Small Aperture - True or False

[Paul in OKC]

As it pertains to the question I asked about the focus problem on my D70, is the following statement true or false: If you want a "razor-sharp" focus, use the smallest aperture you can get by with.

[Peter_AUS]

Yes and no. There are other factors that come into play as well. What is the general sweet spot of the lens as well. You need to have the camera on a tripod, use a shutter cable and have mirror lockup selected as well. The subject needs to be lit properly, not moving as well.

All this will generally get you the shaprest image.

Anyone of these factors out of play and the game changes to take into account that missing element.

[Chunk]

As it pertains to the question I asked about the focus problem on my D70, is the following statement true or false: If you want a "razor-sharp" focus, use the smallest aperture you can get by with.

Generally no as I understand it. The sharpest resolution of of a lens is closer to it's middle apertures than it's extremes.

Focusing with the smallest aperture at the hyperfocal distance for that lens DOES give you the greatest depth of field available to that lens but not the sharpest picture.

[Sebastian]

Chunk is correct, diffraction will cause the smallest aperture to soften the image.

[Chunk]

Chunk is correct, diffraction will cause the smallest aperture to soften the image.

Thanks Seb. I think I may not be completely correct. After writing that response I felt kinda guilty for spouting off remembered stuff from articles I had read many years ago when first trying to make sense of all this.
A google search on "lens sharpness" included this tutorial on the Luminous Landscape site.
http://www.luminous-landscape.com/tutorials/sharp.shtml
Since I enjoy the humor and mix of technical and practical advice on that site I went there first. In this article he says "Prime lenses usually have larger apertures than zooms, and the laws of optics say that all other things being equal (which they often aren't) a wider aperture lens will be sharper than one with a smaller aperture — the issue being diffraction effects."
Does that mean that a given lens is sharper at f1.4 than f8 or just that an f1.4 lens will be sharper at f11 than an f4 lens will be at f11? I had always thought that a lens was sharpest in the middle of it's aperture range but maybe I'm wrong and it's sharpest wide open? That doesn't seem logical to me with all the aberations hanging around the outer edges where the light is tightly bent.

Anyway I think the article is worth a read. He also points out several of the points that Peter has already made in his reply. There are some links to other related articles on the bottom of the page.

I guess I could read some other more technical sources, but I already have my headache for the day.

[Franglais]

As it pertains to the question I asked about the focus problem on my D70, is the following statement true or false: If you want a "razor-sharp" focus, use the smallest aperture you can get by with.

Plunging into my collection of Chasseurs d'Images lens tests the story for definition (sharpness) is pretty much the same for all 24x36 and APS-C lenses, including the 18-70 DX you've probably got on your D70 :

- the lens starts with "good" performance at full aperture
- two stops down from full it reaches a peak (very good)
- performance drops off a bit at f11 but 's still very good
- at f16 its gone down to "good" due to diffraction starting to appear
- at f22 a sudden slip to "mediocre" with diffraction going full blast

This lens gets very good results. Contrast is consistently high, even at full aperture. Its only weakness seems to be vignetting at 18mm and f3.5

Charles

[another view]

I agree that sharpness and DOF are two different things, and personally I rarely use f11 or higher (smaller aperture). At smaller apertures, usually everything appears a little soft - even though it is in focus. The other thing is that by having areas that areoutof focus, the area in focus will appear sharper - it's more of a subjective thing.