BTZS - Spot Meter conversion to Densitometer

[Cletus]

So I just got my copy of Phil Davis BTZS (thanks Sully75!) and in quickly thumbing through, I saw the remark about converting your Pentax Spot Meter into a Trans/Refl Densitometer. Instructions which are in the Workbook. Which I do not possess. No help there.

I'm interested in looking into this, but would like to hear your experience with doing something like this, or your ideas on how to do it. Does anyone have a copy of the workbook, or a file they could post detailing the plans for this little trick?

Phil Davis says the spot meter densitometer is every bit as effective as the real deal. Isn't there also some kind of conversion matrix or table required to translate EV values into density values or something?

I'd very much appreciate hearing your comments or suggestions on this idea. I'd like to finally see what all the BTZS fuss is about, though I doubt I'll plan to take some of the fine tuning and testing as far as the book recommends.

[Robert Bowring]

I do not know how "correct" this is but it seemed to work for me. I used my enlarger as a light source and removed the lens. I placed an unexposed negative in the enlarger and took a reading through the negative with my spot meter to determine film base plus fog. Then I put exposed negatives in the enlarger until I found the one that gave me a reading of 1/3 stop difference from film base fog. This gave me my EI or film speed. If I remember right 1/3 stop=0.1 density. Once you determine your EI you can expose some film at that speed and read the negatives to determine your development time. I exposed the film by taking picture of a grey card. Hope this helps. Like I said it seemed to work for me. It is probably easier to do than it is to explain it.

[ic-racer]

So I just got my copy of Phil Davis BTZS (thanks Sully75!) and in quickly thumbing through, I saw the remark about converting your Pentax Spot Meter into a Trans/Refl Densitometer. Instructions which are in the Workbook. Which I do not possess. No help there.

I'm interested in looking into this, but would like to hear your experience with doing something like this, or your ideas on how to do it. Does anyone have a copy of the workbook, or a file they could post detailing the plans for this little trick?

Phil Davis says the spot meter densitometer is every bit as effective as the real deal. Isn't there also some kind of conversion matrix or table required to translate EV values into density values or something?

I'd very much appreciate hearing your comments or suggestions on this idea. I'd like to finally see what all the BTZS fuss is about, though I doubt I'll plan to take some of the fine tuning and testing as far as the book recommends.

I used to use my spot meter to make H&D curves all the time in graduate school and it worked fine. These days, you should be able to pick up a working densitometer for not much money.

[Cletus]

Robert, IC-racer,

Thanks for your comments, I have noted the EI to Density conversion you suggested, which makes sense and should be a good starting point. I do have an RH Designs Analyzer Pro, which purports to have a densitometer function, though it seems unintuitive and I keep thinking the spot meter trick, using my light table for illumination, might be a bit more straightforward.

I used to have a color transmission densitometer, an X-Rite 8xx(?) if I recall, but it was not only very large and heavy for my small darkroom, but I couldn't get consistent results with Pyro negs so I quickly sold it.

As I mentioned in my OP, I don't really plan to go all-in for the BTZS testing and ultra fine tuning regimens, I'd just like to cover some basics and try to get a better understanding of film and paper curves, CI and some of the other principles.

[Jim Noel]

The method spelled out in BTZS works very well. I made one when the book first came out and used it for years until I finally bought a real densitometer.

[Nathan Potter]

I did this many years ago (when I was poorer with a family) using my old analogue Pentax meter. I made a stand for the meter out of lab clamps and aluminum lab rods such that the meter could be supported about 4 inches above the negative while making measurements. I used a short focal length achromat attached to the front of the meter (about 100 mm focal length as I recall) so that the meter would be in focus about 4 to 6 inches from the negative.

Calibration was done using a Kodak step tablet transilluminated with a stabile light source. Made a plot of EV vs step density, then used that to measure the negative density directly. In order to obtain good accuracy I employed a nominally 2 mm. aperture just above the negative to reduce scattered light and made sure the room lights were darkened or off. The aperture was simply a hole punched in a piece of black paper mounted on a piece of 6X6 inch glass.

The whole contraption worked very well and was highly accurate when precautions were taken to reduce scattered light.

It helps for stability to rig a lever system for depressing the read button on the Pentax while taking a reading. Need a bit of fabrication skills to do this successfully and reduce the hassle factor.

Nate Potter, Austin TX.

[Cletus]

Thanks for the info Nate, this is exactly what I was looking for. I think I can pretty much picture the setup now, but could you elaborate a little on the "short achromat" you used for close focusing? I'm imagining something like a close-up filter, or some lens used as such, attached to the thread on the meter's lens, then constructing the frame, or holder, or whatever you call it, to hold the meter to whatever height it will focus on the "floor" at.

Does this sound about like the right idea? Seems to me the whole point is to use something that will make the meter come into focus at 1 to 2" from the floor or ground, then the fixture to hold the meter at that point (and perpendicular to that point) for taking measurements. I can understand the 2mm aperture "plate" and the importance of controlling extraneous light around the measured point.

I have several transmission step tablets to calibrate from and I assume I can just get a reflection step tablet to calibrate this meter rig for a reflection densitometer as well?

Does all this sound about like the right idea? Any specific suggestions for the close-up lens attachment? Something available and known to work for something like this?

Or even better yet a drawing, spec, or excerpt from BTZS Workbook detailing this contraption?

[Ed Bray]

It's available here!

[Cletus]

Ha ha! Yeah, I found that too Ed, thanks. If I understand you, I should probably take the hint and ease off on asking folks to post copyrighted info (however old it is) on the forum, just so I can have an easier time making my little darkroom doo-ma-bob.

Actually, I think I have enough to go on from the generous responses already posted, so I'll probably take it from here and see what works. Already looking at some little 40.5mm close-up filter sets for cheap, which should do the trick for the close focusing part.

[Nathan Potter]

Cletus, yes you have the idea. Your interpretation of what I said is right on. I used an achromatic lens from Edmund Scientific. I think it might have been a 75 mm FL.

A supplementary lens would be ideal but they are of insufficiently short focal length to be useful.

The diopter value of a supplementary lens = 1 meter/ lens focal length. So a 75mm FL lens would be equivalent to a +13 diopter lens. You'd have to stack 3, +4 diopters to get about 75 mm. And using an achromat yields a better image.

The diameter of the lens should be about the diameter of the filter thread on the Pentax but if not just use tape to hold the lens in place. BTW I used a cable release just clamped in the proper position to actuate the read button on the Pentax. That way I wasn't shaking the Pentax around the table while making the measurements.

Actually I'm not sure that you need any critical focus on the aperture and negative to read the transmissivity using the Pentax. You might want to check that out before getting too finicky.

Nate Potter, Austin TX.