Video darkens during zoom with fixed aperture lens

Bob Hart · December 18th, 2010, 02:10 PM

My wild guess is that it might be something to do with the digital magic that happens when a megapixel sensor image is reproduced at a lower resolution for video.

The moire problem associated with these cameras in video suggests to me that fewer pixels across the entire sensor width are being scanned.

I wonder if in stills mode where every pixel is counted, there is some blending of light falling at more acute angles across to neighbouring pixels by the anti-aliasing filter which might have an apparent gain effect. If in video mode there are fewer pixels contributing to the image, then there would not be enough diffusion of light across the gaps by the anti-aliasing filter.

At a wide angle, the angle of incidence of light falling onto the sensor might vary more than when the lens is zoomed in with maybe a tendency for darker corners which is accentuated by 35mm groundglass adaptors.

Does the camera have an inbuilt compensation for this? I understand that HDCAM cameras have a user selectable function that maps the brighter and darker areas of certain lenses and compensates.

If the camera does have a compensation process, then fixed mapping for darker corners would be wrong for the long end of the zoom where the brightness across the image is more even and the entire image would be darkened as the corners are seen to be overbright by the mapping process.

I am likely barking at shadows in the manner of a knowall who knows nothing. It is just a guess that a combination of sorts may be going on.

It would be interesting to see if leaving the lens on wide and travelling the camera itself forward to reproduce the same framing as the zoom-in creates, exhibits the same darkening effect.

I would not regard this effect so much as a defect as something you may need to live with. There are limits to how much work can be done by the processor in these remarkable cameras. If it is any compensation, you may observe a corner darkening effect in the image of purposed videocameras when zoomed in and darkening overall as some of these do not have constant aperture zooms.

John Wiley · December 19th, 2010, 02:24 AM

Quote:

Originally Posted by Karl Edwards

it seems that that canon's lenses are locked down in video mode cannont "scale" the aperture as the zoom moves through its range. this effectively makes the fixed focal length zoom a variable aperture zoom druing any given shot.

If this is the case then it would stand to reason that a manual lens used via an adaptor will not exhibit this behavior - as these lenses would need to be able to correct this without communication with the camera about whether it is in "stills mode" or "video mode"

Wacharapong Chiowanich · December 19th, 2010, 09:01 AM

Yes, if this is the case, a comparable fixed zoom such as the Nikon 17-35 f/2.8 AF-D (with aperture ring) mounted via an adapter will easily solve the problem.

Karl Edwards · December 19th, 2010, 10:12 AM

hey guys,

like i said this looks like a specific problem with canon lenses on canon cameras. not a malfunction so much as the physics and mechanics of the lens. if you put the same canon lens on a manual adapter and back on a canon camera i'd suspect you'd see the problem in still mode as well.

i've done some more tests and here's what i've found:

looks like starting and stopping the shot doesn't "re-cajigger" the aperture back to it's correct size depending on the position of the zoom in video mode. the correct aperture value at any other point in the zoom's range must be tied to the shutter on canon dslrs. my tests have shown that exposure definitely isn't affected by zooming when shooting photos.

what i have discovered is that the issue ISN'T PRESENT when the lens is wide open. this would explain why you don't see it in live view in photo mode (it's always wide open until the shutter is triggered). weird that it only happens when stopped down but could have something to do with the mechanics of the lens.

THE BAD NEWS:

unfortunately this seems to mean that you'll need to adjust your exposure every time you change your focal length with a canon zoom lens in video mode on canon hdslrs. my tests showed approx. 2/3 stop from 17 mm to 40 mm on my 17-40 f4 L lens. haven't tested any other lenses to find the exact exposure difference.

the other issue seems to be that this change in exposure isn't regular and has an odd spherical shape to the decrease in exposure. you can see this shape in the posted video - almost resembles the way the 17-40 elements move inside the lens barrel... i could just be making that part up but it looks that way to my drunken eyes.

it's too bad - the versatility of having mulitiple focal lengths available (i don't zoom during video... hate that!) during a single shot is really handy. especially for run and gun shooting where you might want to keep the clip running as you quickly change angle perspective and sort the shots out in post.

THE GOOD NEWS:

hdslr video has its own character and produces undeniably beautiful images quality if handled correctly. the entire hdslr revolution seems to be about working around the fact that these damn things weren't really meant to do this but do it really well. just another example i guess!

time to get some primes!

k

Bruce Foreman · December 19th, 2010, 10:03 PM

I looked at his footage and noticed that when he shifted the camera's view to where the "mix" of light and dark" tones stayed predominantly "light" the exposure change effect was less.

In Feb this year we had an unexpected and very nice snowfall. I used this same lens (17-40mm f4L) on my 7D and shot some video footage from just off my front porch. For one sequence I zoomed slowly from the 40mm end to full wide (17mm) and there was NO CHANGE in exposure. The mix of light and dark changed only slightly (began to include a few neighbor's trees - dark tones) with the slightly subdued white snow tones continuing to predominate.

In the OP's video as he zoomed from wide to tele, dark tones began to dominate the "mix" and subsequently his image got a bit darker as his composition was beginning to require more exposure. It's a lighting and exposure issue, the guy is wearing a dark tee and jeans, seated on a light colored couch with what looks like slightly lighter wall in the background.

I can zoom with my 17-40 f4l and experience no exposure change so far but I tend to look for lighting and tonal quality issues before shooting.

Karl Edwards · December 20th, 2010, 09:15 AM

hi bruce,

this isn't an exposure issue as the camera is on full manual. there is no automatic exposure adjustment at all.

karl

Paul R Johnson · December 20th, 2010, 11:58 AM

I've just tried this on a DSLR and focused on a solid colour wall at wide angle, with the shutter speed fixed (1/45 for info) On wide angle the correct exposure was f4, zoomed in f6.7 on an 18-55mm and
f4.5 wide, f5.6 zoomed in on a 70-300. With aperture priority, then the shutter speed changes instead.

When the Canon is taking video, the shutter speed is constant, so if the setting doesn't allow the aperture to change OR the shutter speed, then the brightness is the only thing that can change as the light level drops. Or have I missed something here? I've tried 2 lenses which both let different amounts of light through as the focal length changes from an evenly illuminated surface.

Bruce Foreman · December 20th, 2010, 12:57 PM

Quote:

Originally Posted by Karl Edwards

hi bruce,

this isn't an exposure issue as the camera is on full manual. there is no automatic exposure adjustment at all.

karl

It is an exposure and lighting issue even in manual mode inasmuch as we must evaluate exposure requirements and visualize regardless of being in auto or manual mode. In the last position I held we shot thousands of color slides using Nikon F3s, you had to have the exposure "dead on" to meet presentation standards and I often saw exposure requirements change when zooming in close just because the predominance of dark or light image tonality changed.

Here is the video I mentioned using the same lens. Shutter 1/60th, aperture f5.6 and I forget what the ISO was. The zoom isn't the smoothest but there is no exposure change.

Quote:

Paul R Johnson

I've just tried this on a DSLR and focused on a solid colour wall at wide angle, with the shutter speed fixed (1/45 for info) On wide angle the correct exposure was f4, zoomed in f6.7 on an 18-55mm and
f4.5 wide, f5.6 zoomed in on a 70-300. With aperture priority, then the shutter speed changes instead.

Assuming the lens you used is the 18-55mm "kit" lens, that is not a fixed aperture lens. The manual warns against relying on zoom during a video take as the physical aperture will change as the lens is zoomed. The discussion above is around a 17-40mm lens that has constant aperture throughout the zoom movement.

Karl Edwards · December 20th, 2010, 06:37 PM

again, the lens used was the 17-40 F4 L series lens - definitely a fixed aperture lens.

what you're saying just doesn't make sense bruce - it's not how photography works. when your exposure is locked in manual mode the contents cannot and do not affect exposure. exposure is only affected by iso, shutter speed and aperture.

all things being equal it just doesn't happen the way you describe.

let's even say that auto-exposure was somehow influencing the scene - the predominance of darkness in the frame would cause the auto-exposure to lighten the image, not darken it. auto exposure typically (for the sake of simplicity) tries to equalize the contents of the frame to an overal 18% grey tone. show it dark, it makes the darks lighter - show it light it makes the lights darker in an effort to expose them at 18% grey.

it's possible you're referring to the fact that filling the frame with dark elements will create a dark image because you're photographing dark elements. i'm referring to the exposure actually changing during the zoom - again, see the posted video. that's not added in post. definitely there and definitely happening.

also - i only saw one short zoom in your sample video... and it came on the heels of a fade from black. the amount of detail in the scene doesn't show the effect for sure. try your camera pointed at a white wall with no other elements. set your exposure when fully zoomed out then zoom in and out during the video. you'll see the effect - been verified on 4 seperate cameras and three different photographers. not a mistake but something going on with the canon dlsrs and video.

k

Karl Edwards · December 20th, 2010, 07:12 PM

i whipped up a quick test against a white wall that really shows that this issue is not being created by the contents of the frame:

i'd think my lens was the cause but i've verified the same issue on another 17-40 f4 L and a 24-70 f2.8 L and i've also seen it on 2 different 7D's and a 5Dmkii.

the crazy thing is that this does not happen when the lens is wide open. it's only present when the lens is stopped down. also stills shot at the different zoom positions all have matching exposures which tends to incline that it's not a lens issue. this really is limited to the video mode only.

okay, i need to walk my dog! LOL

karl

Karl Edwards · December 21st, 2010, 09:48 AM

you know, i'm not sure this whole "not readjusting the aperture" explanation is the whole story. in fact i think this might be more related to the lens just not scaling the aperture correctly.

if you look through the front of the lens while zooming you can actually see what's going on. the aperture seems to be a fixed size but a second diaphram is coupled to the zoom and opens and closes slightly depending on where you are in the zoom range. i think this is how canon is accounting for the varying amount of light required to keep the aperture value constant through the zoom range - essentially giving the wider focal length less light to match the longer focal length's inability to gather light as efficiently.

i'm leaning towards believing that this whole mechanism is misaligned somehow but when shooting stills you just don't see this varying exposure when moving through the zoom range. maybe the mechanism that triggers the aperture to close at the moment of shutter release is more accurate than the mechanism that scales the aperture when the lens is stopped down? it remains mystery to me.

remember the camera is in FULL MANUAL mode. shutter speed is locked and iso is locked - the only other variable that affects exposure is aperture. makes sense that the lens isn't getting it's own built in aperture correction for the varying amount of light required to keep aperture values constant across a wide zoom range. this of course i sall conjecture as i really have no idea what's going on!

i think i'm going to take this lens into canon for calibration and see if it boils down to the lens itself. of course, i've seen this on 2 other different 17-40 f4 L series lenses and again on a 24-70 f2.8 L (to a lesser degree). difficult to think it's just "my lens" but maybe it boils down to a bunch of lenses being out of whack? i'll keep you posted as i find out more.

k

Karl Edwards · December 21st, 2010, 12:04 PM

stumbled upon this video which describes a similar issue with a different lens and mentions the 17-40 f4 L as having the same known "darkening" issue i'm seeing.

YouTube - 8. Canon 7D (zooms) Advanced Lens Selection - Video Mode Interactive Tutorial

he mentions the 17-40 at around 1:45. it's that secondary aperture deal that i was talking about. glad to finally see that i'm not just imagining this whole thing - maybe the 17-40 is just a bad lens for video? grrr...

karl

Ryan Hargrave · December 21st, 2010, 02:21 PM

interesting tidbit to check:

EOS 7D Firmware Update Version 1.2.2

Firmware changes

Firmware Version 1.2.2 incorporates the following fixes.

Fixes a phenomenon in which the set aperture moves when shooting movies in manual exposure mode using some Canon lenses (such as macro lenses).

Karl Edwards · December 21st, 2010, 03:30 PM

hey ryan,

the camera has version 1.2.2 installed. i don't think the aperture is moving - this looks to be a problem with the lens design itself. check the video linked in the post directly above yours...

karl

Ryan Hargrave · December 21st, 2010, 03:43 PM

yeah I did... just wanted to add that to the conversation just in case. and also for anyone else searching through the issues.

December 18th, 2010, 02:10 PM	#16
Bob Hart Inner Circle Join Date: Dec 2003 Location: PERTH. W.A. AUSTRALIA. Posts: 4,476	My wild guess is that it might be something to do with the digital magic that happens when a megapixel sensor image is reproduced at a lower resolution for video. The moire problem associated with these cameras in video suggests to me that fewer pixels across the entire sensor width are being scanned. I wonder if in stills mode where every pixel is counted, there is some blending of light falling at more acute angles across to neighbouring pixels by the anti-aliasing filter which might have an apparent gain effect. If in video mode there are fewer pixels contributing to the image, then there would not be enough diffusion of light across the gaps by the anti-aliasing filter. At a wide angle, the angle of incidence of light falling onto the sensor might vary more than when the lens is zoomed in with maybe a tendency for darker corners which is accentuated by 35mm groundglass adaptors. Does the camera have an inbuilt compensation for this? I understand that HDCAM cameras have a user selectable function that maps the brighter and darker areas of certain lenses and compensates. If the camera does have a compensation process, then fixed mapping for darker corners would be wrong for the long end of the zoom where the brightness across the image is more even and the entire image would be darkened as the corners are seen to be overbright by the mapping process. I am likely barking at shadows in the manner of a knowall who knows nothing. It is just a guess that a combination of sorts may be going on. It would be interesting to see if leaving the lens on wide and travelling the camera itself forward to reproduce the same framing as the zoom-in creates, exhibits the same darkening effect. I would not regard this effect so much as a defect as something you may need to live with. There are limits to how much work can be done by the processor in these remarkable cameras. If it is any compensation, you may observe a corner darkening effect in the image of purposed videocameras when zoomed in and darkening overall as some of these do not have constant aperture zooms. Last edited by Bob Hart; December 18th, 2010 at 02:19 PM. Reason: error

December 20th, 2010, 07:12 PM	#25
Karl Edwards Regular Crew Join Date: Apr 2010 Location: Toronto Canada Posts: 38	i whipped up a quick test against a white wall that really shows that this issue is not being created by the contents of the frame: 7D + 14-40 f4L White Wall Test on Vimeo i'd think my lens was the cause but i've verified the same issue on another 17-40 f4 L and a 24-70 f2.8 L and i've also seen it on 2 different 7D's and a 5Dmkii. the crazy thing is that this does not happen when the lens is wide open. it's only present when the lens is stopped down. also stills shot at the different zoom positions all have matching exposures which tends to incline that it's not a lens issue. this really is limited to the video mode only. okay, i need to walk my dog! LOL karl

December 19th, 2010, 09:01 AM	#18
Wacharapong Chiowanich Major Player Join Date: May 2008 Location: Bangkok, Thailand Posts: 400	Yes, if this is the case, a comparable fixed zoom such as the Nikon 17-35 f/2.8 AF-D (with aperture ring) mounted via an adapter will easily solve the problem.

December 19th, 2010, 10:12 AM	#19
Karl Edwards Regular Crew Join Date: Apr 2010 Location: Toronto Canada Posts: 38	hey guys, like i said this looks like a specific problem with canon lenses on canon cameras. not a malfunction so much as the physics and mechanics of the lens. if you put the same canon lens on a manual adapter and back on a canon camera i'd suspect you'd see the problem in still mode as well. i've done some more tests and here's what i've found: looks like starting and stopping the shot doesn't "re-cajigger" the aperture back to it's correct size depending on the position of the zoom in video mode. the correct aperture value at any other point in the zoom's range must be tied to the shutter on canon dslrs. my tests have shown that exposure definitely isn't affected by zooming when shooting photos. what i have discovered is that the issue ISN'T PRESENT when the lens is wide open. this would explain why you don't see it in live view in photo mode (it's always wide open until the shutter is triggered). weird that it only happens when stopped down but could have something to do with the mechanics of the lens. THE BAD NEWS: unfortunately this seems to mean that you'll need to adjust your exposure every time you change your focal length with a canon zoom lens in video mode on canon hdslrs. my tests showed approx. 2/3 stop from 17 mm to 40 mm on my 17-40 f4 L lens. haven't tested any other lenses to find the exact exposure difference. the other issue seems to be that this change in exposure isn't regular and has an odd spherical shape to the decrease in exposure. you can see this shape in the posted video - almost resembles the way the 17-40 elements move inside the lens barrel... i could just be making that part up but it looks that way to my drunken eyes. it's too bad - the versatility of having mulitiple focal lengths available (i don't zoom during video... hate that!) during a single shot is really handy. especially for run and gun shooting where you might want to keep the clip running as you quickly change angle perspective and sort the shots out in post. THE GOOD NEWS: hdslr video has its own character and produces undeniably beautiful images quality if handled correctly. the entire hdslr revolution seems to be about working around the fact that these damn things weren't really meant to do this but do it really well. just another example i guess! time to get some primes! k

December 19th, 2010, 10:03 PM	#20
Bruce Foreman Trustee Join Date: Nov 2006 Location: San Angelo Texas Posts: 1,518	I looked at his footage and noticed that when he shifted the camera's view to where the "mix" of light and dark" tones stayed predominantly "light" the exposure change effect was less. In Feb this year we had an unexpected and very nice snowfall. I used this same lens (17-40mm f4L) on my 7D and shot some video footage from just off my front porch. For one sequence I zoomed slowly from the 40mm end to full wide (17mm) and there was NO CHANGE in exposure. The mix of light and dark changed only slightly (began to include a few neighbor's trees - dark tones) with the slightly subdued white snow tones continuing to predominate. In the OP's video as he zoomed from wide to tele, dark tones began to dominate the "mix" and subsequently his image got a bit darker as his composition was beginning to require more exposure. It's a lighting and exposure issue, the guy is wearing a dark tee and jeans, seated on a light colored couch with what looks like slightly lighter wall in the background. I can zoom with my 17-40 f4l and experience no exposure change so far but I tend to look for lighting and tonal quality issues before shooting.

December 20th, 2010, 09:15 AM	#21
Karl Edwards Regular Crew Join Date: Apr 2010 Location: Toronto Canada Posts: 38	hi bruce, this isn't an exposure issue as the camera is on full manual. there is no automatic exposure adjustment at all. karl

December 20th, 2010, 11:58 AM	#22
Paul R Johnson Inner Circle Join Date: Nov 2005 Location: Lowestoft - UK Posts: 4,015	I've just tried this on a DSLR and focused on a solid colour wall at wide angle, with the shutter speed fixed (1/45 for info) On wide angle the correct exposure was f4, zoomed in f6.7 on an 18-55mm and f4.5 wide, f5.6 zoomed in on a 70-300. With aperture priority, then the shutter speed changes instead. When the Canon is taking video, the shutter speed is constant, so if the setting doesn't allow the aperture to change OR the shutter speed, then the brightness is the only thing that can change as the light level drops. Or have I missed something here? I've tried 2 lenses which both let different amounts of light through as the focal length changes from an evenly illuminated surface.

December 20th, 2010, 06:37 PM	#24
Karl Edwards Regular Crew Join Date: Apr 2010 Location: Toronto Canada Posts: 38	again, the lens used was the 17-40 F4 L series lens - definitely a fixed aperture lens. what you're saying just doesn't make sense bruce - it's not how photography works. when your exposure is locked in manual mode the contents cannot and do not affect exposure. exposure is only affected by iso, shutter speed and aperture. all things being equal it just doesn't happen the way you describe. let's even say that auto-exposure was somehow influencing the scene - the predominance of darkness in the frame would cause the auto-exposure to lighten the image, not darken it. auto exposure typically (for the sake of simplicity) tries to equalize the contents of the frame to an overal 18% grey tone. show it dark, it makes the darks lighter - show it light it makes the lights darker in an effort to expose them at 18% grey. it's possible you're referring to the fact that filling the frame with dark elements will create a dark image because you're photographing dark elements. i'm referring to the exposure actually changing during the zoom - again, see the posted video. that's not added in post. definitely there and definitely happening. also - i only saw one short zoom in your sample video... and it came on the heels of a fade from black. the amount of detail in the scene doesn't show the effect for sure. try your camera pointed at a white wall with no other elements. set your exposure when fully zoomed out then zoom in and out during the video. you'll see the effect - been verified on 4 seperate cameras and three different photographers. not a mistake but something going on with the canon dlsrs and video. k

December 21st, 2010, 09:48 AM	#26
Karl Edwards Regular Crew Join Date: Apr 2010 Location: Toronto Canada Posts: 38	you know, i'm not sure this whole "not readjusting the aperture" explanation is the whole story. in fact i think this might be more related to the lens just not scaling the aperture correctly. if you look through the front of the lens while zooming you can actually see what's going on. the aperture seems to be a fixed size but a second diaphram is coupled to the zoom and opens and closes slightly depending on where you are in the zoom range. i think this is how canon is accounting for the varying amount of light required to keep the aperture value constant through the zoom range - essentially giving the wider focal length less light to match the longer focal length's inability to gather light as efficiently. i'm leaning towards believing that this whole mechanism is misaligned somehow but when shooting stills you just don't see this varying exposure when moving through the zoom range. maybe the mechanism that triggers the aperture to close at the moment of shutter release is more accurate than the mechanism that scales the aperture when the lens is stopped down? it remains mystery to me. remember the camera is in FULL MANUAL mode. shutter speed is locked and iso is locked - the only other variable that affects exposure is aperture. makes sense that the lens isn't getting it's own built in aperture correction for the varying amount of light required to keep aperture values constant across a wide zoom range. this of course i sall conjecture as i really have no idea what's going on! i think i'm going to take this lens into canon for calibration and see if it boils down to the lens itself. of course, i've seen this on 2 other different 17-40 f4 L series lenses and again on a 24-70 f2.8 L (to a lesser degree). difficult to think it's just "my lens" but maybe it boils down to a bunch of lenses being out of whack? i'll keep you posted as i find out more. k

December 21st, 2010, 12:04 PM	#27
Karl Edwards Regular Crew Join Date: Apr 2010 Location: Toronto Canada Posts: 38	stumbled upon this video which describes a similar issue with a different lens and mentions the 17-40 f4 L as having the same known "darkening" issue i'm seeing. YouTube - 8. Canon 7D (zooms) Advanced Lens Selection - Video Mode Interactive Tutorial he mentions the 17-40 at around 1:45. it's that secondary aperture deal that i was talking about. glad to finally see that i'm not just imagining this whole thing - maybe the 17-40 is just a bad lens for video? grrr... karl

December 21st, 2010, 02:21 PM	#28
Ryan Hargrave New Boot Join Date: Jun 2010 Location: Houston, TX Posts: 14	interesting tidbit to check: EOS 7D Firmware Update Version 1.2.2 Firmware changes Firmware Version 1.2.2 incorporates the following fixes. Fixes a phenomenon in which the set aperture moves when shooting movies in manual exposure mode using some Canon lenses (such as macro lenses).

December 21st, 2010, 03:30 PM	#29
Karl Edwards Regular Crew Join Date: Apr 2010 Location: Toronto Canada Posts: 38	hey ryan, the camera has version 1.2.2 installed. i don't think the aperture is moving - this looks to be a problem with the lens design itself. check the video linked in the post directly above yours... karl

December 21st, 2010, 03:43 PM	#30
Ryan Hargrave New Boot Join Date: Jun 2010 Location: Houston, TX Posts: 14	yeah I did... just wanted to add that to the conversation just in case. and also for anyone else searching through the issues.