View Full Version : Stouffer step wedge and film speed

20-Jan-2012, 04:51
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?

Ken Lee
20-Jan-2012, 06:46
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.

Larry Gebhardt
20-Jan-2012, 06:55
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.

Jay DeFehr
20-Jan-2012, 07:37

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.

Nathan Potter
20-Jan-2012, 08:59
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.

20-Jan-2012, 10:27
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.

Brian Ellis
20-Jan-2012, 10:51
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.

Jay DeFehr
20-Jan-2012, 11:59
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.

20-Jan-2012, 16:21
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]



Stephen Benskin
20-Jan-2012, 16:55
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.

20-Jan-2012, 17:14
:) Hi Stephen - step 19 reads 2.80 [zeroed on white light] but could you elaborate on your question and based on your obvious understanding of this topic I will be all the better able to understand this.

Stephen Benskin
20-Jan-2012, 18:32
Hi Steve,

It's a pretty straight forward process. Opacity is the antilog of Density and Transmittance is the reciprocal of Opacity. You have a density of 2.80, the transmittance is 1/10^density or 0.0016. Multiply transmittance by the illumination and you have the transmitted light value.

While we can't be certain of anything without everything being calibrated. I believe this may at least give you an general idea. The exposure meter wants to make everything 8/ISO. The value 8 is an exposure constant which I've occasionally written about. Most recently on APUG.org in a thread about the relationship between film speed and exposure meters.

For a 125 speed film, the exposure value at the metered exposure point is 8 / 125 = 0.064 lux sec. Multiply that by 2^5 for five stops and you have an exposure of 2.048 lux sec.

Multiply the transmittance value for 2.80 density into 2.048:

0.0016 * 2.048 = 0.0032 lxs.

The equation for black and white film speed is 0.8 / Hm. Hm being the value of the exposure at the point where the density of the film is at 0.10 over Fb+f. In this example that would be:

0.8 / 0.0032 = 250

This means that for the testing method you described, a 125 speed film will come out with a film speed of 250. The method produces speeds one stop off from the expected speed, but just about in line with Zone System tested EIs.

You can still use this methodology, but pick another aim density. For a 125 speed film, we want an exposure value of 0.8 / 125 = 0.0064. To find the density required, we just work backwards.

Log(1 / (Transmitted light / incident light)) or log( 1 / (0.0064 / 2.048)) = 2.50.

Let’s see if that works. 1/10^2.50 * 2.048 = 0.0065

Plug it into the film speed equation:

0.8 / .0065 = 123

20-Jan-2012, 19:21
As always Stephen I love the math you introduce. I confess to understanding very little of it but will try to apply it this afternoon with the testing I had in mind. Again thanks to all those that contributed.

Bill Burk
20-Jan-2012, 20:03
But if 0.0032 lxs hits a 125 speed film ... Then the film isn't going to develop up to 0.1 density ... So you won't "get" 250 speed.

But if I get what you're saying... Use Step 17 (2.50) instead because the meter set to 125 will end up putting 0.0065 lxs on the film under that step (which is the amount of light a 125 speed film needs to get to 0.1).


Stephen Benskin
20-Jan-2012, 20:17
But if 0.0032 lxs hits a 125 speed film ... Then the film isn't going to develop up to 0.1 density ... So you won't "get" 250 speed.

You're right I wasn't perfectly clear on that. I should have said that you would need to have a 250 speed film for that exposure to produce a 0.10 density. What will happen if using that exposure and the 2.80 is that you would have to open up a stop to produce the 0.10 density which would give you an EI of 64 (and that's in line with Zone System testing - the approximate one stop difference in speed).

21-Jan-2012, 08:04
Dear Steve,

There is a good article on Paul Wainwright's website named "Use your eyes" which describes a similar method. I use it a lot of time. 2 points with the step wedge : Paul have modified its step tablet by punching a hole in it to permit a circle of light to reach the film without any attenuation at all, and by placing a piece of light-proof black tape over a small area to completely block light from reaching the film.
This area on the negative will then be its “film base plus fog” reference.
This article is a good way to test your material but do not forget to use your eyes on actual prints !!!

21-Jan-2012, 15:36
Thanks philbed. A very interesting read.