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#1
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Depth Of Field
Is it true that when photographs are taken with a camera (using manual
focus - set before taking the photos and not changed until after taking the photos) at a wide aperture (lower number - about 3.6) - the depth of field should affect the same parts of the image regardless of the orientation of the camera (assuming the photos were taken in the same spot at the same angle to the subject)? What I mean is that if I took one shot then rotated the camera 180 degrees and took another shot - should they both have the same parts of the image out of focus? Or could the out of focus part of the image rotate 180 degrees too? Thanks for any replies. |
#2
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Depth Of Field
"Matalog" wrote in message
... What I mean is that if I took one shot then rotated the camera 180 degrees and took another shot - should they both have the same parts of the image out of focus? Yes. |
#3
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Depth Of Field
Or could the out of focus part of the image rotate 180 degrees too?
I think of depth of field as a spherical region around the camera. The edge of this sphere is the same as the focal point, and its thickness is is calculable by focal length and aperture. May sound silly, but I envision it as an invisible ball around the camera, with a varying thickness and diameter. Regardless - it's spherical, so rotation of the camera will have little effect on it. |
#4
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Depth Of Field
BD wrote:
Or could the out of focus part of the image rotate 180 degrees too? I think of depth of field as a spherical region around the camera. The edge of this sphere is the same as the focal point, and its thickness is is calculable by focal length and aperture. snip Regardless - it's spherical, so rotation of the camera will have little effect on it. Assuming a subject of a perpendicular plane, is the focus different at the left & right edges of the sensor compared to the center of the sensor (or focal plane) [horizontal] because of their increased distance from the center of the lens? |
#5
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Depth Of Field
I would say that it depends on your depth of field, and thus your
aperture size. I don't have any 'evidence' to back myself up, but let me kind of think out loud for a sec: I did a portrait shoot not long ago, and mucked around with aperture size; I put the aperture wide open (which for that lens was f/1.8), and at 7 feet away, I got a DOF of maybe a few inches.In that shot, the subject's eye was in perfect focus, but at her ear was a little soft. Makes a nice effect, actually. So if the subject were holding up a perfectly flat piece of cardboard which filled my entire field of view, I expect the center would be in focus but the edges would not, given that the edges are further away from my lens than the center is. Obviously, the smaller the subject or the further away, the less of a difference this will make. BUT - if I increase my f-stop, thereby shrinking the aperture, the depth of field will become wider.... this is the 'thickness' of that sphere in my silly analogy. Where it had been a couple of inches thick at f/1.8, it might be closer to a foot thick at f/4. The depth of field would still be spherical in shape, but its thickness would allow for both the center of the cardboard and the edges to be within the range of the depth of field, and therefore the entire piece of cardboard would still be in focus. One other interesting topic, while we're on the subject: hyperfocal distance. Basically, as I said, the depth of field can vary in thickness depending on your aperture - the smaller the aperture (or higher the f-stop), the thicker the DOF. Now - say you're in a biiig room, taking pictures of stuff at the back wall. If the 'center' of your DOF is right at your focal point, then there will be things that are closer to you than that focal point which will remain in focus at a given f-stop, and there will be things *BEHIND* the focal point that will still remain in focus (this extra 'room in your depth of field' is wasted, because it's behind the wall). What a person can do, to take advantage of the full range of your DOF, is to actually focus on a point *closer to you* than the subject. So say you have a room that's 100 feet end-to-end: if you focus on a point that's 30 feet closer to you than the very back, your subject at the back can still be in focus because of your DOF. Why would someone do this? to allow _more_ of your scene to be in focus at the same time. The closer to your your focus point is, the closer that objects can be while still remaining in focus. If your DOF is thick enough, that is. Kind of a convoluted ramble, I know - but there are actually charts available that can tell you what your hyperfocal distance is, given a certain focal length and f-stop. Clear as mud? GOOD! For me as well!! ;-) |
#6
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Depth Of Field
Ryan wrote:
Assuming a subject of a perpendicular plane, is the focus different at the left & right edges of the sensor compared to the center of the sensor (or focal plane) [horizontal] because of their increased distance from the center of the lens? In general, yes. The effect is called "field curvature". It is not related to depth of field per se. For further discussion: http://www.vanwalree.com/optics/astigmatism.html |
#7
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Depth Of Field
wrote:
Ryan wrote: Assuming a subject of a perpendicular plane, is the focus different at the left & right edges of the sensor compared to the center of the sensor (or focal plane) [horizontal] because of their increased distance from the center of the lens? In general, yes. The effect is called "field curvature". It is not related to depth of field per se. For further discussion: http://www.vanwalree.com/optics/astigmatism.html Oh, excuse me, yes, there would be a DOF effect in the scenario you describe because of different subject distances. Still, read the referenced URL anyways; all of the optical goo at http://www.vanwalree.com/optics .htmlis worth digesting. |
#8
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Depth Of Field
Except that its not.
Then what say you on the topic of 'field curvature'? |
#9
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Depth Of Field
"BD" wrote in message
ups.com... I think of depth of field as a spherical region around the camera. The This would be good except the depth of field is more like a plane tangential to a sphere. In your view, if you had a very tall creature, and focussed on its midsection you would expect its feet and head to be out of focus. This is not the case. Regardless - it's spherical Except that its not. so rotation of the camera will have little effect on it. This we agree on. -Andrew |
#10
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Depth Of Field
BD wrote:
Or could the out of focus part of the image rotate 180 degrees too? I think of depth of field as a spherical region around the camera. The edge of this sphere is the same as the focal point, and its thickness is is calculable by focal length and aperture. May sound silly, but I envision it as an invisible ball around the camera, with a varying thickness and diameter. Regardless - it's spherical, so rotation of the camera will have little effect on it. I love the explanation. I will say that there should be an allowance for the fact that with most lenses it is not exactly spherical. As an extreme example is the flat field of most macro lenses. -- Joseph Meehan Dia duit |
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