Autofocus effected by colours ?

[Dolby]

Is the autofocus effected by colour lights at all?

I've often had an issue with red stage lights and blow out - but Friday I went out and all but one were slightly off. The was taken in white light and everything else was LED pink.

I know sometimes reds/pinks can blow out - but would it effect latching on?

I thought my camera/lens was dropped by the maid ... but it seems ok

Speaking of blow out ... did I do something wrong here?!

 

[Dolby]

Bit more on it ... http://www.pentaxforums.com/forums/...-led-stage-lighting-photography-problems.html

 

[rrh]

Autofocus normally likes strongy, contrasty lines to use for a focus point e.g. the black microphone stand, which is perfectly in focus

 

[genetic]

That definitely doesn't look like Autofocus problems. Looks like yu have a problem with blown out hotspots instead. Just drop your exposure a few stops.

The best is to use manual settings - stick to manual white balance. In my experience, indoor (3200k-400k) seems to render LED colours more accurately. You should also shoot RAW to adjust possible white-balance issues later in post.

 

[bwana]

It's blown out and you should have used single spot focusing instead of single shot with expansion.

Unless you're using DPP or shooting JPG there's no point in setting your WB in camera. Rather make a point of noting something that you can use as a reference in post.

 

[koffiejunkie]

As bwana said, spot focus is your friend. The regular single point focus is deceptive - the sensors are much taller than the boxes are high. I stumbled upon an article where someone had mapped them out, and was quite shocked.

With regards to exposure, in my experience, 1/100 & f/2.8 at 6400 ISO will blow out with most stage lighting. On a reasonably well list stage (particularly with LED lighting) I tend to stop down a bit and us 1/400 or 1/320 at a push, if I'm using the 70-200mm or 200mm prime, and I'm seldom hitting 6400 ISO.

Also, I find 1/100 too slow for most musicians moving along to their music, and certainly too slow for anything at 130mm.

Lastly, this might be a silly question, but did you meter on the guy's face? The 7D's spot metering doesn't follow the active focus point.

 

[Dolby]

I can't recall if that was before - or after - I reset the factory defaults.

Half way through I thought 'something is wrong and reset', but can't remember if this was a before / after. I'll check the EXIF at home.

Thanks!

 

[koffiejunkie]

With regards to exposure, in my experience, 1/100 & f/2.8 at 6400 ISO will blow out with most stage lighting.

I should qualify this. On a well lit stage, the level of light is usually high enough that you can get away for 400 ISO or therabouts. Even with only one of those LED lights (small private gig in someone's garden), I barely reached 6400 ISO. Not saying that doesn't happen - I've been to plenty of gigs where the venue and its lights date from the 1960s and the light is weak, even with all the lights on.

Anyway, another thing to remember is that the dynamic range your camera can cope with, drops as the ISO goes up. So does the colour sensitivity:

This may or may not matter depending on what you're shooting, I suppose it depends on what you intend to do to it afterwards.

 

[bwana]

For those who can't, or choose not to, read a histogram there's always black and white.

 

[fvdbergh]

Is the autofocus effected by colour lights at all?

Yes. Different wavelengths of light will converge (focus) on slightly different focus planes, i.e., only one wavelength will actually be exactly in focus. This is where achromatic lenses or apochromatic lenses come in; they use special combinations of different lens material (crown glass vs fluoride glass) to fudge the different wavelengths to focus in the same plane. Most modern lenses will be at least achromatic, i.e, two wavelengths will be exactly in focus on the sensor, but other wavelengths may be slightly off (http://en.wikipedia.org/wiki/Achromat). Since the AF sensor is on a different physical light path, this effect could be worse for the AF sensor than the main sensor, if you have a slight offset between AF sensor focus and main sensor focus.

It may be possible that you have discovered a particular combination of light colour, lens, and AF sensor that maximises this effect. It is well-known that certain bodies (e.g., Nikon D7000) should not be AF calibrated under Tungsten lighting, since this causes the calibration to be off under most other lighting conditions. The good news is that AF-fine tuning (or AF-micro adjust as Canon calls is) could be used to elliminate the offset temporarily.

 

[Dolby]

Thanks ... will try again using manual focus on Friday under the same lights