1d Mark III poor low light AF

Started Feb 13, 2008 | Discussions thread
child of enki New Member • Posts: 4
Re: five

M Stewart wrote:

child of enki wrote:

... the bottom line is this. Neither the 1D MK III or MK IV that I'm testing are able to focus down to -1ev as advertised.

I'll go along with that at ISO100 for my Mark IV. Any idea at what EV the AF does start working?

Now for the second problem. This problem only effects the MK III. Prolonged usage in low light also effects the camera's ability to lock focus. This however won't really matter since your focus accuracy disappears the more the camera struggles in low light so you probably will not even use it.

I don't understand what mechanism there can be for such an adaptive effect?

The good news is that there is a solution for both problems. Use some form of AF assist and you should be okay. the camera will not respond as quickly as it would if it were working properly and you likely won't want to use it to track anything in low light but at least you will be able to use it in dim light like outdoors at night.

Anyway, that's what's wrong.

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M. Stewart
Milton Keynes, UK

It will lock focus at 1ev with {roughly} 20% accuracy. At about 1.5 EV the hit rate jumps to (again {roughly}) 60%. 2 to 2.5 EV is about where you need to be to achieve close to 100%.

The game however does change a little if you use a lens with a maximum aperture of 2.8 or greater because the camera activates a second vertical array of sensors in the center which does indeed improve performance (said to be by a factor of 3). The extra light is of course welcomed and that will allow you to shoot in dimmer light but with the same lens placed on the 1D Digital (aka) Mark I, It will not only lock focus but also be in focus all the way down to 0 EV which is acceptable for most people where as the MK III and IV are just hunting.

I suspect this behavior is caused by the alignment of the virtual focusing plane or maybe even the angle of the main mirror. I’m going to try an experiment in a few days by adjusting the sub mirror to see if that’s the problem; if that yields no change I’ll try adjusting the main reflex mirror and report back with my findings.

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