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Lens focus The place to talk about your camera's glassware.

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Old 27th September 2013
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Auto-focus question - help please!

Hi.

I took a few moon shots a few days ago, and was a little disappointed with the results. I know there are many variables - mostly outside my control at that - but there was something I noticed with the lens which slightly baffled me.

The lens we're talking about is the Zuiko 70-300 F4.0-5.6 ED.

I had it on full extension and the AF appeared to do the job of sizing up the focus for the shots. However they were far from sharp.

So I tried MF instead but was surprised, when the extending the focal length, to 'overshoot' the focus on the subject. When the focus went to infinity, the shot was noticeably back out of focus.

How far away does something have to be to hit infinity with one of these lenses? Is 250,000 miles not enough?

I notice, from looking through the instruction manual and the utility menu options on the camera, that there is a facility to change AF settings ("AF FOCUS ADJ.") depending, I guess, on which lens is plugged in at the time. There are options to set from default data, and lens data.

Please can someone tell me if this is a kind of re-calibration technique, or is it intended for a completely different purpose? And in either event, would it help in my situation?

Any help would be gratefully received. The manual mentions this facility, but does not explain why you might want to use it.
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Old 27th September 2013
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Re: Auto-focus question - help please!

It is not unusual for lenses, particularly long ones, to focus "beyond infinity". I think the theoretical reason is that the physical dimensions of the lens change with temperature so it has to have some extra built in to allow for it. I suspect mechanical tolerances play a part as well. The usual advice for astro shots is to focus manually on a clearly-defined object that is a long way away then point at the sky.

I don't think you need to worry.

Ciao ... John
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Old 27th September 2013
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Re: Auto-focus question - help please!

Try taking your protective filter off.
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Old 28th September 2013
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Re: Auto-focus question - help please!

It wasn't on!
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Old 28th September 2013
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Re: Auto-focus question - help please!

To answer your original basic question, the " AF FOCUS ADJ" is indeed a means of re-calibrating a lens which is for some reason front or back focusing. This normally needs to be done with a focus test chart a bit closer than the moon.

It's possible your moon shot problem is not due to focus issues but technique.

Wrotniak's site has a useful article on photographing the moon here: http://www.wrotniak.net/photo/moon/index.html

You will also find a lot of good information on E series cameras and lenses on his site as well.

Regards.
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Old 30th September 2013
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Re: Auto-focus question - help please!

Thanks Peter.

That's enough of an answer for me. Enough to know that I don't want to go messing about in that part of the menu anyway!

The lens seems to perform in all other scenarios quite satisfactorily, so I shall put it down to technique/outside influences, and leave the lens calibration settings well alone!
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Old 30th September 2013
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Re: Auto-focus question - help please!

My experience is that focusing on the moon is best done manually whatever the lens

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Old 30th September 2013
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Re: Auto-focus question - help please!

Also be aware you need a fairly fast shutter speed, the thing is moving quicker than you might think.
There is a good how to somewhere on the forum
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