BTZS, Zone System, Everyone Else... Film Speed ?

[Mark Barendt]

Users of large format cameras have tended to double the rate their film at about 1/2 of box speed. The main reason is that there seems to be a preference among view camera users to get good detail and contrast at the very bottom of the toe in the open shadows, and the only way to do that is to increase exposure so that Zone II details falls on the straight line section of the curve. This practice was wide spread with Tri-X 320, which with most developers has a notoriously long toe.

Some of the modern T-grain films produce by their nature a much more linear curve and can be used a box speed and still give good detail in the shadows.

Assuming the same type of metering practice this issue is, strictly speaking, not a BTZS issue but one that relates to the characteristic curve type of films/developers.

Sandy

Yep, agree fully.

[D. Bryant]

That is not the same info or a summary of what the thread I linked to offers.

Quite!

[D. Bryant]

Just visit http://btzs.org/....Phil lays it all out.


In post number 18, I've already quoted verbatim from his book, listing the actual page number. In post number 1, I summarized his points. I have already laid out - twice - what he lays out.

What he has laid out, is what I'm asking about.

Ken my point is that based on your question the exposure scale of the print material needs to be accounted for and that's what Phil explains. I never half my film speed for TMAX films. I'm guessing you maybe using HP5. BTZS testing doesn't always lead to half film speed. Are you using BTZS software? Using that and having Phils database is very helpful.

If we are just simply going to scan negatives then you need to process your film to match the capability of the scanner you are using. Most modern consumer flat beds have limitations about how well they will scan highlight areas. So in a sense the scanner becomes your digital paper.

Scan a step tablet to discovery what your scanner really can't do.

[Ken Lee]

Thank you all, for your patience and helpful answers. I have confused a number of ideas that are in fact only loosely coupled.

Sandy, thank you for dispelling my confusion for the Nth time.

Don, you're right: I have been using HP5+, and the BTZS test results which Fred Newman just performed for me (D-23 developer) gives an Effective Film Speed of around 270 for an SBR of 7.

I will scan a step tablet.

[Peter Yeti]

Inspired by this interesting thread, I wanted to know for sure and set up a test that is independent of subjective impression. Of course we all need to know effective film speed of our b/w material but that's certainly not a single number. It depends on the entire work flow. We all know that film speed is different for N or N-1 or N+1 development, it depends on the developer, on the enlarger head and so on. So I think that using b/w film or C-41 for testing meter calibration, zone V for middle grey, and reflectance of a grey card is a dead-end.

Here is my test set-up: Two Elinchrom flash heads with equal soft boxes set at 25 Ws each, outer edges 2m apart. Kodak 8x10'' grey card at 2.5m distance in the middle, resulting in extremly uniform illumination. Measurements were done with a Gossen Variosix F and repeated for 10 times each.

Results:

1.) Incident metering with flash
Repetition accuracy within +/- 1/10 f-stop

2.) Incident metering w/o flash, only modelling light at 100%
Repetition accuracy without measurable deviation

3.) Reflectance metering with flash
Repetition accuracy within +/- 1/10 f-stop. Depending on distance between meter and grey card there is a deviation of up to 2/10 when getting too close to the card because of the shadow of the meter.

4.) Reflectance metering w/o flash, only modelling light at 100%
Repetition accuracy without measurable deviation. Depending on distance between meter and grey card there is a deviation of up to 2/10 when getting too close to the card because of the shadow of the meter.

Comparison between 1.) and 3.) +/- 2/10 f-stop at most, neglegible on average
Comparison between 2.) and 4.) +/- 2/10 f-stop at most, neglegible on average

Well, if Kodak tells the truth about their grey card, then at least my Variosix F is definitely calibrated on 18% reflectance.

By the way, I shot thousands of chromes using this meter and incident measuring and got very reliable results. Since E-6 is standardized and chromes are quite unforgiving, this may give further confidence that the 18% calibration is used and reliable. And I never had problems to adjust my standard N development for various b/w films to obtain the specified speed.

Peter

[Heroique]

I hear it approaching like a thunder storm – a “precision vs. accuracy” discussion.

[Ken Lee]

Precision versus accuracy indeed.

Well, if Kodak tells the truth about their grey card, then at least my Variosix F is definitely calibrated on 18% reflectance.

How do you know that it's not 17%, or 7 % for that matter ? All you know is that you got the same value from your light meter, over and over, no ? That just tells you that your meter is consistent, which is what we'd expect from a decent tool anyhow.

You could have metered any object with a matte finish, and unless your equipment were faulty, gotten consistent readings. It could be a sheet of white polystyrene packing material, for example.

[Peter Yeti]

If incident and reflective measurements give the same value, as they do in this experiment, then you found the reference. In this case a Kodak grey card for which Kodak gives a reflectance of 18%. Measurements are within about 10%. So, 17% reflectance would be in the error bounds, 7% wouldn't.

[Ken Lee]

"If incident and reflective measurements give the same value, as they do in this experiment, then you found the reference. In this case a Kodak grey card for which Kodak gives a reflectance of 18%. Measurements are within about 10%. So, 17% reflectance would be in the error bounds, 7% wouldn't."

If your meters agree, that doesn't mean that they are both calibrated to the 18% standard, only that they are calibrated to the same standard as one another.

If your meters agree, it doesn't mean that 18% is the correct value to use for establishing film speed and other sensitometric concerns. It only means that your meters agree with one another.

The question entertained by this thread, is whether the 18% standard is an appropriate value.

[Larry Gebhardt]

"If incident and reflective measurements give the same value, as they do in this experiment, then you found the reference. In this case a Kodak grey card for which Kodak gives a reflectance of 18%. Measurements are within about 10%. So, 17% reflectance would be in the error bounds, 7% wouldn't."

If your meters agree, that doesn't mean that they are both calibrated to the 18% standard, only that they are calibrated to the same standard as one another.

If your meters agree, it doesn't mean that 18% is the correct value to use for establishing film speed and other sensitometric concerns. It only means that your meters agree with one another.

The question entertained by this thread, is whether the 18% standard is an appropriate value.

Ken, I think you are missing that Peter measured the grey card with reflectance and also using the incidence meter got the same results. We know the incidence meter is supposed to give a middle grey exposure, so for his meter it also means that the reflectance meter is calibrated to give middle grey based on an 18% card (assuming his card is 18% grey as Kodak says).