keller,


There are many reasons to prefer a fast prime over a slightly slower one. There are only a few primes that have apertures as fastor faster than1.4. It does make a difference and my advice is that you go to Bryans ISO 100% comparison chartsand see for yourself the change in sharpness of different prime lenses as you change from wide open to stopped down. If you have certain lenses in mind then go to his lens review and peruse the sample images. Do likewise at other lens review sites like photozone.de and slrgear.com. pbase is another good site for "real world" examples.


The DOF control is one of the major creative features of lenses with large apertures and as you study photos created by pro photographers you will see very clearly how this creativefeature is used. Using a lens like the 50 1.4 or 50 1.2 wide open on a FF body is hit and miss because the dof is so razor thin. When it hits the results are stunning. When it misses just a little the results are rather disappointing.


The"low light" capabilitity is the other creative feature of a lens with a large aperature. F1.4 as mentioned is 4 stops faster than f2.8. Fortunately the DOF for any given distance isonly 2times as shallow. See DOF Master to calculate. That's a good thing. Also compare the DOF on a FF and crop body cameras to understand better how the DOF changes between these formats. I find on my 5D I will stop the same lens (50mm 1.4) down more because of the shallower DOF created by the FF format for the same field of view. Remember, 16ft shooting distance on 1.6X crop = 10ft shooting distance on FF for the same basic field of view. The FF image will include more background @ infinitybut the subject will be aprox the same size in the frame. The 1.6X crop will have the same field of view as an 80mm lens on the FF. That's also worth comparing as a useful study. The DOF for equivalent fov setups is a ratio of aprox 1:1.6.


The best feature in my opinion of a lens with a large aperature is the amount of light available for the AF system and this is regardless of the set aperture. All lenses AF wide open. It is not until you press the shutter release that the lens stops down. I find thatfast lenseslike the 50 1.4 and the f2.8 zooms AF very accurately do to the larger default apertures.