setup for 4x5 sheet film testing

I am planning to do some (4x5 sheet) film testing and for that I would like to construct a "test rig". My idea is to (partially) sacrifice a sheet film holder by cutting a window in one of its' darkslides. I then plan to attach a step wedge covering the window. The procedure would then be:

1) Load a sheet film holder with the film under test. Use a unmodified darkslide to cover the film.
2) Set up the camera in front of an evenly lit surface.
3) Take a light reading on the surface and calculate the exposure such that the surface is placed into zone X (?).
4) Insert the film holder, remove the "plain" darkslide and insert the modified one with the step wedge.
5) Make the exposure, remove the modified slide and insert the plain darkslide.
My idea is that I should now have a single sheet with exposures from zone X and downwards.

Any comments on this procedure?
My biggest concern is that with a test rig as described above, the step wedge will not be in perfect contact with the sheet film, but there will be a small gap of approx 2mm between the film and the darkslide/wedge. Will this gap cause too much scatter and stray-light etc. for the approach to become improper?

All comments welcome!

The way I test a new film type is to find a scene that requires an even exposure .
Set the camera up , take a spot meter reading at ISO 50 ( for e.g) of a mid grey part , weatherd rock or something , move the dark slide by 1/5th of it travel , take a shot .
Then move it by another 1/5th and take another .
Until you have a piece of film with 5 exposures on it .
The last one therefore with just one expose on it has been exposed at ISO 50 , the one with 2 shots is effective ISO 100 and so on .
When i develop the film I can see how the film will behave shot a different ratings looking at the density of whats on it .
I expect that would give the same result as using a step wedge .
I leave shutter speed and aperture the same so each expose add an extra equal exposure to what has already reached the film .

As far as combining a step wedge with a dark slide , unless one is the same thickness as the darkslide , you won't be able to remove it from the film holder to swap it for a solid one .
You would have to set everything up in the darkroom , guess your focus point , take a shot , backinto the darkroom or changing bag to take it out .

Or have I misunderstood your reason for doing this ?

Save yourself the time and trouble of carving up a darkslide and just tape your step wedge directly to the film when loading. Remove it before developing. Much easier and your step wedge will be in contact with the film too.

Keep in mind that testing this way will not show the effect of flare in the image, since it's basically just a contact print of the step wedge. Your shadow values in practice will be somewhat higher than the test will show. This may affect your effective film speed.

Best,

Doremus

Doremus, I realize your approach is probably best in terms of getting a proper contact print. My idea was to have a convenient way of testing new films or devs without having to go into the darkroom and tape wedges directly onto film; this allow me to use holders that have already been pre-loaded with film (I tend to load film holders in batches and then store them at home for easy access). Good point about making sure the modified darkslide actually fit in the filmholders darkslide slot. Thanks!

John Schaffer's (Chemist and former President of University of Arizona who set up the Center for Creative Photography) The Ansel Adams Guide to Basic Techniques of Photography Book 2 provides a detailed method for testing film with a step wedge including 4X5. He just tapes the step wedge to the film in a film holder, the dark slide will slide over easily. You can use film holders already loaded with film, just remove the dark slide tape the film, shoot and process.

I bought 4 or 5 spare dark slides and drilled one hole on each, about 10-12 mm diameter; positioned on one side (left or right) of the film. Each slide then allowed for 2 exposures, based on which side of the slide was towards the lens. Using them sequentially allowed for 8 or 10 different exposures on one sheet along with film-base-plus-fog. But this was not using a step wedge.

Good Morning,

The approach described by Neilt3 is the simplest; I've used it and found it entirely adequate. The step wedge approach may give slightly more precise information, but, for most purposes, how precise do we need to be considering all the other variables in film exposure and processing?

Konical

FWIW, I make film speed tests and zone rulers simply by pulling the darkslide in increments. Four or five stripes on a negative. Three negs total for film speed test and tests for N and maybe one more development scheme. My tests are of real-world subjects with a range of tonalities and discreet areas of tonality than can be easily metered. Resulting negs are enlarged and "proper proofed." This takes flare from metering, lenses, camera bellows and enlarging at least somewhat into account.

As for film speed. Unless you're using a developer that loses you a lot of speed (this is usually mentioned in the accompanying literature), or have some other anomalies/irregularities in your system, you can pretty much just rate your film 2/3-stop slower than box speed for starters and refine as you go using field tests. Flare from the lenses you use, meter and camera bellows makes enough difference in nailing down film speed that just contacting a step wedge often proves inadequate (but at least you'll be overexposing; not underexposing). Keep good notes, check shadow detail to make sure they consistently end up where you planned them and tweak E.I. as needed.

Best,

Doremus