Canon L lens vs. Prime Lens I am very curious and need your advice. Since prime lenses are known to produce very sharp images, but lacks the versatility of zoom, would Canon L zoom lens, say 24-70mm shot at 50mm, be comparable to their non L series 50mm prime lens??? Please help!!!
The 50mm f/1.4 is sharper across all apertures; the two lenses are probably closest in sharpness at the 24-70's max of f/2.8, but by f/4 the 50mm f/1.4 is clearly in the lead. To further cement things, the light falloff of the 50mm f/1.4 is also clearly superior to the 24-70 f/2.8L, most visibly at larger apertures. Depending on your camera, this second item may not be a big issue; when EF-series lenses are used on APS-C sized sensors the pictures benefit from a reduced light falloff due to less of the image hitting the lens being captured by the sensor (the infamous "crop factor").
Link to 50mm f/1.4 to 24-70mm f/2.8L comparison: http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=101&Camera=453&Sample=0&FLI=3&API=0&LensComp=115&CameraComp=453&SampleComp=0&FLIComp=0&APIComp=4 (copy both lines of the address into your browser)
So, that's the f/1.4, what about its little sibling, the 50mm f/1.8 II? The L glass does better against the "nifty fifty", but still loses in sharpness from f/4 through f/5.6; at f/2.8 and from f/8 to f/11 the two are about equal. The 50mm again benefits from reduced light falloff, but not as dramatically as the f/1.4.
It really shouldn't be surprising that even L-series zooms can't stand up to prime lenses under certain conditions. Even the 50mm f/1.8 II, currently selling for barely $100, can get away with such prices because 1) the lens makes considerable sacrifices elsewhere (build quality, AF motor, lack of full-time manual focus, etc) 2) 50mm primes, in particular, were once the standard lens and so have had more research & development than just about any other fixed focal length and 3) it's vastly, vastly easier to design a fixed focal length prime lens than to make a complicated zoom, especially a fixed-aperture zoom, and a high-quality one at that, which the 24-70mm f/2.8L is. That's why the 24-70mm f/2.8L is currently $1350, the 50mm f/1.4 goes for $350 and the f/1.8 II for only $100. You get what you pay for. There are other things the L-series lens likely does better, such as color saturation and vividness; I haven't seen any website providing such comparisons online, and you asked specifically about sharpness, anyway.
Think of it this way: zooms, even L-series, are by nature jacks-of-all-trades. The really expensive ones are really, really good. But they still have to make compromises in order to get the best image quality across the range of the zoom. The primes, meanwhile, are single-purpose lenses, so even the cheap ones can equal the performance of a zoom set to its focal length, while the mid-price ones can easily surpass it.