A GL-2 Imaging Bug / Attribute

Aaron Koolen · August 21st, 2003, 02:51 PM

As far as the corners go, that definately looks like the keyhole effect. For the difference in darkness, how far in were you zoomed Ryan? If you zoom in too far with an f-stop 2.4 (I think) or lower you will notice the camera will close down the iris automatically, and hence a larger f-stop and the image will go darker. In auto it might have compensated for this by lowering the shutter speed so the effect might not be as pronounced...Just a guess.

Cheers
Aaron

Jeff Donald · August 21st, 2003, 03:00 PM

You have two effects going on at the same time. Read my post above and you'll know all about vignetting (keyhole effect).

The reason the picture darkens while you zoom in manual is because the lens is a variable aperture lens. You set the exposure (aperture and shutter speed) in manual, but the aperture changes as you zoom (lens design). Depending on how you zoom (wide to tele or vice versa) and set the exposure the picture will become lighter or darker.

Set the exposure to auto when you zoom, or ride the aperture and adjust it as you zoom.

Ryan Krga · August 22nd, 2003, 05:20 AM

OK, I get that now, but can someone explain why this started happening out of no where? I'm looking at all of my old footage and see no darkening or vignetting going on.

Jeff Donald · August 22nd, 2003, 05:39 AM

In all your old scenes, how often did you shoot a white wall while zooming? In most cases, the vignetting will not be seen in a average outdoor scene. Depending on the lighting, you may or may not notice it occasionally indoors. I suspect it has been there all along and you've just not noticed it.

Ryan Krga · August 22nd, 2003, 06:24 AM

I just shot the wall to show you what I was talking about, I didn't zoom in on either of those shots. It is VERY noticeable in an outdoor scene as well. It's a pretty big flaw to not notice, it only started happening when I put a Sony Premium tape in my camera.

Aaron Koolen · August 22nd, 2003, 06:45 AM

Ryan, this is very strange and I can't offer any other suggestions. What happens if you put the old tapes back in? The tape won't have anything to do with it, but to be sure put other ones in. Hvae you bumped your camera at all recently? I don't know if it's possible to put the lens mechanism out of kilter or not.

Cheers

Aaron

Andre De Clercq · August 22nd, 2003, 06:51 AM

See my post on dec 3 2002
Mireille, just read Ken Tanaka's tread. The "keyhole" (portholing) is known with most (all) zoom lenses on their extreme zoom-in position specifically at wide open aperture. The effect belongs to the mechanical vignetting and is caused by the large entrance pupil (at that extreme zoom setting), which becomes larger than the focussing lens group diameter, causing the "keyhole" effect and the "F drop". I wonder if you will find a XM2 which at low F number will not show this effect. If you do a check, keep in mind that the human eye rapidly adapts to brightness non uniformity. So you should use fast zoom-in/out in manual low F-number setting on a uniform gray field. Did you know that most TV sets have a 50% brightness fall-off towards the corners... ever noticed?

Jack Robertson · December 21st, 2003, 09:15 AM

Hi Ken,

I just read this thread and I have also noticed that my XM2 seems to darken the image in low lit conditions near the far end of the zoom.... but I think that the manual actually says this on page 78...

"The brightness of the subject may change if you zoom during exposure lock."

I'm not sure if this is a bug or whether this has already been addressed in the many replies (which I haven't had the time to read), but a work around would be to zoom in all the way and observe the f-stops... note what reading is at the end of the zoom and then zoom back to wide and re-adjust your f-stops to the ones that appeared at the "end" of the zoom.

That way when you zoom in there will not be any darkening of the image.

Cheers, and let me know what you think.
Jack

Joe Sacher · December 21st, 2003, 12:02 PM

<<<-- Originally posted by Jack Robertson : Hi Ken,
I'm not sure if this is a bug or whether this has already been addressed in the many replies (which I haven't had the time to read), but a work around would be to zoom in all the way and observe the f-stops... note what reading is at the end of the zoom and then zoom back to wide and re-adjust your f-stops to the ones that appeared at the "end" of the zoom.

That way when you zoom in there will not be any darkening of the image.

Cheers, and let me know what you think.
Jack -->>>

Yes, this apeture setting is the key here, and this has been addressed in the repiles you didn't read. Essentially, unevent light falloff is eliminated when the apeture is closed a little from wide open.

Ales Marlin · November 23rd, 2005, 03:13 PM

Hi, a newbie here!

Using this old thread as I do not want to open a new one about similar problem.

Recently I bought an used XM-1 (GL-1). Today I noticed pretty disturbing light falloff (is is this the correct term?) at the corners, specially when zooming-in and zooming-out.

As far as I read in this thread, this should be something normal to get used to with prosumer camcorders. However, I think that in my case it could be a defect.

I made a clip, capturing white wall while zooming in and out. All settings were manual, 1/60, f/1.6, gain +6.

The room was evenly lit with a 100W tungsten bulb.

Here is link to the clip: http://www.privacyblog.net/IAM/test/vignetting.avi

Could somone please comment this? Is this normal or have I bought a faulty cam?

Don Palomaki · November 23rd, 2005, 05:56 PM

Look to me like it could be normal.

See notes above on zoom. Then you zoom in the maximum lens apreture drops from F/1.6 to something like f/2.6 and in manual exposure mode this will darken the image.

ALL, repeat ALL, lenses have corner fall off, especially noticeable with white (gray) flat surfaces, zoom lenses, and at maximum aperture. Corner fall off can be a stop or even more relative to the center. Usually not noticed with averaage image content.

Geoff Holland · November 24th, 2005, 12:05 PM

I had this problem and changed the camera.... the new one had the same problem. It only happened in an artificially lit room. It was noticeable at wide angle and got worse when zooming in. It didn't happen on my old Panasonic NVDA1, which has 17x optical zoom.

There is a fault, no doubt about it.

However, I kept it beyond the "returnable" date and couldn't change it. Pity... I would have gone for the Sony HDV camera.

Never mind, the pictures in general are very good, I avoid indoor filming with white walls (difficult in the Middle East!!) and am waiting until the thing croaks or my kids wreck it so I can get the Sony!! The XM2 is a good, but overhyped camera as far as I can see.

Ales Marlin · November 25th, 2005, 02:31 PM

Thank you for replies. I will mainly use the camera outdoors, so if this is mainly an issue when shooting in rooms with white walls under artificial light, it's not such a problem.

Apart from my very busy schedule it is snowing here almost whole week, so I had no opportunity yet to test it outdoors.

Will post a message when I make an extensive test outdoors.

Ben Winter · November 25th, 2005, 08:56 PM

Looking at the output on a 20" production monitor, I'm seeing very, very little change in the image as I zoom into a white wall, however very close to the end of the 20x zoom (perhaps at 19x onwards) the image gets universally darker. The vignetting is very, very barely apparent. I bought my GL2 about four months ago.

Barry Goyette · November 26th, 2005, 10:54 AM

To those new to this discussion...A big component of this "bug" (and it isn't a bug...it's simply the limitation of providing a 20x zoom in a camera at this price point) is the gain settings, in combination with large apertures that would typically be present when one is inside, photographing a white wall. In a typical indoor lighting situation the automatic gain on the gl2 will naturally go to +12db...what gain does in this situation is to expand the tonal range by moving midtones towards the highlight range...essentially increasing the contrast of the tonal range of the scene...so any variation in the white wall, due to fall-off at large apertures, would be magnified by this added gain. (white balancing incandescent or flourescent light sources add to the problem even further). Ben's comment regarding the overall darkening near 20x is correct as the maximum aperture of the gl2 is smaller at the long end of the zoom.

One would probably never see the vignetting effect in an typical daylight situation, and as long as the light levels are high enough in an interior, and you're photographing your kids instead of the drywall texture in your living room, it's likely that you won't see it there either. Oh, and didn't your momma tell you it's bad manners to play with your zoom like that all the time...

Cheers

Barry

August 21st, 2003, 02:51 PM	#61
Aaron Koolen Trustee Join Date: Apr 2002 Location: Auckland, New Zealand Posts: 1,727	As far as the corners go, that definately looks like the keyhole effect. For the difference in darkness, how far in were you zoomed Ryan? If you zoom in too far with an f-stop 2.4 (I think) or lower you will notice the camera will close down the iris automatically, and hence a larger f-stop and the image will go darker. In auto it might have compensated for this by lowering the shutter speed so the effect might not be as pronounced...Just a guess. Cheers Aaron __________________ My Website Meat Free Media

August 21st, 2003, 03:00 PM	#62
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	You have two effects going on at the same time. Read my post above and you'll know all about vignetting (keyhole effect). The reason the picture darkens while you zoom in manual is because the lens is a variable aperture lens. You set the exposure (aperture and shutter speed) in manual, but the aperture changes as you zoom (lens design). Depending on how you zoom (wide to tele or vice versa) and set the exposure the picture will become lighter or darker. Set the exposure to auto when you zoom, or ride the aperture and adjust it as you zoom. __________________ Jeff Donald Carpe Diem Search DVinfo.net for quick answers \| Where to Buy? From the best in the business: DVinfo.net sponsors

August 22nd, 2003, 05:39 AM	#64
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	In all your old scenes, how often did you shoot a white wall while zooming? In most cases, the vignetting will not be seen in a average outdoor scene. Depending on the lighting, you may or may not notice it occasionally indoors. I suspect it has been there all along and you've just not noticed it. __________________ Jeff Donald Carpe Diem Search DVinfo.net for quick answers \| Where to Buy? From the best in the business: DVinfo.net sponsors

August 22nd, 2003, 06:45 AM	#66
Aaron Koolen Trustee Join Date: Apr 2002 Location: Auckland, New Zealand Posts: 1,727	Ryan, this is very strange and I can't offer any other suggestions. What happens if you put the old tapes back in? The tape won't have anything to do with it, but to be sure put other ones in. Hvae you bumped your camera at all recently? I don't know if it's possible to put the lens mechanism out of kilter or not. Cheers Aaron __________________ My Website Meat Free Media

December 21st, 2003, 09:15 AM	#68
Jack Robertson Regular Crew Join Date: Jun 2003 Location: Sydney, Australia Posts: 125	Zooming and Darkening of Image Hi Ken, I just read this thread and I have also noticed that my XM2 seems to darken the image in low lit conditions near the far end of the zoom.... but I think that the manual actually says this on page 78... "The brightness of the subject may change if you zoom during exposure lock." I'm not sure if this is a bug or whether this has already been addressed in the many replies (which I haven't had the time to read), but a work around would be to zoom in all the way and observe the f-stops... note what reading is at the end of the zoom and then zoom back to wide and re-adjust your f-stops to the ones that appeared at the "end" of the zoom. That way when you zoom in there will not be any darkening of the image. Cheers, and let me know what you think. Jack

August 22nd, 2003, 05:20 AM	#63
Ryan Krga Regular Crew Join Date: Jan 2003 Location: Chicago, Illinois Posts: 116	OK, I get that now, but can someone explain why this started happening out of no where? I'm looking at all of my old footage and see no darkening or vignetting going on.

August 22nd, 2003, 06:24 AM	#65
Ryan Krga Regular Crew Join Date: Jan 2003 Location: Chicago, Illinois Posts: 116	I just shot the wall to show you what I was talking about, I didn't zoom in on either of those shots. It is VERY noticeable in an outdoor scene as well. It's a pretty big flaw to not notice, it only started happening when I put a Sony Premium tape in my camera.

August 22nd, 2003, 06:51 AM	#67
Andre De Clercq Major Player Join Date: Sep 2002 Location: Belgium Posts: 804	See my post on dec 3 2002 Mireille, just read Ken Tanaka's tread. The "keyhole" (portholing) is known with most (all) zoom lenses on their extreme zoom-in position specifically at wide open aperture. The effect belongs to the mechanical vignetting and is caused by the large entrance pupil (at that extreme zoom setting), which becomes larger than the focussing lens group diameter, causing the "keyhole" effect and the "F drop". I wonder if you will find a XM2 which at low F number will not show this effect. If you do a check, keep in mind that the human eye rapidly adapts to brightness non uniformity. So you should use fast zoom-in/out in manual low F-number setting on a uniform gray field. Did you know that most TV sets have a 50% brightness fall-off towards the corners... ever noticed?

December 21st, 2003, 12:02 PM	#69
Joe Sacher Regular Crew Join Date: May 2003 Location: Indianapolis, IN Posts: 177	Re: Zooming and Darkening of Image <<<-- Originally posted by Jack Robertson : Hi Ken, I'm not sure if this is a bug or whether this has already been addressed in the many replies (which I haven't had the time to read), but a work around would be to zoom in all the way and observe the f-stops... note what reading is at the end of the zoom and then zoom back to wide and re-adjust your f-stops to the ones that appeared at the "end" of the zoom. That way when you zoom in there will not be any darkening of the image. Cheers, and let me know what you think. Jack -->>> Yes, this apeture setting is the key here, and this has been addressed in the repiles you didn't read. Essentially, unevent light falloff is eliminated when the apeture is closed a little from wide open.

November 23rd, 2005, 03:13 PM	#70
Ales Marlin New Boot Join Date: Nov 2005 Location: Ljubljana, Slovenia Posts: 7	Hi, a newbie here! Using this old thread as I do not want to open a new one about similar problem. Recently I bought an used XM-1 (GL-1). Today I noticed pretty disturbing light falloff (is is this the correct term?) at the corners, specially when zooming-in and zooming-out. As far as I read in this thread, this should be something normal to get used to with prosumer camcorders. However, I think that in my case it could be a defect. I made a clip, capturing white wall while zooming in and out. All settings were manual, 1/60, f/1.6, gain +6. The room was evenly lit with a 100W tungsten bulb. Here is link to the clip: http://www.privacyblog.net/IAM/test/vignetting.avi Could somone please comment this? Is this normal or have I bought a faulty cam?

November 23rd, 2005, 05:56 PM	#71
Don Palomaki Wrangler Join Date: Sep 2001 Location: Northern VA Posts: 4,488	Look to me like it could be normal. See notes above on zoom. Then you zoom in the maximum lens apreture drops from F/1.6 to something like f/2.6 and in manual exposure mode this will darken the image. ALL, repeat ALL, lenses have corner fall off, especially noticeable with white (gray) flat surfaces, zoom lenses, and at maximum aperture. Corner fall off can be a stop or even more relative to the center. Usually not noticed with averaage image content. __________________ dpalomaki@dspalomaki.com

November 24th, 2005, 12:05 PM	#72
Geoff Holland Regular Crew Join Date: Aug 2005 Location: Montreal, QC Posts: 60	I had this problem and changed the camera.... the new one had the same problem. It only happened in an artificially lit room. It was noticeable at wide angle and got worse when zooming in. It didn't happen on my old Panasonic NVDA1, which has 17x optical zoom. There is a fault, no doubt about it. However, I kept it beyond the "returnable" date and couldn't change it. Pity... I would have gone for the Sony HDV camera. Never mind, the pictures in general are very good, I avoid indoor filming with white walls (difficult in the Middle East!!) and am waiting until the thing croaks or my kids wreck it so I can get the Sony!! The XM2 is a good, but overhyped camera as far as I can see.

November 25th, 2005, 02:31 PM	#73
Ales Marlin New Boot Join Date: Nov 2005 Location: Ljubljana, Slovenia Posts: 7	Thank you for replies. I will mainly use the camera outdoors, so if this is mainly an issue when shooting in rooms with white walls under artificial light, it's not such a problem. Apart from my very busy schedule it is snowing here almost whole week, so I had no opportunity yet to test it outdoors. Will post a message when I make an extensive test outdoors.

November 25th, 2005, 08:56 PM	#74
Ben Winter Trustee Join Date: Jun 2005 Location: Hollywood, CA Posts: 1,675 Images: 1	Looking at the output on a 20" production monitor, I'm seeing very, very little change in the image as I zoom into a white wall, however very close to the end of the 20x zoom (perhaps at 19x onwards) the image gets universally darker. The vignetting is very, very barely apparent. I bought my GL2 about four months ago. __________________ BenWinter.com

November 26th, 2005, 10:54 AM	#75
Barry Goyette Trustee Join Date: Oct 2001 Location: San Luis Obispo CA Posts: 1,195	To those new to this discussion...A big component of this "bug" (and it isn't a bug...it's simply the limitation of providing a 20x zoom in a camera at this price point) is the gain settings, in combination with large apertures that would typically be present when one is inside, photographing a white wall. In a typical indoor lighting situation the automatic gain on the gl2 will naturally go to +12db...what gain does in this situation is to expand the tonal range by moving midtones towards the highlight range...essentially increasing the contrast of the tonal range of the scene...so any variation in the white wall, due to fall-off at large apertures, would be magnified by this added gain. (white balancing incandescent or flourescent light sources add to the problem even further). Ben's comment regarding the overall darkening near 20x is correct as the maximum aperture of the gl2 is smaller at the long end of the zoom. One would probably never see the vignetting effect in an typical daylight situation, and as long as the light levels are high enough in an interior, and you're photographing your kids instead of the drywall texture in your living room, it's likely that you won't see it there either. Oh, and didn't your momma tell you it's bad manners to play with your zoom like that all the time... Cheers Barry