Gamma in a digital file is exactly like gamma in film. Technically, gamma describes a specialized change in transition of tone as a deviation from a gamma of 1.0 (linear), which you will recognize in Photoshop’s Image>Adjust>Curves dialog box when it first opens. Note that our curve does not default to a straight line every time you attempt to apply a curve function; we remember the last curve application and display it every time (as a labor saving device for high volume workflows). Very handy under carefully controlled light.
A gamma of 1.0 displays like the illustration to the left where the straight line (from corner to corner) describes a linear transition to tone; there is no difference in the width (separation) of any individual tone as compared to any other tone between highlight and shadow. Gamma values in excess of 1.0 are displayed to the left as the curved lines running from corner to corner; higher gamma values have larger bends. Gamma has a characteristic where there is an increase in contrast from the shadow to the apex of the bend, and a decrease in contrast from the apex of the bend up to the highlight. This is an important characteristic of gamma, it gains contrast in the shadows and it gives that contrast away in the highlights.
Let’s consider the ramifications of the contrast characteristics of gamma. Our eyes see very small nuances in low values only when the overall illumination is low, like on a moonless night, but we can’t see lots of low values at first. It takes 90 minutes to acclimatize our eyesight to very low light levels, but we can see relatively well after we adjust. That same ability to discriminate small differences in reflectance do not exist on a bright, sunlit day because our pupils have “stopped down” to the point where small differences are essentially below threshold, which is why we cannot see extremely low valued transition in bright light unless the contrast of those low values is greatly expanded. That is why in general, we want to add contrast to areas of the reproduction that are low valued reflectances.
In addition to our eyes' inability to distinguish subtle separation in low values under reasonably bright illumination, our ability to visually detect transition of tone in low valued reflectances is further hampered by the printing press’ inability to render single percentage point separation starting at 100% on down to 94% and this inability progresses farther toward the highlight end of the scale as the paper quality diminishes. This inability to render what our eyes have difficulty discerning in reasonable illumination further recommends a contrast increase in the shadows. Rather than transition at: 100%-99%-98%-97%-96%, a low valued transition might better read: 100%-97%-95%-93%-92%. Normal eyesight would have a better chance of seeing shadow separation in the second progression. Hence, the gamma’s characteristic gain in low valued separation falls conveniently to hand as a valuable tool in the shadows.
High values generally do not render well with large transition gaps. Because the threshold reflectance on the paper target is white, we like to see smooth transitions, single digit gains render best. In areas of high reflectance (especially when viewing 2.0 D and less) our eyes easily perceive small nuances in reflectance. Larger gaps in accumulating density are perceived as holes, overly abrupt and therefore, generally undesirable. Because gamma’s characteristic smoothening of transition into the highlight becomes more pronounced as the value increases, it becomes sort of a self adjusting tool at both ends of the transition from highlight to shadow.
Now that you know how gamma works, let’s explore how to use it with the T2 software. General gamma values should fall between 1.8 and 2.5. Gamma has been used successfully as high as 2.7 but noise generation is distinctly possible at such an elevated value. Try the 2.7 so you’ll know what you can get away with. Gamma cannot be generalized into a value that you always use any more than generalizing lighting that you always use; lighting changes in order to optimize the shot. A good starting gamma value is 2.2.
Let’s shoot a test to help you understand how gamma can be used to advantage when shooting images. Make a capture of a scene that contains low valued reflectances that you’d like to separate. Black cordura is an excellent choice for this test. Shoot a low valued area and place the reflectance at 90% (zone 1). Evaluate the transition of tone in the lowest area of the reflectance you’ve chosen to test. Using the sample size of 1, zoom in until you can see the image start to pixellize. Write down the highest value on the cordura weave and the lowest value on the cordura weave.
Shoot another capture of the scene with a gamma of 2.7. The first thing you’ll notice is how bright (and seemingly flat) the scene renders. Most observers will assume that gamma flattens and brightens the overall contrast of a scene, which it does not. Make several Meter captures, dropping the aperture until you get the original cordura reflectance back to 90%. Evaluate the local contrast in the same manner as the first time you sampled values. Note that the local contrast has increased, but the f-stop had to move in order to bring the original reflectance back down so you could see it. Pushing the gamma will almost always require an f-stop change because gamma changes the exposure index. In shooting film, changing the development changes the gamma and as a result, the exposure index.
Gamma values will in general change according to the light you’re using. If for instance, you’re shooting very full light, such as a large light box on top and two medium boxes for fill, close to the set, you’ll usually want a gamma in the neighborhood of 1.8. If you’re shooting furniture sets and you don’t have enough light you’ll end up closer to 2.7. You may want to change the gamma when you change your lighting style. Evaluate the very lowest transitions and make gamma changes accordingly.
The T2 Capture Curve can provide the same effect as gamma. The Capture Curve works in addition to gamma so you can choose to use a lower gamma and a more aggressive Capture Curve to get the same result as a less aggressive Capture Curve and a more aggressive gamma. Be aware of making gamma and Capture Curve adjustments that are in conflict with one another. Because it’s inefficient, you don’t want to use a huge gamma because you have an insufficient Capture Curve for the scene you’re shooting. Balance is the key, use a gamma that allows sensible Capture Curves. Remember, when low valued noise becomes objectionable, look to the Capture Curve and the gamma value after you’ve ruled out temperature related noise. Also, do not push up gamma and/or Capture Curves because you don’t feel like lighting the scene properly.