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 Post subject: Explaining DOF
PostPosted: Tue Nov 09, 2010 8:44 am 
Don't worry - this will not be "how to achieve blurry background" type of thread, I understand the f-stops, small f = big blur big light small DOF, big f = small blur small light big DOF
:lol:

What I want to know is - why. I'm trying to explain it to my girlfriend, but I really can't get a decent quick explanation why a small aperture makes everything in focus.

I've tried wiki but the text there is too much on the subject, I'm looking for a quick nice definition.

I tried to explain it to her by comparing humans pupils to the f-stop. If you squint your eyes you can "sharpen" your field of view.

But when you open your eyes wide...you don't lose focus or DOF...so the human pupil / aperture comparison didn't work.

So...any ideas how to explain this other than "the lens just work that way" :D


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 Post subject:
PostPosted: Tue Nov 09, 2010 9:07 am 
I dont really understand the physics of the whole thing.

But I suppose just get her to try it. Take the same photo at f2.8 and at f16 and get her to see for herself what the difference in practice is.


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 Post subject:
PostPosted: Tue Nov 09, 2010 9:59 am 
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Consider reducing the aperture to an absurd level and think of a pinhole camera which has a ridiculously high f-number and no (curved) glass at all. DoF is about as big as it gets. The more you increase the aperture then, to stop the image blur increasing, you have to introduce a lens (curved glass) to bring the scene into focus. The bigger the aperture the stronger (more curved) that lens has to be, everything else being equal. The stronger the lens the tighter the range of distances in the scene being photographed which that lens can bring into sharp focus becomes. If that concept is difficult then think how easily the human eye can compensate for an error in focus when looking through a weak spectacle lens held at arm's length compared to looking at the same scene through a magnifying glass held at arm's length. :shock: :)

Bob.

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 Post subject:
PostPosted: Tue Nov 09, 2010 2:06 pm 
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Lets see if I can explain this graphically.

Have a look at the digram below,

Image

These two "pictures" where taken at the same focal length at the same distance from a subject. The black vertical lines indicate the aperture opening. The red lines are sight lines from the top and bottom of the aperture opening. As you can see, they approach the subject at a shallower angle. When they meet (or get close to meeting) - they show a focused subject.

In the bottom graphic they are closer longer - thus, a longer depth of field.

Now, I know what your thinking - they only meet at one point in both situations. This is true. There will only be one point in the photograph that will be exactly in focus. However, your brain has a tolerance.

If you notice the lines are closer together in the bottom figure than they are in the top for a longer time. Your brain interprets "close to being in focus" as "being in focus"... again, with a tolerance. Your eyes just don't have the precision to detect see the minor loss of focus - so it still looks in focus.

Trevor


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 Post subject:
PostPosted: Tue Nov 09, 2010 5:35 pm 
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Trevor, I was gonna post that illustration, but you beat me to it! :P

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 Post subject:
PostPosted: Tue Nov 09, 2010 6:27 pm 
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Actually, you do have less depth of field when your pupil is opened wide (at night), our brains are just compensating really well and we have blazing fast "autofocus"

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 Post subject:
PostPosted: Tue Nov 09, 2010 7:17 pm 
Nice graphical representation, cheers guys!


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 Post subject:
PostPosted: Tue Nov 09, 2010 10:09 pm 
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@Citruspers

Good question, I've often wondered that myself. Now to find an optometrist who's also a photographer. Anyone?


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 Post subject:
PostPosted: Fri Nov 12, 2010 2:55 am 
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Found an answer... it's actually true. Your eye's DoF is more shallow in the night. This is why people with bad vision (i.e. inability to focus well) have more trouble during seeing during the night. The DoF is shallower and thus your eye's "autofocus" demands more precision. Huh.


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