Video darkens during zoom with fixed aperture lens

[Karl Edwards]

hey guys.... hoping you can help me figure out this problem:

Unusual Canon HDDSLR Video Behavior on Vimeo

basically, using any fixed aperture zoom lens (i'm using the 17-40 f4L in the video) and filming a brightly lit scene with a 7d. Everything on manual all the way nothing auto anywhere near my camera. Zooming causes the camera to record video about 1/2 stop darker than fully zoomed out.

Canon 7d firmware 1.2.2 on full manual.
14-40 f4 L lens (fixed aperture not variable)
only happens when shooting video - does not happen with live view in stills mode.
camera set to neutral profile with sharpness and contrast flattened.
have recreated on 2 different 7ds and a 5d mkii at vistek.
have recreated using 2 different 17-40 f4 and a 24-70 f2.8 (ruling out a lens issue)
iso expansion on/off same result.
highlight tone priority on/off same result.

now, i know - you're not likely to do this kind of snap-zoom thing during a shoot but it brings up an interesting problem. the camera seems to arbitrarily adjusting the exposure based on the contents of the frame even though everything else is set to manual.

ever hear of this before? hoping to track down what i'm doing wrong or the cause...

let me know what you think! really appreciate your help!
karl

[Erik Andersen]

I've noticed the same thing on 5D2 with the 24-105 F/4.0. I'm very interested to see if anyone can suggest a solution. It's not fun to adjust exposure whenever you zoom in or out.

[Chris Soucy]

Don't think there's any great mystery about what's happening, at all.

Explaining it is not quite so easy, however.

OK, take the zoomed out picture. Aperture and shutter speed set for the amount of light reaching the sensor in general, and each individual pixel in particular.

In this mode, each pixel is "seeing" an area of "X" which is throwing off "X" amount of light, the average across the entire sensor being used to set the shutter/ aperture.

Now zoom in.

Each pixel is now seeing an area "Y" which is significantly smaller than "X", and as such is throwing off significantly less light than "X".

Thus, if neither the aperture or shutter speed changes, the picture must darken, as overall, it is receiving less light.

This is one of the reasons macro shots need their own nuclear generators to get enough light onto the subject (and hope not to cook it to a crisp in the process) as the subject area is so minute.

If this is (apparently) not happening to you when in stills mode, I would subject your methodoligy to some serious scrutiny, it is happening, but in stills mode there is no way to make a comparison, as the still is just one shot, not a series that shows the transition.

Does that explanation make any sense?


CS

[Karl Edwards]

hi chris,

the issue is definitely not happening when in live view for stills mode. this is not something you need to "pixel peep" to see - check the video. it's an apparant approximate 1/2 stop of light loss in video mode only. you can see it in live view AND it's recorded to video. switch to live view (stills mode) and the problem simply isn't there. not comparing still image to still image but live view to live view (video vs. stills mode). this is definitely tied to the video mode on canon hddslrs.

start up your own 7D with a fixed aperture zoom and you'll see what i'm talking about - this one isn't made up or pulled out of thin air. it's been observed by many people today with demonstrations and in-person shooting.

i have to say i've been shooting for over 25 years (and have a BA in photography from ryerson in toronto) and i've yet to hear of zooming over 30mm of focal length requiring 50% more light. an f-stop is an f-stop - this is why handheld light meters work!

karl

[Wacharapong Chiowanich]

The problem could be due to the reduction of the EFFECTIVE or true aperture of the lens. You have to search online to check the true specs of the particular supposedly "fixed" aperture zoom lens you plan to use in shooting which requires the exposure to be absolutely constant or, at least, not noticeably change.

For instance, your Canon 17-40 may have a stated aperture of f/4.0 to 4.0 where in fact it may really be f/3.8 at the wide end to f/4.2 at the tele end or any similar increasing numbers. The fact that your clip gets darker when zoomed in rather than when zoomed out indicates the lens is not a true fixed aperture zoom. True (and ultra-expensive) cine zoom lenses do not exhibit the problem because every one of them has the aperture that is absolutely constant down to 0.0X or 0.00X.

[Karl Edwards]

hey there,

again, this is not present when viewing the image in live view in stills mode and only present when in video mode. both live views use the same screen and the same lens -- and live view in the stills mode uses the maximum aperture of the lens. if the effective maximum aperture was changing i think you'd see it.

remember this is a fixed aperture L series lens. top of the line canon optics and widely regarded as the finest lens in canon's lineup - i don't think that this is tied to canon misrepresenting the lens as fixed aperture when it's not. also the same effect was seen using a variety of fixed aperture zooms but only in video mode. the same effect (to the same degree was seen when using the 24-70 f2.8 L series lens... so this isn't tied to this lens in particular).

not trying to prove there's a problem - just trying to find out what the problem is... not sure it's an issue with the optics to be honest. this really looks like it's tied to the video engine on canon hddslrs.

any other ideas?
k

[John Wiley]

You need to do a bit more testing before you can conclude that it is the camera and not the lenses causing the issue.

What happens when the lens (or lenses) is stopped down to say f/8? Does this behaviour still occur?

What about when you are using manual lenses via an adaptor that have absoultely no 'dialogue' with the camera? If it is the camera adjusting something in the settings when the lens zooms, then theoretically it should not change the exposure if it does not know the lens is zooming.

Are you comparing still photos taken with live view or are you just looking at the LCD display during Live view? Because the LV on many DSLR's will adjust to diplay the best preview (regardless of manual settings) so you can compose your shot - the only applying the selected settings once the shutter is triggered. This can usually be switched off in video mode so you can see what is being recorded. So it could be that there is less light reaching the sensor (or each individual pixel) due to the explanations given here by others, but as you zoom in and the light level decreases, the camera's LCD display is compensating to give you the same live view preview image.

[Karl Edwards]

hi john, thanks for replying!

when the lens is stopped down to f8 or any aperture the same behavior occurs. the sample video is at f7.1 to be exact.

granted i haven't tried a manual lens via an adaptor -- as i don't have such an aparatus. but please remember this ONLY happens in video model and DOES not happen in live view in stills mode. on a 7d the live view displays the image as live video on the lcd just like the video mode. the live view in stills mode however DOES not show this issue in any way shape or form. if there was some "auto-toggle" that was being overlooked you'd expect the same issue to be present live view stills mode. on the 7D live view stills to live view video is simply physical switch and all other camera parameters stay the same.

this seems to be rooted in the video mode itself. it's possible that canon's video codec is affecting the video image and compressing the highlights after processing - not sure.

i've tested this on multiple cameras (2 7D's and a 5D mk ii) with multiple lenses (2 17-40 f4 and 1 24-70 f2.8) and the results are the same when using video mode.

not saying it's my camera - i'm leaning towards thinking it's tied to canon's implementation of video somehow but really i'm asking if anyone else has seen this same problem. ie: i'm not asking "is there a problem" i'm asking "has anyone else seen the same issue"

there's another post in this forum from eric that seems to be similar to what i'm experiencing...

update:

i'm comparing live view to live view. ie: live view in video mode (which is recorded) to live view when shooting stills. the point is that stills mode isn't exhibiting the same problem - only the video mode. i'm sorry but pixels are pixels - when you zoom you're not zooming in on the pixels, you're focussing the light (so to speak) on the same number of pixels. it just doesn't work as described earlier.

light is light - the light falling on the scene is reflected onto the sesor at a given aperture and shutter speed. the aperture and shutter speed determine the exposure - not the focal length of the lens.

turn on your 7d point it at a well lit subject in manual mode and zoom in with a fixed aperture lens. does the exposure drop by half a stop? now switch to video mode. you'll see the issue i'm describing and the issue that's recorded in the posted video.

karl

[Wacharapong Chiowanich]

Karl, try stopping the lens down to f/8 or f/11 to see if the problem persists. My guess is IF it does, the difference in luminance between the wide and tele end will be smaller and may be hard to detect without close examination. It is also possible the problem just goes away as the variation in effective apertures at medium stated apertures of most zoom lenses (cheap and expensive alike) is normally so small as to be insignificant.

You can go to the Popular Photography website. They have a good history of still photographic lens tests and in each of the tests you will find published figures of the lens' actual focal length and aperture. None of the lenses has a truly constant aperture and more than a few "constant aperture" zooms have aperture that varies almost 1/3 of a stop from end to end.

[Chris Medico]

Try turning off "Lens Peripheral Illumination Correction" and see if that has any affect on what you are seeing.

I suspect this issue may have something to do with how the imager is read during video mode vs photo mode.

I will see what happens on my camera this afternoon with the 24-70F2.8L.

[Karl Edwards]

thanks guys,

again, everything that can "possibly" affect exposure is turned off including lens peripheral illumination. also the behavior is the same no matter the f-stop.

the effective size of the diaphram at a given aperture value is different at different focal lengths (accounting for the fact that much more light is needed at longer focal lengths). f4 is still f4 in terms of the amount of light that aperture lets in but on a 400mm zoom an aperture of f4 would need to be a much larger opening than f4 on a tiny 28mm. this might also explain why fixed aperture zooms are so damn expensive!

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.

what's interesting is that every video camera (proper video cam no hddslrs) has accounted for this since the dawn of time. i think the fact that most guys shoot fast primes with hddslrs has caused this issue to be largely unseen.

live and learn i guess - curse you, physics!
k

[Olof Ekbergh]

An interesting note here.

Many cinematic lenses have T numbers not F numbers.

T's are actual measured numbers, not theoretical. F numbers are theoretical arrived at by calculations.

Typically a f1.7 lens will be rated T2.

Any still zoom lens will probably have slight variations as elements change positions and it does not really matter for stills.

But for cinematic lenses this is unacceptable, so they are built differently, and are much more expensive. And most cinematic lenses are primes.

Cinematic lenses also have extremely accurate witness marks (focus distance marks) and the focus ring travel is far larger for more accuracy.

You certainly can use still lenses for video. But they are not really meant for it. So there are compromises.

[Karl Edwards]

i'm with you there olof.

the workaround in this case of course is to start and stop the shot when you change your angle of view. this seems to give the canon hdslr a chance to "reset" the aperture in the lens to maintain the f-stop at the new focal length.

bit of a learning experience with this one - but good to find out what's going on there!

karl

[Erik Andersen]

Wow this pretty interesting. Has this not been noted before ever? I'll have to try the stop/start method when zooming. One more thing to think about!

[John Jay]

Interesting, have you tested this theory?

If you start the shoot zoomed in and then zoom out does the exposure get brighter by half stop?