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10-20-2011, 12:09 AM   #16
Lowell Goudge's Avatar

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Like others have suggested, a fast 70-200 like an F2.8 will be sharper at f4.5 than a consumer lens wide open at F4.5, but asiude from shallow DOF there are other reasons to get a fast F2.8 70-200 (depending on which one, and what is available for tele converters)

I have the origonal Sigma APO 7-0200F2.8 EX (Non DG non macro)

this is arguably the sharpest of the sigmas, with perhaps the exception of the HSM 2 lens.

I use it with sigmas somewhat special Tele converters, to get 300/4 and 400/5.6.

The tele converters are designed to be used on a specific range of sigma (and some old pentax) long fast tele's and tele zooms, due to the unique design and protruding front element.

Since I use the lens on it;s own in extreme low light but also use it with a TC, I need the fast lens because adding a TC to a consumer F4.5-f6.7 lens will cost me AF. AF simply won't work if the combined maximum aperture is above F6.7 that is all there is to it.

Aside from that, the other reason to shoot fast lenses is in many instances ISO performance is simply not enough. I shoot wild life, and in the woods under the cover of leaves, it is always dark, and a slow lens will cost 2 or more stops where it is really needed most.

10-20-2011, 08:09 AM   #17
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A little more on DOF: It's the range of distance from the camera lens (actually from the lens' optical center) where objects are acceptably in focus. DOF is a complex product of photography, presentation, and perception: beside what you do with the camera, how the image is displayed and how people see it makes a difference. But you can't control presentation and perception when you shoot. There ARE factors you CAN control:

* For THICKER DOF, use a shorter lens and/or tighter aperture and/or further subject-lens distance

* For THINNER DOF, use a longer lens and/or a wider aperture and/or closer subject-lens distance

Those rules tell us a few things. A short focal-length lens has thick DOF, and when its aperture is stopped down, everything from near to far looks sharp. Most P&S cameras have very short lenses, often fairly slow (tight apertures), thus their crisp-looking pictures. And shooting macros at very close distances gives very thin DOF. So when shooting teeth, it's very hard to get more than one tooth in focus unless you stop-down the aperture quite a bit. This often requires flash, to have enough light for a fast handheld exposure.

You can also influence DOF by subject-background placement, and lighting. DOF seems thicker if the subject and background are close, and thinner when they're further apart. And DOF seems thicker if subject and background are about the same brightness, and thinner if the subject is lit and the background is dark. These are tricks of subject isolation, to make the subject stand out from their surroundings.

As mentioned, stopping-down to a tight aperture isn't always a good idea. Light bouncing off the edges of the aperture's iris blades causes diffraction, an increased fuzziness. This might not be noticeable is the image is displayed small but becomes pronounced with enlargement. Lenses on our dSLRs are usually sharpest at f8-11, acceptable sharp up to f/18 or so, and rather fuzzy from f/22 onwards. Again, how well the subject is lit can make an apparent difference.

Lens choices have already been covered above. I'll just note that a zoom with a constant f/3.5 maximum aperture is only slightly slower, with only slightly less DOF control, than a f/2.8 zoom, but may be considerably less expensive. I'm a cheap bastard; I notice stuff like that. But f/3.5 AF zooms are rare. Oh bother.

Last edited by RioRico; 10-20-2011 at 08:15 AM.
10-20-2011, 08:14 AM   #18
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Good explanation Rio. Better than I could have probably come up with!
10-20-2011, 09:36 AM   #19
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Thanks! Another few points:

* I like lenses with thin DOF because I can SURGICALLY ISOLATE a subject, cut them right away from their surroundings. This takes practice to do right. Practice practice practice...

* You can roughly compare the DOF of different lens choices by computing the simple DOF Index or FL/AP -- divide the focal length (FL) by the aperture (AP). The higher the number, the thinner the DOF. 50/2.8= 18 and 100/2.8= 36, so a 50/2.8 lens' DOF is quite thicker than a 100/2.8 lens.

* Thin DOF looks artsy, doesn't it? Look at advert and fashion photos, magazine photos, all sorts of commercially published photos. Note how many have thin DOF vs those where the entire image is sharp, with thick DOF. I have an oversimplified rule: Thin DOF is for art, thick DOF is for money.

* Old manual lenses include DOF scales to let you know how to set aperture and focus for desired DOF. Can't do that with new AF lenses. Those scales can be used with dSLR when adjusted by a bit over 1 stop. So if I set a lens to f/11, I read the scale for a bit under f/8, maybe around f/7.

* Related to that is HYPERFOCUS, the focus point and aperture where DOF extends to infinity. I'll set my 21mm lens to f/11, set infinity to just inside the f/8 mark, and I'll see that the focus point is at 2m, giving me a sharp DOF from 1m to infinity. Very useful for street shooting.

And that's my mini-tutorial on DOF. I'll maybe think of more after I've chuged more depresso (decaf espresso). Damn, waking up is tough...


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