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Canon XH Series HDV Camcorders
Canon XH G1S / G1 (with SDI), Canon XH A1S / A1 (without SDI).


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Old April 23rd, 2008, 01:18 AM   #1
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Slow Focus in 30F

Recently I start shooting in 30F. I noticed it takes longer than to focus while zoom-in in 30F than 60i. The scenario is if I have the camera in the widest zoom settings. Then zoom in all the way. The AF takes a very long time to get focus. Sometimes it just won't focus at all. The same situation it focuses a lot faster in 60i. I don't know if I should contact Canon for service on this.

I also have the Canon WA lens adapter so I can't use iAF.

Anybody has a similar experience?
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Old April 23rd, 2008, 02:00 AM   #2
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Taky,

Do you know if the aperture settings were the same in your comparison?

I've found that if you stop down the auto focus seems to misfire.
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Old April 23rd, 2008, 02:02 AM   #3
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It's in the [A] mode.
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Old April 23rd, 2008, 02:46 AM   #4
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If my memory serves there is a mention in the manual that focus is slow on Frame Mode shooting. Just something even Canon can not do much about apparently.
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Old April 23rd, 2008, 02:59 AM   #5
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Probably best to zoom in and focus manually, then zoom out and start shooting. Then the subject should stay in focus all the way. I have found that autofocus is really bad in this kind of situation.
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Old April 23rd, 2008, 03:06 AM   #6
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I have been using the focus ring to adjust. I also found out I was using Hi Speed zoom. That makes it even harder for the A1 to find focus. I turned it off and using normal zoom speed.

So if that's the case, I don't need to send the unit to Canon for service, right?
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Old April 24th, 2008, 11:12 PM   #7
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Okay, found it. I was browsing Canon support site and search for any focus issue. Here's what I found. Now I feel better as I don't have to send my A1 to service.

- When recording under bright conditions, the camcorder sets a small aperture value, and the picture may appear blurred. This may be more noticeable at wide-angle. When you are using a lens with built-in ND filter, turn it on/off according to the screen display.

- When recording under dark conditions, the focusing range narrows and the picture may appear blurred.

- In 30F and 24F mode, autofocus takes longer than in the 60i mode.

- Autofocus may not work well on the following subjects. In such case, focus manually. Reflective surfaces, Subjects with low, contrast or without vertical lines, Fast moving subjects, Through dirty or wet windows, Night scenes
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Old April 24th, 2008, 11:17 PM   #8
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Taky,

That's a great find.

The reason I asked about your aperture settings in my earlier post is for this reason. I've had this experience before. Using the ND filter and thus being able to open up the aperture helps in two regarads:
1. The picture is far sharper
2. The auto focus works better (quicker).
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Old April 25th, 2008, 01:02 AM   #9
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A contrast AF system works by changing the focus a little and then checking the next frame from the sensor to see if the contrast between neighboring pixels increased or decreased. If it decreases, you then focus in the other direction, if it increases then you continue a little more in the same direction. Then on the next frame, you measure again and repeat. At 60i, you can measure and adjust focus 60 times per second, but at 30p you can only perform half as many measurements and adjustments per second, so it takes longer to achieve focus.

At 24p, the motion blur and low sampling rate can make it very difficult to perform contrast AF. That's why the Panasonic DVX100 was MF-only at 24p, and it's probably the reason that Canon put the active IR focus system on their recent cameras that do 24p.
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Old April 25th, 2008, 01:04 AM   #10
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Jason,

Great explanation!

Thank you.
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Old April 25th, 2008, 01:47 AM   #11
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I do not see the same issue with GL2 Frame mode. Also HV30 also has 24p and 30p. I don't notice any focus delay. I suspect the slow focus in 24F and 30F is related to the extra steps behind to convert the interlace signal to progressive while the internal processor is not fast enough to handle these extra tasks. But that's just my guess :)
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