issues with autofocus (Nikon)

Hugh_C

Hi All,

I've been having an issue with some of the portrait/beauty stuff I do (Nikon D300, 50mm f/1,8) whereby the system sometimes can't autofocus. I'm generally about 1 to 2 metres from the subject in a dimly lit room using AF-Single (S mode) and Dynamic-area AF (middle position) with manually selected focus points on the rocker switch at the back. I use a couple of strobes for lighting.

My question is: Is the camera responsible for the dependability of AF or does it also depend on the quality of the lens?

I'm aware that the 50mm f/1.8 is cheap as chips and the build quality mightn't be all that it should be, I'm also aware that I don't have a complete understanding of the AutoFocus system on my camera - the manual is all over the place.

Would I be better off upgrading to a 50mm f/1.4?

Any thoughts?

Thanks,

Hugh

magicbastarder

does the autofocus assist light come on?

Hugh_C

occasionally. Can't remember how I have it set, don't have the camera nearby.

magicbastarder

i assume it only comes on when the AF system judges it to be necessary; i'm not sure if that's based on light or contrast levels.

when you say the lens can't focus, i assume you mean it hunts rather than it just sits there dead as a post?

eas

Both camera and lens play a part in how fast/accurate is will focus.

The 1.8 is a fine lens, unless there's a fault with yours it's not part of the problem here.

your camera will start to struggle in dim light particularly if the selected focus point is towards the outside of the array. I'd suggest keeping your focus point in the center and recomposing if required. Also, as had been mentioned use the focus assist light on your body or strobes.

Thecageyone

Have you tried Manual focus in this situation? If it's for portraits you're as well off using MF. Or, try putting the cam into release priority over focus priority. You'll get the shots, and judge yourself if they're in focus or not. Must be some kind of light conflict.

Hugh_C

magicbastarder wrote: »

i assume it only comes on when the AF system judges it to be necessary; i'm not sure if that's based on light or contrast levels.

when you say the lens can't focus, i assume you mean it hunts rather than it just sits there dead as a post?

Thanks mb,

I think this is the case, yes, only comes on when the camera thinks it's necessary. And yes, the lens hunts and doesn't latch onto anything.

@eas: I use the strobes off-camera so I can't use their AF assist lamps. The 1.8 is pretty much my workhorse a brilliant lens, I occasionally use an old MF 85 1.4 which is a great lens but is awkward in the fast-paced production environment I work in.

I suppose I'm wondering if the 50mm 1.4 would help ...

Hugh_C

Thecageyone wrote: »

Have you tried Manual focus in this situation? If it's for portraits you're as well off using MF. Or, try putting the cam into release priority over focus priority. You'll get the shots, and judge yourself if they're in focus or not. Must be some kind of light conflict.

Yes, I override to manual in cases where it hunts (but doesn't find). To be honest I haven't tried the release priority because there's a lot of pressure in my sessions to turnover fast, so I have to rely on AF to get me through the session.

Thecageyone

Unless the 1.4 focuses a lot quicker there's little difference really. Both have the same minimum focusing distance.

Fajitas!

Tbh, it just sounds like you don't have enough light in the studio. I'm going to assum you don't have modeling lights on the strobes, so there's two relatively easy ways of getting around this - First one, get an LED flashlight, use it to focus with AF, turn it off, and you're good to shoot - The second (which I'm not sure you can do on your D300, and requires a tripod) is turning on liveview, getting your focus, when you're happy, go back to the viewfinder.

Tbh though, I'd rather the investment into having modeling lights.

The 50 1.4 will have better build quality, I'm sure, it might have better IQ, and it might focus in good light ever so slightly faster, but I don't think it's the answer to your problem. Whether it's a 50 1.4 or an 85 1.2, if there's not enough light to focus, it won't focus.

eas

As Fajitas said - the 1.4 will focus a bit quicker and quieter in good light, but will hunt just the same if there isn't enough light.

Before making any purchases - try my tip above about keeping your focus point dead center and recompose if required. I can focus in very dark conditions as long as there is something for it to grab onto and the focus point is in the center.

Hugh_C

eas wrote: »

keeping your focus point dead center and recompose if required.

Thanks eas, I think this'll be what I'll try first.

Hugh_C

Fajitas! wrote: »

Tbh, it just sounds like you don't have enough light in the studio. I'm going to assum you don't have modeling lights on the strobes, so there's two relatively easy ways of getting around this - First one, get an LED flashlight, use it to focus with AF, turn it off, and you're good to shoot - The second (which I'm not sure you can do on your D300, and requires a tripod) is turning on liveview, getting your focus, when you're happy, go back to the viewfinder.

Tbh though, I'd rather the investment into having modeling lights.

Thanks Al,

I have liveview but it's so friggin slow to use in the studio when the model is flailing round pulling poses that's it's just not even a consideration. You'd lose the moment and so many of them ...

Must try the led flashlight idea, I've read that people don't bother turning them off, unless you're wide open the strength of the flashlight is completely overpowered by the strobe.

Fajitas!

A bit of gaffer tape/no more nails and you'll have a hotshoe mounted LED flashlight too, which saves a hand. And yep, as you said, you're often going to be overpowering it by your strobes. Sent you a PM about the 1.4 too.

eas

Hugh_C wrote: »

when the model is flailing round pulling poses that's it's just not even a consideration.

If your models are moving fast and often, it may also be a good idea to switch from AF-S to AF-C. Actually, for people AF-C makes more sense to me 100% of the time.

Hugh_C

eas wrote: »

If your models are moving fast and often, it may also be a good idea to switch from AF-S to AF-C. Actually, for people AF-C makes more sense to me 100% of the time.

Interesting thought, I've tried the AF-C option a quite often for flaily (?) models, but it still hunts and fails.

eas

yeah - not a solution to your problem with focus hunting in low light - but imo a better option in general for the type of work it sounds like you're doing.

Borderfox

The light levels must be really low for the lens to hunt, the 85mm f1.2 I have will focus in near darkness even on the 5d. I have moved from the 50mm f1.8 to the f1.4 and find the focussing more accurate but it still hunts in the same low light situations that the f1.8 would have.