Stouffer step wedge and film speed
Is this correct or close?
Expose a sheet of film sandwiched together with a stouffer step wedge to an even light source in the camera over exposing by 5 stops. Read step 19 for zone 1 "speed" and step five for zone 8?
When using the densitometer do you zero on the light without the exposed film or zero on the actual film base?
Anyone use this method to get a film speed? And zone 8. Care to share your full methodology?
Re: Stouffer step wedge and film speed
There are 3 different Stouffer step wedges. Which number is yours ? How many steps ?
In the wedges with 31 steps, each step represents 1/3 of an f/stop.
In the wedge with 21 stops, each step represents 1/2 of an f/stop.
Re: Stouffer step wedge and film speed
I do not use that method. I use the step wedge under the enlarger just to plot the curves for contrast. To determine my film speed I expose a gray card (placed on zone 1) at different speeds and look for the shot that gives me .1 above the film base.
Re: Stouffer step wedge and film speed
Steve,
If you're going to all the trouble to expose a step wedge, why not invest in the WinPlotter software and use the BTZS system? You'll retrieve a lot more useful information with a minimum of effort.
Re: Stouffer step wedge and film speed
Yes, I do something similar using the 21 step Stouffer. I have a holder with the step wedge taped inside permanently so frequently expose an extra sheet (4X5) and send that off with each batch of E6 to be developed; the B&W I do myself. I expose the Stouffer to a white matboard that I also spot meter on to accurately record the EV value. Yes I over expose by 5 stops to capture a full 10 stop range on film that I can measure using a densitometer. Densitometer is zeroed without the film in order to capture a base fog reading. Seems complicated but after a few times it's pretty fast.
Good way to assess outside lab control.
Nate Potter, Austin TX.
Re: Stouffer step wedge and film speed
Re: do you zero on the light without the exposed film or zero on the actual film base.
A new densitometer should come with reflection and transmission calibration devices. I bought a Heiland densitometer from RH Designs in the UK. It came with a piece of paper for reflection and a piece of cardboard with a hole in it containing a piece of film for zeoing out the densitometer before making a reading.
If you bought a used one that didn't have the calibration gizmos, who knows? If you use the unexposed film for zero, you'd get very different absolute readings if you used Tri-X vs T-Max.
Re: Stouffer step wedge and film speed
When using a transmission densitometer you read the actual processed film base (to account not only for the base but also for chemical fog, i.e. "film base plus fog"). That is then nulled into the readings from a calibrated step wedge, either automatically by the densitometer or manually by you. At least that's how it was done with my Macbeth transmission densitometer IIRC.
Re: Stouffer step wedge and film speed
First you calibrate the densitometer, according to it's specified procedure, then you read the processed FB, then you zero it on the processed FB, then you read the steps. Zeroing on the FB is like a tare weight on a scale, it eliminates the part you don't want to measure.
Re: Stouffer step wedge and film speed
Jay I actually have and have used for about 7 years now the WinPlotter and ExpoDev software I got from Fred at the View camera store - however - I wanted to test the method I described for a friend.
The step wedge is the standard 21 step 5x4 stouffer.
My densitometer is calibrated according to instructions and using and understanding it is not an issue.
Nathan seems to use a similar method I have described so if I process the exposed sheet - zero the densitometer on film base +fog and read step 19 should that theoretically be zone one with the 21 step wedge.
Ok supplementary question. On the Stouffer wedge exposed film which physical area is regarded as film base + fog? I assume the [on the wedge] black central area between steps 1 - 11 and 11 - 21 ??
Once the step 19 value is .1 - .15 can I then use step 5 to determine a development time to give a density of 1.25 - 1.3 [diffusion]
Thanks
Steve
Re: Stouffer step wedge and film speed
What is the density for step 19 on the stouffer step tablet? I'm asking because the idea with contacting the step tablet within the camera is for the camera to act like a sensitometer. If I knew the density of the step tablet at step 19, I could determine the exposure at that point.