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-   -   auto focus: sometimes missing focus (https://www.dvinfo.net/forum/avchd-format-discussion/234359-auto-focus-sometimes-missing-focus.html)

Malcolm Hamilton April 30th, 2009 02:32 PM

auto focus: sometimes missing focus
Hi there,
Just got a Canon HF-S10, and set it up as a second camera for an interview. Put it on a tripod a few feet from my subject, for a tight shot from the side, with the subject's face predominant in the frame. First time using it, and I had to attend my other camera, so I left it in autofocus.
Now I'm screening the stuff... and the shot is in focus sometimes, and out of focus other times...
Can anyone enlighten me as to why this would be?
(next time I'll just go manual focus).
Cheers, Malcolm

Rick Bolton April 30th, 2009 05:31 PM

Malcolm - I have experienced some of the same thing with the Canon HF 100. The autofocus appears to drift even though the subject is nearly static and dominates the frame. There is minimal movement in the frame.

There is an initial soft focus and over 4 - 6 seconds the focus sharpens. At some point the focus drifts out and then back in. So so AF in my opinion.

Robert Young April 30th, 2009 11:37 PM

I think it is just the nature of the beast, not particularly Canon, and not particularly small consumer type cams. It is inherent with autofocus, and seems particularly notorious with head shot/interview set ups.
I get the most consistant results by zooming in, get the sharp focus, turn autofocus off, recompose the shot, then let it roll. Even if the subject moves around a bit, I think this works more reliably than having autofocus suddenly decide to go for the wallpaper 8 feet behind the subject, or do the wierd pulsing, seeking behavior.
The problem is that you can get away with autofocus just often enough to start feeling confident, then, wham- it ruins an important shot.

Malcolm Hamilton May 1st, 2009 05:54 AM

Thank you both. That's exactly the problem I experienced, Rick.
Robert, I will now use your modus operandi.
Cheers, Malcolm

Ron Evans May 1st, 2009 07:24 AM

The problem with most consumer cameras is that the approach of zooming in to focus then reframing doesn't always work because of the backfocus of the consumer lenses if the zoom range is not close. This is where the Sony "Spot focus" is really useful in that one can frame then set focus on the subject where ever this is in the frame.

Ron Evans

Robert Young May 1st, 2009 12:01 PM

a good point and a good alternate stratigy.
Do you know for sure if backfocus is a significant issue with the top end Canon & Sony AVCHD cams?? I've only used the Sony SR12 w/ W.A. adaptor and haven't really noticed a problem with backfocus so far.

Ron Evans May 1st, 2009 02:43 PM

All the consumer cams I have ever had have a backfocus problem. I currently have FX1, XR500,SR11, HC96,TRV50,PC10,TRV740,VX3. The problems with the FX1 and VX3 are minimal but the others are significant. IF one zooms in to focus( on something 60 feet away), take some video then lets the camera focus at the wider framing needed and compare the outputs on tape the wider shot will always look more in focus. Some of the issue may be that the iris stops down on lots of the consumer cams as the zoom reaches the max. When the camera focuses at the wider angle the camera is able to provide more depth of field using a more ideal focal point for the actual framing. I have used the mainly consumer cams as backup to the VX3( when I was shooting Hi8) and then the FX1. I learned to use manual exposure then the "Spot focus" at the framing I wanted with the average stage lighting, as the best solution to focus for these wide shots on an unattended camera.

Ron Evans

Robert Young May 1st, 2009 09:49 PM

Thanks for the info.
I usually shoot with EX1, which, with peaking, easy manual focus, no real backfocus issue, is a no brainer. But now I'll take a closer look at my SR12- particularly the spot focus, which I have never used before.

Ron Evans May 2nd, 2009 05:47 AM


Originally Posted by Robert Young (Post 1136225)
Thanks for the info.
But now I'll take a closer look at my SR12- particularly the spot focus, which I have never used before.

Spot focus is one of the best features of the Sony's. There is spot exposure too and on the new XR500 a menu item that combines spot focus and spot exposure in one screen. Just touch where you want the subject to be and focus and exposure are set for that point. Essentially this is what the face detection uses to identify faces and then set focus and exposure. In the case of face detection it also sets WB for faces and assigns more bandwidth to this face area in the encoder. It is these sort of features that make testing a little different from actual experience. A camera may not test as good as another in lab tests of test charts but on actual video of people( why most people buy a video camera) may be better. Standard test are good for testing the imager but this is not what the camera records as the features use the DSP to modify the imager output based on the image( auto scene detection , face recognition etc). The standard test pattern is unlikely to invoke these DSP responses. The tests therefore are not really a reflection of actual user performance.

Ron Evans

Robert Young May 2nd, 2009 07:05 PM

Ron- when you activate the spot focus/exposure by touching the screen, does it lock the focus and exposure for the duration of the shot? Or does it reset if the subject/camera move off of the "spot"? If it's the former then it sounds like a very useful feature.
Your point about the bench tests not measuring the cam's actual "smart" performance is well taken. Apparently the SX 500/520 have gotten very smart indeed from what I read.
I just ordered the SX 520. I'll see soon enough.
I've been so impressed with the image quality of the SR12, editing with Cineform DI, and delivery in BD- if the SX is even better (particularly for low light, low noise) it's going to be near competitive with my EX for a lot of projects.

Ron Evans May 2nd, 2009 08:17 PM

Yes the Spot focus or Spot exposure or the new Spot focus /exposure stays until they are changed. I use the SR11 for a fixed camera full stage shot. The focus is set framed for the full stage and using spot focus with the expected main lighting for the show in manual exposure. This stays for each part of the show. If I put on standby power I know the exposure stays as set but I usually reset the spot focus after a power down. I have done some more testing of the XR500 and it does do a good job of maximizing the focus /exposure in any situation expecially when there are people around in the shot that it locks onto. For my use in the theatre I will use the same as the SR11 since with the lights going up and down to black I do not want the exposure to hunt and in the same circumstance auto focus just doesn't work. As an extra camera where this is not important it works great using all the smart features in full auto even in this theatre environment as I think the auto scene select manages the lighting. As a family cam it will be great. A little smaller and lighter than the SR11 with much less grain and a little better low light.

Ron Evans

Dave Blackhurst May 3rd, 2009 02:29 PM

Robert - you'll find the XR to your liking, it is a surprisingly able little camera. I'm still learning it's ins and outs, but the more I play with it the more I'm convinced they actually got the "auto" stuff pretty solid.

And I'm pretty sure that none of the firmware magic would show up in most testing, and might actually be a negative in canned "tests". If you're measuring "accuracy" while the camera is auto adjusting to be what it considers to be accurate, are you getting a valid test? I've noticed one review saying the camera is a bit "dark" or underexposed, yet I'm finding it to be either spot on or still in need of a minus 1-2 AE shift to get what I want. Using the knob for exposure seems to provide adequate image control.

The OIS and low light improvements are worth the price of admission. I'd sure like to see the sensor and overall performance in a slightly larger package with more manual control, BUT, it ain't bad as it sits - you can concentrate on framing and composition instead of tweaking.

Robert Young May 3rd, 2009 03:19 PM

Honestly, that's exactly what I am looking for. I use the SR12 for a travel cam & other situations where a big cam is not practical. Usually these are run n' gun set ups, handheld (monopod on a serious day). Autopilot with some focus tweak is going to be all that there is time for. I recently did a 50 min travel doc with the SR12. Delivered on BR, the imagery was absolutely pro level (the clients were certainly impressed)- except for a little jiggle & wiggle on the handheld shots, occasional focus issues, and pretty poor low light/noise performance. If I can improve on those items with the XR, then I'll be thinking that it's as good as it needs to be. When I need more & better, well, that's why Sony invented the big cameras.

Dave Blackhurst May 3rd, 2009 10:00 PM

The new OIS is almost to the point that you can get away with handheld if you're not too zoomed in.

I've got a couple small light monopods with belt clip (can't get the wobble out of a monopod to the ground to save my life, but with a belt clip type setup, I get pretty good results!), and they are probably sufficient with the improved stabilization. It all fits in a small camera bag, along with extra batteries and a WA lens... even room for one of their bluetooth wireless mics and a Sima 20W LED light with a small folding bracket, and a wall charger!

Hard to argue with the portability! And I suspect 99.9% of viewers wouldn't see a lot of difference between what the XR can produce and a camera 5x the price...

Robert Young May 3rd, 2009 10:44 PM

Depends on your delivery format too.
Here are two 5 min shorts for web, shot at the same location:

Shot with EX1 1080i HQ on tripod

Shot with SR12 on monopod
The Ceremony

Overall, they are pretty close to my eye.

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