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Old October 3rd, 2002, 10:55 AM   #1
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Depth of Focus question

Which xl1s lens has a depth of focus closest to a 35mm lens. I noticed how shallow a 35mm lens could be. Can any xl1s lens get somewhere near that? I also wanted to ask which lens for the GY500U or the 5000 U has closest depth of focus to 35mm.
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Old October 3rd, 2002, 12:19 PM   #2
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I believe you just might have to spend $8000 dollars on a 35mm adapter, and then $2000 (at the very least ) for a 35 mm lens. All the available lenses for the XL1 have whole hunka depth of field.

I wonder if someone out there could tell this gentlemen how an EOF adapter plus a still camera lens could help him. I know you have a much wider angle of view with those, but how's the depth of field?
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Old October 3rd, 2002, 02:14 PM   #3
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To better understand Depth of Field please read this post http://www.dvinfo.net/conf/showthread.php?s=&threadid=3926 and if you have any questions post back in that thread so I can keep it all together.

Jeff
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Old October 3rd, 2002, 04:07 PM   #4
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See http://www.urbanfox.tv/production/p14dvfilm.html and my earlier posts on the subject.
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Old October 3rd, 2002, 06:20 PM   #5
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:I believe you just might have to spend $8000 dollars on a 35mm
:adapter, and then $2000 (at the very least ) for a 35 mm lens.
:All the available lenses for the XL1 have whole hunka depth of
:field.

Well, not exactly, there are some techniques which you can make use of with any standard setup. However, the solution you mention (P+S Technik mini35 digital adaptor) does work quite effectively when properly used. 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. This is why the GL1/2 cams have a 20X optical zoom range, due to the smaller 1/4" CCD's. Slap that lens on the GL2 in front of some 1/3" CCD's and it would probably have a focal length somewhere around 12X or so. In short, the mini35 creates an actual 35mm image plane, which is resolved onto a high quality piece of spinning ground glass. That image is then recorded onto the CCD's via a second relay lens which has an additional exposure control to help maintain a wide open setting on the lens used in various lighting conditions.

I agree with Jeff Donald that there is a wealth of information on DVINFO.NET regarding D.O.F., and that by doing a Search in the Forums on "Depth Of Field", you will find many useful tips, opinions and articles.

You can also check out my article on the making of "Exclusive" which describes some techniques I used to constrain the D.O.F. in some shots I had to take in very bright sunlight. Check out the image samples while you are at it.

http://noisybrain.com/onloc1_01.html -or- on dvinfo.net in the "Articles" section at http://www.dvinfo.net/articles/index.php

:I wonder if someone out there could tell this gentlemen how an
:EOF adapter plus a still camera lens could help him. I know you
:have a much wider angle of view with those, but how's the
:depth of field?

Josh, I think you meant to say narrower field of view? The EF adaptor is not an "optical" adaptor like the mini35 is, if you will, it is mechanical - and any focal length lens you use with it is magnified by a factor of 7.2x, which does not really allow for many wide angles at all. Certainly, using a more extreme telephoto focal length can help to create a shallow D.O.F., but again D.O.F. is affected by other things as well, such as size of image plane, iris setting, focal length, distance from subject to background and critical focus.
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Old October 3rd, 2002, 08:01 PM   #6
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I thought when you put the still camera lenses on an video camera, you got a huge field of view because of that 7.2x business. Do I have it backward? Or does the 7.2x not affect the angle of view?
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Old October 4th, 2002, 01:09 AM   #7
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The 7.2 times difference between the 1/3 inch chip and a 35mm piece of film affects everything. However, a wide angle lens has a wide angle of view, a telephoto lens has a narrow angle of view. When the effective focal length or magnification changes by 7.2 times (increase) the angle of view narrows. The relationship between Telephoto and angle of view is inverse. As the magnification increases the angle of view narrows.

Jeff
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Old October 4th, 2002, 01:13 AM   #8
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Alright, I guess I thought it was the angle of view that was modified by a factor of 7.2, and not the focal length. My bad.
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Old October 4th, 2002, 01:24 AM   #9
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Yah, it confuses a lot people. But think of it in relationship to looking through your lens. Play with the zoom. As you makes things bigger, less things are in the field of view (narrow field of view).

Jeff
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Old October 4th, 2002, 02:01 AM   #10
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Right. . .since we're on it, one more: 'longer focal length' means more zoomed in, whereas 'shorter focal length' means less, correct? Or is that backwards.
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Old October 4th, 2002, 06:32 AM   #11
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Zoom in, zoom out are sometimes confusing to people because of the way some zoom lenses work. The push, pull style of zooming (rather than twisting or rotating) sometimes works backwards. You pull the zoom in, put the image zooms out or in depending on your perspective (very confusing). Longer and shorter are terms used to describe focal lengths of lenses. It corresponds to the length of the lenses in millimeters (mm). The standard 16x lens is 5.5mm to 88mm. The widest angle of view is the 5.5mm position. The 88mm position is the narrowest angle of view, but the most magnification. The zoom range is 16x. This corresponds to the range the lens can zoom over, or through (5.5 X 16=88). The image size will also change by a factor of 16 as you zoom.

Around 7mm in focal length the lens has about the angle of view of the human eye when relaxed. Any focal length smaller (shorter) than 7mm is a wide angle. Any focal length longer (higher) than 7mm is a telephoto.

Jeff
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Old October 5th, 2002, 12:32 AM   #12
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Thank you sir. I've held off asking long enough. . .what do the millimeters refer to? What exactly is 5.5 millimeters?
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Old October 5th, 2002, 05:49 AM   #13
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The focal length is an actual distance and it's commonly measured in millimeters. But first, some terms.

Nodal Points - Nodal points are reference only points. The design of the lens determines the points exact location. Nodal points refer to a locations along the lens axis (center of the lens) where the the light rays are displaced lateraly (think of it as the ray being bent). The rear most at which the light is displaced is the rear nodal point.

Focal Plane - Plane on which the lens forms a sharp image. Usually perpendicular to the lens axis and where the CCD (or film) is positioned to acquire a sharp (in focus) image.

Principal Focal Point - When the lens is focused at infinity, the point along the lens axis at which the light rays focus on the Focal Plane.

Focal Length - Distance between the rear nodal point of the lens and the principal focal point when the lens is focused at infinity.

Reasonably accurate laymens definition - the distance between the focal plane (CCD) and the last point the light is bent when leaving the rear of the lens.

Back Focus - Distance from the rear lens surface to the focal plane when the lens is focused at infinity.

People with the manual lenses need to adjust the back focus in order for sharp focus to be maintained throughout the zoom range.

Jeff
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