Film Processing and Equipment Main Page
Film tests can vary according to your particular equipment and technique. The method I used incorporates densitometry rather then actual printing tests simply because I happened to have a couple densitometers idling in my lab. Printing tests are the better method since it incorporates your enlarger as a variable.
Notes on Densitometry and staining developers: When using emulsion staining chemicals such as PMK, Pyrocat, and even some Metol developers a color densitometer must be used if printing densities are to be measured accurately. The chart below shows how staining developer density compare under different color channels. Non-staining developers wouldn't have this separation and all color channels should be similar as in the second image below.
Film Test: General Method
1) I generally perform film test during my normal shooting activities so lenses, targets, and lighting conditions will tend to vary. I generally use a smooth flat gray painted column located near my apartment. Focus on infinity I keep the lens 5 or 6 inches from the surface and find a shutter/aperture combination which will read "correct" (Zone V) on my camera's light meter and allows me to adjust at least 3 stops in each direction. In other words I attempt to hit Zone V on f/5.6 which allows me to step down to Zone II and up to Zone VIII using the same shutter speed. See Chart below. I also find it useful to take a shot without any exposure to see what Zone 0 looks like. This measurement will represent the film base plus fog density for the film. Of course this is not always possible on sunny or cloudy days, so one set may be at 1/500 and another set be at 1/125. It is also useful to check the linearity of the camera: shoot Zone V at different f-stop/shutter combinations, you may discover not all shutter combinations are equal. Variation may be attributed to both aperture and/or shutter. I found some combinations to be up to 20% different from others. The point is there will be some degree of error. Some error causes poor accuracy, some error causes poor precision. By using the same camera, lens, film, and developer for a while it is easier to move from the not accurate not precise to the accurate not precise for 35mm. If you want to get to the accurate/precise area then large format is the way to go because development has to vary for each negative which is not possible for roll film.
2) On Example Chart I the below the "correct" meter reading represents Zone V for that shutter/aperture combination. The measured density is indicated.
3) I develop the film normally and measure the densities of each zone. I measure the density emulsion side facing the light source of the densitometer. Some suggest that the direction of the film when measuring is meaningless as long as your consistent. I noticed there is a slight density increase when measuring emulsion side towards the densitometer light source, which is below. This may be a result of light scatter by the plastic base. Subtracting out the base density plus chemical fog (bd+f) will leave you with density numbers which will correlate with the printable range of paper.
A measured density of 0.1 for Zone I is generally considered the speed point of most films, which means the minimum amount of incident light hitting the film which causes a printable density. Most film characteristic curves show this value in LogE in Lux-seconds, but I prefer to use relative log exposure which is the density that correlated to the Zone of interest. Each increase stop should increase the film density by a value of 0.3 density units.
Referring to the chart, the first test roll was developed for 15 minutes. The Zone I density was 0.02 meaning there was 0.02 density units greater then film base and fog, or pretty much no recorded density above film base and chemical fog (FB+f). Zone II measured 1.0 which is where I want Zone I to be. The conclusion is that there needs to be more overall exposure. The easiest way to do this is to decrease the ASA of the film from 400 to 200 which correlates with one stop or 0.3 density units. The next test roll was shot at ASA 200. The Zone I density value is now 1.0 exactly what should be expected. This indicates I loosing one stop of effective film speed.
Now that I know I should be at ASA 200 I need to evaluate how the highlights are developing. If I look at the first test the Zone VIII is 0.91 which is about one stop too low. The next test at ASA 200 the Zone VIII is 1.04 which is only a half stop increase. Even know I saw a one stop increase in the shadow density there was only a half stop increase in the highlights. The FX2 is compensating the highlight development meaning it progresses slower then shadow development. This is also typical for developers when they are highly diluted or weak. This indicates that I need more development time for a normal density range typical for a grade 2 paper. The third film test I developed for 30 minutes. The shadow densities remain consistent which is a good characteristic for a developer and the highlight densities increase almost one full stop, pretty much where I want them to be for an N development time. When graphed the shape of the curve is an inverted "S" which will have a lower contrast through the mid-tones compared to a normal "S" shape which shows higher contrast in the mid-tones. This milder midtone gradation is considered more picturesque. Other developers can give high contrast through the mid-tones, HC110 for example.
Example Chart I
Film Test: TriX in FX2 (actual results)
Density Measurement using Macbeth TD-502Bd+f = 0.30
|Relative log exposure||0||0.3||0.6||0.9||1.2||1.5||1.8||2.1|
|ASA 400 15 minutes||0.02||0.10||0.47||0.91|
|ASA 200 15 minutes||1.0||0.24||.66||1.04||N-1|
|ASA 200 30 minutes||1.0||.25||.74||1.24||N|