Color shift with zoom

[Robert Turchick]

Shot this weekend at a hanger which had halogen lights and skylights. When I started to go through the footage, noticed that when I zoomed in all the way, the color shifted...zoom back and the color went back to original. I also noticed some color shifting when it was stationary but attributed that to the partly cloudy day. Lighting was changing in the hangar.
It's most apparent with the skin tones.
Could this be a problem with the camera or the fact I have a Tiffen UV filter mounted? Or am I missing something else?
Attached are untouched stills from the QT Pro Res file.

[Andy Tejral]

Auto Tracking White Balance or from a white card?

If auto, well, it just doesn't work well all the time.

If you did a white, you didn't do it where the guy's face was? Or the combination of light changed from the time you did the white?

Just a couple of guesses...

[Robert Turchick]

WB from a card right where he was standing! full manual on camera. Another friend came up with the white balance as a possible source of the issue. It is possible I missed the button or something dumb. It wan't hard to fix in post but I will be playing with the camera tonight to make sure everything works!

[Steve Wolla]

Looks like as you're zooming you are losing quite a bit of your ambient lighting. It looks slightly under-exposed. Was the exposure on automatic? Try putting it on manual iris, and open it up a few stops next time if you can.

I do not believe from this, that the cam is defective--this has happened to me before too. When it does, I just zoom out slightly and keep shooting. Then I can zoom it in slightly in post, if need be.

When that route does not work, (I use Premiere Pro CS4) I go into Video Effects....Adjust....Shadow and Highlight, and fix it there. I am sure other NLE's have a similar feature.

[Robert Turchick]

thanks Steve. I was on manual iris and that's why I'm a bit baffled. I was full open for most of this shoot. Think the UV filter could be cause?

It was no problem in FCP to fix but a bit annoying. This is the first time I've seen this and the only major difference from the other shoots I've done was I was purposely far from the subject so I could get a good DOF look which worked great. The filter has been on the lens from day one so that's a constant. I'm gonna play with it tomorrow and get to the bottom of this.

Thanks for the ideas!

[Robert Turchick]

Did another shoot today and had no issues...must have "dumb-thumbed" a setting on the air museum shoot! SO....a big WHEW!!! Me...not the camera!

[Mike Sertic]

The HMC40's lens has a maximum aperture that varies as you zoom. Max aperture at full wide is f/1.8, max aperture at full telephoto is f/2.8, so if you start zoomed out with the iris fully open, then zoom in all the way, you should expect to lose one and a third stops of light.

Not sure if this explains the color shift, but if you had the gain locked down this is probably why you were seeing the exposure change that Steve noted.

I believe (someone please correct me if I'm wrong), the solution for the exposure issue would be to manually set the iris to f/2.8 or higher and then set gain/shutter speed for proper exposure, which should then be consistent across the zoom range.

[Tom Hardwick]

I'm solidly behind Mike on this one - it's simply the fact that the full tele shot is under-exposed compared to the wide shot (look at his shirt whiteness / brightness).

It's not the UV filter Robert. It's the fact that you haven't let the gain compensate for the lens ramping, the loss of lens speed (you say you were on full manual).

I think the next shoot (where you didn't meet this problem) was possibly in a better lit place? In which case your full wide and full tele could have been shot at f/4, say, and all would have been well.

tom.

[Robert Turchick]

I see! Yes last night's was actually a theater show so much better lighting. Plus I was at about f/4 for proper exposure. Makes sense!
Thanks guys!

[Bob Diaz]

Originally Posted by Mike Sertic

The HMC40's lens has a maximum aperture that varies as you zoom. Max aperture at full wide is f/1.8, max aperture at full telephoto is f/2.8, so if you start zoomed out with the iris fully open, then zoom in all the way, you should expect to lose one and a third stops of light.

Not sure if this explains the color shift, but if you had the gain locked down this is probably why you were seeing the exposure change that Steve noted.

I believe (someone please correct me if I'm wrong), the solution for the exposure issue would be to manually set the iris to f/2.8 or higher and then set gain/shutter speed for proper exposure, which should then be consistent across the zoom range.

No, that would only have an impact on exposure and should not change the color.

By the way, all zoom lenses act the same way. At wide angle, the F-Stop is a lower number than at maximum telephoto.

You are correct, if the F-Stop was at F2.8 or higher, there would be no change to the exposure.


Bob Diaz

[Tom Hardwick]

But if you look carefully Bob, it's only the exposure that's changed, and if Robert was in manual (and I assume that means colour balance too) then brightening the full zoom shot should make the colours match more evenly.

You say all zoom lenses ramp (lose speed as you zoom towards telephoto). This is only true of lenses that are designed this way. It makes them cheaper, lighter, smaller and they can still be advertised as a 10x f/1.6 zoom (whatever). But of course we could have 10x zooms that maintain their f/1.6 aperture throughout the zoom range - it's just that few of us are prepared to pay for them.

tom.

[Chris Soucy]

Originally Posted by Mike Sertic

I believe (someone please correct me if I'm wrong), the solution for the exposure issue would be to manually set the iris to f/2.8 or higher and then set gain/shutter speed for proper exposure, which should then be consistent across the zoom range.

It's a great theory, shame it isn't how it works.

The aperture setting is basically a percentage of the available light. As the lens is zoomed in and loses light, that fixed aperture percentage remains constant, so less light makes it through to the sensor.

In other words, if everything is locked off (shutter, iris, gain - no matter what the setting) and the exposure is perfect, if you zoom the lens and lose over a stop, then the picture must get darker.

The only way to compensate is to either reduce the shutter speed, open the iris or increase the gain.

As Robert has stated a couple of times, everything was locked, so if the zoom was racked in, the picture must get darker.

BTW (not being familiar with that camera) I presume you have the ability to lock the aperture and let the shutter speed look after itself?

If the camera had been in that mode, it is possible that a colour shift could occur if, left to it's own devices, it went from 1/120 say, to 1/60, due to those Halogen lights.

Far safer all round, when shooting under any sort of discharge lighting, to set the shutter manually to 1/60 and leave the aperture to do it's own thing.

It would have solved the problem (above) as well.


CS

[Guy McLoughlin]

The HMC-150 f-stop a the wide end of the zoom is f 1.6 which closes down to f 3.0 at the long end of the zoom, thus if you did a full zoom you are losing more than 1.5 stops from wide to tele.

To eliminate the f-stop change, always determine what the minimum f-stop is when zoomed in to what ever your tight shot is ( let's say it's f 2.6 in tele mode ), then zoom out all the way to full wide and manually adjust the iris to be the same as your tight tele shot. ( in this case you would adjust your iris to be f 2.6 when zoomed out all the way ) Now when you zoom the f-stop will stay exactly the same for the full zoom.

[Tom Hardwick]

I may not be reading you right Chris, but I beg to differ. You say, ' if you zoom the lens and lose over a stop, then the picture must get darker.' But this only occurs when you're shooting at an aperture greater than the maximum aperture at the telephoto end of the zoom.

So, if you're filming wideangle at f/4, everything locked down tight, and you zoom to full tele you'll still be shooting at f/4. The exposure will remain constant, as will the colours, contrast, grain, everything.

If you're shooting wide at f/2 and zoom to full tele with everything locked off then yes, your tele shot will be a stop under-exposed. If you knew you were going to be filming in this situation and needed everything locked down (excellent photographic practice) you'd up the gain by +6dB and film everything at that setting.

Much better to do this than to allow the shutter speed to roam btw.

tom.

[Tom Hardwick]

Originally Posted by Guy McLoughlin

To eliminate the f-stop change, always determine what the minimum f-stop is when zoomed in to what ever your tight shot is

You're right in what you say Guy, but I think you mean maximum when you write minimum (above).