Exposure: Film and Digital

Exposure is the change caused by light when it strikes a photosensitive material. It has two factors—time (or duration of exposure) and intensity (the volume of light striking that material.) In both film and digital systems the time of exposure is indicated by the shutter speed, the amount of time that the gate through which light travels after it leaves the lens remains open. The volume of the light that gets through in that time period depends on the size of the diameter of the opening in the lens, known as the aperture number or f-stop. Exposure is like turning on a faucet and letting water flow through a pipe. If we leave the tap open for a longer period of time more water will flow through. If we enlarge the pipe more water can run through in that same period of time.

Exposure is calibrated with a system of stops, or the modern equivalent, EV (exposure value). If the total amount of light doubles or halves there is a change of one stop, or one exposure value. A 1 stop or 1 EV change can be made by halving or doubling the exposure time (shutter speed, for example changing from 1/15 to 1/30 second, or from 1/30 to 1/15 second). It can also be changed by opening or closing the lens diameter by one f-stop (for example, by going from f/11 to f/8 or from f/8 to f/11).

This system allows us to expose with the same amount of light and alter image effects by balancing the changes we make in aperture and shutter speed. If we use a faster shutter speed we can freeze motion; if we use a narrower lens opening we can increase depth of field. As long as we maintain a balance between the two (change the shutter speed in proportion to how we change the aperture—one going up one stop and the other down one stop) we maintain the same overall exposure. This setup is called “equivalent exposure” and is the basis for many creative techniques used in photography.

This exposure system holds true for both film and digital photographic systems, as do the image effects they create. As mentioned, the physics of photography do not change because we have gone from a film to a digital medium.

Digital SLR Photography

Your decision to work with a digital SLR (DSLR) might just change the way you make, process, store and print your images. True, both film and digital photography capture light and require processing steps to see that recording as an image. But digital imaging differs in that it begins as an electronic signal generated by light and ends up as binary code that describes color, brightness and tonality. It is in how you deal with those codes to create an image that makes digital so different.

At first, DSLR cameras can seem familiar. Placed side by side, film and digital cameras often resemble one another; in fact, most DSLRs are built inside the frame of a 35mm SLR body. There are also similar buttons, dials and controls. Terms such as shutter speed, aperture, autofocusing and exposure are all quite familiar to the film photographer. But it is in the behavior of the digital sensor where the two mediums diverge. This is especially true in the image record--the digital image file. It is so unlike the film record that it’s easy to become confused by familiar terms that mask the need for very different operating procedures.


Digital images require considerably more “housekeeping” than film. Both types of image record must be stored properly, be kept safe from damaging influences and must be archived in a way that makes them readily accessible. But you can hold a piece of film up to the light to see the image. Not so with the digital image file. It is “virtual” (mathematical, really) and requires a good deal of computation to be read. It cannot be seen without the aid of a computing device. It requires the intercession of yet more machines and mediums to be maintained, and is always tied to the “grid”—sometimes called the “digital imaging infrastructure.”

This grid is dynamic and can change over time. It can render some cameras and devices obsolete in short order. It can drop image-recording mediums entirely. This pace of change means that there has to be a fairly strong awareness of changes in the computer and imaging industry at large and how some modes of recording and reading systems will change. As you get more involved you’ll see that copying, backup and why an awareness of how recording and playback systems and formats may change over time is important.

The housekeeping extends to archiving, storage and, ultimately, to you taking in hand the actual processing of the image itself. This does not mean you have to do the math or complex programming—the camera’s on-board image processor or your computer handles that. But to get that image onto a print, or corrected or refined to meet your standards, often means your intercession will be necessary. It is this hands-on demand that can make digital so different for many photographers. Granted, this is where your creative process as a photographer comes into play, and where the real potential of this medium resides. But digital is anything but a “you push the button and we do the rest” process.

This is not meant as a cautionary tale to scare you off the digital path. It is presented in the hope that this will become an eyes wide-open endeavor for you.