Low light AF issue with 35mm f/1.8 DX lens.

A couple of weeks ago I wandered into a 'tech exchange' shop. The sort of place that's plastered wall-to-wall with used mobile phones. Anyway, sitting in a display cabinet was an unloved-looking 35mm f/1.8 DX AF-S lens with no price ticket.

Long story short, I walked out with it after paying a relatively meagre amount for it.

I'm knocked out by the image quality it delivers. Less impressed by its focussing ability though.

Strangely, it seems to have problems locking onto a reasonably high contrast subject in Live-View in low light. Despite the fact that I can easily focus it manually a darned sight better in the same conditions. Viewfinder focus is as good as any other lens I possess. It's just Live-View it seems to have a problem with.

Anyone else notice a Live-View issue with this lens?

I'm not thinking of returning it. Just curious why it appears to be reluctant to focus in LV mode. The image hunts around good focus and then settles 'a mile out'. AF fine tune does nothing to affect LV it appears.

Oh, yes, BTW. Camera is a D7200 that usually delivers good focus, The lens glassware looks in V.G. condition; just one miniscule coating scuff off-centre of the front element. Mechanically a bit untidy, but no sign or feel of any real damage.

Live view does not start at a wide open aperture, but at whatever aperture was set before switching to LV. I'd see if it makes a difference whether or not you put the lens at 1.8 before switching. In good light LV focuses pretty well regardless, but in dim light it can be pretty bad, and misses pretty badly when the aperture is stopped down.

If you can see the lens diaphragm from the front you can confirm this by setting it stopped down, and then switching to LV. You'll see that the aperture stops down when you do.

I noticed on the D3200, as well, that although it is technically always in focus priority, when in Live View the program is changed, and although it always tries to focus first, it will fire after a second or two whether it hits or fails. It looks as if the D7100 behaves the same. Since it is very slow to focus in dim light, then if you're using shutter button AF, it might be firing before it's finished, even though you have focus priority set.

Thanks Dieter. I was using the lens wide open, or close to (f/2 -ish). Auto ISO was in the region of 4000 @ 1/60th to give some idea of light level.

I only noticed the combo's reluctance to AF in LV a few days ago. I need to find the time to see if it's light-level related, or subject specific, or lens specific. The lens and camera seemed to have no trouble locking onto the fine mesh of some net curtains - unfortunately, when I pointed the lens at what I really wanted to photograph it wouldn't lock on properly. The subject was inside the room BTW, not the other side of the net curtains or window!

I don't have that lens, but experimenting a bit with a 50/1.4D and a D7100, I note that even in decent light it hunts in Live View when it does not normally. That lens is reasonably quick to focus in the viewfinder, and almost always goes straight to the focus point, but in the same light and same settings, in Live View it hunts. And even when it ends up in the right place, that hunt can take a couple of seconds.

Trying to get a similar exposure level suggests that it's pretty dark. I found it locked on to a door frame fine, but in other circumstances, it didn't do so well. Using back button focusing, so it has time to finish, it would hunt around for a couple of seconds, then stop, but leaving the focus square red.

edit to add: I just tried with back button focusing off, and found that the behavior is just about exactly as you describe. In poor light, the AF gives up before it's settled, and the shutter fires. In decent light it usually gets where it needs to go, but at something similar to the setting and light you describe, I got a blur even aiming at a nice contrasty subject. I think it's just a shortcoming of the Live View AF system, combined with an impatient cancellation of focus priority. It works better with back button, though I've only fiddled a bit with it. But it seems that although it also gives up then, it tries for a little longer, and finishes a bit more often.

OK. Thanks guys!

I dug out an old screwdriver AF 50mm f/1.8 Nikkor, and that did exactly the same as the AF-S 35mm. So I guess it's just a Live View AF phenomenon, and nothing to do with the new-to-me lens.

I'd not noticed the LV AF shortcoming before, and you know how it is when you start playing with a new bit of kit. You get paranoid about any little glitch that you find, and immediately blame it on the piece of gear you're informally 'testing'. I should have learned by now to question the first conclusion to present itself before jumping to it.

Anyhoo. Here's the subject that the lens refused to focus on. This time in slightly better light, and a lot closer to.