Even in theory there are differences in the way digital sensors and films record light that makes ISO values only approximate. But these differences are usually fairly subtle and theoretically exposure should be more or less equal if you use the same ISO, aperture, and shutter time. For more about this, please see: Why are these film photos brighter than digital photos taken at the same time with the same settings?

In practice there are even greater differences that may affect each of these basic components of exposure.

ISO: Since digital sensors have a linear response to varying brightness levels of light and film has a more logarithmic response, comparing an ISO value for a particular digital sensor and an ISO value of a particular film is only approximate. This value is usually closest in the mid-tones but will vary more in the highlights and shadows.

Keep in mind that not all films with a specific ISO/ASA rating have the exact same response curves. Some may have deeper blacks and brighter highlights for higher contrast while others may have lighter shadows and more restrained highlights for less overall contrast. These curves can also be manipulated by modifying exposure times and then compensating by altering the developing time. That's basically what Ansel Adams' 'Zone System' was about.

Compound that with digital cameras that actually use different ISO values internally than they are labeled in the settings. They usually do this specifically to preserve highlight detail in the raw image data collected.

So digital cameras tend to have their actual ISO sensitivity for a particular setting rounded up. On the other hand, film manufacturers tend to round the sensitivity of their films down to the next nearest "standard" value.

With exposures for film longer than about 1 second the Schwarzschild effect, sometimes referred to as reciprocity failure, must be taken into account. The sensitivity of films at longer exposure times is not linear. This must usually be taken into account when exposing film for longer than one second. This can very significantly impact exposure times, and it varies by the specific film in question. The manufacturer of your film should be able to provide information regarding how much compensation is needed for longer exposures. Many film manufacturers include such information on publicly available data sheets.

Aperture (Av): Different lenses labeled with the same aperture value may not be equally bright. This is partly due to differences in transmission loss through the various elements of each lens. But at maximum aperture it is also due to the values of each lens being rounded to the nearest or (usually) next wider standard f-number.

The differences due to transmission loss are carried across the entire range of aperture settings. The differences between stated and actual aperture when wide open tend to be reflected in successive apertures settings as well in order to preserve the differences in stops between the maximum aperture setting and the others. Sometimes the further one moves from the maximum aperture the more "honest" the actual f-number is with regard to the actual diameter of the entrance pupil relative to the lens' focal length. By the way, focal lengths are also approximated and rounded to the nearest "standard" number in the most favorable direction!

Here are the actual transmission measurements for three different Canon "L" lenses with an "f/4" maximum aperture. Even when using each of the respective lenses on the same camera, the exposure values would need to be adjusted slightly to give the same brightness of exposure.

The EF 24-70mm f/4 is essentially an "honest" f/4 lens throughout its zoom range. The EF 17-40mm f/4 is one-third stop slower at about f/4.4 and the EF 24-105mm f/4 is two-thirds stops slower at around f/5.1.

Shutter Time (Tv): Like the other two basic components of exposure, shutter times are only approximate. Even the numbers we assign to them are rounded to easy to use values.

Of ISO, Av, and Tv, the latter is usually most consistent across digital and film platforms if the camera has an electrically controlled physical shutter or a purely electronic shutter. If the film camera has a mechanically controlled focal plane shutter or iris shutter, all bets are off as to which exposure components (ISO, Av, Tv) will be most and least accurate.

Beyond the differences between various films or digital sensors, there's also the difference between how various built-in meters work in different camera models.

Most modern cameras, film and digital, have more than one metering "mode" that affects how the information collected by the light meter is weighted. The same light meter in a digital camera such as my Canon EOS 5D Mark III will read differently based on the scene contents and the metering mode selected. This is particularly the case if there is a wide variation in brightness between different areas in the scene.

• 'Spot Metering' measures only about 1-3% of the entire fame, usually in the center.
• 'Partial Metering' does the same thing with a slightly larger area, usually about 10-15% of the total frame.
• Center-weighted Averaging' gives more weight to areas near the middle of the frame and less weight to areas near the edges and in the corners.
• 'Evaluative Metering' will compare the information provided by the various zones of the light meter to an internal database and then base the calculated meter reading on which profile in the database most closely matches the current scene.

If the camera is pointed at a scene that is dark on the borders and very bright in the center, the meter reading will change based on the metering mode. Spot and Partial metering modes will ignore the periphery and meter only for the bright area in the center. Center weighted averaging will give more weight to the center, but will also be influenced by the darker edges, resulting in a meter reading that will recommend a brighter exposure. Evaluative metering will try to determine why there is such a wide difference in various parts of the frame and recommend exposure based on what kind of scenario it thinks the camera is pointed at.

Consider this example: You're shooting a landscape underneath a bright sky using Evaluative metering. Nikon calls it 'Matrix Metering'. If the scene is framed so that it is very bright in the upper 1/3 and darker in the lower 2/3 of the frame the camera will probably assume the photographer wants the lower 2/3 of the frame to be properly exposed and will allow the upper 1/3 to be overexposed. On the other hand, if the scene is framed so that the upper 2/3 is very bright and the lower 1/3 is darker, the camera might assume you want the sky to be properly exposed, leaving the lower 1/3 of the frame underexposed.