There’s this famous urban legend about a woman who has a lump growing on the side of her face and when she finally goes to the doctor, he cuts it open with a scalpel and millions of baby spiders crawl all over her face. Then there’s the other urban legend about F8 being the sharpest f-stop on every lens.
The first was made up to scare the crap out of us about spiders and the second may have been true for lenses built last century, but certainly doesn’t apply to digital lenses.
The truth is not all lenses are created equal and the “sweet spot” or aperture setting that will create the sharpest image will vary from lens to lens. (If you really want to geek out on this subject and find out the science behind lens sharpness, check out Ken Rockwell’s blog)
So here’s the non techy lowdown on lens sharpness
So just to clarify, by sharpness I mean actual focus sharp not depth of field sharp.
Most lenses are sharpest 2-3 stops from either extreme. My workhorse lens is the Canon 70-200mm L IS. I use it for approximately 70% of my shoots and know after taking hundreds of thousands of shots that at full zoom of 200mm this lens is sharpest at f5.6 and f11 ½ and starts to lose a bit of sharpness at f161/2-F22 and could be a bit hit and miss at F2.8
If I apply the 2-3 stop formula on my F2.8 70-200mm lens, I end up with a sweet spot of f5.6-f8 if I want to shoot on a wider aperture and f11-f16 if I want to shoot at a narrower aperture.
Having said that, all lenses are slightly different so it’s worth investigating your own lenses’ sweet spots for future reference. Other influencing factors will be proximity to subject, zoom factor, light quality and shutter speed. Make sure when you test that all the variables are consistent.
Oh, and just as a side note: the lenses I’m working with today are sooooo much sharper than the ones I was using 10 years ago. So unless you are planning on shooting diamonds for billboards or super high end million dollar macro shoots, I wouldn’t worry too much. A $100 lens today is better than $1000 lens I was working with last decade. For the record I shoot the majority of my portraits at f2.8 (wide open) and rarely have any issues.