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| raptar 108 W A | |
| Author | Message | ||
|---|---|---|---|
| 1banjo Joined: 16 Nov 2008 Posts: 492 |  Posted: Fri Sep 02, 2011 6:50 pm Post subject: raptar 108 W AI 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?? | ||
| 45PSS Joined: 28 Sep 2001 Posts: 4081 | Posted: Fri Sep 02, 2011 8:12 pm 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 | ||
| 1banjo Joined: 16 Nov 2008 Posts: 492 | Posted: Fri Sep 02, 2011 8:50 pm 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?? | ||
| 45PSS Joined: 28 Sep 2001 Posts: 4081 | Posted: Fri Sep 02, 2011 9:36 pm
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. | ||
| 1banjo Joined: 16 Nov 2008 Posts: 492 | Posted: Fri Sep 02, 2011 11:25 pm that don't change it any it still look like 12 mm | ||
| 45PSS Joined: 28 Sep 2001 Posts: 4081 | Posted: Sat Sep 03, 2011 1:13 am 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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