I gave a friend of mine an 85 1.8D as a gift portrait lens. It was new then, but it's been a couple years now. After recently buying a new body, he's discovered through careful and exhaustive testing with this and other lenses and different bodies that the lens is not sharp at 1.8, or even 2.8. It starts getting crisp at f 4.0 which makes it pretty useless as a portrait lens. Details like hair and skin are clearly soft despite how the lens is focused (MF or AF). We are both certain that this isn't a matter of operator error. Could there be some "copy variation" that would account for this? What could be wrong with it? I realize of course I could just send it to a repair center and spend additional money to "maybe" have it fixed.
Can you provide a sample image at a couple of f/stops. Just from the scenario you give the best I can recommend is to take the lens to a qualified repair center and see what happens.
It would be useful to know what bodies you tried this on, how you determined focus, and what was the focus target. You might also try just the traditional "brick wall" test from a tripod just to eliminate questions about using a three dimensional surface. It's not very useful to know that hair/skin are soft if we can't visualize where these objects are in relation to the desired focus point and the f/stop used.
Roger It's still, ISO, aperture and shutter-speed, right?
Aperture, shutter speed and distance from subject are also important to have along with a sample image. Quite often a thin depth of field (wide aperture at short distance) or camera shake (from a too slow shutter speed) are confused with the lens not focusing.
Have you checked for front/back focus problems? They are more apparent at open apertures where dof is thin and, depending on the severity of the problem, may seem to disappear at smaller apertures. I have a 85mm 1.8D too, mine has a back focus issue (easily fixed through AF fine tune). It is pretty sharp at f1.8 or f2.0 but not quite as much as my 24-70 overall.
Sat 05-Jan-13 10:21 PM | edited Sat 05-Jan-13 10:22 PM by vancwa
My friend printed a resolution chart and mounted it to a flat surface. (This was admittedly not the "real" ISO standard resolution chart, but good enough for subjective comparisons.) The camera is a D90 and was on a tripod and carefully placed at 90 degree angle to the chart. I really don't know the distance but he probably came close to filling the frame.
He shot several sequences - 5 at f/1.8 using AF, 5 at f/1.8 using MF, 5 at f/4 using AF, 5 at f/8 using AF. All shots within a group of 5 look identical so no point of posting more than 1 here. The AF vs MF look the same at 1.8. The f/8s were, of course, very sharp.
I don't believe there is any way to adjust a D90 for for front or rear focus. What do you guys think?
It helps to get better light/exposure on the test chart... a quick correction in PS Curves helps, see attached...
Clearly at f1.8 the corners are softer, but for portraiture that's desirable so no worries there. As for the centre, Well there's definitely a difference between f1.8 and f4 but I don't see it being all that problematic. No lens is at its best wide open and if this one at f4 is acceptable to your friend I'd say that the f1.8 sample is relatively acceptable too.
To my eye the corners in the AF version are slightly sharper than MF