Film Testing with Step Wedge for Dummies

[bernard_L]

Thanks for the reply. I had similar thoughts as to the white target. I have modern flash strobes with adjustable and stable color temperature. I'm planning on using a large 84 inch umbrella to light up my target.

You don't really need a target at near-infinity, meaning large target. As far as photometry is concerned you only need to ensure that your camera is focused at infinity. A uniform light source will meter the same from close on as far away, as long as it covers the acceptance angle of the meter. It's an optical theorem about invariance of intensity. So, what I'm getting at is that a small (say 10x10cm) diffuse source (light box, opal glass) would be OK, with lightmeter and/or camera "with their nose on the glass".

Thanks for the reminder that the density values differ between contrast and diffusion.

Don't worry too much. The Callier effect comes into play when the light source is collimated (point source condenser enlarger) and the imaging system has a small angle acceptance (the enlarger lens aperture as seen from the film). With the setup you propose, step wedge flush against film, in-camera, the illumination angle is small but the acceptance angle (film flush against step wedge) is large. So, your effective density should be close to the nominal densities provided by Stouffer. I my case, the Stouffer values match within +/-0.02 what I measure with my Macbeth densitometer.

silveror0 The target density for VIII is 1.30 above fb+fog for diffusion enlargers like mine; it's 1.20 above fb+fog for condenser enlargers (which inherently produce higher contrast).

These values are not Eternal Truths. Doing a quick-and-dirty contrast index calculation
(1.20-0.1) / [(VIII-I)*0.3] = 0.52
I find this on the low side, allowing to fit a typical outdoors scene into the scale of normal paper with minimal manipulations, but with lowish tone separation. Irrespective of my taste, or what you may read on forums, do your own experiments, and find out whether you do not prefer to develop to a C.I. 0.6, obtain better separation, and dodge/burn as needed to fit the tones into the scale of normal paper (whatever that is for you, 2, 2.5, 3, or whatever).

 

[Bill Burk]

Nice to have a complete set of tools, thanks.

I like Ralph’s spreadsheet, if you can figure out how to make it work it would be a great way to save time. The green column is the original step wedge (it’s establishing the horizontal axis and is optional because he gives nominal values). Note he uses 31 steps spaced by 0.1 density while many people use Stouffer scale T2115 which has 21 steps in spaces of 0.15

 

[bascom49]

Nice to have a complete set of tools, thanks.

I like Ralph’s spreadsheet, if you can figure out how to make it work it would be a great way to save time. The green column is the original step wedge (it’s establishing the horizontal axis and is optional because he gives nominal values). Note he uses 31 steps spaced by 0.1 density while many people use Stouffer scale T2115 which has 21 steps in spaces of 0.15

When I first purchased Ralph's book I did give his process a go. I purchased the Stouffer 31 step wedge, in 8x10 size, but did not know about sandwiching the step wedge into my 8x10 film holder. I may revisit with 8x10 film after getting familiar with the 21 step in 4x5 and hand plotting the data. Thanks for the additional info.

 

[bascom49]

You don't really need a target at near-infinity, meaning large target. As far as photometry is concerned you only need to ensure that your camera is focused at infinity. A uniform light source will meter the same from close on as far away, as long as it covers the acceptance angle of the meter. It's an optical theorem about invariance of intensity. So, what I'm getting at is that a small (say 10x10cm) diffuse source (light box, opal glass) would be OK, with lightmeter and/or camera "with their nose on the glass".

Don't worry too much. The Callier effect comes into play when the light source is collimated (point source condenser enlarger) and the imaging system has a small angle acceptance (the enlarger lens aperture as seen from the film). With the setup you propose, step wedge flush against film, in-camera, the illumination angle is small but the acceptance angle (film flush against step wedge) is large. So, your effective density should be close to the nominal densities provided by Stouffer. I my case, the Stouffer values match within +/-0.02 what I measure with my Macbeth densitometer.

These values are not Eternal Truths. Doing a quick-and-dirty contrast index calculation
(1.20-0.1) / [(VIII-I)*0.3] = 0.52
I find this on the low side, allowing to fit a typical outdoors scene into the scale of normal paper with minimal manipulations, but with lowish tone separation. Irrespective of my taste, or what you may read on forums, do your own experiments, and find out whether you do not prefer to develop to a C.I. 0.6, obtain better separation, and dodge/burn as needed to fit the tones into the scale of normal paper (whatever that is for you, 2, 2.5, 3, or whatever).

This video is interesting and is similar in concept:

 

[bascom49]

Kodak E-64 Curve Plotting Graph Paper:

Thanks

 

[ic-racer]

I'd be skeptical of using flash (very brief duration) for test exposures unless you're looking for data applicable to use under studio conditions; Reciprocity occurs with both very short exposures as well as very long exposures. AA suggested a shutter speed around 1/25 sec, as it's usually most accurate around that speed. I used 1/25 but checked it to be certain it's spot on.

The target density for VIII is 1.30 above fb+fog for diffusion enlargers like mine; it's 1.20 above fb+fog for condenser enlargers (which inherently produce higher contrast).

Sensitometer light type does not matter for control strips and comparative testing. If one is setting up an ISO testing lab then it may matter.

https://www.photrio.com/forum/threads/the-great-sensitometer-shootout.95837/page-3#post-2003141

 

[fix-erUpper]

5) Spot meter target, open up five stops for Zone X. Check

I'm a bit lat to this thread, but hopefully someone will still answer this question: Why do you expose the step tablet to zone 10? Why not zone 5? Or, for that matter, any other zone? thanks.

 

[Mr Bill]

I'm a bit lat to this thread, but hopefully someone will still answer this question: Why do you expose the step tablet to zone 10? Why not zone 5? Or, for that matter, any other zone? thanks.

Hi, when you expose through a step wedge, it holds back progressively more light from each step. So in order to get a wide exposure range you need a generous amount of light to start with. Now, consider if you just exposed according to the meter - the amount of light reaching the film would only be a nominal mid-tone, a Zone V. So at best your step wedge would show this and lesser exposure.

If you want to see, on the wedge, higher "zones" or whatever you want to call them, you need to increase exposure by a similar amount. A common step wedge covers an exposure range of about 10 f-stops; if you're using one of these then the +5 stop ( over the meter reading) sounds about right to include the "toe" of the film curve. But if you want to see higher up on the curve, you'll need to increase the exposure even further, probably losing the toe in the process.

So things are not necessarily cast in stone as to how you expose. Once you actually start doing it you'll see how it works. As a note, your lens will have some fall-off away from center, as well as reducing transmission somewhat. So these things can affect your results (you can make estimated corrections by adding a "corrective density value" to the step wedge patches).

 

[fix-erUpper]

Hi, when you expose through a step wedge, it holds back progressively more light from each step. So in order to get a wide exposure range you need a generous amount of light to start with. Now, consider if you just exposed according to the meter - the amount of light reaching the film would only be a nominal mid-tone, a Zone V. So at best your step wedge would show this and lesser exposure.

If you want to see, on the wedge, higher "zones" or whatever you want to call them, you need to increase exposure by a similar amount. A common step wedge covers an exposure range of about 10 f-stops; if you're using one of these then the +5 stop ( over the meter reading) sounds about right to include the "toe" of the film curve. But if you want to see higher up on the curve, you'll need to increase the exposure even further, probably losing the toe in the process.

So things are not necessarily cast in stone as to how you expose. Once you actually start doing it you'll see how it works. As a note, your lens will have some fall-off away from center, as well as reducing transmission somewhat. So these things can affect your results (you can make estimated corrections by adding a "corrective density value" to the step wedge patches).

Thanks for the answer. I'm understanding that we are just trying to 'center' the wedge exposure so that we don't lose the toe or the shoulder in the exposure. But what I want to do is expose a series of step wedge negatives and input the step densities in the Way Beyond Monochrome eXcel Spreadsheet. But, if I expose to zone 10 vs zone 5, the individual measured step densities will be different between these two lighting situations. So won't my film speed, developing times, etc., all be different depending on which zone I set the grey/white card lighting to?

 

[Bill Burk]

Manufacturers have to properly calibrate their exposures for testing because they need to honestly tell us what the film speed is. We can make “any” exposure and work backwards from the test results to figure out what “our” exposure was that got us there.

Ralph’s spreadsheet does that.

Mr Bill is right but I like to tell it like a story. The step wedge has a practically clear patch which is going to get a full blast of test exposure, and is going to be really black on the negative. But the darkest patch only lets 1/1000th as much light through to the film. A good exposure for the test will make the darkest patch practically clear on the test negative.

So you want to give “1000 times the amount of light that will barely affect the film.”

p.s. The version of Ralph’s spreadsheet from his sample chapter linked in this thread doesn’t calculate Exposure Index (film speed) or traditional Zone System N-times.

I believe Ralph has a more sophisticated spreadsheet where those things are included.

 

[Nodda Duma]

To Bill’s point about calibrating exposures as a manufacturer, my setup for characterizing my dry plates is a little different. I use a calibrated 4x5 Stouffer wedge placed on top of a plate, and expose like a contact print under my enlarger where I can control both exposure time and light value to a high level of accuracy. I have a calibrated lux meter to measure light at the wedge/plate, with light values constant across the plate to within +/- 1% lux or +/- 2.5% of a stop as read by the meter. Since the plates are orthochromatic and unsensitized, I can’t perform true ISO measurements, but I have correlated the exposure times to a nominal outdoor lighting condition and meter reading so I can plot the characteristic curve and calculate the actually speed of the plates. In that sense it’s a relative measurement.

Obviously shortcomings to my setup, but it’s repeatable and measurable, and not dependent on weather, camera, shutter speed, etc.

-Jason

 

[fix-erUpper]

Manufacturers have to properly calibrate their exposures for testing because they need to honestly tell us what the film speed is. We can make “any” exposure and work backwards from the test results to figure out what “our” exposure was that got us there.

Ralph’s spreadsheet does that.

Mr Bill is right but I like to tell it like a story. The step wedge has a practically clear patch which is going to get a full blast of test exposure, and is going to be really black on the negative. But the darkest patch only lets 1/1000th as much light through to the film. A good exposure for the test will make the darkest patch practically clear on the test negative.

So you want to give “1000 times the amount of light that will barely affect the film.”

p.s. The version of Ralph’s spreadsheet from his sample chapter linked in this thread doesn’t calculate Exposure Index (film speed) or traditional Zone System N-times.

I believe Ralph has a more sophisticated spreadsheet where those things are included.

Thanks for the reply, I'm beginning to understand. I do have a spreadsheet that does the film speed calc. Thanks.

 

[Nodda Duma]

To add to the conversation and provide a real example, here is the characteristic curve I generated for my latest batch of emulsion for my ASA 2 speed J Lane Dry Plates.

Base fog is 0.09
Gamma is about 1.05. That's at a developer temp of 22C, so I'd expect it to be a little lower at 20C.