There's a belief that focal length magnification helps crop sensor cameras get more “reach” than a full frame camera shooting with the same focal length. It's a bunch of hooey.
Focal Length Magnification Explained
What's the difference between a 200mm lens on a full frame camera vs. a crop-sensor camera? Does the crop sensor really give you more reach, or a focal length magnification over the full frame camera body?
Nope. The focal length is the focal length, no matter which sensor you're using. You aren't getting any more reach when you use a crop-sensor camera body than if you used a body with a full frame sensor. If you stand in the same spot and look at the same subject using a crop-sensor and a full-frame camera at identical focal lengths, the subject will not appear any farther or closer in either camera.
Instead, what you see is that there is more width to the frame on the full-frame camera. On the crop-sensor, the sides are literally cropped off.
Now, that gives you more pixel density of your subject on the final image if both cameras are shooting at the same megapixels per image. However, shoot with your 12 MP crop sensor compared to my 36 MP D800, and I have much more pixel density even after I crop the image to look like the crop-sensor image.
In any case, you don't have additional reach with a crop-sensor camera. That's a misnomer that's grown legs, but it misrepresents the truth.
Other Differences Due to Sensor Size
I started off with crop sensors on my Nikon D70 and D200 cameras. When I got my first full frame lens, the Nikon 70-200mm f/2.8 VR lens, I was in heaven. This is a spectacular piece of glass. One day I decided that I wanted to go completely full frame. I bought a D700 and sold every piece of DX (Nikon's designation for crop-sensor) gear that I owned. Finally, I'd be able to use my full frame lens the way it was intended.
What the hell was this vignetting on my photos now? As it turns out, that model of the 70-200 has some pretty heavy vignetting when used on full frame sensors. In other words, it worked better on my DX gear because they effectively cropped out the area of the lens with vignetting and shot through the sharpest part of the lens in the center. Nikon ended up creating a revised version of the 70-200 (which I've yet to purchase) to deal with the vignetting problem, and also upgrade the vibration reduction.
There are benefits to shooting a full frame sensor, such as a greater field of view for composition and better performance at high ISO in low light. However, don't dismiss crop sensor bodies as I did. It's clear to me now that I was getting better performance from my lens by using the crop sensor camera that didn't reveal the vignetting issue. It just didn't magically provide magnification of my focal length.
Depth of Field is another factor to consider. Without getting into technical details, you can achieve a shallower depth of field with a full frame sesnsor than with a crop sensor. Whether that is a good thing or not depends upon your preference and skill. I spent this weekend testing my new 85mm F/1.4 lens to see how that shallow DOF worked. Suffice it to say that it offers an extremely slim area of focus. That works for some subjects, but not for all.
Here's an example of a shot where I had one thing in mind, but found the DOF at f/1.4 had something else planned.
When I saw this image on the little screen on the back of my D700, I was happy and making noises like “Oooooh!” Then I got home and looked at the image in much greater resolution. Then I made noises like “Eeewww!” Here's a 100% crop to show you why.
That area just above the eye is in focus, but the rest of the area is already outside the depth of field at f/1.4. Had I shot this with a crop sensor camera, I suspect I would have more depth of field, but it would have cut out portions on either side of the image. It's literally a crop, not a magnification effect.
I keep finding that everything in photography is a compromise. You can take beautiful images with most any camera these days. The trick is to decide which one makes it easiest for the type of photos you want to make.