I have a t2I. I've always been very happy with it, but I would like to buy a new camera. Basically so I don't have to switch lenses as much. I shoot pretty much anything. Mainly wildlife, nature scenes, and old buildings. I thought if I'm buying a new camera maybe go for the 5dmrk2. I worry about losing reach on my wildlife pics. Would I really lose reach or can I just crop all the way down to what I would get with my t2I?
As far as lenses go, I have a canon 24-70 2.8, 100-400L, 10-22, and 100 2.8 macro. I think if I go with the 5dmrk2 my 24/70 would be great, but id also have to find an equivalent ultra wide angle. Are the low light capabilities of the 5d2 really worth me adding that camera, keeping in mind I still plan on carrying my t2I on my hikes?
The short answer is yes, as a practical matter the t2i will provide a meaningfully superior image compared to a 5D2 image that has been cropped to the same FOV -- when the ISO is relatively low, the lighting is good, and the image properly exposed, etc. This is because in this situation the t4i "puts more pixels on the image". I mentioned all those disclaimers because - as the available light decreases and ISO goes up, there will be some point where the cropped 5D2 image will be better (even though the final result may not be worthy of a large print)
It escapes me who provided the comparison on this forum (hats off to whomever that was), but it has been established with reasonable credibility that the 7D (which uses the same 18MP sensor as t2i) provides "just noticeably better IQ" (my interpretation) compared to a 5D3 image cropped to the same FOV -- in optimum lighting conditions where the 7D shines. So -- given the small IQ improvement of the 5D3 over the 5D2 (see http://the-digital-picture.com/Reviews/Canon-EOS-5D-Mark-III-DSLR-Camera-Review.aspx
) I would say yes you can expect photos from the T2i to be better than those of the 5D2 cropped to the same FOV -- again where the lighting is good, the image is properly exposed and you are shooting at 400 ISO for example.
My observation, however, is that adding the 5D2 to your bag would be more about adding new capabilities to your arsenal, not about loosing reach. They are different cameras optimized for different situations. As the above review shows, the 5D2 can provide significant IQ improvement over the 18mp crop (7D/T2i) esp in high iso situations and if you CAN move in closer. Especially if the light is poor, then 5D2 is a no brainer advantage.
Bottom line is that the combination of the two bodies will allow you to optimize IQ in many more situations, avoiding the use of the t2i when it is disadvantaged, and reaching for it when the situation calls for it. In well lit distance-constrained situations where you can't get closer, reach for the t4i. As ISO increases, though, the 5D2 will start to become more attractive, and especially when you are NOT distance constrained (say, old buildings in low light) it will produce better IQ than the t2i.