The 600 Rule

How-to have  'points of light' stars,
                           photos with very little star movement. 

"Night Sky Landscapes" images with both the stars and the earth kept sharp requires the length of the exposure to be short enough that the stars do not appear to ‘trail’ due to the earth's rotation.

It mostly depends upon the focal length of your lenses.

For an 8x10 inch or smaller image the old 600 Rule works just fine.

It allows for the most seconds of exposure with the least amount of noticeable star movement.  For larger prints using a number of 500 or 450 is even better – less time to notice the Earth is rotating during the time you expose your image.

Divide the focal length of your lens into 600 and write it down.  Then do the same with the numbers 450 and 500.

24 into 600 = 25 seconds maximum to have little star movement show on 8x10 inch print 

24 into 500 = 21 seconds maximum to have little star movement 8x10 inch print or larger

24 into 450 = 19 seconds maximum to have almost no star movement large prints

That 6 second difference means you may need to increase the light on your landscape by  increasing the ISO or aperture settings…or adding ambient light.

Lenses with a longer (telephoto) focal length is like a magnifying glass --- showing more star movement than with wider angle lenses.

So, if you want a few extra seconds of exposure time, for images at 8x10 inches or less (like smaller online images) "the ol' 600 Rule" works...

Using the 600 Rule for astrophotography and Night Sky Landscapes:  Before you go out at night, calculate the maximum exposure time your lenses will allow you to shoot before the stars begin to make trails in your photos.

 

divide the number 600 by the focal length of your lens

Remember to calculate for the crop factor if your camera has an APS-C sized (crop) sensor.  

Canons with APS-C sensors ‘crop factor’ is 1.6x,
Nikon and Sony are 1.5x. 

So, if your camera has a APS-C sized sensor, divide 600 by your ?? mm lens (times) the crop factor.

examples here:

                                                  

                                         Cameras with crop factor                  600 ÷(divided by) your lens = max # of seconds

Canon with APS-C  sensor  50mm lens x 1.6 = 80mm focal length                600 ÷ 80mm     =           7.5 secs

Nikon   with APS-C  sensor  50mm lens x 1.5 = 75mm focal length                600 ÷ 75mm     =           8.0 secs
 

		1.6 crop sensor
14	mm	27	seconds
24	mm	16	seconds
50	mm	8	seconds
		1.5 crop sensor
14	mm	29	seconds
24	mm	17	seconds
50	mm	8	seconds

Cameras with Full Frame (FF) sensors, simply divide 600 by the ?? mm lens

                                                     Cameras without crop factor

14	mm	43	seconds
24	mm	25	seconds
50	mm	12	seconds

                                                         50mm lens = 50mm focal length              600 ÷ 50mm        =       12 secs

  


600 Rule 

"calculating for star point images,
                photos with very little star movement"

for this photo, APS-C camera, 50mm focal length,

600 ÷ 50 x 1.5 = 8 seconds is the longest exposure for a 50mm lens in order to have “Star Points” with very little movement

for this photo, FFcamera, 50mm focal length,

600 ÷ 50 x 1.5 = 12 seconds is the longest exposure for a 50mm lens in order to have “Star Points” with very little movement

~~~Keep Looking UP

return to:    Robert Howell Photography.com