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02-10-2008, 08:54 PM   #16
m8o
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It's a rare lens though that's at or atleast near max sharpness wide open... I only have one of them in my arsenal, and no is't not my 300 f/2.8. Thought it's a close 2nd. Yep, the * Pentax f/4.5 long lense are in a class by themselves for how sharp they are wide open.

To the point of usefulness of 'wide open'... But my two main advantages to a f/2.8 as I see it and why I'd continue to chose that rather then a f/4 if I have the chance...

1) Generalizing, when you stop a f/2.8 lens down to f/4, it's enough to sharpen up to near max rez, and usually is maxed before f/5.6 as well as to get the edge sharpness up near center sharpness. Still generalizing, this ratio is usually what you find of an f/4 lens, meaning (other then the remarkable FA* 300mm f/4.5 and others made of the same cloth ) it's f/5.6 to sharpen it up, max before or at f/8 with edge sharpness again lagging. CAs and Vignetting usually follow the same 'formula'. I appreciate that extra stop of light to get to the lens's sweet spot as I find I rarely get to shoot widlife in sunny conditions; clouds are common, and moreso the canopy of trees. [I love that sunny 16 formula ]

2) If you have a look @ the three pages of bird shots here [ 070820 - Leavning the Serengeti and Day Room @ Kilimanjaro (KIA Lodge) album | m8o | Fotki.com from image #8 thru to the end ] you'll note they're all f/3.2 or f/2.8 with the majority the latter, taken over the course of 1 hour. They were all handheld, with some of the last few leaning against a fence post as the bird ate the fruit, as the conditions were -really- dark by the time I was shooting those shots. It was dark, gray, misty, overcast the whole time and drizzly much of the time. A lot are motion blurred with those hummingbird like birds the first two pages quickly on the move as you can tell when a flower @ the same depth plane is sharp. Granted I missed focus on a good many too.

...But there's a bunch at f/3.2 or f/2.8 that are nailed and work to prove that even within feet of the subject f/2.8 or f/3.2 can offer very workable DOF. The 1st one below is f/2.8 and second f/3.2. They are crops, with subject I think about 6 - 9 feet (2 - 3 meters) away in these two here.

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...at least for 3rd party prime lenses or all zooms that need to be stopped down to get to max sharpness, I'll always look for f/2.8 to start with. Those A* or FA* f/4.5 lenses are quite in a class by themselves in sharpness off wide open tho, and knowing what I learned over the months here that would have been a good alternative for me (if I wasn't intending to use TCs... but I was )

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