Best DSLR or ILC for Night Sky Imaging?

Started Nov 8, 2017 | Questions thread
kiwi2 Veteran Member • Posts: 4,398
Re: If you want the best, a dedicated cooled CCD cameras are far better than all the DSLRs and ILCs

1DSmII wrote:

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.

It does become the light source as it is projecting an image circle created by its aperture.

How could I be "going wrong" as this is the basic principle of using f/ratio to calculate exposure for last 150 years of photography at least. Photographers don't need to work out the aperture size when calculating exposures. They just use f/ratio and film/sensor sensitivity and shutter speed time. The f-ratio is already the aperture relative to the focal length.

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.

Have a look at the EXIF info of those two photos.

Both f/5.6, ISO 400, 1/640 sec, taken within a minute of each other by the same camera/sensor in the same light. The only difference was the focal length and aperture. Yet they were both exposed more or less the same.

So how do you explain one had an aperture of 1.7mm and the other 89mm yet a similar amount of light reached the sensor?

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