an illuminated solid angle of 53 degrees that is the same brightness as the subject [with a freshman physics level of simplification], there is nothing "1:1" about this.
"In the early years of photography the lens makers unfortunately chose to describe their lenses by the ratio of equivalent focal length to the entrance pupil diameter and then to write this as an inverse fraction! This must rank as one of the sillier decisions in the history of optics." Optics, M. H. Freeman
Last edited by Nicholas Lindan; 02-16-2011 at 04:31 PM. Click to view previous post history.
Originally Posted by Paul VanAudenhove
If you are measuring at the film/paper plane then you are making the measurement with the lens stopped down to the exposing aperture - it is the same as making a stop-down measurement with a TTL camera light meter. The f-stop of the lens is automatically taken into account as it is attenuating the light falling on the film/paper/gray card/metering cell.
The f-number that you use to read-off the exposure time is just the fudge-factor that allows you to set the meter to the ASA of the film being exposed. The fudge-factor will be different depending on how you are doing the metering.
The best fudge-factor/f-stop setting is best found by doing test strips. Expect the number to be different for B&W negatives and color transparencies.
By the time you get all of this calculated so that it is accurate I wil have been able to make at least a dozen enlarged negatives via the good old tried and true test strip method, nd simultaneously made a chart which in the future will enable me to make such negatives w/o test strips.
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