Best DSLR or ILC for Night Sky Imaging?

Started Nov 8, 2017 | Questions thread
1DSmII Contributing Member • Posts: 793
Re: If you want the best, a dedicated cooled CCD cameras are far better than all the DSLRs and ILCs

kiwi2 wrote:

1DSmII wrote:

kiwi2 wrote:

What Roger is leaving out in his formulas, is the inverse-square law. Light fades the further it has to travel. A longer focal length lens may have a larger aperture, but then the light also has to travel further over the focal length of the lens.

I can't see how this can be correct. The inverse square law says that if you double the distance then you are left with 1/4 of the light (if memory serves).

Lets say you are imaging a star that is 100 light years away, then you would have to move a further 100 light years away from that star to be receiving 1/4 of the light that you were receiving at 100 light years, all other things being equal. The length of the lens is irrelevant, especially if the sensor remains at the same distance from the star.

A few inches, feet, or even miles difference between the first/front element and the sensor is not going to have any significant impact on the light intensity. Obviously, the light transmission of a huge optic would play a part, so for the experiment to work, the longer lens would need to have the same light transmission characteristics as the shorter lens you are comparing to.

If what you said was true then it would imply that if you image the same star in January, and again in August, the brightness of that star would be significantly changed comparing one image to the other, since we have moved millions of miles nearer to or further from the star, and this is simply not the case.

The lens is creating a projection onto the sensor. So it is that projection that now becomes relevant as it has a finite amount of light from the aperture. The projection of the 500mm lens was 50 times the distance over the projected distance of the 10mm lens.

That's why the 1.7mm aperture of a 10mm lens can put the same amount of light onto the sensor as the 89mm aperture of the 500mm lens and the f-ratio is a constant across all lenses of any focal length...

I think I see where you are going wrong here. It sounds like you are treating the lens as if it were the light source, which is not what it is.

I'm not sure that a lens with 1.7mm aperture can be compared with one of 89mm, whatever the focal lengths involved, in the way you are suggesting. I don't see how those two lenses can gather the same amount of light unless compensating by using a longer exposure with the shorter lens.

Hopefully someone who understands the physics better than me can step in here. I'm still trying to get my head around the subject too, so I could well be missing something.

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Leo S.

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