I am always striving to be a better photography instructor. I constantly revisit popular techniques and guidelines to see if there is another way to present information in an easier format. Some guidelines stand the test of time, while others change as technology advances in the photo industry. I remember memorizing multiple exposure formulas with my film camera…not any more, just let the camera figure out exposure.
Night photography often uses the 500 rule. Divide 500 by the focal length of your lens to determine the longest shutter speed you can use before you record star moment. For example, I like to use my 14mm lens for star photography. 500 divided by 14 is 35…so I could use a 35 second exposure and still not record movement. But wouldn’t it be nice to have a ‘rule’ that not only adjusted for star moment, but also gave you a guideline on basic exposure for a star shot? With that in mind I came up with the 25/25 rule.
It is really simple. On a dark night, set your ISO at 2500, your shutter speed at 25 seconds, and shoot at F2.8. As long as your lens is 20mm or wider you won’t get star movement, and this exposure should be close to correct. You may have to adjust a little, but this should put you in the ballpark. Really simple. 25/25 at F2.8. Give it a try, no math calculations needed!