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Old October 3rd, 2002, 09:32 PM   #1
GreenRubberPlant
 
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Now about image plane...

Now that I've asked about d.o.f., I have questions regartding what the heck image plane is and how i can get a large image plane on an xl1. I have a 2000-3000 dollar budget for a lens and I'm wondering which lens has the best shallow depth of field and large image plane. I'm not going to buy a p+s technik, because that's just outrageously pricey for me at the moment.
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Old October 4th, 2002, 07:01 AM   #2
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The image plane is an imaginary plane infinite in size (in a theoretical sense). The subject (what you focus the lens on) resides in the image plane. For practical purposes the image plane can be thought of as being limited in size by the angle of view of the focal length of the lens. The image plane is parallel to the focal plane (in most cases). The focal plane is where the light comes to focus on the CCD inside the XL1.

Every lens has shallow DoF and deep DoF. The DoF depends on the 5 factors in this post http://www.dvinfo.net/conf/showthread.php?s=&threadid=3926 The image plane will change as you zoom and refocus your lens. Wide angle lenses have the widest angle of view, thus the largest image plane.

Jeff
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Old October 4th, 2002, 08:30 AM   #3
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Chicken or egg?

I think GreenRubberPlant is keying off comments by Don Berube in the previous thread titled "Depth of Focus Question". Don explains that larger CCD's will make it easier to acheive shorter DoF. Here is an expert from Don's post:

<<<-- Originally posted by Don Berube : :
"It has more to do with the actual size of the image plane than the lens used. In general, the smaller the image plane, the more inherent depth of field you will realize. That is why it is so much easier to realize a lower depth of field with 2/3" CCD's over 1/3". The size of the image plane also affects the effective focal length and zoom range you get from any given lens used."

So... Does the focal length change the image plane, or does the image plane change the focal length? I suspect that this is in part a semantic issue - how do we correctly use the term "image plane"? We see it being used to refer to the size of the CCD, the imaginary theoretical plane of infinite size, and the finite area on which the image is actually focused based on the angle of view of the lens.

I'm confused!

lyd

EDIT:
Ok, I just reread the DoF thread,
http://www.dvinfo.net/conf/showthread.php?s=&threadid=3926
and I think part of at least my own confusion was the difference between "target size" and "image plane". So the size of the CCD is definately the former. I am still unclear on the way focal length interacts with image plane in the non-theoretical, finite sense.
lyd
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Old October 4th, 2002, 09:26 AM   #4
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Go back to the same post and read the definition of DoF. People sometimes use image plane as a plane in front of the lens (which is correct - see definition) but also as the plane where light comes to focus behind the lens. The later is incorrect. Where light comes to focus is the focal plane (hence focal plane shutter). Target, or target size can be used as either the image plane or the focal plane.

Jeff
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Old October 4th, 2002, 09:37 AM   #5
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Sorry, missed your last question. In almost every instance the image plane is parallel to the focal plane. In the real world, the image plane is then defined by the limits of the field of view. Quite simply the edges of your scene or what you see in your viewfinder. When you zoom your lens the image plane will increase or decrease as the angle of view changes.

Jeff
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Old October 4th, 2002, 10:38 AM   #6
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Okay... thanks tons, Jeff.
If I understand correctly now, the key is carefull understanding of the terms related to target, as they can refer to several different things. Is this correct?

Image Plane: paralell to the focal plane. When speaking of the "size of the image plane" you are essentially talking about your field of view.

Target Plane = parallel to the image plane. When speaking of DoF you are indentifying 2 of these, at the near and far limits of acceptable focus.

Target (in front of the lens) = your subject

Target (behind the lens) = Your media (film, prism to CCDs, whatever)

Focal plane = point behind the lens at which light is focused.

Target Size = Size of the subject relative to your angle of view, or absolute size of the film, CCDs, etc. with the context being in front of or behind the lens, respectively.

Does this parse? I'm sorry to belabor the topic, but I'm struggling.

lyd
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Old October 4th, 2002, 10:38 AM   #7
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I don't think the question of the lens having "the best shallow depth of filed and the largest image planel" is relevant. The image plane (theoretically unlimited in the geometric optics theory) is not related to DOF. Only the mechanical constructiobn of real life optics determine the image plane and whatever DOF can be realised independant of the image plane size. In general the image plane is not made larger than needed for a paricular target under different zoom conditions. We (Barco) designed several oversized image plane optics for LCD and DLP projectors in order to allow "off axis" projection.(up to 120%)
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Old October 4th, 2002, 11:14 AM   #8
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lyd,

You've got it perfect. You've done a great job sticking with all the new knowledge your acquiring.

In the real world target size behind the lens (chip size, film size) is fixed. You can't change 1/3 inch CCD's for 2/3 inch CCD in your XL1 or whatever you shoot. So for practical purposes only changes in front of the lens affect DoF. Dre... points out that in many cases the target size (subject size) is fixed. Go back to the other thread and reread the part about News Anchors (talking heads) and product shots. The target size is fixed now at both ends. Changing the focal length and distance to subject will cancel each other out. Law of Reciprocity. They will not, and can not change DoF in this example. Therefore the only contol you have over DoF (in this example) is F Number. Larger F Numbers will increase DoF and smaller F Numbers will decrease DoF.

So, the answer to GreenRubberPlant's question is the lens won't affect DoF if he uses a camera with a fixed CCD and fixed target size (subject size). A scene in his movie calls for a head to be a certain size. He's now got a fixed target. The only thing that will affect DoF is the F Number. All the lenses that have XL mounts have virtually the same range of F Numbers. Therefore they all have the same DoF given the fixed target size and same F Number.

Jeff
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