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Old Feb 9, 2009, 9:25 PM   #11
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so could someone shed a little light . . .
shouldn't a lens be its sharpest at the smallest diaphragm (in this case f22)?
especially a good wide angle that has a limited DOF.
if f11 is the sharpest for this lens, why even have f22?
you might get more dof but it will be softer throughout, right?

most confusing to me.

thanks for your considered answer.

ohillary


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Old Feb 9, 2009, 9:59 PM   #12
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ohillary wrote:
Quote:
so could someone shed a little light . . .
shouldn't a lens be its sharpest at the smallest diaphragm (in this case f22)?
especially a good wide angle that has a limited DOF.
if f11 is the sharpest for this lens, why even have f22?
you might get more dof but it will be softer throughout, right?

most confusing to me.

thanks for your considered answer.
Hi ohillary,

Welcome to the Pentax/Samsung DSLR forum!

The "sharper as the aperture gets smaller" thing has some limitations. As apertures get smaller, a lens will get sharper until the lens/sensor combination reaches its diffraction limit -- when the bending of the light (diffraction) because of the aperture blades causes a loss of resolution because the aperture is too small for the lens/sensor combo. I'm no expert here, so excuse me if this doesn't make much sense. . .

My understanding of this is that a smaller sensor hits this limit at larger aperture openings than a larger sensor, so many APS-C sensored cameras will see loss of res at larger apertures than 24x36mm sensored cameras using the same lens. If you look at some small sensored P&S cameras, you'll note that many don't even offer smaller apertures than f11 because that's already past the diffraction limit. 35mm lenses usually offer aperture values of f22-f32.

Scott

Edit: Now I think I remember -- Smaller sensors use scaled down lenses to achieve the same equivalent Field of View, and use appropriately scaled down apertures -- the openings are actually physically smaller for an equivalent f stop value. This causes the diffraction limit to be reached at larger f-stop values on these smaller sensored cameras.
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Old Feb 9, 2009, 10:02 PM   #13
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I'm not a technical person, but I've discovered that no lens I own is sharpest at its smallest aperture, or it's largest aperture for that matter. There's so little light coming through that I would imagine it is hard for the lens to be sharp (some time try looking through a lens stopped down to it's smallest aperture by putting a lens with an aperture ring to it's smallest setting, then use the optical preview button to temporarily stop down the lens). The only lens that I use often at smaller than say f16 is a macro lens, and it's designed to be sharp at all apertures. Most of the time I use f8 or f11 if I want the sharpest picture and don't care about the aperture/dof, but different lenses have different characteristics and are sometimes sharper at different points (the reason why Pentax offers the option to set the camera to choose a lens' "sweet spot" for the program line).

So why would you use a lens at something when it's not its sharpest? For the same reason you'd use a fast lens wide open when it's not as sharp - either for the shutter speed or the depth of field.
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Old Feb 10, 2009, 3:19 AM   #14
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Great first series with this lens Greg,

I've had that lens now for a small year and I must say that it's one of my favorites (meaning i don't leave home with a camera without it)

the only thing I don't understand is your need to go to f22, even f11 is something I rarely need.

seeing that most of your shots are at 10mm

10mm f8 and focussing at 1m the lens is in focus from 0.36m -> inf.
10mm f11 ... 0.31mm -> inf.
10mm f22 ... 0.18mm -> inf.

the smaller the focal length, the larger the area in focus.


Also I found that using autofocus it sometimes focusses at things I'm not focussing it, so most of the time I just go to manual focus and turn the focus ring to about the distance I wan't it to focus (ex 1m).

ps: really like the last shot

Ronny


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Old Feb 10, 2009, 3:50 AM   #15
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Obviously a very good lens used by a very good photographer. not much distortion here....maybe I need to start looking more at the short side of the lens world more.....Nah!!!! Well not yet anyway.

Dawg
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Old Feb 10, 2009, 9:52 AM   #16
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very nice shots, that place looks very familiar, is it along the pennine way near crowden by any chance?

P.S i also NEED one of these lenses lol
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Old Feb 10, 2009, 2:15 PM   #17
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ohillary wrote:
Quote:
so could someone shed a little light . . .
shouldn't a lens be its sharpest at the smallest diaphragm (in this case f22)?
especially a good wide angle that has a limited DOF.
if f11 is the sharpest for this lens, why even have f22?
you might get more dof but it will be softer throughout, right?

most confusing to me.

thanks for your considered answer.

ohillary

Just remembered having seen an article about diffraction.
About the easiest one I've ever seen (:-) the only one also :-))

http://www.cambridgeincolour.com/tut...hotography.htm


Welcome to the forum ohillary
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Old Feb 11, 2009, 1:39 AM   #18
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Thanks for the link, Ronny. Very interesting and educative.

Kjell
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Old Feb 11, 2009, 3:25 AM   #19
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Greg ... excellent pics, area looks familiar to me too, but I can't place it. ... J

Thanks for that link Ronny, I just had a brief browse ... need more time to absorb the info, will look back later, when I've more time. ... Jack
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Old Feb 11, 2009, 8:04 AM   #20
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Hey Greg,
These are fantastic! that lens is on my short list also.

hummmm... lens hood,
I didn't even know you had bright sunny days in Manchester...
GW:bye:
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