I was recently asked at an event what really is the difference between full frame and a smaller 1.5x crop camera. This person knew the sensor was smaller, but was more curious why people were getting more telephoto power from the smaller sensor. Is it really true a 1.5x sensor makes your lens a longer telephoto? A 300mm is turned into a 450mm? Let’s just answer that straight up. No, a smaller sensor doesn’t change the optical telephoto power of your lens. A 300mm is a 300mm lens. It becomes a longer telephoto if you add a teleconverter.
What happens with a smaller sensor is the angle of view is reduced, and so the camera captures less but the actual subject is larger. See the above side by side comparison. A D800 (full frame) and a D300s (1.5x) shot side by side at 200mm at 5.6 at the same subject. The D300s subject is bigger.
If I put the D800 into 1.5x crop mode, notice how now the subject is the same size in both shots. But what is interesting is even in 1.5 DX crop mode, the D800 file is 44mb versus the 34mb from the 12MP D300s. Starting with the huge 36MP file from the D800 has some advantages!
This last image shows the original D800 file, and the D800 1.5x crop file side by side. It is important to note that even though the subject is larger using the 1.5 x crop mode, this doesn’t duplicate the bokeh you get from using a longer telephoto lens which will compress the scene.