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I recently bought a Marumi variable ND filter and I've been testing it on my 24 - 105mm F4 and 70 - 200mm F2.8 canon lenses. It seems that my lenses have a hard time focusing when the ND filter is attached and even when it's focused I lose a lot of sharpness in the shot. Is this normal? I'm shooting with a Canon 6D.

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Thanks! I did put the density to minimum but i'm sure my aperture was too low. I'll try it on f/8 + and see if i get better results. I appreciate your feedback. –  Bernard May 28 '13 at 9:22
    
The aperture is set only for a moment of the shot. Focusing is always done using the widest aperture available. Unless you have aperture ring on lens and set the aperture manualy (physically). –  Petr Újezdský May 28 '13 at 10:06
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2 Answers

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Do you by chance have the Marumi ND2-400 Filter? We have a blog post with a review on it that may help you - http://photo.blogoverflow.com/2012/03/marumi-nd2-400-variable-nd-filter-review/

With that in mind I have a few questions -

  • Are you in Live View mode or not?
  • Are you using live view to focus or not?
  • Could you post a sample image with the sharpness issue?

If you are using it on the strong end of the filtering I would highly recommend focusing in live view at the maximum magnification, and also of course using a tripod and shooting using the hyperfocal distance(see: What is "Hyperfocal Distance"?).

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Great comments thanks. I do indeed have the Marumi ND2-400 and the article was pretty helpful. I guess I just have to play around a bit more until I get the sweet spot. Shooting in Live view seems to be another good tip. I'll try that as soon as we get some sunshine again in Cape Town! –  Bernard May 28 '13 at 13:56
    
@Bernard - I do think that the other answer presented here by MikeW is accurate - You may be seeing the result of diffraction, so make sure you look into that as well. –  dpollitt May 28 '13 at 15:07
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The main advantage of the variable filter is that you can turn it so that it is at the minimum setting (letting the most light through). At that point it's about the same as having a normal polarizing filter, so should be easy to focus. Once focused, then carefully adjust the filter to darken to the desired density.

In order to get long exposures in the daytime, you are probably using the filter near maximum density and also lowering your aperture to f/22 or smaller. If so, your lack of focus is most likely blur caused by diffraction. It will vary by lens, but you probably want f/8 or f/11 for maximum sharpness.

For long exposures, obviously use a sturdy tripod, and you may want to use a remote shutter release and mirror up.

I've been using these sorts of ND filters a lot over the past year. I started out using normal AF, f/22 and a self-timer, and I'd say I had one good shot in ten. Now I use f/11, live view focusing, mirror up, and take more care to make sure the tripod is stable (weigh down with sandbags when it's windy) and my shots are pretty much 100% sharp all the time.

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I wouldn't say that a 10 Stop ND filter is easy to focus. It is if you are in live view, if you aren't, you can't even see out of the viewfinder. –  dpollitt May 27 '13 at 19:57
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Not when it's variable. You dial it down and it's like having a 2 stop filter on. You can easily see through it. –  MikeW May 27 '13 at 20:21
    
Yeah good point. I would imagine your idea about diffraction being the issue here also might be correct. –  dpollitt May 27 '13 at 21:13
    
In my experience it was the main factor, shutting down the aperture to get the longest exposure, shooting for 20-30 seconds for a waterfall, when 2-3 seconds would have done. –  MikeW May 27 '13 at 22:47
    
Sure it can be. I've shot the same type of stuff at f/16 without problems, so it might not be what the user is experiencing. This was at f/14 for example - 500px.com/photo/35788148 –  dpollitt May 28 '13 at 0:19
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