One sixth of a stop??? Do you think that anyone can, or needs to be that accura....
In another life, all of the studios doing product photography had their lenses marked in 1/10 of an f stop.
We were doing all of the work using tranny film and as the end product was a printed colour catalogue, really accurate metering was required as the film had to have a highlight to shadow range of 3 stops maximum.
This range resulted in extremely superior colour reproduction in a four or five colour offset printing process.
Trust me when I say if you wish to photograph white or slightly off white fluffy towels, using a white or very light background as requested by the client. Then by keeping the exposure absolutely spot on at the start of the process, you could see the different fluffy white towels clearly in the catalogue.
FWIW All of the Hasselblads were very easy to work with, one metered exactly to a known standard, applied black or white reflectors to kill or lift light, adjusted the lens to the correct 1/10th of a stop then shot.
With the RB/RZ most photographers somewhere along the line forgot to allow for bellows extension. You could see immediately on the lightbox the difference.
The basic idea for this came from an american photographer who preached total control to save money. His name may have been Dean Collins, not sure. However we looked seriously into what he had to say, thought it could be worth it and set up one studio (out of fourteen) this way.
Initially it took about a week to get things going correctly. Going on memory, it was about 20 years ago, there was an immediate savings of film in the vicinity of 20%. With colour stats used for paste ups and layout there was a savings in time and stuffups.
Once the trannies were to be scanned we found that as they were so close in density, saturation, etc, etc, we could get the scanning done with far fewer problems.
The printers had a ball as the highlight to shadow dot didn't blow out or bog down, the client was over the moon as their main competitors catalogue looked pretty ordinary compared to theirs.
The total savings amounted to about 15% in actual costs and throughput increased dramatically once the film had been shot.
We were so impressed with this that we started looking at other ways to improve accuracy in exposure. Sinar had their meter probe for the GG on our 4x5 cameras. We tried them but it didn't really work out as hoped. Apart from that they were hopelessly overpriced and we just couldn't justify them. We did teach the Sinar rep how to use the probes a treat though!
Originally Posted by Mick Fagan
I had been immersed in "Light meter" calibration and accuracy ... and establishing "Accuracy Evaluations" for them. Our targets were the top of the State of the Art equipment available ... using Standard Lamps - Intralaboratory Standards cycled though the NBS.
Our efforts were mainly directed at equipment using Cascade Photomultipliers ... you haven't lived until you take a jolt from the primary energy supply from one of those; ~ 1800 volts or so. Unforgettable.
But, I digress. I'll avoid a discussion of the various problems inherent in measuring anything to determine "exposure" and merely ask about the equipment used that would be accurate enough to determine the amount of light accurately enough to enable confidence in an exposure to +/- one tenth (1/10th) of a stop. What was used?
Also ... "f/stops"? Weren't most of the really accurate lenses calibrated and marked with "t/stops"?
Ed Sukach, FFP.
Ed, The basic idea was to calibrate the whole studio.
Firstly, the two lenses for each camera were colimated by a scientific optical works. Once the lenses had been re-assembled they had their effective apertures measured at (I think) three focusing settings, infinity being one of the settings.
Each lens had lines engraved on a non moving part and a corresponding set on the aperture ring. There was a printed chart for each lens that was permanently attached to each camera stand.
The idea sounds complex but in fact it was extremely simple. The photographer arranged the product, set and focused the camera, decided on the f stop required, checked the focusing distance off the lens, consulted the chart and moved the aperture ring the required amount to get a true aperture.
The photographer also used a flash spot meter which had a specially made black tube attached to the end. This was to ensure that flare didn't play any part in getting a false reading.
Once our test studio was set-up and running it took about three days to get what we wanted. The film was sent off for scanning, colour seps done, then 4 colour chemical proofs were made.
The results were pretty stunning. I can remember standing in front of the colour corrected reflected light box, viewing the proof prints. Pretty much everyone there asked the same question:- "how much does it cost to get a set of lenses calibrated"?
The photographers all liked the system, once they got into the habit of doing things one step at a time, life was a bit easier for them.
I understand all that, but my question was directed toward measuring the amount of light ... accurately enough to make concern about one-tenth of a "stop" reasonable. What brand/ model Flash Meter was used / is supposed to be that accurate?
Originally Posted by Mick Fagan
My Gossen Ultra-Pro has a resolution of 1/10th stop; Gossen only claims an accuracy of +/- 1/3 "stop".
My next question would be directed to Photo Engineer: What are the design parameters / manufacturing tolerances of ISO film speed, for either negative or "reversal" film? It is difficult enough to "worm" exposure meter tolerances out of the manufacturers ... I can only imagine how tight-lipped they are about film speed.
Ed Sukach, FFP.