Dustin, here's my experience. I came from a D5100 with the AF 70-300 IS. Being crop, I perceived 450mm and got used to that. It was indeed a huge and exciting step up from the olden days with my F1 and 200mm. When I bought the D5100 I didn't even know about sensor size, i just knew it was expensive (LOL) and that it should take pretty good pictures. Then after a year I was getting very excited and a wee bit disappointed and since my good friend had been shooting for some years with the same lens but a D300, it was his influence that got me thinking big and Canon, a most ridiculous thought except inheritance money had just come into the picture and my wife was encouraging me (I bet others are envious
At this point I'm burning with interest and reading everything under the sun, all the arguments about crop and tele reach etc. There are a fair number of very educated individuals in photography who regularly ague and debate and it's really hard to decipher the total truth. As an electronics engineer I have half a hope with sensors but beyond that it's Greek.
I believe I have confirmed that cropping FF does not equate to using crop (you know how all the factors are supposedly dealt with in this comparison as per Clarkvision etc.). Thus, at this point in time having got a taste of the best equipment, I like many on this forum, am scheming regarding a higher level camera than the 6D (my wife is waiting to inherit it). I'm prepared to wait on this decision til next year with either the most expensive 1Dx or a new 7D II being the likely contenders. I presently believe that a the reach advantage of a crop, provided it's just 1.6 is a true advantage for bird photography but the 6D is making me well aware of the low light advantage I'm enjoying right now. Daily I'm getting educated on the AF disadvantage of the 6D, even though I won't knock it.
We I go out shooting birds I just can't seem to bring myself to drop back to 420 after having shot for a year at EQ 450. That's right now given all the exciting bird activities that I just don't want to risk missing. There is also a slight advantage with the greater reach in just viewing the birds at the greater distances for ID.
Not sure here but I'm willing to bet that 300 X2 is preferred over 300 X1.4 with cropping to enlarge. That's my judgment, anyway.
What I need to do is head out to the pond, maybe tomorrow with the X1.4 and get as many shots as possible that are similar, for comparison. If the X2 is indeed inferior, it's only a $500 experiment and not worth fretting about.
I'd love to hear any and all assessments of the opinion I've just expressed and other's personal experience but maybe this belongs in a differnt thread??
6D 300 X2 1250th F5.6 ISO 1250