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I've just bought a ND3 filter (-10 stops) and was looking for recommendations on how to focus the camera.

Basically many recommendations are around focusing WITHOUT the filter, then switch to manual focus, screw in the filter, set the shutter speed and take the shot. The reason is that it would be impossible to focus with the ND filter on (the viewfinder is very dark).

However, I can see that LIVE VIEW works very well with the ND filter ON. It even seems to be able to focus correctly.

So what's the drawback of using this method? I do understand that the metering might be a bit off (but this can be compensated with some trial/error on shutter speed I guess?).

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up vote 4 down vote accepted

The disadvantage to leaving the ND filter on is simply that whatever focus system is being used (phase detect sensor, contrast detect, your eye) as less light and therefore less information to work with.

Live view uses the main sensor and thus has the advantage of being able to use hardware amplification of the signal when light levels are low. There will come a point though were the light level is too low, and noise starts to influence the AF result. Sounds like you're not quite there.

When it comes to AF, the proof is always in the pudding - if you focus with the ND filter on and it never misses, then there's no disadvantage to you focussing through the filter.

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My DSLR focusses with a 10-stop ND filter and normal, fast, phase detection AF perfectly fine... – Mike Aug 19 '13 at 15:33
I agree - I've taken a few shots yesterday and my camera (canon 550D), with the filter on, was still able to focus properly. It was a bright day though, maybe that has given it an advantage. – Patrick Aug 20 '13 at 13:30

The drawback is that live view focusing (contrast detection) is slower than the dedicated AF mechanism (phase detection)... But I doubt this would be a problem in your case (as you would probably be using a tripod with such ND filter).

About the light metering, yes, if there's little light, any errors measuring would be magnified when normalizing (in lieu of a better word) the value.

You can also measure (or estimate the distance) and use the focusing guides on the lens.

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One drawback is that you might unintentionally move the lens focus when you screw on the ND filter.

I've been told to use MANUAL FOCUS with the ND filter on. You can still use LIVE VIEW while manually focusing the lens. Just digitally zoom in (not with the lens, but use the "+" for LIVE VIEW) on the area you want to ensure is sharp.

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