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Alternative Imaging Methods
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Old March 4th, 2006, 09:59 PM   #46
Inner Circle
Join Date: Dec 2003
Posts: 4,299
This is a long shot, pertains to operating practices and probably invalid but here it is to waste some server space anyway.

Autofocus with the videocamera works for quickly setting up the relay focus but is not always foolproof. Manual focus is preferred. Whe you set up a lot of soft area in your shot, the videocamera is going to chase that which appears sharpest to it, in this case anything in the image path which can be made to be sharply defined, in this case possibly specks which otherwise might not be apparent.

I suspect you may find that when these blemishes are most apparent, overall sharpness may have also gone off, meaning the videcamera focus has moved off the groundglass. When you reset the SLR (front lens) focus to sharp, the videocamera may well go back to whatever can be made to appear sharpest, which may be the groundglass image. Or you may have manually reset the videocamera relay focus yourself before switching back to auto.

With my arrangement, the PD150 and a 2 x 90degree prism path, the path is very prone to getting bits of grit and dust because of my own untidy construction methods with the prototype. If I leave the videocamera on autofocus after initial setup, as soon as I play with focus pulls to create soft focus transitions, the videocamera focus wll crash to the closest sharply definable mark. Sometimes this is a bit of dust on the front glass of the videocamera itself. Focus will stay there and I manually over-ride it.

Whilst specks in the image path are not good and may emerge as out-of-focus dark areas in certain lighting conditions, locking off your manual focus might be a best practice to minimise the defect. I would favour taping over the focus and zoom rings ring so they cannot be disturbed.

My own personally favoured operating practices :-

After relay focus setup, mask over the focus and zoom ring controls to prevent accidental adjustment.

Set video gain to 0db.

Attempt to maintain videocamera aperture setting at no smaller than f5.6. ( if this is a selectable option.) - helps keep specks soft and the autofocus out of temptation's way. This setting also limits diffraction issues relating to small-CCD video cameras.

Use in-camera ND filter selections to keep to this aperture setting or add pieces of neutral density filter gel between the SLR lens and groundglass to bring light levels within a manageable range.)

1/50th sec shutter speed is preferred.

Try to avoid high light levels being predominent in your out-of-focus backgrounds. Any blemishes in the path which coincide with the soft bright out-of-focus zones will be more apparent in these shots.

(The higher contrast caused may also aggravate the tendency for autofocus to chase specks if autofocus really must be remain selected on. - This is a real problem for me when I use the device without the groundglass to aquire via aerial image through really long lenses. I don't use the groundglass with these lenses because they are most often of an aperture smaller than f5.6 which causes the groundglass texture to become apparent.

I use the autofocus to track objects moving towards and away from my viewpoint. Aircraft against a strong bright background sky wll normally hold via autofocus but if the subject momentarily exits the frame or an insect flies through, this is enough to set off an autofocus crash to the closest sharp object, the speck in the relay path.).

Ignore at will as I am no expert. The handling notes for the P+S Technik devices are valid for the alternative devices.
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