How Does Aperture affect sharpness?

^Joe

^Joe

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I've wondered this for a while now..

I have had a search and see that, for example, a shot @ f1.8 is not as sharp as f2.2.

Also someone shot at (eg) f4, and was reccomended to shoot at f8 (on tripod) to increase sharpness.

I am happy with how exposure works but I was wondering how the aperture affected the sharpness, is it because at lower f numbers, too much light is being sucked in? :thinking:

Cheers
 
Most lenses perform at their best when stopped down a bit, the central part of the glass is probably slightly better than the periphary in manufacture, so stopping down uses the better part of the lens. However go too far and diffraction then becomes an issue, f8 is usually considered the optimum.
 
Most lenses have a "sweet spot" usually around 2 stops from maximum (f2.8 lens sweet spot f5.6). When a lens is stopped down too much diffraction causes less sharpness.
 
Sharpness does increase up to a point as stated above the sweet point is usually 2 stops narrower than max (or thereabouts).

So for a lens that is f1.8 the sweetspot would be around f3.5 or f4. For a lens with a max aperture of f4 it would be around f8.

That said your images should still be pretty sharp even wide open at max aperture. What will change is how much of the image is in focus as narrow apertures allow large depth of field so a lot opf the image appears sharp and large apertures like f1.8 have very narrow depth of field so focus has to be much more accurate. This also depends on the magnification of the subject in the frame too.
 
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^Joe said:
I've wondered this for a while now..

I have had a search and see that, for example, a shot @ f1.8 is not as sharp as f2.2.

Also someone shot at (eg) f4, and was reccomended to shoot at f8 (on tripod) to increase sharpness.

I am happy with how exposure works but I was wondering how the aperture affected the sharpness, is it because at lower f numbers, too much light is being sucked in? :thinking:

Cheers
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You may be confusing sharpness with depth of field. DoF is the zone of acceptable sharpness from near to far within a scene, and it gets deeper as the f/number rises (it also changes with distance and focal length).

Ultimate sharpness is something different, and this usually occurs around f/5.6 on a crop format DSLR, or f/8 on full frame as it's dependent of format size. Compacts for example, because of their tiny sensors, peak at much lower f/numbers, but in no case is it automatically 'a couple of stops from max' or whatever - that's just a coicidence. If you had an f/1.4 lens for example, it would keep getting sharper well past a couple of stops.

The aperture of maximum sharpness is where the reduction of aberrations made by raising the f/number, are overtaken by the losses caused by diffraction which increases at the same time. Most improvement is around the edges and corners - central sharpness is always pretty high. All lenses, regardless of quality, are affected by diffraction just the same.
 
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Ollieeee said:
Something i've read many a time now yet still forget to stop down a bit, need to drill it into my head!
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Stop down for the depth of field you require, don't worry too much about the absolute lens sharpness, nailing the focus is far more important.
 
redddraggon said:
Stop down for the depth of field you require, don't worry too much about the absolute lens sharpness, nailing the focus is far more important.
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:thumbs: Yes, good advice. And make sure the shutter speed is high enough to avoid any motion blur, either camera-shake or subject movement.
 
Good advice from Hoppy and reddraggon.

Also on the stopping down point about it using the center of the lens, as that tends to be shaper which is true, this is why the same lens can look sharper on crop camera .
 
acetone said:
Good advice from Hoppy and reddraggon.

Also on the stopping down point about it using the center of the lens, as that tends to be shaper which is true, this is why the same lens can look sharper on crop camera .
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That would be unusual. If you swap the same lens, on full frame the centre will always be sharper because the image has to be enlarged less for the same size output.

The edges are a different question because they're not seen on a cropper, but on any decent lens they should be pretty good too and so, overall, the full frame image usually wins quite easily.
 
I meant the edges, for the reason that the cropper will as you say not use the very outside of the lens.
In real world use I doubt we would notice unless pixel peeping.
 
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acetone said:
I meant the edges, for the reason that the cropper will as you say not use the very outside of the lens.
In real world use I doubt we would notice unless pixel peeping.
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Sorry to disagree again, but you don't have to pixel peep to see that full frame is better. Sure, the advantage in sharpness is not that obvious in smaller outputs (or on screen) but it certainly is in an A4 print. Better sharpness, less noise, cleaner colours. The sensor has more than twice the imaging area.
 
HoppyUK said:
Sorry to disagree again, but you don't have to pixel peep to see that full frame is better. Sure, the advantage in sharpness is not that obvious in smaller outputs (or on screen) but it certainly is in an A4 print. Better sharpness, less noise, cleaner colours. The sensor has more than twice the imaging area.
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I can't disagree :p
 
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