Film speed test - use just 2 sheets of film - no densitometers
Film speed test using Ilford FP4 to establish your personal film exposure index and normal development time - no need for densitometers with this visual check using a graphic arts step-wedge tablet.
What I am going to explain below is a quick overview for one method of film speed development test and how to obtain your normal development time. It is particularly handy for 4x5 users as it minimises the amount of film used down to potentially two sheets to give you the necessary information.
A more in depth reference to this film speed test and where to obtain graphic arts step tablets are listed at the bottom of this page.
Step 1 Load step tablet and unexposed 4x5 sheet into a film holder
This film speed test uses a Stouffer step wedge as your calibration tool. Modify your step tablet by placing an opaque paper dot sticker above step 21 on the tablet. This will prevent any light being received by the film under the dot, giving you a clear film base reference point later.
The 21 step tablet or step wedge is a calibration device traditionally used in graphic arts applications. It has been carefully processed under laboratory conditions to give 21 steps of grey in 0.5 stop densities. Using a step tablet is one way to avoid the need for densitometers.
In a darkroom or film changing bag load an unexposed sheet of film overlayed with the Stouffer step tablet. The two are sandwiched together and carefully pushed into place within the sheet film holder. Due to the combined thickness of two sheets this can be a little difficult but can be achieved with patience and care. Treat the test tablet with the same care as film.
Step 2 Expose the film with step tablet with a Zone 10 exposure
A Zone 10 exposure is 5 stops more exposure than what your meter is indicating. Observe the following points:
- Use a white card as your exposure subject and fill the viewfinder completely. If you use a darker object the exposure time required will be longer, hence use white.
- Choose a normal focal length lens - wide lenses suffer from light fall off at the frame edges and can affect the test
- Have your lens focused at infinity. The test object will not be in focus and focus is not desirable for this test.
- Choose a day with consistent light, cloudless, south facing is best if in the southern hemisphere.
Step 3 Develop your test film in the developer of your choice
My current developer is Ilford LC-29. You should choose for your test the developer that you prefer or want to use with your film. Prepare it as you would normally would and be consistent with your film processing procedure so you can repeat the results.
For my processing set up, Ilford HP5 takes about 11 minutes to develop normally. Looking at the Iford tables for FP4 suggests that it develops in slightly less time that HP5. So I cut my time from 11minutes down to 10 minutes. Why? 10 is an easy number to work with. If I need to reduce or increase my development times further I will usually adjust by about 10% increments (1 minute) or 15% (90 seconds) or 20% (2 minutes).
Step 4 Analyse your results to get your personal film speed index
The first piece of information in a film speed test is at what ISO speed should I be exposing brand X film with my camera gear and development procedure.
This is where placing the opaque sticky dot near the 21st step is so helpful, circled in red. It gives you a clear film base reference where no exposure has been received. Step 21 on the wedge is also Zone 0. If it is the same density as the red circled clear film base then the film is not overexposed. If step 21 is equal to the circled area and there is consistent tonal separation between steps 20 to 11, then your exposure index is within the ball park. I exposed this test film at 64ISO and I am pretty happy with the tonal placement.
If steps 21, 20, 19 had been clear with no tonal separation between them, then I know the film was underexposed. I would need to reduce my ISO number by about half a stop and try the test again.
Conversely, if steps 21, 20 and 19 had definite grey tones I would need to decrease my exposure by increasing my ISO rating for the film.
Step 5 Proper proof time
From the previous step I am confident that an ISO of 64 is close enough for my camera and developer combination. Now I need to find out what is the normal development time.
A second sheet of FP4 which remained unexposed was also developed at the same time as the step wedge test for 10 minutes.
For all contact printing I have a standard enlarger head height I use in combination with one lens, f stop and neg carrier. My enlarger filtration is set for grade 2 normal contrast paper. I focus the light onto my enlarger base and then make tests strips from the contact print of the developed but unexposed film.
What I am looking for is the first almost black tone which shows very little discernible tonal difference to the next strip after it. This is in the region of the minimum print exposure required to print through the unexposed film to yield a black print value.
In my case above, I could see clear differences between the dark greys at exposures of 8.6 and 10.4 seconds. At 12.4 seconds I could see no appreciable change in blacks between 12.4 and 15.0 seconds. So I choose the 12.4 seconds as the minimum time to achieve close to maximum black.
A note of caution, this is a visual test and it is easy to be overly enthusiastic about achieving maximum black thereby overexposing when establishing your minimum time for close to maximum black.
Here we are working at an extreme end of the paper sensitivity where small changes in exposure can give large changes in density, so take care not to overdo it.
Remember that you want the exposure time just before there is no real appreciable difference in the black with next exposure time which follows.
Step 6 Establishing normal development - printing the step wedge.
This is the final stage, establishing what is a normal development time for your film, camera and developer combination. We need to make a contact print of the actual Zone 10 film exposed with the image of test tablet. Using the exposure determined above for the minimum time for maximum black (in my case 12.4 seconds) I contact print the negative onto the photographic paper. The result is seen below.
In the image above I have shown in red letters the various print value zones and their respective step tablet numbers. Remember each step is 0.5 of a stop. Step 1 on the tablet is equivalent to a Zone 10 print value, Step 2 is half a stop lower at Zone 9.5 and Step 3 is Zone 9.0 respectively.
I like to have a good separation of tones from Zone 2 print value (Step 17) through to about Zone 8 print value (Step 5). After Step 5 - Zone print values 8.5 to 10, the scale remains paper base white. This is a normal contrast range. If the light grey scale ended earlier at say Step 7, this would indicate the film has been developed with higher contrast than normal of about one stop. This is referred to N+1 development. If the grey tones extended all the way down to Step 3, or Zone 9 print value, then this would indicate the film contrast is softer than normal. This is represented as N-1 development.
In this case above 10 minutes development has produced results which show normal negative contrast.
Film speed test Ilford FP4 in LC-29 developer conclusion
Using just 2 sheets of Ilford FP4 film I was able to determine that my personal exposure index is 64 ISO. Normal development is achieved (for me) at 10 minutes using my usual dilution and agitation methods consistent with my film processing procedures.
I have only used my results here as an example of the process. Your results may vary significantly and that's to be expected.
Having conducted my film speed test for my personal exposure index and normal development I can now go about photographing with my new stock of Ilford FP4. This gives me a genuine basis to compare results of my images with other films I have used and to observe characteristics particular to this film and developer combination.
Conclusion: Film speed tests are necessary to understand how to best manipulate your creative materials - your photographic film and paper.
References Calibration and Transmission Step Wedges
Film Speed Test References.
Paul Wainwright has written a nice little pdf which you can download which goes into greater detail the technical details behind this visual test. Go to this page, scroll to the bottom third of the page “Use Your Eyes, Zone System Testing Without a Densitometer” and download the pdf off the link.
Visit Stouffer Graphic Arts for details regarding transmission step wedges and photographic scales, the tools I rely on to avoid the need for using laboratory equipment like densitometers, often referred to in the Zone System film exposure and development method. Keep your references clean, handle with care (like negatives) and store carefully, these will last you for years and well worth having.