Best DSLR or ILC for Night Sky Imaging?

Started Nov 8, 2017 | Questions thread
1llusive Senior Member • Posts: 1,571
Re: If you want the best, a dedicated cooled CCD cameras are far better than all the DSLRs and ILCs

kiwi2 wrote:

Selene wrote:

Thanks and thanks again Roger and Landscaper for starting this thread. I am very new to this. As I live in a very light polluted place with lots of trees, I do most of my night sky photography elsewhere. Thus, being able to use a camera and lens is convenient for me. I am intrigued by astrotrackers, but I am still very much a beginner who has learned a lot from this discussion even though many on here understand technical details much better than I do. I like to kind of think that I am using night skies as parts of scenes I want to shoot. I am sure others have done many of the same shots, though sometimes I have been in places that aren't so heavily traveled. I appreciate that some of you who know so much, like Roger, are willing to help some of us learn. This is the great benefit of these kinds of forums--I can ignore the ones that are way over my head, while trying to get as much as I can from those I can follow. Thanks again for a most interesting and informative thread.

Don't be fooled. 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. For diffuse light subjects, it comes back down to f-ratio as the gauge of brightness.

That's why photographers and camera metres use f-ratio to calculate exposures rather than calculating aperture.

f/5.6 on any lens, be it 10mm or 500mm, is roughly the same amount of light reaching the sensor.

ie. Have a look at the EXIF of these two photos I have just taken out on the back lawn with the same shutter speed and ISO and f-ratio of f/5.6 and shot within minutes of each other...

The 10mm has an aperture of 1.7mm and the 500mm shot had an aperture of 89mm.

So where did the massive amount of more light from the larger aperture go as both photos are similarly exposed? (if anything, the larger aperture shot is a bit darker and underexpose)

In the wider angle shot, the light after passing through the front element only had to travel 10mm to the sensor. (loosely speaking. from the point of convergence technically) In the telephoto shot, the light had to travel 500mm to the sensor. That's why f-ratios are used as the constant/relative brightness of a lens.

This is true for brightness, but, correct me if I'm wrong, most of what Roger talks about relates to the time to achieve a decent signal to noise ratio. As aperture increases on telescopes, we see more detail.


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