Depth of Field and dirty lenses????????? at DVinfo.net

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Old December 10th, 2002, 11:00 AM   #1
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Depth of Field and dirty lenses?????????

I do what I can to keep my lens free of dust, smudges and scratches, but I can't get rid of them all. This presents a problem when shooting with a small aperture and the associated depth of field. The aforementioned items are very noticeable at f-16 and I don't always have the time to add an ND filter. This is especially a problem when the lighting changes and I can't lock down the exposure. I could just switch the internal ND filter on but then the aperture drops from f-16 to f-2. I was disappointed to see the XL-1s did not fix this. This camera needs at least two internal settings. A two stop and a four stop would be a big help.

Has anyone else noticed this with their XL-1?

Greg Matty
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Old December 10th, 2002, 03:19 PM   #2
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Can't you change the shutter instead? Also, try to use a good lenshood which reduces the visibility of dust at lateral incident (sun) light on the lens. FYI "lenspen" is one of the best cleaners.
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Old December 10th, 2002, 07:01 PM   #3
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Andre,

Your solution is very simple and I will probably experiment with it as I have thougth about this before. It would still require me to stop taping and make an adjustment which brings me back to just using an external ND filter.

I also have one other concern. If an XL-1 records 30 frames per second at sixty fields, doesn't that correlate to the 1/60 of a second shutter speed? If I drop down to 1/120, won't that make the image appear to stutter? I don't know how best to explain this but I will try. If 60 fields are exposed at 1/60 of a second, I capture all the available motion. If I go to a 1/120, won't I miss half of it? I am considering 60 exposures in one second at 1/120 of a second per exposure. That means I am only capturing one half of a seconds information (60 exposures/second x 1/120 = .5). For example, if a person was running across the field of view and moves say 1 foot in 1 full frame but I am using a 1/120 shutter speed, I will only have time to record half a foot of motion. The next frame would be the same way and I would get stuttery playback as their will three inches of motion missing from each end of the frame.

I am sure I am thinking about this too much.

Greg Matty
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Old December 10th, 2002, 08:12 PM   #4
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Using a faster shutter will reduce motion blur/smear within the field. This may give it a bit different look for moving subjects, but most folks will not notice or object to it. However, going to 1/30 will result in noticeable artifacts for moving objects.

Try it as see if it works for you.

BTW, 1 foot in one frame is 60 feet per second, or about 41 miles per hour, a rather fast runner!
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Old December 10th, 2002, 09:38 PM   #5
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Motion will appear to stutter on some subjects. Birds in flight, for example, appear unnatural because of the stuttering wing beat. The eye is used to a more fluid motion. Water, waves, takes on a strange effect. It just depends on the subject, some will look fine, others won't. The alternative is to put on your ND filter, or go to -3db on the gain (if your not already).

Jeff
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Old December 11th, 2002, 12:13 AM   #6
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Also, someone had written on another post that you lose sharpness if you stop down more than f8. Your first post mentions you shot at f16.

Not to be obnoxious (which I sure I often am) but I thought depth of field and dirt worked the other way; if you have more depth of field, you see more crap from your lens. YOu can shoot so wide under certain conditions that dust on your lens becomes visible.

Are you using a UV filter? I've taken mine off, and feel much more secure about the cleanliness of my lens. Some people feel it's a security risk, others don't.
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Old December 11th, 2002, 04:31 AM   #7
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Greg, I tought the coarse ND step was your problem. Shutter settings allow smaller increments. Your feeling about shutter settings is correct but pretty complex to get it fully elaborated. Did you know that film mostly has a 50% (180 degree shutter) optical dutycycle which corresponds with your (1/120 shutter) example.
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Old December 11th, 2002, 10:34 AM   #8
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<<<-- Originally posted by Josh Bass : Also, someone had written on another post that you lose sharpness if you stop down more than f8. Your first post mentions you shot at f16.

Not to be obnoxious (which I sure I often am) but I thought depth of field and dirt worked the other way; if you have more depth of field, you see more crap from your lens. YOu can shoot so wide under certain conditions that dust on your lens becomes visible.

Are you using a UV filter? I've taken mine off, and feel much more secure about the cleanliness of my lens. Some people feel it's a security risk, others don't. -->>>

I did not explain myself as well as I should have. That will teach me to post in a hurry while at work!

When you say stop down, I don't know if you mean smaller aperture/bigger numbers, or bigger aperture/smaller numbers. I think you mean the former. I have heard that a camera like the XL-1 is best used at f-8 or larger aperture/smaller number. Going to f-11 or f-16 will result in a less sharp image. I think this is what you were saying and I think we agree on it. Also, I did not intentionally shoot at f-16. I moved from indoors to out and had to remove the locked down exposure or everything would have been over exposed. Flipping on the ND filter would have dropped me to f-2 or whatever it is. That is why I hate the built in ND filter. It drops down too many stops.

I also think we are on the mark on your second comment which is exactly what my original post was all about. As the aperture gets smaller (bigger number?) and less light enters the camera, the depth of field increases. The area in focus in front of and behind the subject increases. At f-16 or f-32, that range includes the lens filter (but maybe not the lens itself?) and voila, specks of dust, smudges and smears are now in focus. I will try shooting without the UV filter as the dirt is probably on that filter.

Andre,

I know very little about camera optics but hopefully one of my applications to USC, UCLA or FSU will result in me being in film school this time next year. And yes, my problems did arise from shooting at f-16 and the built in ND filter. I just wish it didn't change by so many f-stops. Three would be much more practical than six or whatever it is.

Greg Matty

P.S. Now I am confusing myself. An f-16 stop lets less light in than f-2 right? That means the aperture is open more at f-2 than at f-16 right? That is why we squint to see things off in the distance. As we squint we let less light into our eyes and we get a larger depth of field.
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