Setting the correct exposure time value for filming
When filming with DSLRs the question raises, what exposure time should be selected. Closely related is the question of the correct automatic settings: Aperture priority (Tv, A), Shutter-priority (Av, S), full automatic (P) or fully manual setting (M)?
Unlike stated in some books and internet forums, there is a clear answer to both questions. And that is what I want to show and demonstrate below.
Except for special applications (such as slow or fast motion), it is usually desired for filming to obtain a natural looking image impression. The exposure time should therefore be selected so that it matches the visual sensitivity of the human eye as close as possible.
Hollywood movies are shot with 24 frames per second. What many people do not know: At a theater there are not only 24 frames per second displayed. This would lead to intolerable flicker and most people would have to leave the room after a short while fleeing with a severe headache. Rather, each captured image is displayed three times, each interrupted by a short dark period. Therefore, 72 virtual images are shown per second, which is perceived by our visual system as pleasant and largely free of flicker.
In the classic, analog television 50 or 60 frames per second are recorded and displayed. This has earned the media the infamous nicknamed "boob tube". Apparently, 50 or 60 frames per second are just at the limit of what the human eye can perceive. In later years, there were particularly good tube TVs, which doubled the frame rate to 100 frames per second. This removed visual flickering, because 100 Hz is above the threshold of human perception.
We note, therefore, our eyes can perceive movement up to about 50 or 60 frames per second.
And this is why the "right", natural looking exposure lies in this area, i.e., at some 1/50 to 1/60 sec. Shorter exposure times often appear choppy and unnatural. Already a 1/100 sec can make some critical scenes look noticeably ugly. Longer exposure times (eg, 1/30 s) also seem ugly, because of unnatural motion blur.
This rule applies regardless of the recorded scene: The 1/50s or 1/60s is equally suited for quiet landscape shots as for fast Formula 1 races or football games - our eye doesn't change its exposure time as well.
Occasionally, it is stated that natural exposure time for cinematic films were a 1/24s because movies are recorded at 24 frames per second. This is plain nonsense, because after exposure the film strip has to be transported to the next frame and this will require additional time. Actually, the exposure of cinema cameras is carried out with double the conveyor speed, which is 1/48 s. Obviously, this is not significantly different from the already recommended exposure time of 1/50s. Since a rotating disc is used for exposure, which is moved by the drive motor with 24 revolutions per second, the process is also referred to as a 180° exposure: The rotating disc is half black and half transparent.
Now we come to the second question, the appropriate camera setting. Since exposure time is to be held at 1/50s or 1/60s, Shutter-priority (Av, S) and fully automatic setting ( P ) are unsuited. Automatic aperture setting (A) is also less desired for filming when creative use of the shallow depth of field of a DSLR is intended. Thus, the only viable setting is the manual (M).
Set the camera to full manual (M). Fixe exposure time at 1/50 s or 1/60s (that applies to all frame rates like 24, 30, 50, 60 etc) and choose the aperture to taste and scene. You may want to engage automatic ISO to have at least some comfort. Noise performance when recording film is usually less critical compared to still photography (as resolution is lower).
Ps: Another rumor that remains stubbornly says you can not set shutter speeds longer than the frame rate. For example, you can not set 1/10 second exposure time at 50 frames per second. With traditional film cameras and DSLRs that will not be possible, indeed. In traditional video cameras, however, this is usually no problem. The function is called "slow shutter". It realizes a kind of moving average and actually exposes each frame individually (i.e., not 5 images combined). However, all movements are severely blurred. It all looks pretty funky and is suitable for special effects only.