Other New lens.. focusing issue or user error?

I got a new Nikon AF-S DX 35mm f/1.8G a few days ago, and so far I've been having a hard time with the focusing (on D3100). I understand that at f1.8 depth of field can be very narrow, and also that the focusing points aren't necessarily confined to the dot you see in the viewfinder, and the camera will focus on something contrasty within a certain area surrounding the focus point. I took some photos of people, focusing on their eyes, and sometimes it focused in front, sometimes behind. I took some more controlled shots to rule out subject movement and such. These were taken at f1.8 on a tripod, with a remote shutter release, under consistent lighting, and the subjects were static. I feel like the lens/camera should be able to nail focus with ease under these conditions. (the ones with the red rectangle were shot with autofocus, the red rectangle representing the focus point as displayed by Nikon's software, the others were focused manually)








In liveview it focuses better, but not always as accurately as manual focus.

This last one was shot handheld at f2.8, but at 1/320s so hand holding shouldn't have induced any lack of sharpness, and at f2.8 and being a few meters away, the depth of field shouldn't be razor thin. It looks very soft for f2.8 on this lens, to me.

 

It's possible that the lens isn't properly calibrated and will need serviced, but it's also possible that you're not going about it the right way in terms of your AF setup.

A word of advice: always use the centre focus point when autofocus is enabled. If you want to focus on something outside of the centre, simply focus on that thing and then recompose the shot, keeping the shutter release half-pressed. The centre AF point is usually the strongest one, especially on cheaper DSLRs (like the 3100) and you absolutely must nail the focus if you're shooting at f/1.8 and similarly small f-numbers. Forget about the other focus points... they are there to make the spec sheet look pretty. (I don't mean to sound like Ken Rockwell but hey, I think with AF there's a line to be drawn...)

I sold my Canon 50D not because it wasn't a good camera; it just didn't have very good AF (but then I was comparing it to a 1DsII at the time...hmmm). With reasonably good glass like the 35 1.8G, you want to pair it with good AF.

 

Thanks I used single point af mode with the center focus point for every shot except the motorbike I believe. And you are correct, the center focus point on the D3100 is a cross type focus point whereas the others are not. The focus system on the D3100 is probably quite poor, but those shots were under friendly circumstances. If it's so hard to focus when both the camera and subject are static then I think I will have problems with the lens xD

 

Yeah it's definitely the lens, which is a shame. Rather than have the lens fixed you should be able to do a simple swap/replace with the store you bought it from.

 

Ok, thanks for the help I think I will return it to Amazon, if the replacement still isn't right or if it's a body issue I can always send it to a Nikon service center I think, they can probably calibrate it. Thanks

 

Looking at the pictures of the microphone (?) it seems like the lens is front focussing as focus is on the black ring not the dial when using AF.

You can test this by using a focus chart, it will show you if your lens is front or back focussing.

Here's a decent how-to: http://regex.info/blog/photo-tech/focus-chart

Unfortunately the D3100 has no AF micro-adjustment so you cannot correct this in-camera.

 

It is indeed a microphone It definitely seems to be front focusing in some shots but, confusingly, it seems to back focus too Like in this shot, for example (shot on a tripod at f1.8, 1/800s, ISO100)



Maybe something in the lens could be misaligned or something? I don't know

I will try the focus test chart today, thanks

 

I tried some shots of the focus chart

AF with center focus point


AF with liveview

 

I can't see past the purple fringing, so the focus problem is moot. The small focus issue can be corrected but bad optics is bad optics.

Also, and I'm in the minority here, but the shallow DOF is never a good time to be nit picky on auto focus. Stop it down to f5.6 and then tell us if you think things are still wonky.

 

Haha, indeed I think cheapish fast primes are susceptible to that purple fringing in the out of focus areas though, especially wide open. They're called longitudinal chromatic aberrations if I remember correctly. If you ramp up the saturation you can see more clearly where they shift from magenta in the foreground to green in the background. The plane of focus should lie where those colors transition, I would assume.

saturation is bumped up on this one, for emphasis


And sure, at f5.6 the problem, if there is one, will be masked because focus isn't as critical. Whatever needs to be in focus will probably fall within the much wider depth of field. And the LoCA's should be significantly reduced. But my kit lens can shoot 35mm at f5.0. This lens has a fixed focal length, and doesn't have VR (not that I use it much), and was still more expensive than the kit lens, so I'd just expect a lot more from it I guess. I know there's more to the picture than sharpness and such, but I'm happy with the sharpness when the focus is correct. And what good is fast prime if it's not really usable until it's stopped down to f5.6?

 

You don't buy the f1.8 to shoot at f5.6, do you? Well unless you're very fast.

The same lens on my D3100 does very well, only problem I have is that the autofocus isn't very fast in low light*, which is what I bought the lens for...

*in near darkness (where the lens will still produce proper pictures) it doesn't reallly do AF at all.

 

Thanks for the info Xir

I sent some of the images to Nikon support, as well as some images shot with the kit lens, and they said there is a focusing issue and that it was likely an issue with the lens, and that I should return it to the retailer. I returned it for a refund, haven't decided if I want to try a new copy yet Thanks for the help everyone

 

Some advice from experience as this has been my job for the last decade.
Always choose the focus point yourself. Don't let the camera choose it in ant automated way, you may get away with it at f18 but at f1.8 your camera had better be focused on the thing you want sharp!

Get out of auto modes and live in M or A, you're the boss but you can still have the camera do done work for you.

I have the 35mm 1.8 and it's a good lens but the 50mm 1.8 is alot better, if you're working in an environment where that goal length is OK for you.

All lenses have a sweet-spot, the aperture where it's sharpest and it's not 1.8! But we but fast glass to shoot fast but be aware you're not getting the very best from the lens.

I'm around elsewhere in the forums if anyone ever needs any specific advice.

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Awesome advice, thanks I use the center point most of the time, and A or M mode. It's nice to have the camera take some of the workload in A mode, while still being able to dial in compensation as needed.

And yeah, there are some sacrifices at 1.8 Although I think with the primes performance is already looking quite a bit better by 2.8, which still isn't too slow. But when you need speed you need speed, and some sacrifices are acceptable

In what way is the 50 better than the 35? I've actually been eyeballing the 50 since returning the 35. I have a 50mm e series, but manual focus can be very difficult on the D3100, especially for moving things, and there is no metering, so it's only practical under certain circumstances.

Thanks for the advice

 

The 50mm is regarded as one of the best lenses Nikon ever made, personally I like it because it's pin-sharp and it doesn't really have any issues at all. I tend to use it in outdoor locations due to it's focal length being quite long on a DX sensor (the 35mm is equivalent to 50mm on full-frame), and the 35mm in indoor locations.
Although neither of these is my most-used lens, I most often use a Sigma 2.8 17-70mm.
If light is an issue I'll switch to one of the fast lenses before pushing the ISO too far.
For completeness I also have a Nikon 70-300mm and a fisheye addon for the sigma or 35mm and that completes my collection of glass (for that camera).

 

Thanks, that's very helpful 50mm is a nice focal length, even though things can get a bit tight on DX. Have you noticed any difference in autofocus speed between the 35 and the 50? The Sigma looks like a very versatile lens, I can see why it's your most used

 

hmm, not really, they are both pretty much instant in the right conditions, sometimes the trick can be putting your focus point onto something that it's easy for it to see, of course I tend to go for these lenses in low light which makes AF harder and at 50mm distances the AF lamp doesn't help much, but in comparable lighting I'd say they're similar in AF speed, even though the 50mm I have uses the camera body AF motor and I'm pretty sure the 35mm has one built-in.

 

Thank you, that's very useful information I sometimes use a flashlight to focus in the dark when the AF lamp doesn't reach, but yeah, I generally don't expect much from the autofocus in low light

 

The hotshoe flash I have emits a nice laser-looking grid for af-assist but I barely use it on-location.

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That sounds pretty awesome though