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rg828
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High F-stops & Digital

Thu Sep 22, 2005 6:57 am

Weather permitting I usually use apertures of arounf f8-f11.

Lately I've tried using much higher f-stops of around f22, but noticed that despite the decrease in shutter speed, the images dont get any sharper.

My question is whether there is a limit as to how much 'sharpness' a Digital sensor can absorb, if there is a relation between pixel size, sensor resolution, megapixels, etc.

Apologies if this has been asked before
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ChrisH
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RE: High F-stops & Digital

Thu Sep 22, 2005 6:59 am

The pictures wont get sharper per se, all you're doing is increasing the depth of field. In fact, due to diffraction, you shots might decrease in actual sharpness at such high f-stops.

Look for some photo tutorial that explain the basics of aperture and what it does, and you'll figure it out!
what seems to be the officer, problem?
 
JeffM
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RE: High F-stops & Digital

Thu Sep 22, 2005 7:11 am

Quoting RG828 (Thread starter):
I've tried using much higher f-stops of around f22, but noticed that despite the decrease in shutter speed, the images dont get any sharper.

As mentioned, sharpness is not a function of f-stop. DOF is increased, which brings more of the area in front and behind the focal point into focus.
 
dendrobatid
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RE: High F-stops & Digital

Thu Sep 22, 2005 7:19 am

Quoting ChrisH (Reply 1):
In fact, due to diffraction, you shots might decrease in actual sharpness at such high f-stops.

Chris is dead right here. Diffraction will almost certainly reduce the sharpness at very high f numbers.
Most lenses work at their best a couple of stops down from their maximum aperture. On a telephoto/telezoom, unless you are very rich, this is likely to be precisely where you have been, F8/F11

Quoting JeffM (Reply 2):
As mentioned, sharpness is not a function of f-stop. DOF is increased, which brings more of the area in front and behind the focal point into focus.

Being pedantic, this is not stictly correct. Only one plane (not as in aircraft) is ever perfectly in focus. As you stop down the area of acceptable focus, the depth of field, increases....note the use of the words perfect and acceptable.

Mick Bajcar
 
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rg828
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RE: High F-stops & Digital

Thu Sep 22, 2005 7:34 am

Quoting ChrisH (Reply 1):
The pictures wont get sharper per se, all you're doing is increasing the depth of field.



Quoting JeffM (Reply 2):
As mentioned, sharpness is not a function of f-stop. DOF is increased, which brings more of the area in front and behind the focal point into focus.

Thanks guys, I forgot about DOF. However for whatever reason I always associated higher f-stops with higher sharpness. An obsession I suppose!

Quoting Dendrobatid (Reply 3):
Chris is dead right here. Diffraction will almost certainly reduce the sharpness at very high f numbers.
Most lenses work at their best a couple of stops down from their maximum aperture

So its a matter of lens quality then?
If I take the perfect lens ever made, focused on the same plane of a subject and took two shots - one at f11 and another at f22 - besides the extra DOF there wont be any increase in sharpness?

I guess what I want to know is how sharp can a picture get.
I dont know the key to success, but the key to failure is trying to please everyone
 
dendrobatid
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RE: High F-stops & Digital

Thu Sep 22, 2005 7:42 am

Quoting RG828 (Reply 4):
So its a matter of lens quality then?
If I take the perfect lens ever made, focused on the same plane of a subject and took two shots - one at f11 and another at f22 - besides the extra DOF there wont be any increase in sharpness?

Not quite so !
What Chris is saying, and quite correctly is that the image at f.22 is likely to actually be less sharp, though the depth of field will have increased.
That is not a factor of lens design but of the laws of optics. Without getting too technical, the actual aperture, the iris, the physical blades and such, start to interfere with the waves of light at the very small apertures leading to a loss of resolution.
You have been doing the right thing and probably getting the best out of your lens at f8 or f11
Mick Bajcar
 
Newark777
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RE: High F-stops & Digital

Thu Sep 22, 2005 1:25 pm

Don't forget too that the shutter speed will decrease (longer exposure) the higher you go, so some camera shake may come into play at the higher f stops.

Harry
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rg828
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RE: High F-stops & Digital

Fri Sep 23, 2005 1:20 am

Thanks for the explanations, all of you.

Brilliant summary Mick!
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