Need help with theory

[J.davis]

I have a question -

The Problem -
I was taking backlit photos of my Grand daughter, and my ancient 85 1.8 Nikkor was having trouble focusing.
I was in a darkened part of the rumpus room and my GD was in the centre of the room with a far light behind her head
Settings where ISO 100 @ F2 - spot metering. Nikon D7000.
I would press the shutter and it would not fire on every second shot.

The Question -
Would increasing the ISO have any effect on the lowlight focusing.
(Had I thought of it at the time I would have tried it out.)

Any thoughts?

[fess67]

Hmm. I am not sure to be honest. I know that during focusing the lens is always wide open and stops down only when taking the shot so that would not affect the focus. Focus needs contrast so if it is dark and there is no obvious edge to pick out the focus will hunt and likely miss. More light on the subject is required.

I am not sure if an increase in ISO will help since it is not casting more light on the subject - I suspect the ISO is not a factor here.

What if you could use a flash but set the flash to not fire....the flash puts out a beam of light to help focus so that would solve that issue - then if it can be stopped from firing (or set to a very very low setting) then that might help............................Of course I may be full of it but at least I am thinking :D

[WhoDo]

D7000 has a focus assist light for just such an occasion. Turn it on. ISO adjustments won't help.

[ricktas]

focusing is contrast based, so if you focus on an edge you will find it will lock focus better. Focusing on dark, light, plain colours (blue sky) is hard work for AF systems

[bconolly]

Another trick that I've seen somewhere on the web that works very well is to have them hold a light source (mobile phone, small torch etc) somewhere close to where you want to be. Your auto-focus will lock onto the bright light source quickly. AF assist only has an effective range up to about 3m so depending on the size of the room may not help. It's also a bit intrusive I find for portrait shoots (not really nice to have a bright white light shining straight into your subjects eyes if you can avoid it ;)

[DesmondD]

Using spot metering forces the camera into the older TTL flash mode. TTL-BL is designed for back-lit situations and you would have ben better using matrix or cw metering.

I have a question -

The Problem -
I was taking backlit photos of my Grand daughter, and my ancient 85 1.8 Nikkor was having trouble focusing.
I was in a darkened part of the rumpus room and my GD was in the centre of the room with a far light behind her head
Settings where ISO 100 @ F2 - spot metering. Nikon D7000.
I would press the shutter and it would not fire on every second shot.

The Question -
Would increasing the ISO have any effect on the lowlight focusing.
(Had I thought of it at the time I would have tried it out.)

Any thoughts?