Stasrting from Scratch with a densitometer
I have just purchased and am awaiting delivery of my first transmission densitometer--a Tobias TBX. All I know about it is that it comes with a power cord and will light up. I didn't pay a lot for it, but probably too much.
I own a 31-step Stouffer wedge. Where do I go from there?
I don't have a manual; I haven't the foggiest notion of what else I need to calibrate the thing or how to go about it.
I photograph with black and white film.
Any guidance or information will be appreciated.
Please don't transmit guffaws!
Are chortles OK? How about snickers? Giggles?
Originally Posted by wskmosaic
The question you need to answer is "Where do you want to go with this?"
Normally this combination of densitometer and step tablet is used for "testing film" and "testing exposure".
All that testing is only needed if you are going to immerse yourself in the Zone System.
The result of many weeks of Zone System testing always seems to result in the finding that Tri-X 400 should be shot at 320. Which should be no surprise as the Zone System defines film speed in a different manner than the ANSI/ISO standard used by Kodak. But then it is always best to be generous with exposure and rating a film at 1/2 to 3/4 of box speed won't hurt.
But then again, one man's ISO320 is another man's ISO 200. Or 650. Depending on so many uncalibrated variables such as what camera is used (and is it calibrated? Are its speeds exact or off by a percentage?), what lens is used (are the f stops exactly accurate or are they off by a percentage at any given aperture?), is the processing rigorous (like what's your shaking procedure? Is your thermometer absolutely exact or it is one degree Celsius off? Do you stop counting the time when you start pouring off the developer or when you start filling the Stop? That's like 30 seconds difference right there) and so on.
All in all, there are so many different variables that I simply rate it at 400, 800 or 1600 and try to be as consistent as possible over the years so I can duplicate MY OWN results from one time to another (the key words here are "Duplicate my own results from one time to another".
And please don't get me started on "A film's true speed". That is theoretically very nice and all, but in real life it's utter BS. One only induces great confusion in internet forums when one starts to talk about true film speed because this invariably and instantly wrecks all the recommended development times given by the manufacturers for their own films.
Like, for example, when Ilford recommends HP5 in Ilfosol for 8 minutes at 20c and Joe comes in and says "That's BS, Ilford is a true ISO 250 film". Well, who cares if it's a true ISO 250. What is this supposed to mean? Do I still develop for 8 minutes but expose at 250 or do I expose at 400 and develop for 10 minutes?
Basically, follow what's on the box and you'll be fine. All the rest is pretty much nonsense and a matter of one's own gear calibration that he isn't even fully and accurately aware of. And Ansel Adams is dead, anyways. And Capa didn't rely on the zone system to capture essential scenes that changed the face of photography for ever. And what's the difference between a center weighted measurement and a super duper Matrix metering measurement when both measure one same scene at 1/60th@f8?
And in the end, all this deep accuracy talk means nothing if the image is unappealing, badly composed, badly timed, badly processed and overall flavorless to start with... which is about the norm (just as in any field).
So do like anyone else that's been doing this long enough: Rate your film at any given ISO and stick with that so your entire process, even if sub-optimal or perfectly perfect, is always consistent.
Originally Posted by Nicholas Lindan
Do you work in color, or only b&w. I find my old macbeth quite useful for determining corrective filtration in old dupe film stock, and crossed curves in some interneg film, and that let me know if pushing my e-6 films in home brewed processes is really effective.
A densitometer is a handy tool. A 31 step step wedge is a powerful tool all on its own though. With the wedge your eye does the work subjectively. With the denitometer you can assign numbers and plot H-D graphs. Not everyon'e cup of tea. but it can be quite useful.
Using my Macbeth let me get a box of 100 8x10 tech pan film calibrated with a home brewed developer so that I now have a stable expsoure and development scenario to use on this film that I bought at a swap meet on a flyer for $5.
my real name, imagine that.
I spoke at length to a Tobias technician about calibrating the densitometer without having their check plaque. If you take a reading with nothing in the densitometer and adjust the small screws on the side to zero you will have the low reading correct. Then, use a 3.0 ND filter to take and adjust the densitometer for the high reading.
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I understand the first step; could you elaborate a bit on the high readings part? Isn't there a whole scale I should be looking for?
Originally Posted by Greg Davis
The densitometer is (comparatively) linear along the scale from low to high. So if zero reads zero and 3.0 reads 3.0, everything in between should be good.
Originally Posted by wskmosaic
It's the film and paper that takes a curvy path from one end of the scale to the other, and that's what you use the step wedge for.
In theory a densitometer only needs to be zero'd. Usually you would zero it on B+F of the film margin.
All density readings are a ratio of the zero reading. If the reading is 1/2 the zero reading then the density is 1 stop/0.3OD, if the reading is 1/4 the zero reading the density is 2 stops/0.6OD.
Older densitometers converted the readings to density with a 'logarithmic amplifier' - these are prone to all sorts of drifts and so old densitometers also need to be calibrated with a known density. Modern densitometers take the logarithm in software and drift is essentially zero so there is no need for a second calibration point, and linearity is close to perfect with modern A/D converters (but not so with the old log amps).
If you twiddle trim pots to calibrate the densitometer it is more than likely an older design with log amps. Caveat emptor.
Last edited by Nicholas Lindan; 04-13-2012 at 03:11 PM. Click to view previous post history.
Giggles are good. Their kind of sympathetic. Chortles and the rest tend not to be.
Anyway, you've all given me bits and pieces to put together. I just found that a users manual is available from the manufacturer and, between you and them, I think I'll get there.
I do do zone system black and white and find that my best system speeds are always a lot slower than the manufacturers' speeds. It's true for all the film I've tried.
I hope the densitometer enables me to do what I've been doing, but do it with more precision.
So, I respectfully disagree with just about all of the post below.
But to get to the heart of the matter, if you want good shadow detail, you will want to test for film speed. Ansel Adams sure thought that it was important. Hmm. He obtained great results.
And, that's a good place to begin with your densitometer. But to really get the best from your densitometer, which is to get the best from black and white film, one must also test to determine development times for N, N-1, N+1, N-2, and N+2 contrast ranges.
The strategy is to expose for the shadows and develop for the highlights. Testing film speed enables the former; determining development times enables the latter.
I will say that I agree with the following comment:
"And in the end, all this deep accuracy talk means nothing if the image is unappealing, badly composed, badly timed, badly processed and overall flavorless to start with..."
But, that's about it.
Originally Posted by NB23