05-31-2017 10:42 AM
If you mean non-air show spotting, the Sigma 150-600 sport gives you the best reach and autofocus speed. I had pretty good luck with my 150-600 TAMRON at a thunderbirds show.
I was at an airshow last week and my 18-135 was more than long enough for my APS-C. I put my 70-300 away.
05-31-2017 12:05 PM
There are multiple meanings to the term "plane spotting" so we need to know whether you mean around an airport etc or for those flying at altitude. It's also important to know whether you are using a crop body or a full frame body. Prices are also different in different countries. I'm in Canada & we pay much more than those in the US but less than in Europe etc.
06-01-2017 09:36 AM
One of the most popular air show lenses has been the 100-400 but the newer version isn't a budget lens. The older one was a push / pull while the newer one is a twist to zoom. Sigma has recently put a 100-400 on the market which seems to be a decent lens at roughly 1/3 the price of the Canon. You may be able to get by using one of the various 70-300's out there & in most cases the more you spend the better the AF speed will be along with better image stabilization & sharpness. IS is very useful with the slower shutter speeds you should use when shooting prop planes.
06-01-2017 10:05 AM
"What is a afordable..."
Yes, what is "affordable"? Then add, "...and good lens..."? IMHO, no kit lens fits both limiters. Affordable, yes. Good, not so much.
Again, IMHO, the choices are one of the Sigma/Tamron 150-600mm offerings or the new ef 100-400mm II.
I have not tried the new Sigma 100-400mm offering and I likely will not. I see no good reason the give up my 500 to 600mm advantage in focal length. The new Siggy may be a nice lens, IDK.
Remember as zoom ratios increase IQ suffers.