I was shooting some portraits of my daughter last night and was constantly fighting my camera. I know I was pushing it a bit, but I also expect that the camera could handle the situation. I was shooting in a very dark room, well backlit with a SB-900 on camera bouncing back and right. Shooting a D700 with a Sigma 50 1.4, the lens was constantly hunting (and rightfully so since there was very little contrast to be detected). I was surprised though that the SB-900 did not seem to be helping to catch focus though. Only every once in a while I would see blips of IR from the SB-900. Setup was manual exposure, single point focus, TTL BL flash.

I'm looking if anyone has some suggestions that can help me learn and improve for the next time I have this type setup. My single point focus was always where I wanted focus rather than focus and reframe, and I did notice improved performance using the center cross sensor and reframing. I just wonder if I'm missing something because I would expect a FX body with a 1.4 lens and speedlight to be more capable than this.

The IR assist from the SB-900 will only come on when using AF-S and the centre point. If you move the focus point off centre, you won't get that assist.

With the assist, you should have no problems at all. I can focus in pitch black conditions with my D700/Sigman 50 1.4/SB-600.

Dylan

oh, I should note that without the flash on camera, the built in assis light will behave in the same way.

I use a flashlight to help AF stuff work on the dark. It works with MF cameras too if you and the subject aren't running around.

DTOB wrote:
The IR assist from the SB-900 will only come on when using AF-S and the centre point. If you move the focus point off centre, you won't get that assist.


Dylan, this is simply not correct. With SB 900 IR assist you can use 11 points, not only the center point. More info about this you can read HERE

Mishu01 wrote:
DTOB wrote:
The IR assist from the SB-900 will only come on when using AF-S and the centre point. If you move the focus point off centre, you won't get that assist.


Dylan, this is simply not correct. With SB 900 IR assist you can use 11 points, not only the center point. More info about this you can read HERE



Whoops, looks like I may have mis-spoke.

I am pretty sure that my sb-600 will not do this with anything but the centre point.

Thanks for the correction, this would be a good reason for me to upgrade. I'll double check when I get home.

Dylan

Even the SB800 lets you use off-centre points for IR assist. At least on the D2 bodies. I'd assume newer bodies would be similar.

I am constantly humbled... you learn something new everyday I guess. I never knew the af point mattered for af assist. So I guess the wise move would be to switch the af point selection from 51 to 11, that way I'm always getting assist?

Berschwinger wrote:
I am constantly humbled... you learn something new everyday I guess. I never knew the af point mattered for af assist. So I guess the wise move would be to switch the af point selection from 51 to 11, that way I'm always getting assist?

This is probably exactly what has tripped my up with my SB-600 as well, and why I presumed that anything off the centre was a no-go. Switching from 51 to 11 seems like the sure fire way to go.

I just did a quick test with my D700 and SB-800.

It works on some focus points but not all. Left on 51 points - You can go three over left or right and straight up or down. It won't work if you over and up or down.

Edit: Only the ones that work in 11 point (all of them) will work in the 51 point. Which is the THIRD to the left or the right and the one farthest away. The first and second ones over do not work.

Interesting!

On 11 points, it will work on all of them!

Not every AF sensor can see a lighted target area at every focal length. It varies with flash unit (because of the focus assist pattern), the focal length of the lens and perhaps the model of the lens (because the FL determines where the AF sensors lie in relation to the image frame and hence to focus assist pattern). Therefore you will get a variety of results from different people who are trying it with different lenses and focal lengths.


With my D3s I can get the SB-900 focus assist to work regardless of whether the AF selection set is 51-point or restricted to 11-point. It works for single-point AF, dynamic area AF, and auto area AF. It also works regardless of whether the AF dynamic area is set to 9-point, or 21-point, or 51-point, or 3D 51-point.

It will not work with AF-C mode.

It will not work if the lens is set to MF mode.

It will not work if the AF has been set but subsequently locked.

It will not work if the flash is not ready (the battery hasn't yet delivered sufficient charge to the flash unit).

The D700 has, I think, a custom function to disable the use of the external flash focus assist because it also has an internal focus assist light. So this as well as the other conditions would prevent the SB-900 focus assist from working.

The fact that it works with all 11 points in 11-point selection mode is not guaranteed at every focal length. e.g. from 106mm to 135mm it is reduced to a vertical column and a horizontal row of AF sensors. So you might as well stick with 51-point selection if that is what you would normally prefer.

I have also found that the focus assist pattern can be hard to see through the D3s viewfinder but is quite evident when I do not look through the viewfinder. Maybe there is an IR or near-IR filter in the viewfinder.

- Alan

Alan321 wrote:

It will not work with AF-C mode.

- Alan


This is another culprit I did not know about. I was in AF-C as I was near wide open shooting a moving target. I was able to see the IR through the viewfinder very clearly when it did engage. I assume I had switched to AF-S at that point. Maybe AF-C was a bad choice also - if the camera locked focus correctly, it would quickly loose it and start hunting again.