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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 July 19th, 2010, 01:08 PM   #1
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Focus - diffraction blur?

Normally instant autofocus on the XH-A1s works well for wedding group shots outside. However I noticed some footage recently looks soft on the big screen.

I don't know what my settings were on the day - I'm usually mostly manual with the exception of autofocus in some outdoor situations.

Looking in the XH-A1s manual, page 56, I learn that 'F-stops in grey, i.e.F-10 to F22, will cause diffraction blur'. I'd been wondering why some of the lower F-stops appeared in grey on the screen.

So... it wasn't an issue with the focusing, either auto or manual, as such. Looks like I have been combating the bright sun by sometimes closing the iris too much and introducing 'diffraction blur', instead of using the ND filter. I may have done this once or twice recently - fortunately it is not too damaging to the overall work.

This is correct then - to shoot successfully in bright sunshine one should never go down into the 'grey' F-stops (the effect being increasingly blurred focus) but instead use the ND filter which will keep sharp focus so long as the F-stops don't dip into the grey? Guess I should activate the iris limiter, or is there any reason not to?
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Old July 19th, 2010, 04:59 PM   #2
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Diffraction limiting is just the laws of physics talking to ya.

Yes, you should use ND filters instead of small apertures if sharpness is your aim.

Video is so different from the photography I'm used to. I come from a large format (5x4 inch sheet film) background where you don't start to see diffraction limiting until around f/22 or so. Then again, my fastest lens in the LF world is f/4.5 which would give most videographers severe heart burn at the least. ;-)
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Old July 19th, 2010, 06:57 PM   #3
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Rob, you have it right. Adjust the ND filter so that you can keep the iris below f8.

If you need a more automatic mode, Av mode may be helpful as it will vary the shutter speed to accommodate the increased lighting. It's not as jarring on-the-fly as the ND filters that also shake the camera. But the preferred approach is to keep things running at 50fps for pal 50i and move the ND until you run out of range. Also be aware that there are some visual artifacts from an overly fast shutter and they are difficult to see in the VF or LCD but obvious when you look at the footage.

Last edited by Les Wilson; July 20th, 2010 at 08:32 AM. Reason: converted settings to PAL
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Old July 20th, 2010, 12:40 AM   #4
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Here is an instance where I initially forgot to use the ND filter. When filming outside events I use TV mode and check the aperture setting by pressing the Exposure Lock button. If the f number is high, I use the ND filter.
Attached Thumbnails
Focus - diffraction blur?-no_nd.jpg   Focus - diffraction blur?-with_nd.jpg  

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Old August 11th, 2010, 03:21 PM   #5
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OK so I have discovered definitely never to use 'greyed out' higher f-stops as this can cause frankly ghastly diffraction blur - you don't always sense it in the viewfinder (especially on bright days), but at home the big screen tells a different story.

I am aware f-stops affect depth of field, but did not know f-stops can have a direct bearing on focus quality - i.e. the sharpness of the subject (too high and focus softens as proven).

Therefore, is there an optimum f-stop for 'sharpest focus' on the XH-A1S? I found the article below, which suggests it is not simply a case of going to the other extreme and using the lowest f-stop/widest iris (which could be achieved using the ND filter) as this can also distort with 'abberations' (?). Does the content of this article apply to the lens of the XH-A1S?

"Picture sharpness also varies with f-number. The optimal f-stop varies with the lens characteristics. For modern standard lenses having 6 or 7 elements, the sharpest image is often obtained around f/5.6–f/8, while for older standard lenses having only 4 elements (Tessar formula) stopping to f/11 will give the sharpest image. The reason the sharpness is best at medium f-numbers is that the sharpness at high f-numbers is constrained by diffraction, whereas at low f-numbers limitations of the lens design known as aberrations will dominate. The larger number of elements in modern lenses allow the designer to compensate for aberrations, allowing the lens to give better pictures at lower f-stops. Light falloff is also sensitive to f-stop. Many wide-angle lenses will show a significant light falloff (vignetting) at the edges for large apertures. To measure the actual resolution of the lens at the different f-numbers it is necessary to use a standardized measurement chart like the 1951 USAF resolution test chart"
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Old August 12th, 2010, 08:55 AM   #6
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The hard work has been done for yoy. Here's the answer in an earlier thread.
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Old August 12th, 2010, 09:41 AM   #7
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Rob, the clipped bit in italics has been written about focal lengths designed fot the 36 x 24 mm gate, ie huge compared to your tiny 1"/3 chip. So none of that applies to you.

Diffraction losses occur whenever light hits a sharp edged object, and the diaphragm blades are a prime example of this. So wide-open the lens exhibits the least amount of diffraction, but of course miss-centering of elements and vignetting cause more severe losses such that diffraction goes unnoticed.

But with your tiny chips a good system will start to show the effects of diffraction at about f/4.5, especially at the wide-angle (short focal length) end of the zoom. Stay wide open is the motto.

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