FEChariot wrote in post #18072163
You should probably correct that statement. Then why have a 600 or 500 or 100 or what ever rule you want to use that includes focal length in the rule?
Again, the rules that people use to determine exposure length are dependent on what they feel are reasonable amounts of star trailing and are dependent on focal length. Wider focal lengths do not mean more or less star trailing, wider focal lengths mean you can take longer exposures before that significant star trailing becomes apparent. Exposure length dictates star trailing, but exposure length is not the only factor for collecting light and collecting light is the most important thing for producing astrophotography images.
davidfarina wrote in post #18072303
But its not about the light. Its about trailing. And if you use a 50mm lens at 2.8 and 30s to shoot the milkyway, or you use a 16mm lens at 2.8 the wide angle shot will have a lot less starttrailing. I partly agree thats it about exposure time too, but not how you stated entirely. Its a mix of FL AND exposure time.
I dont know why some people must always correct another person when it was clear what he meant. Especially when the correction is not totally correct either.
It's not about the light? The whole point of photography is to capture light, stars produce light, the whole point of doing astrophotography is to take pictures of stars (and other night sky subjects), ergo the whole point of astrophotography is to capture the light produced by stars/other night sky objects. The amount of acceptable star trailing will vary from person to person, some use a "rule of 600", others a "rule of 500", others a "rule of 200", in any case the focal length is used to determine exposure length and exposure length will determine the amount of star trailing. But like I said above, exposure length is just one factor for collecting light, light which is absolutely vital for producing an image. If you collect too little light you won't see anything in your image, if you expose for too long to capture more light you will get star trailing.
Efficiently capturing light will yield better Signal to Noise ratios, that's why you want a lens that is fast and is reasonably wide angle (like the much heralded 24mm f1.4), those lenses will capture significantly more light (or worst case, equivalent levels) with shorter exposure times. Think about it this way, if a 24mm f1.4 lens captures ten times as much light with an exposure compared to an 14mm f2.8 lens you could use a 3 second exposure with the 24mm lens to capture an equivalent amount of light as a 30 second exposure with the 14mm. A 3 second exposure with the 24mm will have significantly LESS star trailing than a 30 second exposure with the 14mm lens, using the rule of 500 the maximum exposure with 14mm is ~35 seconds, max with the 24mm is ~20 seconds. 30 out of 35 seconds is almost 86% of the maximum exposure time, 3 out of 20 seconds is only 15% of the maximum exposure time. You could theoretically capture 10 separate exposures with the 24mm at 3 seconds each in the time it takes to capture one 30 second exposure with the 14mm, you could either stitch those images to produce a wider field of view or stack them to improve SNR and a much better overall final image.