Focusing in low light

[bb40]

Hi, i'd like if someone could explain what dictates the speed at which a camera can autofocus ( more in lowlight than normal circumstances). Is it the quality of the camera , lens or combination of both. Reason for asking is i was taking some pics of my grandson yesterday in my lounge and it struggled on quite a few occasions to get the focus. It was hunting quite a bit to find focus even though i was focusing on the eye, which i would have thought should have decent enough contrast to get focussed. Kit i was using was d300s with sigma 70-200 f2.8 os. Shooting at f2.8.

Thanks

 

[Phil V]

Both camera and lens. Your camera is quite capable, I would work on the assumption; if there isn't enough contrast to take the picture, there probably isn't enough for a decent image. You'll steal more ability with a fast fixed lens.

However you can add focussing contrast with a flashgun or flash transmitter with a focus assist light (you don't have to set the flash to fire).

 

[bb40]

Both camera and lens. Your camera is quite capable, I would work on the assumption; if there isn't enough contrast to take the picture, there probably isn't enough for a decent image. You'll steal more ability with a fast fixed lens.

However you can add focussing contrast with a flashgun or flash transmitter with a focus assist light (you don't have to set the flash to fire).

Thanks for reply Phil. Where i was shooting from he was about 2 feet from a large window. It wasn't an overly bright day but still quite a bit of light coming. I then switched so he was at the other side of the room so his face was catching the light(maybe only another 3 feet from same window). Seemed to focus better from this side of the room. I did compare the same lens later on my son's d7000 & the d300s with different low light subjects and the d7000 seem to be quicker with focussing.

Could you explain the last part of your post please. Thanks again

 

[Phil V]

Attach a flashgun and set the camera to not fire a flash. But the flashgun will emit a focussing beam whe you activate AF.

 

[Phil V]

You know how your eyes are rubbish at judging changes in WB that your camera sees as glaring? And how your eyes adjust automatically to varying light levels.

What might look like a contrasty subject to your eyes is often nowhere near good enough for your cameras AF system.

 

[bb40]

You know how your eyes are rubbish at judging changes in WB that your camera sees as glaring? And how your eyes adjust automatically to varying light levels.

What might look like a contrasty subject to your eyes is often nowhere near good enough for your cameras AF system.

Thanks Phil

 

[deeley_s]

Just asking - did you check which focus point your camera was using? I've been caught out before when I've changed it and forgotten to set it back. Remember also that the size of sensor covered by the focus point is quite small, so you are trying to get a high-contrast edge in that small space

 

[bb40]

Just asking - did you check which focus point your camera was using? I've been caught out before when I've changed it and forgotten to set it back. Remember also that the size of sensor covered by the focus point is quite small, so you are trying to get a high-contrast edge in that small space

I tried both single point and dynamic. Single point a bit more successful than dynamic. So sensor size wise I assume ff better in these conditions?

 

[deeley_s]

Then I'm with Phil. AF isn't magic - it's arithmetic. The camera just checks the difference in the colour value across the edge - the greater the numerical difference between pixels, the greater the contrast. Your eye has an adaptive supercomputer behind it and is far better at seeing edges. Phil's AF assist is the next best trick

 

[bb40]

Then I'm with Phil. AF isn't magic - it's arithmetic. The camera just checks the difference in the colour value across the edge - the greater the numerical difference between pixels, the greater the contrast. Your eye has an adaptive supercomputer behind it and is far better at seeing edges. Phil's AF assist is the next best trick

Thanks for your input.

 

[harryg]

Another way you could accomplish the desired result would be to focus on an area in the same pane with more contrast then lock your focus, this may require +/- compensation depending on the area you have focused on.

 

[juggler]

Probably not directly applicable since your camera focuses differently from mine, but my mirrorless OM-D E-M5 seems to be better at detecting horizontal edges than vertical ones. Or I may have that the wrong way round. Anyway, I try rotating the camera before adding extra light.

 

[Phil V]

Probably not directly applicable since your camera focuses differently from mine, but my mirrorless OM-D E-M5 seems to be better at detecting horizontal edges than vertical ones. Or I may have that the wrong way round. Anyway, I try rotating the camera before adding extra light.

This (a bit) too. Different cameras and different focus points can make a huge difference. It's where Nikon win in the specs compared to Sony, Sony have the same sensor with a better idea of how to get the best from it, but Nikon have better AF tech.

A 7dII lags well behind in the DR and noise tests, but the AF system is comparable with the best cameras money can buy.