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Can someone explain to me (focusing a lens)


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#1 Ronney Ross

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Posted 09 May 2006 - 09:27 PM

On some of the behind the scenes stuff I have seen on tv they are using a tape measure to get distance. Please explain this concept to me because I am not getting it would using a tape measure outweigh the use of a light meter or am I in left field.


Please school me,

-Ronney Ross
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#2 Mike Rizos

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Posted 09 May 2006 - 09:40 PM

Hi, you are even farther than left field!
The measure is used to determine focus, the light meter for exposure.
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#3 John Mastrogiacomo

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Posted 09 May 2006 - 09:42 PM

On some of the behind the scenes stuff I have seen on tv they are using a tape measure to get distance. Please explain this concept to me because I am not getting it would using a tape measure outweigh the use of a light meter or am I in left field.
Please school me,

-Ronney Ross

A tape measure is used to determine the distance where to set the focusing mark on a lens (used mostly for prime lenses). A light meter is used for exposure.

For example, You may measure the focusing distance to be 8 feet. You would then set the focusing distance on your lens to 8 feet. You then measure the amount of light with your light meter. If the light meter reads 5.6 you then set the f-stop on your lens to 5.6.

Search the web or this forum for more information. :)
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#4 Ronney Ross

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Posted 09 May 2006 - 10:11 PM

Thanks for the info, I completely understand the use of a light meter for exposure but by watching the short little two second clips you see on dvd's I wasn't sure if they were using a their distance to get a depth of feild reading and just setting the f/stop from that.
(But taking time to think about it I know it doesn't make sense.)
-Ronney Ross
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#5 Michael Nash

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Posted 10 May 2006 - 07:16 PM

Thanks for the info, I completely understand the use of a light meter for exposure but by watching the short little two second clips you see on dvd's I wasn't sure if they were using a their distance to get a depth of feild reading and just setting the f/stop from that.
(But taking time to think about it I know it doesn't make sense.)
-Ronney Ross


Well there's some basis to your idea, but you had some things confused. The f-stop is always set for the desired exposure, never for the depth of field. If the depth of field needs to be changed via the f-stop, you would typically change the lighting, distance or ND filtration so that you could maintain the desired f-stop.

Perhaps you've learned from still photography, where you can easily change the shutterspeed if you want to change your f-stop for a different depth of field. With motion picture photography if you change the shutterspeed, you get motion artifacts like strobing or blurring.
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