Determining Exposure For Contact Copying Onto Sheet Film

[holmburgers]

Howdy all,

I have here a confounded set of calculations that could benefit from your looking over. Rude-Golberg would be fond of such a "proof", and it is born from my lack of densitometer and a desire to waste as little film as I can in making exposure tests.

What I'm looking to do is copy a 4x5" slide to b&w film by contact under my enlarger, ultimately with tri-color filters to make separation negatives.

I'm able to take an incident reading of the light at the baseboard (EV 1.66), and I'm trying to determine how much light, on average, the slide will block, and consequently how much extra exposure to give above the incident reading (figured from f/1 and EV 1.66 on an exposure table).

The slide I have in mind is low contrast (overcast day) and exposure was on the money, so I can safely assume that it's within the dmin/dmax, or on the straight line portion.

The straight-line portion of Kodak EPP appears to have a density range of ≈ 0.66 to 2.75 (http://www.kodak.com/global/en/professional/support/techPubs/e113/e113.pdf).

So, if I turn the density into opacity, using D=logO, I get about 4.6 and 562 O for 0.66 & 2.75 D respectively.

This is effectively my "filter factor", is it not? So that 4.6 equates to about 2 stops and 562 to about 9 stops, for an average of 5.5 stops.

Is there anything obviously amiss with my calculations or assumptions?

Thanks in advance

 

[2F/2F]

I'd start with the incident meter reading and see what happens. The exposure given by the meter should place a mid-tone on the transparency as a midtone on the negative, and all else should fall into place on the neg. Whether or not the full range of the transparency is captured is another story. Due to the wider density range of the transparency film, you may need to change exposure and development significantly to get a full-ranged negative that holds all the information you want.

I used to do them by contacting as well, until I learned on this Website that projection is better, for quite a few reasons.

 

[holmburgers]

But I'm putting a powerful "filter" (a.k.a. the slide) in front of the film, therefore I need to account for this or it'll be underexposed.

By what criteria is projection better? I'm under the assumption that eliminating an optical system is better (consider the fidelity of contact printing).

 

[MattKing]

The highlight area in the slide is very, very close to transparent.

Do you have a piece of transparency film that is essentially clear (developed leader?)

If so, insert it into your light path and determine how much the incident meter reading changes.

 

[holmburgers]

Good point Greg, I will definitely do that.

 

[nworth]

Some experimentation is in order. I would use 2F/2F's method with a little change. I would measure the light at the easel with an incident light meter set to the EI of the film you are using. Then I would back off 4 or 5 stops and contact print a step tablet. (You could use a properly exposed negative or transparency, but the results would be harder to read.) Then develop the film and adjust the exposure as necessary.

 

[holmburgers]

Hmm, that's odd... I think I saw a shorter version of 2F/2F's response last night, and thus my reply doesn't address all that he said. It must've been edited(??)

At any rate, won't the density range of a slide be significantly shorter than the brightness range of a real-world scene? It seems that the b&w film should have no trouble getting all the information from the slide... but what do I know?

nworth, now when you say "back off 4 or 5 stops", do you mean decrease exposure from the incident reading? Now I'm even more confused, because let's assume I'm doing this under a step wedge, at the metered reading I'm theoreticaly giving the clearest portion of the wedge a medium grey exposure, and thus the darker densities of the step wedge will only get whiter, whiter, whiter... so where is my black?

I think I need to increase exposure from the incident reading to place the medium grey exposure somewhere that corresponds with the middle of the wedge/transparency. This was my reasoning behind figuring out the opacity of an average slide. Sure, the highlight portions of a slide are quite transparent, but a straight incident reading will put the highlights on middle grey, and where does that leave my shadows?

Density in my negative is not a problem, it's the lack of information from underexposure that will be detrimental.

I appreciate the advice so far, but have we thought this through all the way??

 

[nworth]

Holmburger. I think you are right about the exposure. Use the exposure indicated by the incident meter and go from there. I was thinking of the way I use a reflection meter with a light table. You are also right about slide film. But rather than a short range, it more exhibits high contrast when you photograph it. You may have to use minus development to get a good negative. Try to get the exposure right first, then deal with the contrast. Once you get it right for one slide, similar slides should be the same.

 

[holmburgers]

So maybe err towards shadow exposure and then reduce development?... sounds good!

 

[ic-racer]

Sounds like you are asking something pretty simple, like using the midpoint of the slide density as an average factor. That would easily be estimated by D-max minus (base+fog). For example, if your base+fog is perhaps 0.01 and the D-max of the slide film is 2.0, try 1.0, which would be close to 3 stops.

 

[holmburgers]

Sounds like you are asking something pretty simple, like using the midpoint of the slide density as an average factor.

Precisely!

Using the characteristic curve link included in first post, can you confirm or amend my calculation? I arrived at about 5.5 stops.

 

[ic-racer]

Precisely!

Using the characteristic curve link included in first post, can you confirm or amend my calculation? I arrived at about 5.5 stops.

That curve is a little funky (not giving the total density). Other sources put the D-max around 3.5 to 4. So I would agree with your numbers. So 5.5 stops more exposure than the incident meter seems like a reasonable place to center you initial test strips. You used the flat disk on the meter right?

 

[holmburgers]

Actually.... I actually took a reading of a grey card on the baseboard, with my spot meter as close to the axis of the lens. It's not particularly precise, but should in theory give the same reading.

Thanks though, I'll report back once I do this.