Personal Film Speed

Having just taken on a new enlarger (De Vere 504) and a new film developer as standard, it’s a good ooportunity to re-calibrate my photographic workflow. This post describes how I went about this, starting with personal film speed.

Introduction

It’s only in the last two years or so that I have tried to exert more control over my darkroom process. Film, paper and chemicals are getting more expensive, so standardisation and calibration not only increase consistency but also reduce the cost per picture.

This post is about finding a personal film speed. In future posts I will cover testing for film developer time (varying the contrast), finding a proper proof exposure time and standardising f-stop test strips and split-grade printing matrices.

What is a personal film speed? Films are rated by manufacturers for their sensitivity to light. These ratings are known as box-speeds. For example Ilford HP5+ is rated at 400 ASA, a medium speed film. These ratings are measured by manufacturers using specialised kit that tries to emulate typical camera kits. However there is no such thing as a typical camera kit. So, for accurate calibration, we need to do our own sensitivity tests, known as personal speed tests. The manufacturers’ box speeds are just good approximations.

Many film photographers do not take the trouble to find personal film speeds.

That may understandable if workflows involve scanning negatives and then processing images with software. Knowing your personal film speed is probably less important in such situations. Within limits, reasonable results can be obtained from fairly poor negatives.

The same is not true for the darkroom worker. All things start from having a good negative. So we try hard to ensure that negatives are as good as they can be.

Finding a personal film speed is essentially calibrating the in-camera or stand-alone light meter to the film speed ¹.

There are many ways of finding a personal film speed ². It doesn’t actually matter which method you use as long as the logic is easy to follow. A personal film speed only holds for a particular combination of kit and process, so it’s best to test speed with the most commonly used combinations ³.

Here is what I did to check on personal speed:

Exposure

I chose an overcast day to avoid rapidly changing light conditions. I set up my camera on a tripod in front of a 18% grey card on a north facing wall, being careful not to project any shadows onto the grey card. The set up ensured that the grey card filled up the image of each frame. (It’s not necessary to use a grey card, but as I have one, I thought I would try to match the grey card with the greyness of the eventual print as an additional step in the exercise – see below, but this is optional).

I set the lightmeter to half-stop increments to match my lens’s aperture gradations. I then focussed the lens to infinity and spot-metered the grey card from the position of the lens. I set the lightmeter to box speed for HP5+ (= ASA 400). I then took the following images of the grey card:

1. Blank, in case this frame was damaged by reel loading
2. Blank, in case this frame was damaged by reel loading
3. f16, 1/1000 second, lens cap on, hand over lens cap
4. f16, 1/1000 second, lens cap on, hand over lens cap
5. 4 stops down from metered value
6. 1/2 stop open
7. 1/2 stop open (giving 1 stop up from 4 stops down)
8. 1/2 stop open
9. 1/2 stop open (giving 2 stops up from 4 stops down)
10. f16, 1/1000 second, lens cap on, hand over lens cap
11. f16, 1/1000 second, lens cap on, hand over lens cap
12. Metered value at box speed
13. Metered value less 1/2 stop
14. Metered value less 1 stop
15. Metered value plus 1/2 stop
16. Metered value plus 1 stop
17. Rest of film – pictures taken across the remainder of the day

On the same day, I developed the film in Rodinal 1:50 at 20 C. for 11 minutes (the normal development time for 35mm HP5+ rated at ISO 400 at 20 C.).

Theory

Remember the adage:

Expose for the shadows, develop for the highlights

The photographer is normally trying to record a full range of a scene’s tones on film so that shadows are not blocked up and highlights are not blown to white. Film can only capture a limited range, so if a scene’s actual tonal range is greater than that which can be captured on film ( a common scenario), then the photographer needs to decide which end of the tonal range is important or whether to compromise between dark tones and highlight values. This means that the photographer needs to be able to reliably place a a tonal value in the scene to the tonal scale on the print paper, via the negative. This requires knowing how the film behaves with a combination of kit and process.

In general, the darker tones of a scene are captured through adequate exposure (aperture and shutter speed settings), whilst the lighter parts of a scene are controlled through manipulating the film development time.

So in terms of this test, the idea here is as follows:

1. For frame 5, (4 stops down), when we develop the film and print from the negative, we are looking for a dry print that is just discernibly different from complete black. We use frames 3 or 4 to work out the enlarger exposure time to give complete black. Frames 6 to 9 enable us to judge film speed if the test for frame 5 does not give us a result just a whisker off black.
2. For frame 12, we are looking for a dry print whose tonal value matches an 18% grey card.
3. We will worry about the lighter tonal values when we come to test the film development times. That’s a different test.

The point about the second check ( 18% grey), is that the complete black of (1) is difficult to judge without an expensive densitometer. Our eyes adapt to register as black the blackest end of a range of blacks, when in fact that black might just be shy of complete black. Frame 12 provides a sense check as a 18% grey card gives us a reference point. It will also prove helpful at a later stage when I come to recalibrating my proper proof exposure time.

Enlarger Settings and Print Development

The first step is to print off an exposure value scale using the normal paper used for printing. The picture below shows 3 second exposure steps under my 504 enlarger set at a specific height with the lens at f8. I have placed a 18% grey card and maximum black card against the exposure scale for comparison. The purpose here is to find the minimum time it takes for the paper to record maximum black.

From this I estimated that maximum black was registered after 9 separate exposures of 3 seconds each. I say ‘estimated’ because this is not easy to judge. My wife thought maximum black was achieved after 8 separate exposures.

Next I exposed each of the frames of interest using the same enlarger settings for 9 separate exposures of 3 seconds each. Note, this is not the same as one exposure of 27 seconds, because you have to allow for the warming and cooling time of the enlarger bulb.

I then dried the prints using a hair-dryer and compared them to the original exposure scale and the black and grey cards.

The results are shown in the picture below:

The frame that recorded a tone just a whisker short of black was frame 7, one stop more open than frame 5. This suggests that my personal speed is one stop slower than box speed i.e. c. 200 rather than the box speed of 400. That is, to get the kit to record a whisker short of black required the exposure to be greater by one stop, that is the lightmeter needs to be set on a lower ISO.

However, it is difficult to judge, partly because the prints have a slight ‘colour’ when compared to the true black card. However, comparing the frame that most matched the 18% grey card corroborated this result.

Conclusion

For my walkabout kit and darkroom process, my film speed for HP5+ is somewhere between ISO 200 and 400, but closer to 200 than 400. I would be happy to use 200.