Quick & easy film test question

Alex Benjamin · Oct 30, 2025

Plotting my first film curves. Got my film, got my step wedge, got my enlarger (Durst CLS 501 color head), borrowed a densitometer. Going to test T-Max 100 in Rodinal (I'm crazy that way).

Question: do I need an 80A filter under the enlarger lens? Kodak Basic Photographic Sensitometry Workbook says yes, I've read elsewhere that with modern film, not so much.

Follow up: if I don't put one (as I don't have one), how far off are my numbers going to be.

Thanks.

Terrence Brennan · Oct 30, 2025

They may be off a bit. You will probably be using a lamp with a colour temperature of 2800-3200K, which would be closer to available light, in terms of spectral response.

When I make sensitometric exposures, using a modified B-22 enlarger, with the said modificiation being a small electronic flash in place of the regular lamphouse, I use a filter pack consisting of a Wratten 2B filter, a Wratten 81A filter and the appropriate Wratten ND filter to give an exposure of a single "pop" of the flash, with a 50mm lens set at f/8.

The flash is enclosed in a simple lamphouse, made of the same sheet metal used for heating ductwork, and it is constructed using the thin silver duct tape used to seal ductwork.

Doremus Scudder · Oct 31, 2025

If you test with a ~3200K light source, you'll get good results for your film in tungsten/halogen incandescent light.

If you want results for the film in daylight, ~5500K, you'll need a light source balance to that. an 80B or 80A will do the trick (which one depends on your light source; halogen or regular tungsten-filament incandescent. but I imagine either would get you really close, regardless of the exact light source), or, you can use flash, as mentioned above.

A viable alternative without much work, however, is to use your color head to dial in an approximate daylight color temperature. A quick Google search turns up the following:

"To adjust an enlarger head to daylight color temperature, set your filters to the standard daylight equivalent, which is approximately 5500 Kelvin. Start with a base filter setting, often around 50M, 40Y, 00C..."

There's also a bit of a discussion on the subject here: https://www.photrio.com/forum/threads/daylight-filter-settings-with-color-enlarger.167659/

Hope this helps,

Doremus

pentaxuser · Oct 31, 2025

Just out of curiosity what is being tested here in terms of a film?

Thanks

pentaxuser

Pioneer · Oct 31, 2025

I thought he said TMAX 100.

Alex Benjamin · Oct 31, 2025

Thanks all. I was able to borrow an 80A filter at the last minute.

@Doremus Scudder : thanks for the tip on color-head adjustment. I'll bookmark the thread you refered to.

pentaxuser said:
Just out of curiosity what is being tested here in terms of a film?

Thanks

pentaxuser

T-Max 100 in Rodinal 1+50.

pentaxuser · Nov 1, 2025

Alex Benjamin said:
T-Max 100 in Rodinal 1+50.

Thanks but what I meant was what is the reason for concern about an enlarger's light source in terms of temperature?

Sorry if my question was termed in a confusing manner It was just that normally a film and developer are tested for establishment of speed, grain, personal development time where the enlarger's light source is immaterial or so I thought

Thanks

pentaxuser

MattKing · Nov 1, 2025

As most films will differ in their sensitivity depending on the spectral response of the film and the spectral behavior of the light source, it is important to be consistent in the light source.

Doremus Scudder · Nov 1, 2025

Alex Benjamin said:
... @Doremus Scudder : thanks for the tip on color-head adjustment. I'll bookmark the thread you referred to. ...

Alex,

On second look, it seems to me that the color-head settings suggested by Google AI are backward (maybe they were for balancing color negative film exposed under light...). At any rate, you would need to remove red and other longer-wavelength light from the mix, so adding cyan first and then balancing with, maybe a touch of magenta would seem more logical to me.

Best,

Doremus

Stephen Benskin · Nov 1, 2025

Alex Benjamin said:
Plotting my first film curves. Got my film, got my step wedge, got my enlarger (Durst CLS 501 color head), borrowed a densitometer. Going to test T-Max 100 in Rodinal (I'm crazy that way).

Question: do I need an 80A filter under the enlarger lens? Kodak Basic Photographic Sensitometry Workbook says yes, I've read elsewhere that with modern film, not so much.

Follow up: if I don't put one (as I don't have one), how far off are my numbers going to be.

Thanks.

Just out of curiosity, what page does the Sensitometry Workbook mention a 80A filter?

Milpool · Nov 1, 2025

The current Kodak tech sheets for the TMax films are shorter than they once were but if you go back 15-20 years they indicated that there was essentially no difference between tungsten and daylight speeds (assuming no camera filters).

In practice the greater difficulty / source of error associated with using an enlarger as a sensitometer has to do with knowing the amount of exposure the film is actually receiving since both illuminance at the film plane and exposure time need to be known if one is trying to evaluate film speed.

The proposed methodology is more useful in finding a development time for the desired contrast than film speed or EI.

When it comes to film speed, the differences between developers in relation to film speed tend to be greatly exaggerated so while the conventional wisdom is that Rodinal produces less than full emulsion speed, the difference (if it exists) is likely a small fraction of a stop. In addition, since the "personal EI" one is most often testing for is 2/3 stop below ISO by definition, simply using an EI of 50 for an ISO 100 film covers everything.

In the end with a test like this ISO 100 is the known reference point and what is really being tested is the test methodology.

Stephen Benskin · Nov 1, 2025

Milpool said:
The current Kodak tech sheets for the TMax films are shorter than they once were but if you go back 15-20 years they indicated that there was essentially no difference between tungsten and daylight speeds (assuming no camera filters).

In practice the greater difficulty / source of error associated with using an enlarger as a sensitometer has to do with knowing the amount of exposure the film is actually receiving since both illuminance at the film plane and exposure time need to be known if one is trying to evaluate film speed.

The proposed methodology is more useful in finding a development time for the desired contrast than film speed or EI.

When it comes to film speed, the differences between developers in relation to film speed tend to be greatly exaggerated so while the conventional wisdom is that Rodinal produces less than full emulsion speed, the difference (if it exists) is likely a small fraction of a stop. In addition, since the "personal EI" one is most often testing for is 2/3 stop below ISO by definition, simply using an EI of 50 for an ISO 100 film covers everything.

In the end with a test like this ISO 100 is the known reference point and what is really being tested is the test methodology.

That's the direction I was heading. Generally, any difference in spectral response is greatly outweighed by using an enlarger as the exposure device. I couldn't find a reference to an 80A filter in the copy I have. The color temperature of the light source will factor into determining the film speed, but only if the illuminance of the light source is known and controlled like a calibrated sensitometer. The booklet appears to explain the theoretical process without defining the limitations of any specific approach.

Alex Benjamin · Nov 1, 2025

Stephen Benskin said:
Just out of curiosity, what page does the Sensitometry Workbook mention a 80A filter?

It doesn't.

Got the idea for the filter here:

Milpool said:
The current Kodak tech sheets for the TMax films are shorter than they once were but if you go back 15-20 years they indicated that there was essentially no difference between tungsten and daylight speeds (assuming no camera filters).

That's what I also got from Davis' BTZS book. He does mention filters are no longer necessary with modern film. I just felt like doing it ol' school.

Stephen Benskin said:
The color temperature of the light source will factor into determining the film speed, but only if the illuminance of the light source is known and controlled like a calibrated sensitometer.

Was able to measure illuminance in Lux with the Sekonic L-508 lightmeter.

Milpool said:
The proposed methodology is more useful in finding a development time for the desired contrast than film speed or EI.

Desired contrast is a lot of what I'm aiming for.

But essentially, I'm doing this exercice as an exercice, i.e., to understand the process a bit better.

Milpool · Nov 1, 2025

That’s right - the booklet explains the terms and give an overview of the process but assumes a sensitometer is used (ie that the exposure is known).

Stephen Benskin said:
That's the direction I was heading. Generally, any difference in spectral response is greatly outweighed by using an enlarger as the exposure device. I couldn't find a reference to an 80A filter in the copy I have. The color temperature of the light source will factor into determining the film speed, but only if the illuminance of the light source is known and controlled like a calibrated sensitometer. The booklet appears to explain the theoretical process without defining the limitations of any specific approach.

Stephen Benskin · Nov 1, 2025

Alex Benjamin said:
It doesn't.

Got the idea for the filter here:

That's what I also got from Davis' BTZS book. He does mention filters are no longer necessary with modern film. I just felt like doing it ol' school.

Was able to measure illuminance in Lux with the Sekonic L-508 lightmeter.

Desired contrast is a lot of what I'm aiming for.

But essentially, I'm doing this exercice as an exercice, i.e., to understand the process a bit better.

Really quick, the guy in the video says footcandles and lux are the same. They aren't. Lux is the same as m/cd2. Lux is ft/cd2 * 10.76.

What you're doing is fine. Too many people attempt to achieve a precision that's just not possible with the tools at hand. This can be frustrating and more importantly lead to a false sense of accuracy. Mentioning the 80A filter made me believe this might be those cases. Your approach is perfect for contrast determination and assigning a relative film speed that can be used as a reference. The Sekonic exposure meter and an enlarger (with all that is associated with it) are not precise enough to determine an accurate film speed. In addition, the methodology used in interpreting the results is an important factor. But as long as you realize these limitations, you'll be fine.

Alex Benjamin · Nov 1, 2025

Stephen Benskin said:
Really quick, the guy in the video says footcandles and lux are the same. They aren't. Lux is the same as m/cd2. Lux is ft/cd2 * 10.76.

What you're doing is fine. Too many people attempt to achieve a precision that's just not possible with the tools at hand. This can be frustrating and more importantly lead to a false sense of accuracy. Mentioning the 80A filter made me believe this might be those cases. Your approach is perfect for contrast determination and assigning a relative film speed that can be used as a reference. The Sekonic exposure meter and an enlarger (with all that is associated with it) are not precise enough to determine an accurate film speed. In addition, the methodology used in interpreting the results is an important factor. But as long as you realize these limitations, you'll be fine.

Thanks, Stephen.

You're right in thinking that I'm not looking for ultimate precision. My problem right now is figuring out the Photronix Delta III densitometer...

Quick & easy film test question

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