"sweet spot"

[ifurlong]

I am wondering if we can start a list of "sweet spots" (max sharpness) of the various lenses we use.

[roanjohn]

f8

Ro1

[Haifidelity]

Just remember F/8.

-hza

[cowman345]

why f8? is there something special about the physics pertaining to this aperture?


-dave-

[ifurlong]

so f8 for every lens?

[msvadi]

May be there are some exceptions? For example, according to www.photodo.com Canon EF 200/1,8L USM performs best at f/4.

Weighted MTF:
f1,8: 0,82,
f2,8: 0,89,
f4: 0,90,
f8: 0,87.

May be that the reason they don't make 200/1.8 L anymore? ;) (but one can still purchase a lens hood for 200/1.8L, it's $450 at B&H ;) )

[PhotosGuy]

so f8 for every lens?

It used to be considered that max sharpness was about 2-1/2 to 3 stops down from max aperture. From what I've been hearing about autofocus & backfocus problems on some bodies, I suspect that only a lab test with a chart followed by a high dpi print will tell for sure. I'm not sure that looking at a 70-90 ppi monitor will get the job done.

[leony]

most lenses perform best at 2 stops higher than max. (for a 2.8 lens that's 5.6, for 5.6 lens that's 11)

8.0 is a safe bet for just about any lens. as you close down further than 16, quality goes down very quickly because of physics laws - that's the reason small-chipped digitals only go up to 8.0 in their apperetures.

if you're shooting an 8x10 view camera, the negative is so big that you can easily shoot at 64 and still get awesome results.

[Tom W]

F8 isn't a hard and fast rule - its a ball-park figure. Some lenses are better at f/5.6 or even f/4 , while others might peak at f/11. And different conditions might cause variance in what is best.

Shooting into a strong light source, for example, will encourage flair. In general, the wider the aperture, the more opportunity internal reflections have of getting to the sensor. So even if you have a lens that does its best on the test bench at f/5.6, you might be wise to stop down a couple of stops in such lighting conditions to reduce flair.

Of course, at the other end of the spectrum, you get refraction problems. As you stop down to a smaller and smaller aperture, the incidence of refraction (light bending at the edge of the aperture leaves) increases as a proportion of the total available light so that images again become softer and less defined.

Good glass tends to give a wider band of acceptable or high image quality, while less advanced lenses, while they may be great at middle apertures, tend to lose image quality faster as you deviate from the "sweet spot" of that particular lens.