Focusing on a D90

[coombzy]

I'm currently having issues learning how to get the D90 to focus where I would like it to. I selected single focus point, then 3D tracking, but neither made much of a difference and the result was a toss up. The lens is a 35mm f/1.8 which I've had on my D40 for over a year and have never had trouble before. In other words, any slight body movements on my part (which I obviously avoid) after holding the shutter halfway down shouldn't be an issue if it worked with the D40, right? As an explanation on Flickr I was told that the D40 has 3 focus points as opposed to the D90's 11 so less room for error I suppose. But if it's told to focus on a single point, why would it focus slightly behind or in front? I was also told the wider the aperture, the more room for error which again makes sense. But again I wonder why this is only an issue now, after upgrading to the D90. I really do love shooting with f/1.8 and am concerned I have to lower the aperture to compensate for the D90 simply not liking it. I was shooting a little sparrow that was sitting relatively still, only his head moving around, in close range, so I would expect this low action to be something the camera can handle.

I'm still learning the ways of the camera so bear with me. If I have the D90 set to 3D tracking but have it in AF-A does that make the 3D tracking redundant? In other words does the automatic tracking override any other tweaks I make? Does it have to be in AF-C or AF-S? Some said their favourite setting was single focus point with AF-S...

Sorry for all the questions, I'm just a little baffled and hope it's me rather than the D90 being overly finicky.

[OsmosisStudios]

3D tracking is for big sweeping movement, not for twitching birds. For that you'll want AF-C with single point AF, possibly with the burst mode set to High Continuous.

[coombzy]

It did seem like pulling out the big guns, but single point wasn't cutting it. That said, I was likely in AF-A! So by setting it to AF-A I'm handing the reins to the camera to figure out what to focus on, and the fact that single point focus is selected won't matter unless I'm in AF-C?

Thanks for the reply! I was concerned about recalibrating lenses which another person mentioned. And that, to me, sounds like you have to pick a body and stick to it. For now I'm clinging to my D40 like a comfort blanket and have no intentions of putting it away on a shelf anytime soon so versatility is rather critical.

[OsmosisStudios]

Originally Posted by coombzy

So by setting it to AF-A I'm handing the reins to the camera to figure out what to focus on, and the fact that single point focus is selected won't matter unless I'm in AF-C?

Not quite.

AF-A just decides whether to use AF-S (single focus) or AF-C (Continuous focus) by itself based on the subject. If you're in 3D focus with AF-A with a small bird, the bird doesnt move enough to trigger the 3D focusing and therefore isnt enough to trigger AF-A to go from it's default of single focus to the more active continuous.

In AF-A the camera bases it's choice (AF-S or AF-C) on what it sees: movement gets AF-C, stationary subjects get AF-S. With 3D focusing it needs to actually engage the 3D focusing portion to trigger AF-C

As for recalibrating lenses, that depends greatly on what body you have. On the D300 and above, it's possible to adjust focus on individual lenses in camera using AF Fine Tune. THe D700 and D3 series save that information in the camera for each lens (up to 10, i think) but the D300/s does not, and so the setting has to be changed every time you change lenses. While that *can* be a bit of a pain, there are lenses that are notorious for back or front focusing, and really benefit from the settings.

On the D90, to calibrate the lens, you'd have to send both lens and body to NIkon and ask them to calibrate focus for each, to eachother. And youre right, that makes that lens really only 100% useful on that particular body.

The reason for this is rather simple: All lenses and bodies have a certain focus tolerance from the factory to pass Quality Control. If the margin for error is, for example, a fictional +/- 3 units, and the lens is a +2 and the body is a -2, when they're placed together, they negate eachother giving you perfect focus. If, however, the body is a +2 and the lens is also a +2, they'll give unacceptable results even though both items are within QC standards. For the vast majority, this doesnt matter as it's not really examined at that close a level and the requirements are that critical. This is why the AF Fine Tune function is only available in the pro-tier bodies.

[coombzy]

Apparently this camera is a lot more intelligent than I originally gave it credit for, even if I disagree with its decisions sometimes. I'm noticing a dramatic change in white balance too when it sees it fit to alter that.

Thank you very much for this thorough explanation! I'll avoid calibrating in order to keep the lenses versatile and just focus on the autofocusing modes (ie. making sense of them, which this post helped with a lot).

[OsmosisStudios]

90% of photography is practice.

[coombzy]

I think that's why I miss the D40. I know it inside out. With focusing difficulties I was regretting not going for the D300s but if it's not the camera I don't mind learning. I can't help worrying after seeing so many threads on Flickr about D90 focusing troubles.