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Image brightening and darkening when action goes across screen

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Jaeson Jrakman
Image brightening and darkening when action goes across screen
on Apr 16, 2015 at 4:55:53 pm

Hello,

I'm currently using a Canon camera to shoot video and I'm having an issue with the image brightening and darkening, and even changing color a bit, whenever someone walks into frame.

Right now I'm using an auto setting so I'm wondering if this is a white balance issue? Under auto, is it adjusting for light conditions as the person moves into frame?

Thanks in advance for any help anyone can give.


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Todd Terry
Re: Image brightening and darkening when action goes across screen
on Apr 16, 2015 at 5:07:06 pm

[Jaeson Jrakman] " Under auto, is it adjusting for light conditions as the person moves into frame?"

Absolutely...

Old schoolers like me will tell you to never use auto anything. No matter the camera, I always shoot with everything full manual... color temp, exposure, focus, everything. I don't really have a choice as I primarily shoot with the C300 and PL-mount cine lenses, so there aren't any auto features, as is customary for cine-style cameras... but still, that's my choice and what's considered the "right" way to do it.

What is happening is when someone walks into frame they are changing the total amount of lightness and darkness in your frame, so the auto-exposure is compensating for it. Same with your color temperature, the new element in your frame is changing the overall color temp of the scene and adjusting the white balance for you.

If you switch both of these features to "manual" it will eliminate your problem.

I'm not knocking auto features, they help a lot of people, but they can also cause problems. For the vast majority of the history of motion picture cameras there were zero auto features... and if you can learn to shoot fully manual, you can shoot anything, and always know exactly what you are going to get.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Blaise Douros
Re: Image brightening and darkening when action goes across screen
on Apr 16, 2015 at 5:09:27 pm
Last Edited By Blaise Douros on Apr 16, 2015 at 5:11:58 pm

It is likely adjusting for lighting conditions. If the lighting in your foreground is significantly different than your background, then that would usually cause such an adjustment in auto-exposure.

As a side note, I'd recommend never using auto exposure. Most cameras aren't smart enough to do more than just sort of make a guess at which part of the image you want correctly exposed.

[Edit] I see that Todd beat me to the punch with a more detailed explanation. What he said ^^


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Jaeson Jrakman
Re: Image brightening and darkening when action goes across screen
on Apr 16, 2015 at 9:36:17 pm

Thanks for your response guys.

Actually I'm using a Nikon D5300. Sorry for the confusion.

I'm using the manual setting now, With an aperture setting of a f6.3, and a Shutter speed setting of 1".

I'm shooting a closeup of a white piece of plastic, with a light shining directly on it. I have the plastic subject on a lazy susan.

When I turn the lazy susan, when the plastic is parallel to the lens, it looks like there might be some light reflecting off of it into the lens?

I say this because I'm still getting the color and brightness variations. I try to fix them in post, but boy is it a bear!


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Todd Terry
Re: Image brightening and darkening when action goes across screen
on Apr 16, 2015 at 10:19:35 pm

Can you post a sample? Might clear things up.

What framerate are you shooting? And not sure exactly what you mean by a 1" shutter speed?

Most of us shoot most "normal" projects at either 24fps (actually usually 23.976fps unless there is a reason to do true 24fps) or 30fps (usually 29.97fps). A "normal" shutter speed is "one over twice the frame rate"... meaning usually when using 24p a normal shutter speed is 1/48th of a second, and for 30p it's 1/60th. That shouldn't have any bearing on the effect you are seeing, though... I only mention it because you stated your shutter speed and I wasn't sure how to interpret what you wrote.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Jaeson Jrakman
Re: Image brightening and darkening when action goes across screen
on Apr 16, 2015 at 10:33:02 pm

I can't post a sample at this time, unfortunately.

But in the handbook, on page 43, you'll see the controls I'm talking about, the circle on the far left of the display, that indicates "shutter speed" I've set mine so that it reads 1", whatever that might mean.


http://download3.nikonimglib.com/archive1/mIIJL00acvof00aAgRu77efiy804/D530...


I too always thought that shutter speed referred to the 29.97 fps that I normally shoot in. Maybe it's a control for the still photography function of the camera?


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Todd Terry
Re: Image brightening and darkening when action goes across screen
on Apr 16, 2015 at 11:04:33 pm

[Jaeson Jrakman] "I too always thought that shutter speed referred to the 29.97 fps that I normally shoot in."

Noooo noo.... shutter speed and frame rate are two entirely different things, although some people seem to confuse them.

FRAME RATE is how many frames per second you are shooting. I.e., at your 29.97 rate you are shooting about 30 frames each second.

SHUTTER SPEED is, well, the speed of the shutter... how long the exposure is for each of those frames. The shutter speed could be a lightning-fast 1/1000th of a second, but still shooting at a frame rate of 30fps.

Here's where it comes from.... in a real film camera (which all this theory is based on) the film is advanced a frame at a time and then stops for the exposure... 24 times every second. The shutter is a spinning half-disc, rotating 24 times a second. Because it is half a circle, half of the revolution it allow the film to be exposed, and half the time it blocks the light so that the film can be advanced to the next frame. Thus, if it is exactly a 180° shutter (which some but not all are), then at a frame rate of 24fps you have a shutter speed of 1/48th of a second, because it exposes the film one half of the time of each revolution. That's where the calculations come from... and that gives about the right amount of motion blur for our brains to interpret those 24 individual pictures as smooth motion.

Now you can shoot with a faster shutter speed than that, by using a variable shutter in a film camera, or merely cranking up the shutter speed in a digital camera. Crank it up much more than normal and you lose motion blur in each frame, thus giving you what is called the "narrow shutter" look... that choppy strobby effect (see the movie "Gladiator") that is sometimes used to make action movies look more "actiony." People also call this the "Private Ryan" effect, as it was used in Saving Private Ryan extensively.

The instructions on page 43 of your manual are all about that shutter speed and have nothing to do with frame rate. Now, 1" would usually mean a shutter speed of one second (a very long shutter speed if you were taking a still image). I'm guessing that since it is technically impossible to shoot a shutter speed that is longer than the frame rate, that your camera is actually just shooting as slow a shutter speed as it can, and then stopping (which would actually be 1/30th of a second, rather than the real 1-second speed it is set on). At any rate, most people would consider 1/30th much too slow for any footage that contains any action, as the footage would be quite blurry. In that case 1/60th of a second would be the "normal" speed and what would likely look best.

But as I said, all of this should have no effect on your brightening/darkening images and your changing color temps. Without seeing any footage I really can't hazard any more of a guess as to what is going on there.

By the way, even though you now apparently have your iris set to manual, if you still have any auto-exposure voodoo going on then while the iris is locked the camera might be floating the ISO up and down to compensate for exposure changes. The ISO should be at a fixed setting, too... not on "auto."

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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