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FORUMS Photography Talk by Genre General Photography Talk 
Thread started 11 Sep 2012 (Tuesday) 16:11
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Why is DOF greater on larger sensors?

 
hairy_moth
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Sep 11, 2012 20:24 |  #16

TooManyShots wrote in post #14978768 (external link)
Longer lens or walk closer to the subject.

I initially wrote it that way, but moving closer to the subject would change the perspective and it would not be the same (some might even argue, not even similar). So I re-worded it to just "longer lens."


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TooManyShots
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Sep 11, 2012 20:31 |  #17
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hairy_moth wrote in post #14978922 (external link)
I initially wrote it that way, but moving closer to the subject would change the perspective and it would not be the same (some might even argue, not even similar). So I re-worded it to just "longer lens."

The perspective shouldn't change much as long as you aren't talking about a wider focal length. Let's say you want to frame a full figure shot using an 85mm lens on both 1.6x body and a full frame or a medium format. Is obvious that with a 1.6x crop body you have to stand back a bit far to frame the entire full figure. Whereas on the full frame and medium format, you don't have to. In fact, you may need to walk up closer to the subject to fill up the frame.


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Sep 11, 2012 20:46 |  #18

TooManyShots wrote in post #14978955 (external link)
The perspective shouldn't change much as long as you aren't talking about a wider focal length.

I don't know, I think you are coming dangerously close to be told to look at the bear's feet! :)


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TooManyShots
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Sep 11, 2012 20:52 |  #19
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hairy_moth wrote in post #14979018 (external link)
I don't know, I think you are coming dangerously close to be told to look at the bear's feet! :)


Well, I don't care. I always like full frame anyway.


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krb
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Sep 11, 2012 20:59 |  #20

TooManyShots wrote in post #14978955 (external link)
The perspective shouldn't change much as long as you aren't talking about a wider focal length. .

Focal length does not change perspective.


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Sep 11, 2012 21:25 |  #21

DEAR LORD, MAKE THIS STOP!


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frugivore
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Sep 12, 2012 06:11 |  #22

I think some of you are discussing whether the projected image onto the sensor changes in any way when the size of the sensor changes. Of course it does not. The lens is blind to whether you are using a 5D or a 500D.

What we should be discussing, do to help the OP understand the 'why' behind the differences in DOF between FF and crop, is how the principal and incident light rays behave when they enter a lens.

Once the OP sees this visually, I think he/she will understand why subject distance affects DOF. Cambridge in Colour shows this here: http://www.cambridgein​colour.com/tutorials/d​epth-of-field.htm (external link). Canon's Lens Work III has many great diagrams about this - chapter 10 I believe.




  
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Curtis ­ N
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Sep 12, 2012 08:35 |  #23

Ok here it is, in a nutshell.

In the forumla for calculating hyperfocal distance (external link), the focal length is squared. Because of this, changing the focal length in the calculation has an exponential affect on hyperfocal distance. The circle of confusion size (determined by the amount of magnification) is in the denominator and only used once (not squared). Hyperfocal distance is used to calculate DOF.

So...
When you switch to a larger camera format, and increase focal length to obtain the same field of view, the larger focal length will tend to decrease DOF, while the larger format will tend to increase DOF (larger format = less magnification = larger circle of confusion). But since the focal length is squared in the equation, the combination of these changes results in a greater hyperfocal distance (and hence a smaller DOF).

There's a short piece on DOF vs. format size here:
http://en.wikipedia.or​g/wiki/Depth_of_field (external link)

And if you understand this, you understand why consumers who graduate from a P&S to a DSLR often get frustrated. The short focal lengths used with the tiny sensor of a P&S camera give you a lot of DOF. Because of this, focus accuracy is less critical and as long as you focus on something in the same zip code as your subject, you'll be fine. With the longer focal lengths common to a DSLR, the shallow DOF requires more careful and accurate focusing for good results.


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goldboughtrue
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Sep 12, 2012 11:26 as a reply to  @ Curtis N's post |  #24
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Yes, I understand now. Because a longer lens is needed on a larger sensor to get the same perspective as a smaller sensor, then the background appears more out of focus.


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krb
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Sep 12, 2012 11:30 |  #25

goldboughtrue wrote in post #14981421 (external link)
Yes, I understand now. Because a longer lens is needed on a larger sensor to get the same perspective as a smaller sensor, then the background appears more out of focus.

Correct except for the perspective thing. You need the longer lens to get the same field of view. Getting the same perspective happens because you are able to stay in the same spot.


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Sep 12, 2012 11:31 |  #26

goldboughtrue wrote in post #14981421 (external link)
Yes, I understand now. Because a longer lens is needed on a larger sensor to get the same perspective as a smaller sensor, then the background appears more out of focus.

A tidbit for you: Perspective (the relative sizes of objects in the scene which are at different distances from the camera or your eye) is not affected by focal length. Perspective is affected only by the distances between the camera and the various elements of the scene.

That said, I suspect that you really meant "field of view" instead of "perspective". The two are radically different and field of view is, of course, affected by focal length choice as well as the camera format (size of film frame or sensor in the camera).


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goldboughtrue
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Sep 12, 2012 15:59 |  #27
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SkipD wrote in post #14981449 (external link)
That said, I suspect that you really meant "field of view" instead of "perspective". The two are radically different and field of view is, of course, affected by focal length choice as well as the camera format (size of film frame or sensor in the camera).

You're correct. I meant field of view. I guess I'm not current on all the right terms.


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Why is DOF greater on larger sensors?
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