If you are going to shoot weddings (or portraits) for a living, you might want to get different gear. Better ISO performance is a product of larger sensors and newer hardware and software. Full frame sensors generally have larger photo sites so each receptor can get more light, which makes a better signal to noise ratio, for less noise in the final result. Faster lenses can help, too, since you can open the aperture more.
Another benefit to full frame is that for any given focal length, you can stand closer due to the larger sensor and larger image circle of FX lenses. Standing closer gives better bokeh. Some lenses have better bokeh than others, but for now we will think of just one lens for the demonstration, and since I shoot with Canon, my choice of lens won’t be something you will get, regardless of which lens I choose.
Click the images to see the large version and EXIF data on Flickr.
Herewith, a diagram. The line at the top is the backdrop. The X’s are coffee cans with faces on them, B is 6 feet from the closer can. A is 10 feet from the same can. A and B represent camera positions. L is a light and umbrella, and R is a reflector.
So, now we know the setup, here we go, …
This is a Canon 550D/Rebel T2i (APS-C crop sensor) at 10 feet, with a 100 mm lens opened all the way to f/2.8, and focused on the lower right corner of the nose as viewed.
Moving the lens to a Canon 5D Mk III, full frame body, same settings, same focus point on “subject”, and we see my backdrop is too narrow! Oh, well, next time I’ll hang a 12 foot backdrop, the blue one is 8 feet wide.
We also see the face on the green can looks almost identical to the previous image for sharpness.
So, lets move the full frame body to the 6 foot mark (B), so the subject returns to original size in the frame. Same settings, focused on same point on “subject”.
Now the face on the green can is almost gone.
You can get bokeh with a crop body, and even with consumer grade lenses, but if you have lenses that are 2 to 5 stops faster, and a full frame body so you can get closer at the same focal length, you will get bokeh sooner, more bokeh with less effort.