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11-22-2006, 04:53 PM   #1
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Sweet spot?

From what I've read (if I understood correctly), each lens has an aperture that produces superior images compared to other apertures.

That's probably confusing. What I want to say is:

A certain lens at f11 produces better quality shots than at f 4.5, as an example, when various combinations of aperture/shutter speed will expose the photo correctly.

Is this always the case, that there is an ultimate setting for each lens?

If so- what is the best way of determining the "sweet spot"? Take a whole whack of photos of the same stationary subject under controlled conditions?

11-22-2006, 05:11 PM   #2
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you know i Just read something about that in DPR.

something about setting the MTF in the K10..

i gotta go read the thread again, i just kinda skimmed through it
11-22-2006, 05:26 PM   #3
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QuoteOriginally posted by Tom Lusk Quote
From what I've read (if I understood correctly), each lens has an aperture that produces superior images compared to other apertures.

That's probably confusing. What I want to say is:

A certain lens at f11 produces better quality shots than at f 4.5, as an example, when various combinations of aperture/shutter speed will expose the photo correctly.

Is this always the case, that there is an ultimate setting for each lens?

If so- what is the best way of determining the "sweet spot"? Take a whole whack of photos of the same stationary subject under controlled conditions?
I read two things:
one said F11 is the sweet spot for max sharpness
the other side 3 stops from the wide open of the lens

cheers
11-22-2006, 05:51 PM   #4
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QuoteOriginally posted by Tom Lusk Quote
From what I've read (if I understood correctly), each lens has an aperture that produces superior images compared to other apertures.

That's probably confusing. What I want to say is:

A certain lens at f11 produces better quality shots than at f 4.5, as an example, when various combinations of aperture/shutter speed will expose the photo correctly.

Is this always the case, that there is an ultimate setting for each lens?

If so- what is the best way of determining the "sweet spot"? Take a whole whack of photos of the same stationary subject under controlled conditions?
All lenses have an f/stop where they are sharpest. Some Pentax cameras (e.g. the *istD and the K10D) has a program (MTF program) that will select the sharpest f/stop if possible. The FA, FA J, D FA and DA lenses transmit information about what F-stop is their sharpest to the camera. This is a Pentax exclusive, no other make has it.

At large openings like 1.4, 2, 2.8, lenses are not at their best. When stopped down they improve considerably as less of the glass is used for forming the image.

When stopped down too far the opening of the aperture becomes very small and the edge of the aperture mechanism creates diffraction and the sharpness deteriorates.

So somewhere in between, typically 5.6, 8, or 11, lenses are at their best.

11-29-2006, 09:35 PM   #5
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Hi Tom, I did read about the good aperture for F* 300mm f4.5 between f5.6 - f8

There was this story about diffraction as well but that lens is just so sharp wide open!

(p.s. I co owned the same lens with Joele but i do know that he wants to own it himself hence i am searching another one myself)

some of the lenses are just funny like sigma 30/1.4 being sharpest at f2.8 and getting bad after f8.
11-29-2006, 10:42 PM   #6
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All camera lenses have a sort of bell curve to their sharpness, with the x-axis being aperture from wide open to fully stopped down, and the y-axis being resolution of the lens.

Take this for example, taken from the Sigma 17-70mm review at Sigma AF 17-70mm f/2.8-4.5 DC macro - Photozone Test Report / Review



On the y-axis, higher numbers are better. You can see that at wide open, it is kinda soft, and then sharpness increases as you stop down, which maxes at its sweet spot. Then it starts dropping again and if the charts would have showed f/22, the resolution would have been abysmal due to diffraction.

Different lenses have different aperture sweet spots. On a single lens, different focal lengths will also have different sweet spots. Many have them at f/8 or f/7.1 or f/11.
12-03-2006, 02:45 PM   #7
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Hi Tom,

Not sure if you return to old posts or not. The answer to your question about determining the sweet spot is "yes there is software to do this". It's called Imatest, cost 30-80 dollars (US), it's online and the author is a fellow named Norman Koren--he also has a website.

MTF or more commonly MTF50 is the signal processing term for the response of an emf system to a sine wave input; MTF50 is the point of 1/2 signal degradation. Koren's program looks at slanted lines imaged by a camera, applies an industry standard statistical analysis and pops out an MTF curve--slick. There are also modules included for distortion, color, dynamic range. The graphs above come from Imatest.

A few hours with Imatest and you either heave a great sigh of satisfaction for buying good glass, or you seriously consider joining a group of hermit monks.

The Imatest site is geek paradise; you're forewarned!
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