Originally posted by skierd Other than the “it’s nice to know” part. if the exposure comes out the way you expect it why does this matter? Just curious. It doesn’t particularly bother me if shutter speeds are really a nominal speed than actual speed so long as the steps between are repeatable and consistent so I know what I’m getting in to when I take photos.
For 99% of images, you are right. But there are at least a three scenarios where it does matter:
1. Interval shooting: Intervalometers (including the ones built into the K-1) work by "pressing the shutter button" on a schedule of timed intervals. If you set the intervalometer to 30 seconds, then every 30 seconds it will try to take a picture. Now if the shutter speed is set to "30" but the exposure time is really 32 seconds long, then the next intervalometer button press will happen during the previous exposure and do nothing. Instead of getting one shot every 30 seconds, you'll get a shot, then a 28 second delay, then another shot, then a delay , etc. and half the number of shots you expected. Successful use of interval shooting depends on knowing the true exposure time and this work shows that the true exposure time (which is mathematically calculated for accuracy) is different than the dial time.
2. Interactions with artificial lights: Many types of artificial lights (fluorescent, CFL, LED, etc.) flicker stroboscopically in time with the power line or some internal clock. Often this flicker frequency is exactly twice the line frequency (e.g., 2*60 Hz = 120 flickers per second in the US). If you set the shutter speed to "1/60" you'd expect to always get an exposure illuminated by exactly 2 flickers of light. But we now know that the camera is actually shooting at 1/64 so it's not getting two full flickers. Because the intensity of the light source and color of the light source varies during the flicker cycle, shooting at 1/64 in a place lit by a 120 flicker/second light source may have weird color and image brightness variations between the shots.
3. Measurement: In technical photography, you can estimate the velocities of objects from the length of the blur they create in the image. But to get an accurate velocity estimate, you need to know the true exposure time which might be up to 13% different than the dial time.
Besides, it's a interesting foray into science and engineering to try to figure out exactly what the camera is doing. It's fun exercise for the little grey cells to figure a way to accurately measure the true exposure time of the camera as AstroDave has done.