Nikon Focusing

[chase3311]

I do not understand the Nikon focusing system when it comes to nighttime Photography. Why dose the infrared not lock on its target. About 80% of the time they just will not fire. Maybe someone can shine some light on this subject.

[Simon Henson]

It's my understanding that in order for the auto focus to 'lock on' to the subject, it requires some contrast or edge detection. In other words, if the scene has very little tonal differences (like in fog, smoke etc.), the information received by the camera is variable.
Is it possible to manually focus on these occasions?

[The Stig]

Fast lenses generally are better at focusing in low light than their slower counterparts. This delivers more of the available light to the AF system, and makes the job easier for the AF system. As Simon says, if there's little or no contrast the AF system won't have anything to work with and will just hunt for focus without locking on to anything. Manual focus is one option, but I've found with my not-so-fast tele zoom that getting focus lock (or close to it) at the short end of the zoom and then zooming in helps to some extent.

[Dave Humphries]

I do not understand the Nikon focusing system when it comes to nighttime Photography. Why does the infrared not lock on its target.

Hi Chase,

Possibly because all the cameras I know do not use 'infrared' to focus with, not in the way you might be thinking, e.g. like electronic tape measures?

Whilst it is true they have a focus assist illuminator, this is usually just a green or white LED and is a light source that is to provide light a close subject and allow the contrast/edge detection mechanism described by Simon and Stig.

It would help to know what you're trying to shoot, perhaps if you could describe it, or better still give us an example image (one that did 'take').

Cheers,

[Nomadr]

if you use a flash (sb600 etc) and set the camera to single servo it should fire an infra red light to assist autofocus, it works pretty well,cheers martyn

[chase3311]

I am an event photographer mostly classic cars and races of all sorts,
Sometimes with the bigger objects you don’t have the option of getting close enough for the Illumination Assist to do the job. At this point budget is an issue for a new faster lens. Sometimes I will take a small flash light with me to shine so I can get the focus to lock.

The thing though is that candid spur of the moment picture, indoors or out in a low lighting situation, a lot of the time I don’t get the picture. If I do get it to fire the shots come out very clear.

I suppose a more current model flash could help things.
thank you for your feed back.

Nikon D3000 & Dx40. both with 18-55

[Black Pearl]

Try knocking the multiple AF points off and just have the centre one selected. Although Nikon (the others are just as bad) say the camera will track and lock onto all sorts of subjects with all the AF points working they simply won't. A Pro body might as they have hugely powerful and dedicated auto focus processors but a D3000 or even worse a D40x won't stand a chance. Also the outer AF points are less sofisticated than the centre one and if the camera has one of these selected then again it will struggle in poor light.
Can't say if this is your particular problem but it might help lift your keeper rate even a little which means more money in your pocket.