Nikkor 50mm f/1.8 issue

WelliesNBrellas

My D3300 came with two kit lenses, the 18-55mm and 55-200mm. My experience with lenses is limited to these two zooms.

A couple months ago, I got a Nikon (Nikkor) 50mm f/1.8 prime. I expected to be wowed by it's speed, which is indeed nice, but am having some issues with focus. My problem is that I'm not versed enough in lenses to know if my issues are my fault or due to the lens itself. The focus is nowhere near as crisp as I expected; a sharp image is more the exception than the rule, which is very disappointing. I get better, and more dependably sharp, shots with my kit lenses, but is that due to using smaller apertures?

Today I received a set of cheap 58mm Vivitar close-up filters sized to the 50mm, otherwise identical to the 52mm Vivitars close-ups for my kit lens. Now, the set for my kits focuses just fine and the biggest compromise is expected color aberrations (after all, a set of 4 is only $10USD). But the focus point on the 50mm prime with the filters shifts WAY off center, where I set my focus point. Does this mean something is wonky with the lens?

Is it me, or is this something I should send my lens in for repair while it is under warranty? I feel like a brand new driver trying to figure out if I've just bought a lemon car. Not enough experience to judge where the issue lies.

Thanks!
Rose-Marie

bruto

I would guess that the problem when the filters are on is more likely in the centering of the filters. Inexpensive close up filters could easily be offcenter.

As for the lens itself, I'd first try aiming it at something with a complex pattern or texture (brick walls, newspapers taped to the wall, and oriental rugs are good). Stop it down to f/4 or so, focus and take a flash picture. If it's sharp, the lens is at least not soft, and your next mission is to try to figure out if the focus is accurate. There are various ways to do this, some complicated and some less so but also less accurate. For starters, though, you could put the camera on a tripod, open it up to 1.8, and focus on that brick wall or whatever again. Do it first in Live View with a reasonable amount of light. (edit to add: make sure there's enough light, as Live View especially does not do so well in dim light. If need be, use a flashlight to set the focus and then shut it off when done.) That's the default, as Live View focus is error free. Do it a few times to eliminate mechanical variation, defocusing each time first and then refocusing. You can use the flash to eliminate any likelihood of camera shake. A tripod is best here. You can try it at different distances, but for starters try it at about 4 feet, far enough not to have close focusing errors, near enough to see the details clearly. Now switch to viewfinder view and do the same thing. If the images look the same or very close, you're OK. If not, you may have inaccurate focusing, and either the camera or lens (in this case probably the lens) needs to be adjusted.

Along with lens decentering, it is also possible for the camera itself to have its focus points off center, and possible that the slower kit lenses have enough depth of field to hide this. If you're getting good results with the kit lenses, I'd suspect first the closeup filters, and then the lens.

You can usually check a lens for basic decentering if you do the tripod test on a field that is very flat, and that is guaranteed to be parallel to the camera's image plane. Brick walls are the traditional subject for this kind of test. Shoot wide open. You can expect the image to begin to lose sharpness at the edges, as most do, but the softening should be fairly uniform between left and right, and between top and bottom. If one corner is significantly blurrier than the others, the lens may be defective.

Here's a 50mm 1.4D that's reasonably well centered. As you can see, when shot at 1.4 it's quite decently sharp, but not so much that your eyes will bleed. This is on a D7100, similar in sensor to the D3300.

http://jmp.sh/v/DAPmT9ZmkZ2r7U0mJwGO

WelliesNBrellas

Thank you ever-so-much, Bruto. I appreciate your description of the process for doing a focus check. Much less complicated than many I've read (and therefore not tried to do). I'll give it a whirl and report back here.

The filters were quite inexpensive, so I would not at all be surprised to find there is an issue with them. The curious thing is that each one in the set focuses below the center point. But that is for another day. I'll try to get the matter of the lens settled for now because that is the big thing for me.

Since my sweetie pie husband is hinting that he might get me a real macro lens, the close-up filters may eventually become a moot point.