Motion pictures on film have long been shot at 24 fps, so the 24 fps video rate gives what many consider "film-like" motion. The faster 60 fps rate produces a more "real" feel to some viewers. The standard 30 fps rate works well for general use. Experiment with the different rates your HDSLR provides and see which you prefer.
With some early HDSLRs, the frame rates really were 24 fps, 30 fps and 60 fps. But the "real" NTSC frame rates are 23.976, 29.97 and 59.94, and most of today’s DSLRs shoot at those rates rather than 24, 30 and 60 fps. (Your DSLR’s instruction manual will tell you which it uses; from a shooting standpoint, there’s no difference between 23.976 and 24 fps.) Where did those odd numbers come from? In the days of black-and-white TV, the broadcast frame rate was 30 fps, half the 60 Hz rate of U.S. AC power. When color TV came along, there was interference between the color signal and the sound, so the powers that be decided to reduce the rate to 30/1.001 = 29.97 fps to eliminate that problem. The 23.976 rate is 24/1.001, and 59.94 is 60/1.001.
Note that you don’t necessarily have to play back videos at the same frame rate at which they were captured. If you shoot at 60 fps and play back at 24, you’ll get a slow-motion effect. Conversely, if you shoot at 24 fps and play back at 60, you’ll get a speeded-up effect. Some compact digital cameras can shoot lower-resolution video at 300 fps, even 1200 fps, which is good for special effects when high resolution isn’t required.
Progressive Vs. Interlaced
You’ll see a lower-case letter "p" or "i" following the resolution or frame rate in video specs: 1920x1080p 24 or 1920×1080/24p. The "p" means progressive; each video "frame" is drawn line by line from top to bottom at the specified rate: 24 times a second for 24p, 60 times a second for 60p. The "i" means interlaced, with the even-numbered lines of the frame drawn first, followed by the odd-numbered lines, so the entire image doesn’t appear simultaneously; rather, you get half the image quickly followed by the other half. The significance here is that interlaced can produce odd artifacts with moving subjects. (Standard U.S. television is broadcast at 60i.)
Frame rate is the number of video frames per second. Shutter speed is the amount of time each of those frames is exposed. As with still photography, the shutter speed controls how sharp a moving subject will appear in each frame. However, there’s a relationship between shutter speed and frame rate. In the old film days, this was known as the shutter angle, as the shutter was a disk whose opening size (angle) controlled the exposure.
F-stops Vs. T-stops
Still camera lenses have F-stops. Those are calculated, not measured: F/4 means the effective opening is 1/4 the focal length of the lens. But production tolerances, light loss due to multiple elements and imperfect coatings and the like result in less light reaching the imag
e sensor than the calculated F-stop would indicate. So movie lenses are calibrated in T-stops (true stops), which are actually measured for the lens in question, and thus accurate indicators of light actually transmitted to the sensor. For some lenses, the difference between the calculated F-stop and the actual T-stop can be significant—one reason why pro movie lenses cost more than typical still-camera lenses. The movie lenses also have manual-focusing rings that are geared "slower" for easier and more accurate manual follow-focusing while shooting motion.