and 30 seconds, which would be Rule 420 and 1.02X CoC and EV -2 (but 500 Rule allows 36 seconds, but which is only 1/4 EV more). For exposure of stars in a dark sky on a fixed mount, using a 14 mm lens on a 1x sensor, I’d suggest planning to start at least about f/2. This calculator computes for sensor size and CoC and trail length, but the Rule does not specify a desired exposure. However in today's cameras, a trail 5 or 6 pixels long is usually not noticeable unless viewing is enlarged more than usual. Even photographing the Moon requires sunlight exposures because it moves (but it is illuminated by the same Sun that the Earth sees). But if the shutter exposure is longer than a second or two, then the trail length is also longer than zero. And don't miss seeing below about an easy and inexpensive DIY rotation tracker.īut even when factoring in the crop factor (which is sensor size), the original Rule ignores all other factors. A wide angle sky view with a short lens is a better view of the Milky Way anyway (because it is full sky large up there). A short lens works better for this (a short lens is less magnification of the Earth's rotation). For example, 500 Rule / (1.6 crop factor x 12 mm lens) = 26 seconds maximum time for perhaps acceptable absence of trail blur, but 26 seconds may need ISO 4000 f/2.8 for a minimal exposure of stars. Others use it as the 400 Rule for less star trails but less exposure. Some people use it as the 600 Rule because it specifies more time. The 500 Rule definitely does limit actual photographic exposure time. You will also have to use the ISO and f/stop that gives an acceptable exposure, and that gets much harder. The Rule will be more usable (longer exposure) when using a short lens for a wide sky view than for an enlargement of a smaller area. Maximum Seconds = 500 / (Crop Factor x focal length) However, for other sensor sizes, the same allowable movement and corresponding Rule Number is computed as: The 500 Rule assumes 35 mm film popular at the time, or today for a Full Frame 1x crop factor. That was all there was to the original rule. The "Rule" is only about a maximum exposure time for minimum and acceptable trail length due to the Earth's rotation.ĥ00 Rule: Maximum Seconds = 500 / focal length But the original 500 Rule is Not about other sensor sizes, nor about computing a useful exposure time. Greater focal length does magnify the Earth’s rotation. Longer exposure causes the earth rotation to make longer star trails instead of dots. The "500 Rule" was a simple guide (in older days for 35 mm film) calculating the maximum allowable exposure time of star photos due to the Earth’s rotation making star trails when camera is on a fixed tripod.
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