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### AuthorTopic: What is the red dot?  (Read 2704 times)

#### kaelaria

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##### What is the red dot?
« on: October 17, 2006, 12:53:28 PM »

I have this on some of my prime lenses and don't know what it represents.  Is it the optimal aperture for the lens or something?

http://www.photo.net/learn/optics/dofdigital/35mmdof.jpg

#### ericstaud

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##### What is the red dot?
« Reply #1 on: October 17, 2006, 01:30:29 PM »

When shooting infra-red film you visually focus the camera, check the focus marking (which in your sample image is set at  "0.7"),  move the "0.7" to the red dot and shoot.  The longer wavelengths of infra-red light cause the focus to shift slightly which needs to be corrected.
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#### howiesmith

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##### What is the red dot?
« Reply #2 on: October 17, 2006, 01:34:30 PM »

Quote
I have this on some of my prime lenses and don't know what it represents.  Is it the optimal aperture for the lens or something?

http://www.photo.net/learn/optics/dofdigital/35mmdof.jpg
[a href=\"index.php?act=findpost&pid=80873\"][{POST_SNAPBACK}][/a]

It is the infra red focus adjustment.  When you focus with visible light but take image with IR, you sometimes need to adjust the focus.  Some lenses, like some L lenses, don't require this.  The red mark has nothing to do with what you are trying to do.

If you look at the DoF equations, you can see that when the focus distance is near the hyperfocal distance, the DoF is about evenly split fron to back.  As you move in this changes gradually to about 2/3 in front and 1/3 behind the focus distance.  (I did this from memory.  It may be reversed, but you can see that under the correct assumptions, either 1/2 or 2/3 can be correct.  ANoth good reason to understand the assumptions being made by others on your behalf to try to make your life easier.  But sometimes they make just conuse the actual situation.)
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#### kaelaria

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##### What is the red dot?
« Reply #3 on: October 17, 2006, 01:40:33 PM »

I wasn't insinuating it had anything to do with my other thread, hense the new one.

Thanks for the info, very cool!
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