In Chapter 4 it was noted that there are two ways to evaluate exposure. One way to measure exposure is to take account of the amount of light that the camera will allow to reach the sensor—that is, the size of the aperture and the time during which the aperture is open. This is analogous to the size of the opening of the can in the shower and the time during which there is no cover over it. This measure does not explicitly take into account the sensi- tivity of the sensor in the camera or the brightness of the scene.
The other way to measure exposure is in terms of the sensitivity of the sensor and the brightness of the scene. In this situation, we want the range of scene brightness to be such that, for the sensitivity of the sensor, a suit- able image will be obtained. This means that the range of brightness levels of the important key objects in the scene fall on the monotonic range of sen- sitivity of the sensor, meaning, within its dynamic range. Failure to accom- plish this was described in the theater thought experiment. When setting the exposure parameters of the camera, we are ensuring that the range of bright- ness levels in the scene, as modifi ed by the camera’s aperture and shutter time, is such that it is consistent with the dynamic range of the sensor.
In Chapter 4 it was shown that this occurs when the exposure value (EV a ) based on the amount of light being allowed into the camera is equal to the EV b we obtain when considering the sensitivity of the sensor (ISO) and the brightness of the light available. The two determinations of EV are:
2 2 EVa f s ⫽ / 2EVb ⫽0.0934⫻ISO⫻cd m/ 2 Then, setting them equal to each other gives us:
f s ISO cd m / / 2 2 0.0934 ⫽ ⫻ ⫻ which can be rewritten as,
cd m f
s ISO
/ 2 ⫽10.71⫻ /2
⫻
which we will call the exposure setting equation.
To understand the exposure value derived from the scene brightness and the sensor sensitivity, EV b , we need to refer back to the thought experiment. Areas of the scene that are very dark will not be rendered by the sensor if they do not present enough light to the sensor at its low-threshold sensitiv- ity point. The scene brightness is indicated by the term on the left side of the equation. If there is not enough light, additional light must be found, either by studio lights or a fl ash attachment of some kind.
The right side of the equation deals with the question of how much light is enough. If the f/stop is set to its lowest value (as open as it can get), the shutter speed is set to its longest open time, and the ISO (on a digital camera) is set to its highest value, and the equation still does not balance, there is not enough light. Consequently the same thinking can be applied in reverse if the light is too bright.
At the same time, the bright portions of the scene should not present light levels that are beyond the saturation point of the sensor, and the dark levels cannot be below the threshold level of the sensor. In other words, the scene’s range of brightness levels should be such that they fi t within the dynamic range of the sensor. If the scene’s brightness range is less than the dynamic range of the sensor, there is no problem. Adjustments can be made when the image is processed. If it is greater, there is a problem, since certain scene information will never be captured in the fi rst place, so no degree of postprocessing of a single image can compensate.
It is best to reconsider the situation of taking a wedding photo of a bride and groom. She is wearing a white-on-white dress that is extremely important to her, and he is wearing a black-on-black tuxedo of which he is very proud. You need to take a photo that will make them both happy. Now, if the scene’s range of brightness is greater than the dynamic range of the sensor, the photographer will have to make a choice from the following short list:
1 Lose detail in the dark areas in order to preserve detail in the bright areas (tuxedo is all black but the dress detail is preserved so she is happy and he is not).
2 Lose detail in the bright areas in order to preserve detail in the dark areas (dress is all white but the black velvet lapels are distinct from the jacket body so he is happy but she is not).
3 Compromise and lose a little of both the bright and dark areas (probably neither is happy).
4 Take two shots, one per the fi rst scenario and the other per the second and merge the two at a lowered overall contrast ratio. (Will they stand absolutely still for two consecutive shots?)
There are not very many proper solutions. The professional wedding photographer generally will use a “ portrait ” fi lm that has an extended dynamic range, or if he or she is using a digital camera (digital cameras have signifi - cantly less dynamic range than you can get with portrait fi lm in a fi lm camera), will position additional (fi ll) lights shining on the groom. This will make the groom lighter compared to the bride, and thereby decrease the scene’s range of brightness levels.
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Whatever the practical solution, the idea is to line up the scene’s bright- ness range with the sensor’s dynamic range. This is done by adjusting the aperture, exposure time, and ISO (after the additional fi ll light is brought in). Many modern cameras will make the adjustments for you if you use the auto- exposure mode. Unfortunately, cameras are not sensitive to the feelings of the bride and groom, so some degree of manual intervention may be needed.