Looking for some advice. I want to know whether or not i should buy a tc to use with my nikon d7200. I love wildlife photography and use a sigma 150-600 contemporary, but would love to get abit closer! Any help would be appreciated. Thanks.
DeeDee - I'm not sure that you will see an increase in image quality at all. Yes, closer always seems just out of reach ! :-O
Do you shoot your D7200 in the 1.3x in-camera crop mode ? (That's what I do with mine using a Tammy G2 150-600). That will give you around ~1150mm equivalent (given that there is some focus breathing, and a consumer supertelephoto zoom is more like about ~580mm at the long end anyway). Resolution will be 14.2MP and you will cut out a lot of extraneous border sky or foliage. File sizes will also be smaller giving you more capacity, and importantly an increase in speed ! Top speed will then be 7fps in continuous

It's the same as cropping in post, but you get all those extra benefits. If a bird flys close or whatever it's easy to just zoom out.
As Mike said, there are a whole host of drawbacks to a TC - less light, less IQ, loss of AF points, and AF performance, if not altogether. You are also not getting rid of the atmospheric effects by not getting physically closer. It will also be more difficult to use - requiring a step up in technique.
The Sigma C can be a little soft at 600mm and wide open. It can be better to back off to 550mm and f7.1 or even f8. If you shoot in the 1.3x in-camera crop mode as I suggested, then you will be at around ~1100mm equivalent. It is also easy to switch back to normal if you want to take a shot at the wide angle end, or have a super curious and cooperative bird.
It also helps to talk to the birds ! o

Ask them if you can take their photo, and if they would like to help out by coming closer to you
Chosun :gh: