X00 Rule, what's the correct one now?

[MauriceN]

The first time I've seen such a rule it was called the 600 rule. The same procedure as the other ones but with a different result. Which one is correct now? Yesterday I've read about the 500 rule and when I google it there are rules for 300 and 400 as well... Everyone is so careful when it comes to astrophotography and everything has to be very exact - and then this. ?

[Anthonyexmouth]

for static mounted cameras i go with the 500 rule. generally works well enough. 

[vlaiv]

How about doing proper math instead of the rule? It is quite simple math and you will not be asking your self what is the "correct" rule.

Earth sidereal rate is 15.041 arc seconds per second.

You need to know two additional things - pixel size in micrometers and focal length of your lens in millimeters. Next thing that you want to do is calculate recording resolution, and here is formula for that:

206.3 x (pixel size / focal length)

So let's take an example - you are using 50mm lens with Canon 450d. Pixel size of 450d sensor is 5.2um. You can find this information on internet, or you can calculate it - take sensor width and divide with number of pixels (same goes for height and in general case you should get same number - provided that pixels are squared). So 450D is Aps-c having width of 22.2mm and 4272 pixels wide - pixel size is 22200 / 4272 = ~ 5.1966 or 5.2um.

What would be recording resolution for this setup?  (5.2um / 50mm) * 206.3 = ~21.5 arc seconds per pixel.

Now we know that for each second of shooting stars move for 15 / 21.5 = 0.7 pixels.

Let's compare that with 500 rule. 500 rule says that you can shoot for 10 seconds with this setup. This means that "star trail" will be 7 pixels long. How much of a distortion that is? Depends on star size on image (in pixels). If your star is 6-7 pixels wide, then you will end up with a star that is 7 x 14 pixels (7x7 + 7 pixels of motion) - or egg shaped (but not trailing).

What should you aim for? I would say for this type of photography you should aim for 40-50% increase in one axis over the other. Or in another words - you should let drift be about as long as radius (not diameter) of an average star on the image. In above case this distortion is 100% - or longer axis is twice as large as shorter axis. So if star image is for example 8 pixels (diameter), I would limit exposure to something like 5.7s (that would produce 4 pixels of trailing or about that of radius - half of 8 pixel diameter). And in this case it is closer to 300 rule than to 500 rule . But as I mentioned, don't use "rules", use above math and it will give you precise results for your setup.

Star size on image will vary from setup to setup and with shooting conditions as well. So you can use longer exposures with smaller aperture and in bad seeing.