Back Focus Problem
 

Beware. I had to send my V1U back to Sony today for warranty repair because of a back focus problem. I'm surprised because I haven't put that many hours on it since I got it in late December 06. I was surprised when an interview I shot today was soft. I was rolling as I zoomed in to get focus and it was absolutely sharp, but soft when I pulled out to frame the shot. Tests back at the office confirmed the problem. Shooting in DVCam mode.

Lee, I'm wondering whether back-focus also prevents auto-focus from working properly? Or is it only manifesting in scenarios like you described (ie zoomin-in - focusing - zoomin-out)?

Piotr
It's the same issue. Zooming in, letting AF catch the focus, switching to manual, and then zooming out to frame the shot results in soft focus. Setting the shot and invoking AF results in sharp focus.

Lee

Back focus should have no effect on auto focus, in fact if you're in auto focus you'd never know if you had a back focus issue or not.

To check backfocus switch to manual focus, zoom in on a res chart or something that will easily tell you if you're in focus or not. Focus the lens and then zoom back. The focus should remain the same.

The problem comes about much more so on these cheaper lenses as they're retrofocus zooms. This means the focus has to adjusted as the focal length changes and this is done via a servo system. It might also explain why these lenses are so noisy. As you zoom in and out there's one motor driving the zoom, another one driving focus trying to compensate and even the iris is having to be moved to compensate for the changing loss of light through the lens. Probably also explains why zooming in and out a lot drains the battery a bit faster than normal.

That's about the limit of my knowledge of lens design but I can see why it's so difficult to get it right. The focus servo has no feedback to tell it where focus really is, so I guess all it can do it use a look up table. That might be OK except changes in temperature affect the back focus distance as metals expand and contract.

I find this very disturbing - I thought that one of the only ways to get an accurate focus with an HD camera like the V1 without the aid of an external monitor is to zoom in, check focus, and then zoom out again.

And you can do it of course (that's how I focus in critical situations) - I quess Lee's camera is malfunctioning (the back focus problem can happen with all models; it's not inherent to the V1).

Yes, this could and probably does happen on other models and brands, but I was particularly surprised it happened so soon and after moderate use (I've used my V1 on five or so professional shoots since acquiring it in December). My original post was intended to warn the readers of this forum to be on the lookout for this issue and to see if anyone else has had a back focus problem.

When working without an external monitor I find the peaking and expanded focus to be good aids in verifying focus after the shot is set.

You've got me interested - has it been established that the back-focus phenomenon can develop during heavy/long use? I thought this is rather an assembly error, and can manifest in a brand new camera!

I don't think it has been established that there is a back focus problem. We're talking about one user reporting this problem. Right now the problem is only with his camera, and we don't even know how the problem came about. Maybe the camera lens was jarred during one of his shoots and knocked the back focus off. In any case this is the first I've heard about a back focus problem with the V1. Since this is not a known problem with the V1, I wouldn't worry about it until there are a lot more reports.

Don't get too excited about this. Lets wait and see what the repair center says when Lee's camera comes back. I've checked my camera and its fine. Mine was one of the first shipped back in December.

Lee, I hope it comes back ready to go to work.

Chris

I think nobody's getting excited, Chris - I checked my camera and it's also OK. My question was out of curiousity about the general nature of the phenomenon: can it develop in time during normal use in a unit that had perfect focus when leaving the factory?

It was suggested (indirectly) in Lee's original post.

Bob nailed it 100%.

Since the VX1/VX3 -- I think -- the lens SYSTEMs are retro-focus. The focus and zoom controls send info to a computer that also inputs contrast infomation. The computer sends info to the zoom and focus servos. (Maybe also the iris.)

You tell it what you want -- and the computer meets your goals.

You have two choices to focus.

1) zoom in and focus. This is what you should do if you plan to at some point to zoom in.

2) if you don't plan to zoom in, or plan to zoom out, you can establish focus without zooming in. Simply focus after you have framed the scene.

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But the neat thing is that once the computer has got AF on something -- it will track this object. (The first generation Sony's froze focus when you zoomed. Now, as Bob said, when you zoom, the computer automatically will try to keep focus.)

If the object moves out of the frame, the AF will not quickly change AF until something else moves into the frame. This means you can pan from one subject to another in AF. The AF will NOT try to focus on the background between objects.

Likewise, the AF ignores anything that moves in front of the lens once it has locked focus on something.

I spent 4 months testing the AF -- and even at scenes that needed +9dB gain, it was very hard to fake it out.

PS 1: there is actually a problem with zooming in to get focus. When you zoom in really tight on a face -- there are no hard edges to focus on. The AF and Focus Assist fail. (I think this may be way some feel they need a field monitor.) It's better to zoom in on the upper torso so arms and belt are in the frame. Remember, if you don't plan to zoom in further, the focus is fine.

PS2: Up to about Z60 -- the DOF is so great you almost don't need to worry about focus. That's why AF works so well. It doesn't have very much to do. :)

I'm no expert on lens systems, but I think that a backfocus problem could manifest as a lack of ability to infinity focus. That could be a problem whether you use manual or auto focus. Correct me if I am wrong.

Steve, since you seem to be the resident expert on V1 autofocus, how do you feel about the "focus assist" function? Am I being unfair to my camera by leaving it on manual and only pushing the momentary AF when the distance to the subject changes? Are there secrets to utilizing focus assist?

Markus, I don't know about "focus assist", but if you really mean "AF Assist", then it works the other way around (and works perfectly well): when in AF, and you need to change the object/distance it fixed at - just turn the ring slightly and the AF will change its distance accordingly.

What yu're describing sounds more like using Push AF when in manual focus.

As far as I know getting back focus out will never prevent a lens from focussing at infinity. AF should work regardless of backfocus issues. Where you'll start to strike problems is shooting set pieces and you want to selectively focus on a specific subject. The AF system has to decide to focus on something and that something may not be where you want focus to be. In this scenario having a big full res monitor so you can see what you're doing would seem wise no matter what camera you're using and that's for reasons other than just setting focus. The client more likely than not wants to see what they're paying for, other people want to see how things are holding up as well.
One of my clients had to reshoot the first 4 hours of their shoot due to a makeup malfunction, if they'd had proper monitoring they'd probably have avoided this time loss.