PMW-320 focus problems

[Anthony McErlean]

Originally Posted by Paul R Johnson

If you have bits more in focus than others kind of suggests the lens is misbehaving - but the degree could be hidden by depth of field increases as the lens stops down? It's just a stab in the dark?

Hi Paul, of course when I did all you suggested all was well :). Still testing.

It was misbehaving all right.

Thank you.

[Anthony McErlean]

Originally Posted by Christopher Young

The only time I encountered something like this was shooting on a doco in Fiji. Why I mention Fiji is the fact that when I worked out what was wrong I knew had a problem. No service centre.

I only discovered what was happening when I did a slow tilt on the camera. One of the lens internal rear elements had become marginally loose. I could do a back focus and fix the problem but depending on movement of the camera the lens element would move a fraction and of course focus was lost again. I could move the camera a couple of more times and it would come back into focus again. Move it again and it loose it again. Luckily I had a wide angle lens with me and managed to finish a couple of weeks shooting with that. My problem was there was no Canon repair shop in Fiji. It was either Sydney or Hong Kong

It's worth checking out just in case this turns out to be your problem.

Chris Young
CYV Prooductions
Sydney

Hi Chris, that does sound very like the way my lens is carrying on.
OK at times and then far from OK.
Did you get it sorted in the end? Was it the lens at fault?

Thanks Chris.

[Anthony McErlean]

Originally Posted by Daniel Epstein

You should always check the back focus on a lens with the iris wide open to see if it is tracking. It could easily look okay at f4 and above but look soft when the lens is wide open.

Thank you Daniel, so what is the best way to go about setting the back focus?

Thank you.

[Christopher Young]

Originally Posted by Anthony McErlean

Was it the lens at fault?

Sure was the lens. I remember back in the hotel room sitting down with the lens off the camera and having a good look at it when I heard a very slight 'click'. As I rotated the lens nose down nose up an back and forth I could occasionally replicate the clicking sound. When I got back to Sydney I mentioned this to Canon and sure enough they confirmed the noise I detected was indeed the slightest of movements of the suspect element. The element was held in place by a thin screw in retainer ring and this had backed off.

Just remembered a similar incident a year back with one of our Fujinon lenses that has a 16:9/4:3 aspect ratio cross over unit. One day it refused to swing in the ratio converter and it was exactly the same problem. An element screw in retainer backing out enough to block the movement of element in the cross over unit. In this case though the lens element didn't actually move as it had some kind of setting agent applied to it during construction.

So yes it can happen and be rather puzzling when it does.

Chris Young
CYV Productions
Sydney

[Bob Hart]

If ever you have a walk around inside the gizzards of a damaged or broken lens, especially the modern servo lenses, you will discover some of the most exquisite miniature and precise engineering there is to be found in mass-produced engineering.

In older times, it was apparently routine and desirable for periodic maintenance to be done on lenses.

These days, lenses tend to outlast the cameras they are attached to before receiving attention other than a surface clean. Because of their complexity, precision and inherent reliability, as with modern turbine aircraft engines, a periodic dismantlement and re-assembly for checking purposes may even introduce a fault.

Aside from providing the means to monitor and inspect internal life-limited structures which may fail before their mandated replacement cycle, a philosophy of "if it aint broke, don't fix it" with turbine engines prevails.

Glass and metal are strange bedfellows. They behave differently with changes in temperature. A threaded metal ring retainer for a lens element is apparently best not tightened too firmly. The engineering is so precise that the tension of an over-tightened retention ring not only may risk damaging the element, but distort it ever so slightly but sufficiently for the optical characteristic to be altered.

The downside is that with use, vibration, temperature changes, such retainers, even if thread-locked, may loosen eventually or become polished down at the glass/metal surface as to allow a clearance and movement.

Such events are not necessarily representing a quality issue but unavoidable practical limitations that are sometimes imposed in engineering endeavours. So don't feel too grim about something like this happening. Two years without need for intervention is a good innings.

Please heed the comments of people far better qualified to discuss these things than I.

[Anthony McErlean]

Thank you Christopher and Bob for the info and advice.

I'm going to make arrangments to get the lens looked at.

Thanks everyone.