distance and depth of field

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#1 sam williams

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Posted 19 September 2008 - 01:12 AM

hello,

i am confused about how distance relates to depth of field.

if you were to zoom into an object a long way away, would the focus between the object and the next object behind be dramatically different. Because i thought it was only really true if your focused on an object really close to the camera, and then all the objects immediately behind get blurred.

i know its a nooby question, but i cant figure it out.
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#2 Allen Achterberg

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Posted 19 September 2008 - 01:04 PM

this should be in the AC topic. but ignore distance, and lets make this simple. Whatever lens you use, whatever distance. If its a medium shot, the depth of field will be the same on a 50mm as a 100mm lens. but it takes two different distances of your subject to get a medium shot on either lens. subject will be closer on the 50mm than it will be on the 85mm to get the medium shot. think of it like that.

if you are still on the 50mm, and your subjects walks away (further distance) then the DOF increases the further away your subject gets. so distance IS directly related to DOF as you can see.
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#3 Ira Ratner

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Posted 19 September 2008 - 07:04 PM

this should be in the AC topic. but ignore distance, and lets make this simple. Whatever lens you use, whatever distance. If its a medium shot, the depth of field will be the same on a 50mm as a 100mm lens. but it takes two different distances of your subject to get a medium shot on either lens. subject will be closer on the 50mm than it will be on the 85mm to get the medium shot. think of it like that.

if you are still on the 50mm, and your subjects walks away (further distance) then the DOF increases the further away your subject gets. so distance IS directly related to DOF as you can see.

You gotta explain this again, especially since you went from a 100mm to an 85mm.

Yeah--I know it was just a typo, but I don't understand the concept either.

My only experience with this was 35mm still work, with depth of field preview capability. And with digital, no big deal to play around.

But shooting 16mm still film, this is crucial knowledge.

Edited by Ira Ratner, 19 September 2008 - 07:06 PM.

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#4 Allen Achterberg

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Posted 19 September 2008 - 09:42 PM

it's very simple. it stood true even though I had the confusing typo in there. 50mm focused to 5 feet will have a certain depth of field. Focus the lens to 10' and the depth of field increases. Focus it further it also increases. mean more distance in the real world comes into focus, instead of 4'10" to 5'2" being in focus which is 4 inches of acceptable sharp focus at t2 and the lens set to 5 feet. focus it to ten feet you now have 9'5" to 10'8" which is just over a foot of acceptable focus. (this is for 35mm) see how we went from have 4 inches to over a foot of sharp focus by moving your subject and focusing further away?

This happens with all lenses, and all formats.
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#5 Mike Simpson

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Posted 20 September 2008 - 03:28 AM

it makes more sense if you can look at a lens with distance markings too.

Lets pretend on some lens I am making up some Fstop gives 1 inch of depth of field along the distance markings (i mean if you were to actually measure on the lens how far apart two distance markings are). So on the close focusing end of the lens where 3 feet and 4 feet are an inch apart, you have 1 foot of depth of field. But towards the end of the lens, 15 feet to 40 feet might be an inch apart, so then your depth of field is much greater (25ft), etc etc.

Edited by Mike Simpson, 20 September 2008 - 03:29 AM.

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#6 Ira Ratner

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Posted 20 September 2008 - 08:41 AM

Thanks, guys. I have to run out now and buy materials to refurbish a birdcage, but later this afternoon, I'm gonna study your sentences like they were scripture.
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