I recently moved from a Nikon D300 (1.5 crop using a APS size sensor) to a Nikon D800 full frame and wanted to do a comparison. Realizing that the differences would be hard to see with full size images on this website, I used a small crop area, as shown in the above photo.
The difficulty in doing this comparison is the vastly different pixel count between the D800 and D300. If you are making a large print the image of the D300 needs to be enlarged by 1.71 times more than the D800. So I used a smaller crop for the D300 images, then resized to get 740 pixels across.
Since this is not a scientific experiment but just one to satisfy my own curiosity, I decided to do a comparison using only the lens I have for each format size. Therefore the differences can be partly due to the lenses. The size of the crop for the D800 images was 1/10 the horizontal dimension of the original image, producing a 740×740 pixel crop due to the D800’s amazing 36 megapixel sensor (7,360 x 4,912 pixels). The Nikon D300 has a 12 megapixel sensor. Click each image to see enlarged since this blog might resize to fit.
D800 with both Nikon 16-35 f4 and Nikon 24-120 f4 vs D300 with Tokina 11-16 f2.8
Starting with the widest I can go on both cameras (16 mm equivalent full frame) the full frame D800 is clearly sharper.
Going a bit less wide, 24 mm equivalent full frame, I used both new full frame lens set to 24 mm and the Tokina set to 16 mm, or 24 mm equivalent for full frame. The first two images from the D800 look similar, as I expected based on my other comparison post between these two lenses. The 3rd image with the Tokina on the D300 is clearly less sharp.
First at a moderate wide angle (35 mm full frame equivalent). Here the D800 full frame is much better.
Next is a short telephoto (70 mm full frame equivalent). Again a rather noticeable difference It is a combination of the full frame D800, 36 megapixels and a better lens.
Moving out to the longest I can shoot with the D800 (120 mm), the difference is similar.
In summary, if you want to make large prints, then the full frame setup clearly has some advantages, but for sharing photos on the web, the APS size sensor will give you just about the same advantages for less money and less weight.