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Capitain Boar · Joined: 20 Mar 2011 Posts: 18 Location: Moscow

I would like to shoot some Ilford SFX 200 with my Pressman. I think I'll have to make some focus corrections to get sharp image. How can I calculate it?
The lens is 100 mm Tessar with a 720 nm filter.
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1banjo · Joined: 16 Nov 2008 Posts: 492 Location: kansas

take a look at http://www.ilfordphoto.com/Webfiles/2007651134552223.pdf

1banjo · Joined: 16 Nov 2008 Posts: 492 Location: kansas

this is all they say

FOCUSING
With some lenses, red light focuses at a slightly different
point to other visible light. With these lenses there may
be a focus shift when focusing in white light compared
with red light. However, it is almost impossible to focus a
camera with a deep red filter in place.
With short to moderate focal length lenses, this difference
can easily be accommodated by stopping down to the
smallest workable aperture. Some lenses, particularly
apochromatic (APO) designs, may need no correction.

45PSS · Posted: Sun Jun 05, 2011 6:21 am Post subject:

http://photo.net/large-format-photography-forum/00FQq9

the posted links are dead but the most relevant,
and another discussion

http://photo.net/bboard/q-and-a-fetch-msg?msg_id=004J7h

as banjo said, depends on the lens.
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The best camera ever made is the one that YOU enjoy using and produces the image quality that satifies YOU.

Capitain Boar · Joined: 20 Mar 2011 Posts: 18 Location: Moscow

Thank you for the information.
Ok, I think I will take a glow-lamp and focus the lens with the filter on some distances to mark a scale on my camera. I'll focus on the sun to get infinity focus.
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Henry · Posted: Sun Jun 05, 2011 12:51 pm Post subject:

Uh, make that the moon, not the sun, or you'll risk damaging your lens and your eyes!

Capitain Boar · Joined: 20 Mar 2011 Posts: 18 Location: Moscow

Hmm, The filter is pretty deep (more than 512x), so the sun isn't so bright when I look through the filter.
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C. Henry · Posted: Sun Jun 05, 2011 6:14 pm Post subject:

Don't look directly at the sun, only at the image of it on the ground glass and then only from an off center position.
The filter you are using seems to approximate the "sun screen" used on mariner's sextants used for celestial navigation which required viewing an image of the sun in order to determine the exact angle between the sun and the sea horizon.
BTW I'm the "other Henry" not the same one as posted an earlier reply!

C. Henry

45PSS · Posted: Sun Jun 05, 2011 7:42 pm Post subject:

http://www.msnbc.msn.com/id/3541480/ns/technology_and_science-space/t/how-view-sun-safely/

I use a piece of aluminized mylar, works well.
_________________
The best camera ever made is the one that YOU enjoy using and produces the image quality that satifies YOU.