Test exposure steps in half stop increments.

[Maris Rusis]

I'm planning to place five successive exposures along a 4x5 piece of film by making an initial exposure and then inserting the dark slide to cover some of the film and then making further exposures and advancing the dark slide as I proceed.

For the sake of consistency I'll leave the shutter speed constant and make the exposure changes only with the iris diaphragm.
At moderate shutter speeds there shouldn't be any reciprocity failure or intermittency problems, I think.

Does the following sequence look right to deliver a row of cumulative exposures half a stop apart?
1. Expose at f22 and then push the dark slide so it covers one fifth of the film.
2. Expose at f32 and then push the dark slide so it now covers two fifths of the film.
3. Expose at f32 and then push the dark slide so it now covers three fifths of the film.
4. Expose at f22 and then push the dark slide so it now covers four fifths of the film.
5. Expose at f16 and then push the dark slide all the way to cover the film.

[xkaes]

From my rough calculations, step #4 & #5 are too much.

[Doremus Scudder]

I find it easy to think in terms of "exposure units," where an exposure unit is an arbitrary value that equals the initial exposure. Then you can work with the units arithmetically to arrive at your results. Closing a stop gets you half a unit. Opening a stop from the initial exposure gets you 2 units, etc.

You want stripes in roughly half-stop increments, so the amount of exposure needed for each stripe can be pre-determined in exposure units (EU).

Initial exposure = 1 EU
Second exposure needs to be 1.5 EU total (half a stop more)
Third exposure needs to be 2 EU total (a doubling of the initial exposure)
Fourth exposure needs to be 3 EU total (doubling of exp. 2)
Fifth exposure needs to be 4 EU total (doubling of exp. 3)
Sixth exposure needs to be 6 EU total (doubling of exp. 4)
Seventh exposure needs to be 8 EU total (doubling of exp. 5)

Stacking exposures means you can just add units to get the totals you need.

In your case, Maris, let's have the initial exposure at f/22 at whatever shutter speed represent 1 exposure unit. That means that f/32 would be the half-unit exposure and f/16 would be a 2-unit exposure (double the f/22 exposure). That's all you need for the test.

So, the initial exposure would be f/22
Exp. 2 would add half of that, so stop down to f/32 to get the half unit. 1 unit (exp. 1) + 1/2 unit (exp. 2 @ f/32) = 1.5 units
Exp. 3 should add another half unit to make 2 total, so another exposure at f/32 gets us 1+1/2+1/2 = 2 units.
Exp. 4 should add a whole unit to the mix, so we open back to f/22 and make yet another exposure: 1+1/2+1/2+1 = 3 units
Exp. 5 needs another whole unit to get from 4 to 5 total, so another exposure at f/22 gets us: 1+1/2+1/2+1+1 = 4
Exp. 6 needs two units of exposure, so open up a stop to f/16 for this exposure to get: 1+1/2+1/2+1+1+2 = 6
Exp. 7 now needs two more units to get to 8 total, so another 2-unit exposure at f/16 is called for: 1+1/2+1/2+1+1+2+2 = 8

So, to recap, I get:

Exp. 1 @ f/22
Exp. 2 @ f/32
Exp. 3 @ f/32
Exp. 4 @ f/22
Exp. 5 @ f/22
Exp. 6 @ f/16
Exp. 7 @ f/16

Oh yes, almost forgot. If you want to continue your test on another sheet (you'll notice I have 7 steps...) then you need to determine a new initial exposure for beginning sheet 2.

I find it easiest to just make 4 exposures per sheet. After 4 stripes on one sheet, you need a fifth exposure with 4 EU and, if you're starting a new sheet, you'll need a 4-EU exposure. So, since f/16 is 2 EU, opening up a stop and doubling that would get you f/11, which yields 4 EU. Therefore, sheet 2 would start with a 4-unit exposure made at f/11 for exp. 5. Then you'd go back to adding just 2 EU for exp. 6; i.e., the 2-EU f/16 exposure. If you want exp. 8, then you'd add a 4-EU exposure (f/11) to the mix to get a total of 12 EU on exp. 8.

Hope this helps,

Doremus

[maltfalc]

too much math for me. i'd just keep all the settings the same and vary the number of exposures per step, 1/1/2/4/8, with a half stop nd filter covering half the image.

[xkaes]

For what's it's worth, you were on the right track.

A Triple may not be a Home Run, but it's a lot better than a Single!

[Drew Bedo]

I have had a little trouble with getting the stripes the same width on the negative when testing this way. Three is easier than four. Not sure how I would do five.

Years ago, I did some diagnostic imaging in a clinical setting. Long enough ago that nothing was digital. We captured images off a CRT on 8x10 sheet film with a back that slid around both in both x and y. So four 4x5s on one sheet.

Anybody ever see a back like that on e-Bay?

[neil poulsen]

Out of curiosity, what shutter speed will you be using? What kind of film are you using? What are you trying to achieve with this series of tests? What's the purpose of doing this testing? What are you photographing, and with what lighting?

[Maris Rusis]

Out of curiosity, what shutter speed will you be using? What kind of film are you using? What are you trying to achieve with this series of tests? What's the purpose of doing this testing? What are you photographing, and with what lighting?

Shutter speeds say 1/4 to 1/60 where good shutters run most consistently.
Film? Short scale stuff typically litho, infrared, direct positive paper and the like where one stop increments are too coarse for fine testing and control.
Ambition? Accurate control of exposure, development, pre- and post-flashing, and some possibly interesting densitometry.
Subject matter? Mainly daylight landscapes with unusual films and often strong filtration that works the edges of the film spectral response curve.

One change I will make in the the exposure sequences is to start one stop smaller and expose twice for the first band. This way I can always move the aperture control lever in the same direction and avoid possible backlash or slack in the mechanism.

[Vaughn]

I have had a little trouble with getting the stripes the same width on the negative when testing this way. Three is easier than four. Not sure how I would do five.

Years ago, I did some diagnostic imaging in a clinical setting. Long enough ago that nothing was digital. We captured images off a CRT on 8x10 sheet film with a back that slid around both in both x and y. So four 4x5s on one sheet.

Anybody ever see a backs like that on e-Bay?

I have seen backs that do two 5x7s (approx) on 8x10 -- might come in handy for something like this.,