Back Focus Q

D

Dyrseve989

Well-known member
just curious about how accurate the back focus is on the DVX00A - I have been using this camera for a little while but I feel like when I zoom in after a certain length to focus and then pull back - the focus of my subject is a little soft

-Matt-
 
Somebody claimed it is pretty accurate with this camera. i never bothered verifying that though. it looks fine to me.

the thing is that in cinemode it is pretty hard to judge the focus by eye, espacially if you dont use the EVF function (That really helps a LOT).. so somethign which is in focus may look slightly blurry.
Also, the there's a focus meter active all the time on the display, right? or the 'error' you're talking about is lower than those units, maybe?

Andrea
 
sounds good to me - i just wanted to make sure - because i know alot of digital cameras tend to have horrible backfocusing - although its hard to tell on the DVX I would just assume it worked very well if mayb an unnoticeable amount off

-Matt-
 
well when you pull out even if the back focus is a littel off, the wider angle will add so much depth of field that it shouldnt matter. it could be a foot or two off, and it should still be in acceptable shaprness range.
 
Yeah, I feel like sometimes when I go from a tight and zoom out to a wide, I notice a slight change in focus, almost like the backfocus is off a bit. But then it usually returns (or its just wide enough to be imperceptible.)
 
I just got my camera back from the repair shop (for a problem with phantom power) and now I'm having a focussing problem. I did the following test:

Zoom all the way in on the video cabinet across the room. My focus reading is 74.
Leave the focus on manual and zoom all the way out. The picture is really obviously soft. Unwatchably so, even on the LCD. When I then switch to auto focus the camera adjusts itself and I get a reading of AF31, and everything looks much better.

The camera was definitely not like this before I sent it in for service. Is it possible that they knocked something out of alignment during the repair? The repair guy I spoke to did say that to get to the mic boards he had to take the lens off and take the whole thing apart.

Should I send it straight back, or are there more tests I can do?
 
Send it straight back!
A good Pana authorized service center should NEVER mess up your focus like that. The fix may be as simple as a "software alignment" done properly.
In any case, they should make it right at no additional cost.
Good luck.
Ken
 
X2mag68 said that the extreme depth of field of the lens at wide angle (4.5mm) would minimize the back focus error. That is not correct. It is true that if the back focus is correct, the large d.o.f. makes focusing errors less of a problem.

The accuracy to which the back focus must be set is controlled by the depth of focus- which is the space in the image plane on both sides of the best focus where the image is still sufficiently sharp. This space shrinks as the f number becomes larger (faster) and the focal length becomes shorter.

The formula is the focal length of the lens times the f number divided by 1000.

so the 4.5mm f1.6 wide angel position has a depth of focus of approx .007mm or .0003 "

at tele (45mm f 2.8) the depth of focus is approx .13mm or .005"

One can easily see that the margin for error at the long end is almost 20X what is is at wide.

The lens/ccd combination must be set with .0003 (three tenthousandths ) of an inch to be dead on !!

Removing and re-installing the lens can easily disturb the relationship. It is also possible that inspite of the best efforts in assembly and quality control that occassionally a camera slips thru that is not quite on.

Cameras that exhibit noticeable change in focus (softness) when zooming from tele to wide should be checked and adjusted by a qualified service center. You can see from the numbers this is a very critical adjustment.

Bill Turner
Schneider Optics
Century Division
 
Ok, so I sent the camera back to the repair shop. The technician called me to say that there was nothing wrong with it and that you always have to adjust the focus whenever you move the zoom. Which is not how it was before I sent it in for repair, and not how any other lens I've ever known has worked.

He said the lens on this camera is different to professional lenses and there is no back focus, just a codec which adjusts the zoom-focus relationship automatically. He said if you want to focus manually and then move the zoom, you have to use the button underneath the focus switch (push auto, I think it's called) to make sure it stays in focus. Which seems to defeat the point of having manual focus in the first place, doesn't it?

My impression of the way things are supposed to work is that if I zoom in on someone's face, get focus and hold it in manual, and zoom out without having ever moved the camera, then that person's face should still be in focus. He is saying that this is not the case with the DVX.

I would really appreciate any comments as to whether I'm right or wrong on this one.
 
fritz said:
My impression of the way things are supposed to work is that if I zoom in on someone's face, get focus and hold it in manual, and zoom out without having ever moved the camera, then that person's face should still be in focus. He is saying that this is not the case with the DVX.
Click to expand...

That is a pile of crap, and your so-called "tech" is clueless. Not only is backfocusing perfectly accurate, but the discrete values of the focus scale (i.e., 65, 73, 92 and so on) do correspond to the equivalent in meters/feet.

Let me give you an example that's been verified by me (and many other people on this board) many times : if, on Day One of your shoot, you tape-measure the eyes of your actor at 5.40 meters from the CCDs (that's 84, discrete value), and you set your focus at 84, the focus will be on the eyes. You can zoom in and out as much as you like and it won't change. Then, if you reshoot the scene on Day Two with the talent and the cam in the exact same relative positions (obviously), you set your focus at 84 and the eyes will still be in focus, regardless of whatever you may have done with the camera in between the two takes.

Don't let anyone else tell you otherwise. :thumbsup:
 
Thanks for that guys! It's good to know I'm not crazy.

The technician did call me back a couple of hours later and say that he'd been looking at my camera again and thought that he did notice it going out of focus a bit when he zoomed in. I'm hoping this was his way of saying that he may have been wrong about how it works. He said he'd put it on the computer or collimator (or whatever it's called) next week and see what it says. Apparently there's something you hook the camera up to and it automatically adjusts or tells you it's ok or something. Fingers crossed!
 
Well, this was dramatic. It wasn't even close to being in focus. I spoke to the guy again on Tuesday and he had put the camera on the machine or computer or whatever it was, and he told me that 'a small adjustment' had been made.

Anyway, I got the camera back on Wednesday and it is fine now. What worries me is that I had to argue about it in the first place. But hopefully others can benefit from my experience: if you ever send your camera in for service, check it over thoroughly when you get it back, and not just for the original problem - they might have stuffed something else up.
 
You must log in or register to reply here.
Back
Top