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raptar 108 W A

 
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1banjo



Joined: 16 Nov 2008
Posts: 478
Location: kansas

PostPosted: Fri Sep 02, 2011 6:50 pm    Post subject: raptar 108 W A Reply with quote

I was just looking at a lens I got some time back
and fond out that it has been reshuttered so I was going
to make a F stop scale as the shutter is for a 4.7 not a F6.8
but when I caluate it out @ 6.8 looks like it should be 15.88mm
BUT the largest dimanter can only be 12mm on the front lens element
don't you dived the lens 108mm by 6.8 to get the dimenter of the hole??
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45PSS



Joined: 28 Sep 2001
Posts: 3219
Location: Mid Peninsula, Ca.

PostPosted: Fri Sep 02, 2011 8:12 pm    Post subject: Reply with quote

Lens focal length divided by entrance pupil diameter as viewed through the front lens group equals f stop.

Fl-focal length
Pd-Pupil diameter
f-f stop

Fl/Pd=f
Pd=Fl/f
Fl=Pd*f

108/15.88=6.8
108/12=9
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1banjo



Joined: 16 Nov 2008
Posts: 478
Location: kansas

PostPosted: Fri Sep 02, 2011 8:50 pm    Post subject: Reply with quote

OK that is what I though
the lens say F 6.8 BUT the backside of the fount lens elemet
is only a 12mm diameter hole SO how could it be a F6.8??
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45PSS



Joined: 28 Sep 2001
Posts: 3219
Location: Mid Peninsula, Ca.

PostPosted: Fri Sep 02, 2011 9:36 pm    Post subject: Reply with quote

Quote:
BUT the backside of the fount lens elemet
is only a 12mm diameter hole SO how could it be a F6.8??


How large does that 12mm appear when looking through the front of the lens?

Lens magnification increases the optical size of the opening. Look through the front of the front group and measure the rear opening as it appears from the front of the lens, not the actual physical size.
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1banjo



Joined: 16 Nov 2008
Posts: 478
Location: kansas

PostPosted: Fri Sep 02, 2011 11:25 pm    Post subject: Reply with quote

that don't change it any it still look like 12 mm
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45PSS



Joined: 28 Sep 2001
Posts: 3219
Location: Mid Peninsula, Ca.

PostPosted: Sat Sep 03, 2011 1:13 am    Post subject: Reply with quote

W e l l

Lens mounted in shutter, set for focusing at infinity, aperture lever at its mechanical maximum opening, point at an evenly lit wall.

With focus panel removed from camera point a analog light meter at the center of the lens. Start moving the aperture lever toward closed and stop at the point where the meter needle starts to move. Wiggle the aperture lever to find the exact point. Mark the scale.
http://en.wikipedia.org/wiki/F-number
f stop 5.6, +1/3-6.3, + 1/2-6.7, +2/3-7.1, f8. Now your lens is supposedly a f6.8 so that is a touch over half stop smaller than f5.6 so move the aperture lever so that it reads almost 1/2 stop less exposure and mark the scale. From this point on mark the scale in 1 stop increments. Test with film and carefully metered exposures.

A piece of painters/masking tape can be put over the existing scale prior to making measurements.

Optical measurement is more accurate than mechanical.
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