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09-14-2009, 08:56 AM   #31
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I think making it out to be a black/white issue is a mistake. It's not like there are any real world situations f/2.8 guarantees success and f/4 guarantees failure. It's more a matter of f/2.8 increasing your *odds* of success by a certain amount (by allowing faster shutter speeds, potentially helping with focusing, although you pay for it in reduced DOF). Or, conversely, increasing the quality you get at f/4 odds (by allowing lower ISO at f/2.8 for the same shutter speed, or by giving you better optical quality at f/4 than an f/4 lens).

So in practice, it comes down to knowing your subject matter, shooting style, and IQ requirements, then figuring out how much of an improvement any particular jump in maximum aperture gives you. In some settings, it won't matter at all. In other settings, it will have a slight improvement but might not seem worth the cost (in dollars or size/weight). In other settings, it may increase your odds (or quality at the original odds) by enough to make it seem worthwhile.

For me, I do a ton of concert photography. I know from experience how my odds of success in terms of freezing subject motion change according to shutter speed. I perceive there to be "knees" in the curve around 1/125" and 1/30". That is, speeds faster than 1/125" don't really increase my odds much. Between 1/125" and 1/30", things get progressively harded but 1/30" still gives me "reasonable" odds of success. After that, my odds start dropping rapidly.

I also know from experience that typical nightlub lighting gives me around 1/30" - 1/60" at f/2.8 and ISO 1600 (max ISO supported in hardware by my K200D). So I'm right on the edge. With a slower lens, I'm going to have to tradeoff shutter speed and noise (by underexposing at ISO 1600 and push processing or by going up to a higher ISO - possibly software-emulated - if your camera supports it). If the club is on the dark side of average, I really notice even the half stop I lose going to a f/3.5 lens. In brighter clubs, it only affects my success/quality odds a little. But an f/4 lens almost always pushes my success/quality odds too low, except in unusually brightly lit clubs. And luckily, the larger venues that require longer lenses usually are quite a bit more brightly lit, so I'm actually OK with f/4 in virtually any setting in which I'd need a 200mm lens.

Anyhow, what I'm saying is, you can't answer a question like the one posted here without looking at your shooting at this level of detail, to really understand exactly what benefit that extra stop gives you.

09-14-2009, 02:21 PM   #32
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Well, we were both right and both wrong. Yes, you can get a consumer grade lens faster than f/5.6 at 200mm, but it's f/4.5 not f/4.0.

70-135 = f/4.0
135-210 = f/4.5
210-300 = f/5.6
09-14-2009, 03:55 PM   #33
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QuoteOriginally posted by Spock Quote
Isn't the DA* 60-250mm an f4?
They are. However, you initially seemed to be talking about prime lenses and then slid in zooms. The 55-300mm and 50-200mm are variable aperture well beyond 4 at the long end. Plus, neither is going to be on par with the DA* 200 at 200 and DA* 300mm.
09-14-2009, 04:36 PM   #34
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One stop means double the amount of light that a lens will take in, and that's a lot especially in falling light when you are shooting at the threshold of hand holdability.
It is in this situation where the larger aperture lenses make focusing and tracking the subject easier (brighter viewfinder, narrower DOF).
Plus we all know lenses are usually not the sharpest at their maximum aperture but stopped down. So a lens with a wider aperture has a wider range of usable apertures both in bright and low light conditions. The trade-off is a larger lens diameter and a higher price for all the extra glass. Unfortunately most lens makers have opted on using variable aperture lens designs for their consumer grade zooms for cost considerations. Most don't even offer a budget prime because most consumers want the convenience of a zoom.

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