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10-31-2010, 07:04 AM   #1
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Focus distance / focal length

Yesterday I found a chart on how to calculate the distance needed to focus based on focal length... something like 18mm - 4.4' etc

I can't find my way back to that chart. I don't know that it was Pentax but it should be pretty universal, right? Anybody know where I could find it or something like it again?

10-31-2010, 07:21 AM   #2
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Are you referring to depth-of-field charts? Hyperfocal distance?

Search for "DOF chart".

Here's one from the Articles section in this forum: Online Depth of Field Calculator

H2
10-31-2010, 07:25 AM   #3
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That's not what I had found but it is a VERY helpful tool. I had to work some OT yesterday and saved the link "on my break" but forgot to forward it home.

Thanks
10-31-2010, 07:38 AM   #4
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Maybe this will help. The relationship between focus distance (sensor plane to subject plane), focal length and magnification (m=sensor_width/scene_width) is:

Focus_distance = Focal_length(1+m)(1+m)/m

For scene widths greater than about a meter, m is less than 1/40 and the terms in parentheses can be ignored, so the non-close-up relationship for a Pentax camera is close to:

Focus_distance_meters = Focal_length_mm*Scene_width_meters/24mm

For example, the focal distance for a 2 meter wide scene with an 18mm lens will be close to:

Focus_distance_meters = 18mm*2meters/24mm = 36/24= 1.5m ~ 5'

Dave

PS the more complicated formula should be used for scene widths less than about a meter.

10-31-2010, 07:40 AM   #5
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Ahhhh perfect, thanks a million!
10-31-2010, 09:12 AM   #6
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Simple version!

QuoteOriginally posted by chadci Quote
Ahhhh perfect, thanks a million!
....for Pentax non close-ups....

Focus_distance/Focus_width = Focal_length/24

OR, almost same thing but easier to do mentally,

Distance/Width ~ F/25


Dave

PS -
Handy Table
1:1 ~ 25mm
2:1 ~ 50mm
4:1 ~ 100mm
8:1 ~ 200mm

Q we are 16' from a 4' wide scene, what focal length should be used?
A 16:4 = 4:1 so a 100mm lens is called for (ie 4x25)

Last edited by newarts; 10-31-2010 at 09:18 AM.
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