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Nikon 20/1.8G seems to have focus slipping?


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#1
gstam42

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Hi all, new to the site.

 

I've just purchased a used (from London Camera Exchange) Nikon 20mm f/1.8G lens to use to start some nightscapes/astro shots with my D810. I took it out for the first time last night, and while I got some shots I'm pleased with, I'm disturbed by the fact that focus seemed to keep moving slightly - enough to take stars out of focus - between shots.

 

The temperature was stable at 16-17 degrees (Celsius) I had the lens and camera on a tripod and both (!) in manual focus mode. I would focus on a bright star using Live View mode zoomed in all the way. I'd be quite confident that I got stars to be as pin-sharp as possible after a few moments going back and forth. Then I'd take a shot (usually a 13-second exposure using the 2-second delay), often check it, and proceed to take a few more. It seemed to me that after a couple shots, I was suddenly getting stars across the field to be a bit out of focus, as in the attached example photo.

 

Nothing, I am certain, was bumping the focus ring between exposures. (Reviews praised the focus ring, except for its slight bit of play before it 'catches', but I don't think it's particularly precise or solid feeling at all.) And it certainly isn't too long an exposure or the beginning of star trails. (I used 13 seconds for all shots last night, and a good handful came out sharp; but a good number were lost to this problem, as well.)

 

Is it possible there's something 'loose' and, say, the mirror snapping up and back down a few times has caused the lens to lose focus just enough for stars to start to blur?

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#2
Ron

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While I do have a copy of this lens I've never used it for astrophotography. Actually, where I live, I'd be lucky to see a dozen stars.

 

I'm surprised that you're not using mirror up mode on your camera. That would eliminate the chance that mirror slap is causing your problem. Otherwise, I don't know. I haven't had a problem with focus drift when doing terrestrial photography with this lens.

 

We do have a few users on the forum who do dabble in astro work. Perhaps one of them will jump in with some other suggestions.

 

--Ron



#3
Merco_61

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What tripod and head do you use? The mirror slap might start some resonances in the tripod/head/body/lens system that moves something just a little bit.

 

I would definitely look into getting an MC-30A or MC-36 wired remote and use mirror lock-up rather than the timer. The Nikon remotes have verk flexible cables, unlike the third-party ones.