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Old Mar 22, 2004, 4:59 PM   #1
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Default Depth of Field Question

I'm kinda new to digital photography. I've been reading up on depth of field and about how different aperatures affect it.

My question is, how does focus come into play. Like If I'm shooting a landscape, I know I want a small aperture (high number) to have maximum depth of view to make it all sharp. But, what about focus. I've just been playing with my cameras manual focus ( cannon a60) and I['m really confused as to how it works. What if I set the focus to infinity, but I made a really large apreture? Or vice versa.

Sorry this is so confusing, but I'm really confused myself.
Thanks
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Old Mar 22, 2004, 5:04 PM   #2
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Focus does not effect how large the DOF is. It only effects where the DOF is placed in the scene/viewable area.

I'm not sure if that answers your question, or if you are looking for more info.

Eric
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Old Mar 22, 2004, 5:47 PM   #3
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When you focus on something with a small aperture, everything between it and 2/3 the distance to it is in focus. Also everything from it to 2/3 the distance behind it is in focus. So if you focus on infinity you're wasting the 2/3 behind it because it's infinity. Instead, try focusing on something 1/3 closer to you that way you get the additional depth of field up front. Oh dear, I didn't explain that very well. Lemme make a chart.
A----------B----------C----------D----------E----------F---------G

So if you stand at A and focus on D, everything from C to F will be in your depth of field.
Now pretend G is infinity. If I focus on G, everything from E to infinity will be in focus. That additional depth behind G is wasted because there is nothing behind infinity. It's already in focus.
Now if I focus somewhere between D and E, then everything from a point in between C and D to infinity is in focus. This is how you maximize your depth of field.
Anyone reading this post - am I pretty close to being right or am I way off?
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Old Mar 22, 2004, 8:32 PM   #4
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Ok but on my cameras manuel focus controls, it gives differnt numbers going from about 1 centimeter to infinity. So if I set the focus to say, 10cm....dosnt that mean that everything within 10cm will be in focus and the rest will be outt focus. Isnt this similar to haveing a large apreture (2.8) which would throw the backround out of focus.

I guess this is what I'm trying to ask.
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Old Mar 22, 2004, 9:10 PM   #5
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If you set your focus to 10cm the focus is centered on 10cm. If you have a wide aperture everything from say 9.8 to 10.3 cm would be in focus where if you had a small aperture everything from 8.5 to 12cm might be in focus.

If you want to blur the background you set a wide aperture. Say your subject is 6 feet. Your depth of focus scale says everything from 5.2 to 7.5 feet is in focus. You refocus to 5.2 feet to get as much background blur as you can while keeping the subject in focus.
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Old Mar 22, 2004, 9:25 PM   #6
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Even though it wasn't my question, I think you guys provided a pretty good explanation to abs9986 (hope s/he also thinks so). I was having some of the same issues after taking pictures this weekend of my new house under construction. Couldn't understand why some of them were clear and some were less clear. Just realized what I was doing wrong regarding DOF. Thanks.
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Old Mar 22, 2004, 10:08 PM   #7
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When your camera focus is set to (e.g.) 10m, ONLY items at a distance of 10m will be in focus. Things that are close to 10m away will be very close to being in focus - close enough to correct focus that other issues will be dominant in image degredation. Issues like resolution (pixel count), lens quality, shake, ... Also final print size matters - the smaller the print, the more the image can be degraded without being noticed.

So there is no set "Depth of Field", it depends on how much degredation you can tollerate. The mathematics can be easily found via Google by entering "Circle of confusion" (with quotes). In short: the less fussy you are about being in exact focus, the greater your "Depth of Field".
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Old Mar 22, 2004, 11:04 PM   #8
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Also, something to remember is that the further away your subject is, the larger your focal plane is. IE if I take a picture of something at 1 foot, my focal plane is going to be centered around stuff 1 foot away, thus limiting how near and far the DOF rules LTBerry stated. If you focus on something a mile away, your DOF is going to be MUCH larger, due to a larger focal plane.
I hope that made a little sense at least.
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Old Mar 22, 2004, 11:21 PM   #9
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Default Re: Depth of Field Question

Quote:
Originally Posted by abs9986
My question is, how does focus come into play. Like If I'm shooting a landscape, I know I want a small aperture (high number) to have maximum depth of view to make it all sharp. But, what about focus. I've just been playing with my cameras manual focus ( cannon a60) and I['m really confused as to how it works. What if I set the focus to infinity, but I made a really large apreture? Or vice versa.
Please take a look at the Depth of Field page of my Coolpix 4500 user guide.

CK
http://www.cs.mtu.edu/~shene/DigiCam
Nikon Coolpix 950/990/995/2500/4500/5700 User Guide
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Old Mar 23, 2004, 12:10 AM   #10
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Quote:
Originally Posted by Gandalf065
Also, something to remember is that the further away your subject is, the larger your focal plane is. IE if I take a picture of something at 1 foot, my focal plane is going to be centered around stuff 1 foot away, thus limiting how near and far the DOF rules LTBerry stated. If you focus on something a mile away, your DOF is going to be MUCH larger, due to a larger focal plane.
I thought the focal plane was where the film/sensor was located, as in focal plane shutter which is very close to the film. Could you explain what you mean by focal plane?
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