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Telephoto Lenses with focal lengths longer than 60mm.

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  #1  
Old 16th June 2010
expatscot
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70-300 can't find focus

I was out with my 70-300 this evening and everything seemed to be fine, then the next thing I knew it wouldn't find focus - particularly bad at the telephoto end. The motor could be heard just shuffling backwards and forwards a little bit at a time. I know someimtes the lens seems to throw a wobbler (at least mine did) and not find something to focus on, but this isn't like that, it just rotates about 1/8th of a turn backwards and forwards.

It does ocasionally find what it thinks is focus, but the image isn;t close to being sharp. Also if I'm at the wider end I can sometimes get the focus, but when gradually zooming and re-focusing, it jumps back to the shuddering mode.

I've cleaned the contacts on camera and lens to no avail. My other kit lenses worked fine. When I got home I checked for a firmware update and camera and lens were already OK.

Anyone got any thoughts?
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  #2  
Old 17th June 2010
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Re: 70-300 can't find focus

Apart from the fact that those lenses are not good at low light focus, what body are you using it on?
Also, what is it like in good light?

Ross
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Old 17th June 2010
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Re: 70-300 can't find focus

When using this lens to take theatre shots in available light (in other words quite poor lighting conditions) I have sometimes zoomed out to get an initial lock, and then zoomed in and re-focussed to stop the lens hunting.

I've just tried this now in a dark part of the house and indeed at 300mm, the lens sometimes wouldn't even attempt to focus (Presumably not enough contrast in the frame). I've zoomed out, got an initial lock and then recomposed at 300mm and its focussed fine.

All done on an E-30.

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Old 17th June 2010
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Re: 70-300 can't find focus

Quote:
particularly bad at the telephoto end
Of course, apart from getting in closer to the detail, by zooming out to 300mm it now has a maximum aperture of f5.6 (from f4.0) which will make the subject darker & harder for the camera to focus.
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Old 17th June 2010
expatscot
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Re: 70-300 can't find focus

The body's a e-510 and on the whole it's been OK in good light and poor, but not any more - I've just tried it again and it has the same issue.

I've had the lens for a year and know about some of it's focusing foibles such as the contrast issue and sometimes having to zoom out, focus, then zoom in to give it a bit of a clue. When it's hunting like that it tends to go a long way one way, then the other, then maybe repeat and give up - if I haven't already!

This was totally different - the lens just oscillated backwards and forwards changing direction maybe 10 times a second. Sometimes I could just about get it to focus, but the focus seemed off - not as sharp as I'd expect.

Now I appreciate the sun was low or had even gone down at this point, but the contrast on some of the shots I tried was very high - the Second Severn Crossing towers against the sky shouldn't really be an issue, I wouldn't have thought. For example I got the image below, but struggled to and this was the kind of thing I was getting the problem with:

http://www.flickr.com/photos/28716254@N07/4709087749/
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Old 17th June 2010
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Re: 70-300 can't find focus

I suspect its the repeated patterns of the suspension wires that can cause problems. It may find several positions that seem to match, but only one of them will be the correct focus point. Unfortunately they haven't invented a human brain like camera AF system that says "yes, that's just about it"

Pete
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Old 17th June 2010
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Re: 70-300 can't find focus

How many focus points did you have it set on? I agree with Pete that the number of similar cables to potentially focus on may have confused it. At f5 there will be a relatively small depth of focus compared with the distance over which that frame captures.

Perhaps if you set to only one focus point it would hunt less as it would only be trying to focus on cables spread over say 30m or so.
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Old 17th June 2010
expatscot
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Re: 70-300 can't find focus

Thanks for the thoughts guys, but I don't think it's the cables as this evening it wouldn't focus on a brown wooden window frame against a brick wall across the road.

On focal points I've tried spot and auto - although I admit it was on spot when I frist experienced the problem when trying to take a Heron out on the river. Admittedly the contrast may not have been great there, but then I've tried loads of stuff that shouldn't have been a problem, but were.
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Old 18th June 2010
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Re: 70-300 can't find focus

Quote:
Originally Posted by expatscot View Post
Thanks for the thoughts guys, but I don't think it's the cables as this evening it wouldn't focus on a brown wooden window frame against a brick wall across the road.

On focal points I've tried spot and auto - although I admit it was on spot when I frist experienced the problem when trying to take a Heron out on the river. Admittedly the contrast may not have been great there, but then I've tried loads of stuff that shouldn't have been a problem, but were.
The crossed wires being separated by something like 10m (or 20m) would make the focus point indecisive. Another question before you start thinking about service (but would require it) is, are the lens elements looking cloudy at all? Looking through it off the camera, you may see some cloudiness which, of course, would be the start of mould. If this is the case, by exposing it to ultraviolet light for long enough, it is possible to kill the mould, but it would still need to be cleaned.

Quote:
This was totally different - the lens just oscillated backwards and forwards changing direction maybe 10 times a second. Sometimes I could just about get it to focus, but the focus seemed off - not as sharp as I'd expect.
It's looking like motor or control may need service.

I hope this can be resolved for you.

Ross
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Flickr: https://www.flickr.com/photos/ross-the-fiddler/
Cameras: OM-D E-M1 & Mk II, Olympus Stylus 1, OM-D E-M5.
Lenses: M.ZD12-40mm f2.8 PRO Lens, M.ZD40-150mm f2.8 PRO Lens with MC-14, M.ZD12-50, M.ZD60 Macro, M.ZD75-300 Mk II, MMF-3, ZD14-54 II, Sigma 150mm F2.8 APO Macro DG HSM.
Flashes: FL36R X2, FL50R, FL50.
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