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exposure/focus changes in slow close-ups

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Ruby Gold
exposure/focus changes in slow close-ups
on Apr 1, 2005 at 10:26:54 pm

I'm using a GL2 to shoot slow pans very close-up of paintings. With the camera manually white balanced and focused and the exposure set, I notice that as I very slowly zoom in, at a certain point the exposure goes darker on its own. Sorry for my ignorance here, but can someone clue me in as to why this is happening and how to correct for it? Thanks-


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Bob Hudson
Re: exposure/focus changes in slow close-ups
on Apr 2, 2005 at 1:57:04 am

The speed of the lens is not consistent throughout its range: it lets in more light at wideangle and less as you zoom in.

Bob Hudson
Consultant & Writer
Video, DVD and
Multimedia Production
Overland Park, Kansas USA
http://www.bobhudson.com/


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Ruby Gold
Re: exposure/focus changes in slow close-ups
on Apr 2, 2005 at 7:52:05 am

So how do you adjust for that while zooming to keep the exposure consistent?
thanks-


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Peter Ralph
Re: exposure/focus changes in slow close-ups
on Apr 2, 2005 at 7:32:32 pm

the maximum aperture cannot be maintained when the lens is zoomed in - just set the aperture manually at a higher f-stop and you will be fine


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David Jones
Re: exposure/focus changes in slow close-ups
on Apr 2, 2005 at 2:51:44 pm

Aside from a dolly move instead of a zoom, a cut from wide to tight,
or mega pixel still shot zoomed in computer,
you might switch your camera to aperture priority mode.


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Ruby Gold
Re: exposure/focus changes in slow close-ups
on Apr 2, 2005 at 9:50:56 pm

Thanks-I'll try that. Because I've never used any of the programs, (always keeping the cam in manual), what would keeping it in aperture priority mode actually do?

Also I've got the shutter speed set at 1/60, would changing that help?

thanks again-


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Don Greening
Re: exposure/focus changes in slow close-ups
on Apr 2, 2005 at 11:39:51 pm

AV mode (aperture priority) means that you can tell the camera to keep the iris at a preset opening of your choosing and the shutter speed will change automatically as needed to keep the light constant. AV stands for Aperture Value.

Conversely, you can put the camera into TV (shutter priority) mode which stands for Time Value. This will keep the shutter speed at a preset value and the camera will change the aperture to compensate for any changes in light. The faster the shutter speed the less time there is for light to hit the CCD's, so it's another way to control the light coming in (among other things).

- Don


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Ruby Gold
Re: exposure/focus changes in slow close-ups
on Apr 3, 2005 at 2:29:23 am

Thanks so much for this explanation Don--very helpful. Is there any down side or "side effect" to using a preset like the AV setting?

Also, do you know anywhere I can find a little tutorial or article around the nuances of using these various presets? I was tantalized by the "among other things" reference in your response. Thanks so much-
Ruby


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Don Greening
Re: exposure/focus changes in slow close-ups
on Apr 3, 2005 at 5:55:36 am

Well, let's start with the down side of using AV mode first. We'll keep to something I know you're familiar with. Let's say you're shooting a painting and want to keep the exposure constant while you pan and zoom. Let's also assume you don't have a lot of available light to work with. In AV mode you get the exposure the way you want it and start recording. As you zoom in the light hitting the CCD's gets less and less and your GL2 compensates by decreasing the shutter speed and all of a sudden the smooth motion you were getting is now stuttering because the camera has gone from 60fps to 30fps in order to keep the exposure the same. This means that you have to start with actually more than enough light to give the shutter speed room to drop without going to less than 60fps amd ruining your smooth pan shot. You could dial in some picture gain beforehand but that always leads to a certain amount of picture "noise" in the darker areas of the frame, which I'm sure you'll want to avoid. AV mode will probably increase the picture gain automatically as well if it's not getting enough light. Something else you want to avoid like the plague. Always make sure you've got more available light to start with than you think you need.

TV mode allows you to keep a constant exposure while shooting at lower than 60 fps if you want to do a special effect such as making your footage stutter or look jerky. Emulating a person talking on a video phone with super low frame rates would be a good example. The footage would also look blurry or 'ghostly-like' in the areas of rapid movement. TV mode is also for keeping fast action sharp and in focus by selecting a much higher shutter speed. Sports and fast flowing white water shots come to mind. So to sum up, TV mode keeps the exposure constant while allowing you to capture fast action cleanly or to purposefully create a blurry or stuttering shot.

I don't know offhand of any web pages or books I can point you to for this topic, although I'm sure there's a wealth of info out there. Someone else will have to jump in here. All I can suggest is that you do a lot of practicing with your GL2 and determine what picture "look" and method is best for the type of shooting you're doing. Record some tape and look at it on a monitor. Trust your judgment and your trained eye. Chances are, if it looks good to you then it most likely looks good to everyone else. Remember, you're a heck of a lot smarter than your GL2.

- Don


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