BACKFOCUS problem?

[fanetin]

Hi, I have a DVX100A and have some problems with the focus scale. I thought that in order to focus all along the zoom scale you just had to go to zoom 99, open the iris as much as it gets and get focus. Then you would have perfect focus all allong the zoom scale and with any iris position, just like by broadcast lenses. But I recently got some focus problems and did some test. I put a focus chart and focused there with different zoom. I got these results:

object ca. 50cm from camera

Z F
99 38
90 38
80 38
70 38
60 38
50 38

object ca. 100cm from camera

Z F
99 49
90 48
80 47
70 47
60 47
50 46

object ca. 250cm from camera

Z F
99 75
90 73
80 72
70 69
60 67
50 66

At short range the focus will stay all along the zoom scale, but when I get away from the object the focus would change. When i open the zoom I have to shorten the focus, is taht normal? I did some test as well with the autofocus and more or less got th same result.

Is my camera OK? Does the DVX100A have different focus for diffenret zoom levels? or do I have to take my camera to get serviced? Is this maybe an issue with backfocus?

Help me please, I am a kind of unhappy. Thanks!

[kyle.presley]

Did you shift positions in your setup at all?

[fanetin]

if you mean if I moved the camera in any way or changed the distance from the camera to the object, the answrer is no.

Thanks!

[agcohn]

Aperture (iris) affects depth of field, so unless you have the focus set absolutely perfectly on the plane of the subject, changing the iris could very well change what you see as in focus or not. Increasing and decreasing the focal length (zoom) would make that problem more or less apparent depending on how far zoomed in you are.

edit:
what I'm saying is it doesn't sound like you had the focus perfectly set when you were further away from the subject, which would make sense since if you were judging by eye alone, it would be harder to judge focus if the object is smaller in the camera. The way to really test this would be to set the focus using a tape measure and the charts in Barry's book. That should ensure that you are focused at the correct spot every time.

Using the chart, the focus for something at 100cm should be somewhere b/w 47 and 48 or 3' 2.25" and 3' 4".

At 250cm it should be somewhere b/w 73 and 74 or 8' 0" and 8' 6".

50cm isn't recorded on the chart, and I think it's technically under the minimum object distance of the DVX.

[fanetin]

Well I did not tocuh the iris, it always was fully opened. And I used a focus chart and a monitor so I could see quite good if it was sharp or not. The problem is that once you focused at one distance (with Z99) it should keep the focus all along the zoom, and it des not...

[agcohn]

Well I did not tocuh the iris, it always was fully opened. And I used a focus chart and a monitor so I could see quite good if it was sharp or not. The problem is that once you focused at one distance (with Z99) it should keep the focus all along the zoom, and it des not...
But focus goes by distance between the subject and the focal plane, and you were close with your focus settings when you zoomed all of the way in, but you did not set it exactly correct. You had it at 49, when it should have been somewhere b/w 47 and 48, and you had it at 75 when it should've been b/w 73 and 74.

[fanetin]

the distance to the object is approximately. I can assure you that the focus was ok at Z99 end then it changed at other zooms, like Z80 or Z60. I gooled a lot about this subject and fount that this could be a software problem that can be fixed. This kind of lens have a sensor that, once you make focus at a certain zoom level, tells the lens where the focus should be at all other zoom positions.

[peteohs]

fanetin,

i have the same issue with my DVX-100P. AGCOHN isn't really making sense.

On cameras with real lenses, you zoom in and get your "critical focus" then zoom out and the object you zoomed in on is still in focus. with my DVX, it doesn't work this way. the focus changes when i zoom. its very frustrating.

anybody find a solution to this issue?

[William_Robinette]

No solution, but I have the same problem. Glad I am not alone.

[Barry_Green]

If your DVX isn't holding focus throughout the zoom range, then it should be looked at in the service department.

Zoom in to Z99, set focus, then zoom out to Z00. It should still be in sharp focus. If it isn't, have the repair people look at it.

[lec]

My DVX100b (pal) had the same problem. It was send back to panasonic and (after a few weeks!) it is now okay.

[William_Robinette]

My DVX100b (pal) had the same problem. It was send back to panasonic and (after a few weeks!) it is now okay.

That's just the thing. I can't afford for my camera to be gone for weeks at a time. The problem is not so bad that I cannot shoot around it, but it is a pain. Good thing these 1/3" chips have unlimited DoF...

[peteohs]

my camera isn't under warrenty. where would i send it?