"Where Is My Lens Sharpest?" - A Good Comprehensive Website

[astrostu]

Folks often say that a camera lens is sharpest at 1-2 "stops" below the widest aperture. For many lenses, this is around f/8. However, this often gets construed to mean that lenses are sharpest at f/8, as that is the aperture that is often told to be used for the moon.

While this rule of thumb may be decent, it is inaccurate maybe 1/3 of the time. Last night I found this website. If you click on the review for any of the gagillion lenses that they review, go to the second page, and it shows a sharpness test for every-other f/number. I highly recommend referring to it when trying to figure out where your lens is sharpest.

For example, below is the sharpness chart for a lens I have, a 35 mm f/1.4L Canon. The higher number, the sharper the image. For this lens, if your subject is in the center, then using the lens at f/2 is about as sharp as you can get, though f/2.8 gets you about 1-2% finer resolution. Objects around the border require going to f/8 to get comparable resolution.

For this Canon telephoto zoom lens, 55-200mm f/4.5-5.6 USM II, the sharpest is around f/5.6 at 55mm, but it shifts to closer to f/8 at 200mm.

Here's an example of a Nikkor 300mm f/4D lens. In this case, the sharpest in both the center and edge is at f/8. However, attaching an extender both lowers the overall sharpness and moves it to around f/9.5.

Here's an example from a Pentax SMC-K 135mm f/2.5. It shows the lens is sharpest around f/5.6:

 

[goodoneian]

that's a good website, i discovered it a few days ago myself and have been looking through it a lot

 

[craig]

Excellent link! Thanks.

)'(

 

[Josh66]

Pretty cool.

Kinda confirmed what I already knew (keep it between f/4 and f/8 for the most sharpness), it's nice to see it graphed out though.

I was surprised by how close my lenses are to each other on there.

The drop-off after f/8 on the 100mm Macro surprised me too, I knew it dropped off - but I didn't think it was that much!

 

[JerryPH]

Yeah, that drop off phenomenon is normal on most lenses. When I talk to high end portrait photographers, they get this hard-on about using 2000W/s monolights at full power and using F/32 or even F/64 apertures and about how sharp their pics all come out.

They get all insulted when I tell them that if ultimate sharpness is their goal to crank down the power because they past their lens' best sharpness point 3-4 stops back... lol

Different lenses have different levels, though. For example, my Nikon 18-200 loves F/11-F/13. F/5.6-F/8 for the Sigma 18-50. For the Nikkor 85mm F/1.4, it is wierd. It is tack sharp at anything between F/4-F/16, no real peak, just a large plateau.

 

[AlexColeman]

Thats a good website. Kindof shows F8 and stay shouldnt be a stead fast rule.

 

[ksmattfish]

The softness caused by very small apertures (and these days very small photo receptors) is called aperture diffraction. It's not all about the lens.

Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

 

[JerryPH]

The softness caused by very small apertures (and these days very small photo receptors) is called aperture diffraction. It's not all about the lens.

Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

Oh I knew about diffraction, but was just referring to the actual lens sharpness point of it. It is true that past a certain point, softness will become more a function of diffraction than lens design. Nice site, effectively explained.

 

[JerryPH]

Thats a good website. Kindof shows F8 and stay shouldnt be a stead fast rule.

Well the old "F/8 and be there" rule is just a general rule about DOF and not lens sharpness. Meaning your focus could be off a little, but because of a deep DOF, the picture will still look good printed. It is a valid "rule", kinda-sorta... and as we know, rules are there to be broken when appropriate.