I think Creampuff summed it up perfectly.
Having gone there and done that.
1. The old Tele lenses are huge and heavy. You NEED a good tripod and ball head. Any other tripod cannot take the weight and still be stable. Unless light conditions are good and you can shoot at 1/300 and above.
2. Old coatings (or none), contrast is not as good (but easily adjusted in PP, so not a big deal for me).
3. Very far closest focus distance. The old takumar or SMC-K 500mm f4.5 for example focuses 10m min. This takes away a lot of flexibility.
4. Focal length beyond 135mm are hard to MF. You are looking down a very narrow tube which is shaking badly from hand holding and made worse with manual turning of the focus ring.
Wildlife photography and birding is really hard work. Tropical sun/heat, bugs, leopard crawl to avoid detection, mud (esp. tropical terrain). Wah! I gave up. :sweat:
All said though, an old telephoto is a thing of beauty. An all metal construction that would not be possible (or would cost many times more) if built today. The many bladed aperture and smooth focusing is a pleasure to fiddle with. Not to mention that they are usually 1/3 to 1/4 the cost of an equivalent prime or long zoom. Still "buyable" if its for the occasional playing around.
A more modern telephoto lens has far superior optics and weight (and sometimes price). A 55-300mm for example easily matches these old behemoths at a fraction of the weight and size. Likely better quality pictures as well though a tad bit slower. (I've seen very good samples images from the Tair 300mm though... )
My thoughts is this.
200mm and below, forget the old stuff. Get a 55-300mm. At 190mm, its f4.5, sharp and light. Most old 200mm are like f4 for all the inconveniences, and cheaper but maybe only 1/2 the cost of the 55-300mm.
300mm, there are a few attractive old f4.5 and f5.6 out there. But they get heavier. Around the same price as the 55-300mm. This depends if you can really get a model/copy that is sharp wide open. The 55-300mm is a bit slow at 300mm (f5.8) which is my only minor dislike of it; but still sharp.
400-500mm-1000mm, ok 55-300 can't add a teleconverter w/o suffering noticeable image degradation , and its slow (f5.8 x 1.4 on a 1.4x converter). So arguably these are the focal lengths to play with for old telephoto primes. (Note that I said 'play'; not depend on
)
If you really want superior pictures, invest in a F*300, FA*300 or the new DA*300 and add a teleconveter. A good copy of the Sigma 100-300/4 is scary to behold even when used with a 1.4 teleconverter (can by at less than $1600 new overseas).