Metering for film vs. digital
Shooting film is a challenging process for many new photographers, especially those used to the instant feedback of digital cameras. Modern DSLRs are precision instruments that quite scientifically gather light. Film however is a bit more mysterious. The complete lack of feedback from film, plus multiple film stocks (which all respond differently to light) only adds to the complexity. Additionally, properly exposing color or black and white negative film requires a metering technique that is opposite of exposing a positive image. Permit me to explain.
Your camera’s built-in reflective light meter is very accurate. It evaluates the light reflecting from the entire scene with the goal of avoiding overexposure. This is fine for digital, where our goal is avoiding overexposure (blown-out highlights). Your matrix or evaluative meter is also an excellent tool for exposing transparency film. Again, like with pixels, the goal is to avoid overexposure.
Exposing for negative film
Properly exposing negative film requires a completely different approach. Whilst negative film has tremendous latitude, proper exposure is still important for consistent results. Rather than using a meter that evaluates the amount of light reflecting from the subject, we need a meter that can measure the brightness of the shadow area in our image. We want to allow enough light to pass through the aperture and shutter of our camera to properly expose for shadows, not the highlights. This allows film density to build and prevents underexposure. Failure to allow enough light to reach the film often results in muddy, underexposed negatives and poor quality prints.
The problem with reflected light meters
Light meters come in many forms, the two most important types being reflected and incident. The reflected light meter reads the amount of light bouncing off of your subject. They can be in-camera or handheld. These meters “see” very broadly or very narrowly. Terms like “center-weighted” or “spot meter” help you understand the pattern or angle your reflected meter accepts. However, the reflected handheld meter is similar to your in-camera meter, in that it has no idea what it is looking at, but bases its analysis on returning 18% or middle grey based exposure.
Therefore, the photographer must always interpret the reading given by a reflected meter based on what is actually in the picture. Is your subject a bride in front of a wedding cake on a table draped with white cloth? Or is your subject a coal worker, fresh out of a mine, against a dark brick wall? Neither of these subjects is average, and would easily fool an in-camera or reflected handheld meter, causing gross under or over exposure respectively.
The beauty of the incident light meter
The incident meter reads the amount of light falling upon the subject. Therefore they are much harder to fool. They may be outfitted with a half-dome spherical sensor or a flat disk. In practice, the dome is more useful for taking a general reading of three-dimensional subject matter, like a face. The flat disk is more useful for flat copy work or by more experienced users for analyzing lighting ratios. For live subject matter lit by ambient light, a single reading with the dome facing the camera will give an accurate reading. Due to the latitude of negative film, there is usually enough exposure with this general reading to develop good film density in shadow areas.
So, how do we use an incident light meter? From camera position, if the light falling upon you is the same as your subject, simply hold the meter in front of you with the dome facing the camera. This will yield an average 18% reading regardless of the subject. Is your subject backlit? Is your subject standing on a beach or on a ski slope? No problem. The dome of your meter is reading the amount of light falling upon your subject. Simply transfer the reading to your camera and shoot away. If your subject is in different light than your camera position, the incident meter requires you walk up to your subject and place the meter in the scene. Again, the meter’s dome faces the camera. Be careful not to shade the subject with your body to unduly influence the reading.
Rating film vs. box speed
Have you ever wondered what people are talking about when they say they “rate” their film? In a nutshell, they are using experience to compensate for various factors, often when using a reflected light meter. With a correctly used incident meter, this practice can largely be avoided. The film speed or ISO rating of your film is very scientifically determined by the manufacturer, so by exposing it correctly at its box speed, it should yield good, consistent results.
However, there are many factors that contribute to the density your exposure will yield on your negatives. Rating your film at a speed fractionally slower than box speed (setting your meter for 200 ISO, vs. box speed of 400) takes further advantage of the latitude of negative film and can therefore give a bit of a buffer to help reduce the chance of underexposed areas of a negative.
In the past, judging film exposure was fairly straight forward. When a contact sheet was made (example pictured at top), one could readily see whether the film density was consistent. In later years, commercial laboratories included back-printing on our paper proofs which included film density. This made it easy for photographers to judge the quality of the decisions we made at the time of exposure (albeit after the fact).
Today, most of us rely on commercial scanning to produce our positive film images. We often have no idea how much adjusting was done to create our scans. This can make it difficult to learn from our mistakes and create more satisfying film images. The image above (frame 10 of above contact sheet) was scanned recently on my Pakon F135 Plus. Interestingly this is from a shoot in 1978. Upon examination, we can see that enough exposure was given to this soldier to develop shadow density and result in very satisfying black tones.
If you have your negatives scanned and would like to check your exposure, you can check the density of your negatives by holding them to an even light source. The dense, dark negatives have seen greater exposure whereas the thin, almost transparent negatives have been underexposed. If across relatively similarly lit scenes you are seeing obvious variation in negative density, you might want to hone your metering technique to obtain more consistent exposure from one frame to the next.
My suggestion? Beg, borrow or steal (just kidding) a proper incident light meter. You should find your exposures nice and consistent, which will lead to an increased enjoyment for shooting film.