Diaphragm setting unprecise

[Luc De Wandel]

As a photographer, I prefer to adjust the exposure manually on my XH-A1. But when I do so, it ennoys me that the fine adjustment of the diaphragm, especially in the smaller numbers (16 and smaller) is very rough. The result is a black image (underexposure) with a turn of just a few millimeters at the diaphragm ring. Is there a way to get around this, without using higher shutter speeds?

[Petri Kaipiainen]

You should never use small apertures on (small sensor) video cameras. Lens diffraction starts to make the image soft, generally you should use apertures larger than f8 only, f-stops around 4-5.6 are the sharpest.

As the exposure can not be adjusted with short shutter speed due to strobing side effects, you must use ND filters. For that reason they are built in in every video camera.

[Luc De Wandel]

Originally Posted by Petri Kaipiainen

You should never use small apertures on (small sensor) video cameras. Lens diffraction starts to make the image soft, generally you should use apertures larger than f8 only, f-stops around 4-5.6 are the sharpest.

As the exposure can not be adjusted with short shutter speed due to strobing side effects, you must use ND filters. For that reason they are built in in every video camera.

Thanks for your reply. It's kind of odd to me, because that principle is completely opposite to what I'm used to as a still photographer: all my Canon L-lenses perform better at small apertures, especally the wide angle ones. That's why I always tended to shut down the aperture whenever and as small as possible. Just out of intrest: do you know the explanation for this difference in a videocam-lens?

[Petri Kaipiainen]

The focal length is shorter, the actual size of the aperture is smaller, diffraction sets in earlier. The sensor is small and the picture is enlarged quite a lot. Many video cameras and small digital still cameras do not even have f-stops smaller than f8 or f11.

If the focal lenght of the lens is something like 4.5 mm at the wide end (XH-A1), the actual hole at f22 would be something like 0.2mm, even difficult to arrange accuratelly with a mechanical aperture system.

Diffraction affects also still camera lenses. The best sharpness is a balance between generally low quality at full open and diffraction at small apertures. Most lenses perform the bast at around f8 to f11, not smalle apertures (while depth of field is of course biggest there). The very best lenses start to show diffraction already at f5.6.

[Luc De Wandel]

Originally Posted by Petri Kaipiainen

The focal length is shorter, the actual size of the aperture is smaller, diffraction sets in earlier. The sensor is small and the picture is enlarged quite a lot. Many video cameras and small digital still cameras do not even have f-stops smaller than f8 or f11.

If the focal lenght of the lens is something like 4.5 mm at the wide end (XH-A1), the actual hole at f22 would be something like 0.2mm, even difficult to arrange accuratelly with a mechanical aperture system.

Diffraction affects also still camera lenses. The best sharpness is a balance between generally low quality at full open and diffraction at small apertures. Most lenses perform the bast at around f8 to f11, not smalle apertures (while depth of field is of course biggest there). The very best lenses start to show diffraction already at f5.6.

I understand. Thanks.

[Denise Wall]

Canon lenses are generally sharpest at somewhere around 2 stops above the maximum aperture.

Specifically, there are charts for each lens such as the ones he does:

http://www.wlcastleman.com/equip/reviews/index.htm

[Luc De Wandel]

Originally Posted by Denise Wall

Canon lenses are generally sharpest at somewhere around 2 stops above the maximum aperture.

Specifically, there are charts for each lens such as the ones he does:

http://www.wlcastleman.com/equip/reviews/index.htm

Thanks, Denise. As a matter of fact, I have the 'Canon lens book' for photo camera lenses and it's one of the most interesting books I've ever read on photographic glass. As a photographer I had already tested all my Canon L-lenses thoroughly, and I found that the wide-angles tend to get sharp at around f8. This said, Canon does a much better job in making telephoto lenses than wide-angles. The 300mm f2,8 IS that I use, is one of the sharpest lenses around. You can see some of the results of this lens and of the 70-200 f2,8 IS on my site www.concertpix.be. But I was not aware that the lenses for digital videocams were a completely different piece of cake. I thought that closing down the aperture would improve some of the unsharpness that I noticed in the wide-angle range. Until Petri and yourself pointed out the difference. Never too old to learn...

[Denise Wall]

Nice pics, Luc. That 300 2.8 is a sweet lens. I have several nice L lenses but not that one, unfortunately.

I wonder if anyone has done resolution charts at the different distances and apertures such as Castleman's for the A1 zoom lens? Does anybody know?

Or maybe the video people aren't such avid pixel peepers as the photogs ;)

[Luc De Wandel]

Originally Posted by Denise Wall

Nice pics, Luc. That 300 2.8 is a sweet lens. I have several nice L lenses but not that one, unfortunately.

I wonder if anyone has done resolution charts at the different distances and apertures such as Castleman's for the A1 zoom lens? Does anybody know?

Or maybe the video people aren't such avid pixel peepers as the photogs ;)

Yes, that would be mighty interesting, or Canon could include it's video lenses in the next 'EF Lens Work' book. Then we would also have the exact MTF-characteristics to compare them to EF-lenses. Nothing beats an evening of pixelpeeping (for photogs that venture into video, that is).