Back focus for lens with extender
 

As I don't find a special thread abut 'lenses', I'll ask a question here:

I have a Fujinon wide-angle lens, with extender. I managed to get the back focus perfectly right with the extender out of the optical path, but when I turn in on, the back focus is WAY out of line. There's even no way to get it right, as the adjustment slider physically doesn't go far enough. Is there another adjustment that I'm missing, inside the extender housing maybe?

Make sure that the macro ring is clicked into it's locked position. Once it's locked in you should have no trouble with back focus. If you are still experiencing issues then you may have knocked an element loose in the lens and it needs a rebuild.

I checked and the 'macro' slider is in the locked position. I also opened the covers of the extender and everything looks fine: no loose elements or displaced lenses. I guess I'll have to take it in for service. I'm not in a hurry, because I never use the extender. If I want more tele, I use my Canon 20x zoom (with extender 2x).

Thanks for the reaction.

Yes it sounds like the extender is out of alignment.

That's a possibilty, but then it's quite strange that I can get a perfect focus with the extender, albeit on only one particular zoom-position. If it would be misaligned, I would be surprised I would be able to get a sharp image in any position.

Mechanically, it falls right in position when I turn the handle. So it doesn't look like being misaligned. Anyway, I've been fiddling with it with no result, so whenever I the time is right, I'll taken it in for service. Thanks guys.

Luc,
If the extender is set so the back focus isn't working well but is close you still should be able to focus at any particular focal length with the front element but it won't track when you zoom out. What you also may notice is the the distance numbers are not accurate. When you focus at 5 feet real distance it might be 4 or 6 on the marking. The back focus is set so the zoom lens focus is correct on the chip block through out the zoom range but doesn't necessarily prevent you from focusing at any particular part of the zoom. My guess is you would find places it won't focus properly but these would be at the wider end of the lens not at telephoto.

Yes, Daniel, that's exactly what happens: I can get a perfect focus with the extender at some point, but it goes out of focus as soon as I start zooming. So I'm reckoning the extender lens block cannot be misaligned, otherwise I wouldn't be able to get a good focus in any position of the zoom, am I correct? Is this something I can repair or adjust myself, or should I take the lens to service?

Luc,
If the lens tracks focus without the extender but doesn't with the extender then the extender is the problem area. The fact you can get anything in focus with the extender in use is not an indication that the extender is okay. It is not. Very doubtful you have the correct expertise to fix it yourself. Call up a lens repair shop to get it repaired if you want to use the extender properly.

OK, Daniel, guess I'll do that. As a professional stills photographer I know something about photo lenses, but I'm only a dummy when it comes to videoglass...

That is why it's always strange to me that wide-angle zooms in video are as long as telephoto-zooms. In my photographic mind, wide-angles are physically short and long lense are... long. But I had to rethink that when I started filming ;-)

Hey Luc,
Wise choice. As for Video Wide Angle lenses and their length they tend to have bigger front elements and extenders as well as controls so they are similar in size to longer zooms but they usually don't have as large a zoom ratio as the longer lenses. Also since they have to control the backfocus to work on the video camera they have some of the same design elements.

Yeah, I'm still learning every day. I didn't know that video lenses had a 'sweet spot' either. So in the beginning I thought that the smaller the aperture, the sharper image I would get. Not true, I know now. Great forum this is, for newbies in video like me.
Thanks!