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ZEISS DTI thermal imaging cameras. For more discoveries at night, and during the day.

DSLR Digiscoping (1 Viewer)

I found a 100mm spotting scope to have plenty of light for my APS-C chip. The issue was the aberrations on a cheap scope. The second issue was keeping it steady. That can be done. My SLT has no mirror to slap. I use cable release on a tripod.

I agree it has to be over 600mm fl which I can do with my camera lenses. I would not pay for the duplication. It has to be 800mm and up with a fast aperture.

The 500mm zooms are not great at 500. I have the tammy, it stays in the cupboard as not worth carrying when a top end prime is available. I'm sure the sig is the same. They can not keep up with a prime.
 
I agree it has to be over 600mm fl which I can do with my camera lenses. I would not pay for the duplication. It has to be 800mm and up with a fast aperture.

The fast aperture isn't going to happen with a spotting scope or probably anything even vaguely portable. As has been said there is no magic and a 100mm scope set up for 800mm is going to be f8 at best however you do it, usable but hardly fast.

The 500mm zooms are not great at 500. I have the tammy, it stays in the cupboard as not worth carrying when a top end prime is available. I'm sure the sig is the same. They can not keep up with a prime.

Generally true but a bit sweeping. For instance I've just been fortunate enough to acquire one of the old Tamron 200-500 31a lenses, and it's looking way ahead of my equally old Sigma 400 prime at 400mm, or indeed at any focal length.

I'm also experimenting with an SLR adapter on my 80mm scope giving me 850mm and f10.5. That's very usable and quite comparable to using the Tammy with a converter, but I have a feeling the lens and converter combination is going to win as it has the two basic advantages of being easier to use and having a bigger objective grabbing more light.
 
The 500mm zooms are not great at 500. I have the tammy, it stays in the cupboard as not worth carrying when a top end prime is available. I'm sure the sig is the same. They can not keep up with a prime.

Some of what you say is true, but you would expect a $6-10,000 prime to perform better than a $7-900 zoom. My 600 f/4 and 200-400 are relatively heavy compared to the Tamron and Sigma XX-500 zooms. The key is to know the limits of the zooms and how to make them perform well.

In the case of the Tamron 200-500, you don't want to use it with just a cable release. It requires proper Long Lens Technique - that is, a hand or beanbag across the barrel. It also performs better at faster shutter speeds with subjects inside 150 feet. That makes it a pretty good choice for handheld use or when you need a light weight long lens. It also is better at 450mm than 500mm under most circumstances. I use it as part of a light weight kit for situations that involve hiking or canoeing.

The Sigma 150-500 has its own advantages - the biggest being OS. It also is relatively small in size compared to other long zooms or primes. It is also better with relatively close range subjects.

You can find some older manual focus long lenses for lower prices. In some cases the optics are quite good, but some like the mirror lenses are very tough to create reliably sharp images.
 
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