Q: I have two zooms with a maximum aperture of f/4.5-5.6 so I'm planning to buy a "fast" 50mm lens for low light photography without a tripod. What is the difference (besides price!) between a 50mm f/1.4 and the 50 mm f/1.8? It seems like such a minimal difference but the f/1.4 lens costs about $300 more. Do you think I should buy the more expensive model?
You're right: An aperture of f/1.4 does not sound much wider than f/1.8. In fact, it's less than one stop so the shutter speed in a dark location will not be substantially faster. And yes, the f/1.4 lens (of any brand) is significantly greater in size, weight and price. That's partly because the wider aperture requires a barrel of wider diameter and larger pieces of optical glass. Note too that the lens elements must be more effective at correcting optical aberrations which are more problematic at f/1.4 than at f/1.8. All of this drives up the manufacturing cost.
In addition, a 50mm f/1.4 lens is larger/heavier - and far more rugged, with tank-like construction - than its more affordable f/1.8 counterpart. Some, like the Nikon 50mm f/1.4G, are also weather-resistant, useful when shooting in a dust storm or during inclement weather. The superior construction makes sense, because a 50mm f/1.4 lens is usually bought by professional photographers or by very serious photo enthusiasts. Both expect the very best in build quality.
The difference in shutter speed is not great at f/1.4 vs. f/1.8. This image, made at f/1.4 at ISO 100, required a shutter speed of 1/25 sec. If I had used an f/1.8 lens instead, the shutter speed would have been around 1/15 sec. but switching to ISO 200 would have allowed me to shoot at 1/30 sec. (c) 2011 Peter K. Burian
Even with high-grade optical elements, it's very difficult to make a lens that will provide superb image quality at f/1.4. Often, the 50mm f/1.8 model will provide comparable quality at f/1.8. Hence, you should be perfectly happy with the less expensive lens unless you're certain that you need the slightly wider aperture and more rugged construction. You'll probably be pleasantly surprised to find that the f/1.8 model provides decent image quality at f/1.8 and excellent from f/2.8 to f/11.
NOTE: Peter Burian teaches Mastering the Digital Camera and Photography and Mastering the Canon EOS Digital Rebels BetterPhoto's digital photography school online. Also check out Peter's Pro BetterPholio website: www.peterkburian.com.