Canon XH A1 and soft focus issue
A handful of online comments have asked about a “soft focus” issue with the Canon XH A1, a professional HD camera noted for its sharp lens! Only a few people seemed to encounter the soft focus problem – and usually dealt with it by manually focusing instead of using the auto-focus. But no one seemed to identify the root cause problem and the correct solution.
Yesterday, I encountered soft focus and it took quite a bit of testing today to locate the cause of the problem and to identify a solution.
I was video taping a marching band competition (mostly in the rain – such fun) and noticed that the camera, which was set to auto focus, was usually slightly out of focus. Even when I manually set the focus and then turned auto focus back on, the camera would shift back out of focus just enough to produce a soft image appearance.
In the interim, I set focus to manual and peaking and magnification on and manually set the focus. But when I offloaded to the computer and blew up the images, I could see that the images were still not sharp.
What was going on?
After shooting a bunch of test images on both an XH A1 and an HV30 – and finding the little HV30 to have sharper images, I knew something was not right.
After a few hours of experimenting, I found the problem – and its even mentioned on page 39 of the manual. Sigh.
Here goes – yesterday, I was shooting an outdoor event, under cloudy skies in 24F mode at 1/48th of a second in Tv mode. I had the AGC set to off and gain manually set to 0 db to avoid adding any noise to the image. Previously I had shot similar events in 30p at 1/60th or 1/100th of a second exposure.
The problem turned out to be caused by not using neutral density (ND) filters resulting in a high f-stop setting auto selected by the camera.
What I did not realize is that when the AGC is set to off, the XH A1 does not advise you when the neutral density (ND) filters should be selected. Consequently, I never thought to use an ND filter.
Further, I did not have the exposure information visible on the display and did not realize the camera was trying to shoot at f-9.5 – where the lens sharpness is degraded. (I didn’t have my reading glasses on and for some reason I thought it said f-19.)
Two problems occur at a tiny aperture:
- The auto focus mechanism basically does not work at higher f-stops. In fact, it probably doesn’t work well above f-5.6 or so. If the camera is in full auto mode, it generally chooses excellent settings and you don’t need to worry about this.
- The lens sharpness is severely degraded at tiny apertures like f-9.5.
Consequently, there were two problems happening at the same time – one was the poor auto focus and the second was shooting at an f-stop where the lens produces soft images.
The solution is that when shooting in manual mode with AGC turned off, check the f-stop being set by the camera! Set the ND filter and shutter speed as required. Or, I suppose, shoot in Auto mode, with the AGC set to Auto/On as well. Then you’ll get an ND filter warning as required. You can also test your setting by turning AGC back on and see if the camera warns you to enable the ND filter. Set the ND filter and then turn AGC back off.
In my test shots, once I set the ND filter and dropped my exposure to f3.4 – from f-9.5 – the blurriness went away and sharpness was returned.
I have not yet determined the f-stop having the maximum sharpness but have seen some blog posts indicating f-4 is quite sharp.
Why shoot at 24 fps? When uploading online, this provides the highest quality image for a given bit rate. Although it may not make that much difference due to compression across frames.
Update: July 2010
I recently ran across the same problem with my older Panasonic AG-DVC 30 SD camera. The DVC 30 has a fake progressive mode for taking a “frame” like image versus an interlaced image. The problem is that in this particular camera, there is a resolution drop when using the “30F” frame mode due to how Panasonic scans the pixels to create the frame. If I understand the situation correctly, this could result in a 33% resolution drop in Frame mode. ( To my eye, it depends on the image because it typically seems less than this – and on some test shots, the 30F mode definitely looked better than a high quality de-interlace of the 60i video).
There are two possible work arounds. One is to shoot in normal 60i and get full resolution – and then use DV Film Maker in post to deinterlace the 60i to either 30p or 24p. This works quite well, although its not as good as shooting native progressive to start with.
The second method is to use the camera in 60i but manually set the shutter to 1/30th of a second. This actually produces a true full frame progressive image and is fine as long as you can use the low shutter speed. So far so good.
However, if shooting in bright light, the DVC 30 will push the f-stop as high as it can go – and sure enough, the auto focus fails and the camera is unable to obtain a clean focus.
Consequently, I’d say this problem is probably common across many cameras. The main issue with the XH A1 is that in manual gain setting mode, we no longer see the warning about the need for a neutral density filter.
In the case of the DVC 30, a simple solution would be to use an external ND filter.
Update November 2010
I shot an event, outdoors, under stadium lighting, at night. For some of this event, I used f1.6 aperture and 0 db gain. The image looked fine in the viewfinder. However, after watching on the computer, I discovered some of the shots did quick small jumps out of focus and then back in focus. Really ugly stuff. What’s going on?
In low light conditions, the “Instant Auto-Focus” feature often starts doing a hunt and seek. Therefore, DO NOT use I.A.F. mode in low light. Either use the standard through-the-lens autofocus, or use manual focus.
Some of the event I shot at +6 db gain and I did not seem to have the focus problem there. Again, in low light:
– don’t use I.A.F., use standard auto focus or manual focus
– if you use auto focus, probably best to add sufficient gain
The problem with gain is that it adds some noise, although nothing to worry about at +6 db.
Focus on the XH A1 is a complicated affair!