Using densitometer with grey card to determine effective ISO (expired)?

[GBS]

I've got some expired film to test. Is there a factor or formula for using a densitometer with a bracketed grey card to get a close approximation for the current ISO? (I understand this won't account for color shifts or crossover. I'm simply looking for an educated ballpark.)

In my limited understanding/experience, I would assume it's something like:

[grey card frame densitometer reading] - [base densitometer reading] = [target value]

Am I close? What's the target value?

I'm interested in using something like this for negative as well as slide.
Thanks!

 

[Nicholas Lindan]

Well, heck, after stating that expensive filters were a fetish I may as well continue in the same vein and state that extensive film testing is also a fetish. And this from someone who spends days and weeks testing B&W paper...

ISO/ASA is just a number you set on your lightmeter to make the photos you like the best.

Take a picture of your normal subject matter. Meter at the box speed and bracket a few stops either side. Process, print, and pick the one you feel is the best. Use the result to find the number to set your lightmeter.

If you use a spot meter then set the meter at two stops over the box speed and meter the detailed shadows of your normal subject matter. Again, bracket and pick the print you like the best.

For B&W most people find that exposing at around 1/2 to 2/3 of box speed and pulling the development by ~15% produces the best prints. This is where you will land up with extensive Zone System/BTZS testing. The Zone System is just an elaboration on the old & very sage advice "Expose for the shadows, develop for the highlights."

For slides, box speed, is the way to go. Some like to expose at +1/3 of box speed - expose a 100 speed film at 125, say. It is easy to blow highlights with slide film so err on the side of slide underexposure.

For color negative most err on the side of a bit of overexposure, you can overexpose color negative by a stop or two with no ill effect and maybe a slight increase in image quality.

 

[GBS]

Thanks for the reply. I'm familiar with those basics and methodologies. I'd like to use the process outlined in my question for a number of reasons. Indeed, I will use bracketing and subjectivity as well, but for now I'm curious if there is an accepted reading a grey card should provide on properly exposed frame, and how would I account for the base?

Side note: this is ECN-2 film and I won't be using prints as part of my process.

 

[Mr Bill]

Is there a factor or formula for using a densitometer with a bracketed grey card to get a close approximation for the current ISO?

No simple way that I can see. Here are two things that have to happen when establishing an ISO speed for a general purpose b&w negative film. First you need to develop to a specific "contrast" level (contrast is not exactly correct, but it's fairly close). Second, according to the ISO standard, you need to find an amount of "light energy" that produces a specific small amount of exposure effect on the film. This effect is defined as a density increase of 0.10 above the "clear" base, aka base plus fog. Your densitometer could measure this difference, or you could just say, oh, I see a slight difference from the unexposed part, and this will be pretty close.

The main problem is in knowing the value of that "light energy" that gives the slight "exposure effect." And... the ISO film speed standard does not use exposure meters. However, by looking at both the ISO film speed standard and the ISO exposure meter standard there is a sort of relationship between the two. A member who has studied this sort of thing quite a lot, Stephen Benskin, says that the ISO "speed point," where the film first shows a slight exposure, is defined as being about 3 1/3 f-stops below the exposure meter reading.

So, the suggested method is to 1) find a "correct" development time and temperature, and 2) shoot a series of test exposures that cover a range about 3 to 4 stops below what the meter says. On the developed film you find the frame that is closest to the speed point, meaning that the film is about 0.10 density units above "base plus fog." From your notes (you kept notes, right?) you get the exposure for that frame, then calculate the exposure 3 1/3 stops brighter. This brighter exposure is what the exposure meter is supposed to read. So... adjust the ISO speed on the meter until you get the correct reading. That will give you the correct ISO speed for the film. More or less, anyway. You'll be affected by the precision of setting exposure changes in your camera, as well as any errors in the exposure meter.

Fwiw the gray card's reflectivity doesn't really come into play here. You would get the same result from a white card. This is cuz the meter doesn't know what it's reading. All it knows is the strength of the light it reads, and whatever this is, the meter will attempt to make this record on the film as a mid-gray. And the film speed-point should be 3 1/3 stops down from that point.

 

[ic-racer]

B&W FIlm: Meter any uniform target and expose 4 stops under. Process the film and place the film over exposure meter, looking for a 1/3 stop drop from film base.

 

[dkonigs]

ISO speed does have an official definition. That definition is:
S = 0.80 / Hm
where Hm is the light exposure (in lux-seconds) required to achieve a transmission density of D=0.10 above base+fog on the film.

If you have a sensitometer with a step wedge, and measurements of their behavior, you should be able to figure it out. I think Kodak even has an old workbook PDF that walks through the process. Doing it from an actual film exposure would certainly be more difficult, but may still be possible. (I think the calibration software for the Sekonic meters can do something like this with digital images of their fancy multi-step grey card, combined with camera settings and a meter reading.)

 

[ic-racer]

"Hm" can be any combination of lighting and shutter speed and f-stop. These are the variables the in camera test will be testing to achieve the film density.

 

[Mr Bill]

Side note: this is ECN-2 film and I won't be using prints as part of my process.

Whoops, ok, you wanna use color film. Ok, very similar process. Except now you need to use the specific color of light that the film is rated for. Photographic "daylight" has a color temperature of about 5500 K, as I recall. And you need a color densitometer, status M to be exact. There are three different dye layers, cyan, magenta, and yellow, and the status M densitometer is able to read each of the separate layers. I don't recall the exact calculation method, but it roughly averages the speeds of the three individual dye layers.

One other difference; b&w neg film specifies an aim contrast, of sorts. With color film you have to stick with the process specs, so you don't independently adjust for contrast.

Ps, it may not be obvious why the color of the light is so important. Essentially each of the dye layers have their own speed, and they are designed so as to more or less coincide under the correct light. If, for example, you use a lower color temp light this will be deficient in bluish light. Consequently the blue-sensitive layer will be underexposed, making it appear to have a lower film speed. When in reality the deficiency is only in the light source.

 

[dkonigs]

Ahh yes, for color film there is still a formula. Its just a different one:
S = sqrt(2) / Hm
where Hm is the exposure (in lux seconds) required to produce Status M red, green, and blue density values of D=0.15 above the corresponding minimum density for each color. With this exposure, Hm is:
Hm = sqrt(Hgreen * Hslowest layer)

If it wasn't obvious by now, I ended up getting myself copies of ISO 6:1993 (B&W ISO) and ISO 5800:1987 (color ISO) because I like actually knowing how these things are defined. Trying to gather information from "forum heresay" always gets conflicting advice or a competition of opinions.

But yeah, the easiest way to figure this out for practical purposes is probably to do bracketed exposures of one of those fancy scale grey cards and simply see how far off a sensible middle gray is from where you'd expect it to be.

 

[laser]

There is no need to make this too complicated. Use the sunny 16 rule. Under direct sun photograph as typical scene at a series of shutter speeds. The shutter speed that yields a density of about .8 (red density) of a 18% gray card will be the reciprocal of the EI. Have the gray card at a slight angle so you don't get a spectral reflection from the card.

Alternatively, photograph a typical scene in the same manner that includes a gray scale. The proper exposure have as just perceptible density in the blackest step. In Zone System speak this will establish Zone I. An 18% gray card is Zone 5. Again, this shutter speed will be the reciprocal of the EI.

Some fine-tuning may be advisable but these techniques will get you within a stop.

www.makingKODAKfilm.com

 

[RalphLambrecht]

I've got some expired film to test. Is there a factor or formula for using a densitometer with a bracketed grey card to get a close approximation for the current ISO? (I understand this won't account for color shifts or crossover. I'm simply looking for an educated ballpark.)

In my limited understanding/experience, I would assume it's something like:

[grey card frame densitometer reading] - [base densitometer reading] = [target value]

Am I close? What's the target value?

I'm interested in using something like this for negative as well as slide.
Thanks!

the target transmission densityvalue for Zone V in a negative s 0.72 for Zone I it is 0.1 if you expose and develop normally.

 

[Craig]

Use the sunny 16 rule.

It may depend on where you are, but "sunny 16" has never worked for me. Reciprocal of shutter speed at F11, gives perfectly exposed slides over the years for me.
So for example 1/125 at F11 with 100 speed slide film is right.

 

[GBS]

This is ALL excellent info! Thank you! I was recently gifted an old tube densitometer with step wedges, but first I need to get it working again. In the meantime, my local lab has one I may be able to use for a couple frames.

the target transmission densityvalue for Zone V in a negative s 0.72 for Zone I it is 0.1 if you expose and develop normally.

@RalphLambrecht I assume 0.72 is total density minus base? Will this basic formula work with a color negative total transmission (averaging all channels)?

The shutter speed that yields a density of about .8 (red density) of a 18% gray card will be the reciprocal of the EI.

@laser This would make sense, being the negative of 18% transmission. For clarity, the value 0.8: is that total transmission minus base? Why only the red channel? Just curious.

Thanks! Excited to try some tests.

 

[Mr Bill]

Why only the red channel? Just curious.

That's the channel traditionally used since I first started using densitometers in a large lab back in the mid '70s. So who knows when it started?

As to why, I'm guessing, but the red channel is the most sensitive for skin tones. So a portrait lab trying to screen for proper exposures would get best results on the red channel. Now, if they're already using the red channel for SOME readings, why not just stay there for gray card readings too? So it's been traditional since at least that time frame for the manufacturers of professional color negative films to give aim values for both flesh tones AND gray cards with respect to portrait work. If you look at data sheets for just about any pro negative color films (see the section on evaluating exposure) you'll find general density aims.

@RalphLambrecht I assume 0.72 is total density minus base? Will this basic formula work with a color negative total transmission (averaging all channels)?

Regarding the color negative, "will an average of all channels work?", nope, not likely. If you know what you're doing and have a good reference image you could probably fudge some numbers, but generally speaking, no.

If you want an idea of what you're looking at, take a look at the Kodak Portra film data sheets (they're online). Go to the "characteristi curves" (near the end of the document). You'll see three curves plotted - red, green, and blue. This data is taken from a neutral exposure; the color densitometer is able to see each of the individual dye layers inside of that image. Without this ability to isolate colors you can probably see that you have little chance of figuring out what the red channel is doing.

 

[GBS]

@Mr Bill Thank you for the explanation. I'm new to densitometers and this makes sense. Hoping I can get this old one I have up and running.

In the meantime, I shot 4 of the expired stocks I'm testing today at box, +1, +2, +3, and +4. I included a grey card in an outdoor scene with highlights and shadows. I can generally tell from the negatives which frames are closest. I'm sure scanning will only help more. This is a fine starting point, but I still want to learn to use the densitometer.

Thanks again t everyone who's contributed!