Lauri Kettunen
Well-known member
Here are instructors how to collimate the Birger mount.
1) You need a backfocus chart.
If you don't have one, download a chart from http://www.mediacollege.com/video/test-patterns/back-focus.html and print it to paper. About 1/4th of an A4/Letter size paper is big enough.
2) I recommend to use the EF 24-70mm/f2.8 L lens or equivalent. Put the camera horizontally on a tripod and measure the height of the mid point of the front glass of the lens. Tape the back focus chart on a wall such that the mid point is on the same height as the mid point of the lens.
3) Check that you have the mid cross set on in the LCD/EVF, point the cross to the mid point of the chart and mesure that the sensor is perpendiculary to the wall and 1.0m away from back focus chart. Try to adjust the distance within say 1.0cm.
In practice the sensor lies very accurately on the same plane as the camera plane about 2cm backwards from the Red logo in the front of the camera. (This plane has 8 small screws.)
3) Now, focus of the lens to the 1.0m mark, set the iris wide open, for this adjust the shutter speed accordingly. Set both edge highlight and 1:1 view on in the camera.
4) Remove the front part of the Birger mount, adjust the collimation ring, put the front part of the mount back and go back and forth until the image is sharp. When the reddish ring around the center is the largest, this is also the sharpest image.
5) Zoom to the tele end of the lens.
6) Find which way you have to manually focus until the image is sharp. Say, this is X meters instead of the 1.0 mark.
The goal is to collimate the mount by iteration such that the image is simultaneously sharp both at wide and tele ends of the lens. Quite likely, the focus mark will not be precisely 1.0m.
7) If you have no exprience how to iterate to the proper setting, employ a trivial average approach: Set the focus of the lens to the average of 1.0m and X meter, i.e. to (1.0 + X)/2.
8) Zoom back to the wide end and go back to the 4. step and iterate until the changes are negligible.
To test that the collimation is correct, measure the distance of the front of the lens to the back focus chart. Then put a EOS body to the lens and check whether the auto focus finds the same focus distance.
If you understand about differentiation, instead of the averages read how much the focus distance number changes with respect to the numbers in scaling of the Birger mount and use the ratio (change in focus distance)/(Erik's number) to approximate how much you need to adjust the collimation. This will converge very quickly to the right setting. The average will converge as well, it only takes a few more iteration cycles.
Hope this is helpful.
1) You need a backfocus chart.
If you don't have one, download a chart from http://www.mediacollege.com/video/test-patterns/back-focus.html and print it to paper. About 1/4th of an A4/Letter size paper is big enough.
2) I recommend to use the EF 24-70mm/f2.8 L lens or equivalent. Put the camera horizontally on a tripod and measure the height of the mid point of the front glass of the lens. Tape the back focus chart on a wall such that the mid point is on the same height as the mid point of the lens.
3) Check that you have the mid cross set on in the LCD/EVF, point the cross to the mid point of the chart and mesure that the sensor is perpendiculary to the wall and 1.0m away from back focus chart. Try to adjust the distance within say 1.0cm.
In practice the sensor lies very accurately on the same plane as the camera plane about 2cm backwards from the Red logo in the front of the camera. (This plane has 8 small screws.)
3) Now, focus of the lens to the 1.0m mark, set the iris wide open, for this adjust the shutter speed accordingly. Set both edge highlight and 1:1 view on in the camera.
4) Remove the front part of the Birger mount, adjust the collimation ring, put the front part of the mount back and go back and forth until the image is sharp. When the reddish ring around the center is the largest, this is also the sharpest image.
5) Zoom to the tele end of the lens.
6) Find which way you have to manually focus until the image is sharp. Say, this is X meters instead of the 1.0 mark.
The goal is to collimate the mount by iteration such that the image is simultaneously sharp both at wide and tele ends of the lens. Quite likely, the focus mark will not be precisely 1.0m.
7) If you have no exprience how to iterate to the proper setting, employ a trivial average approach: Set the focus of the lens to the average of 1.0m and X meter, i.e. to (1.0 + X)/2.
8) Zoom back to the wide end and go back to the 4. step and iterate until the changes are negligible.
To test that the collimation is correct, measure the distance of the front of the lens to the back focus chart. Then put a EOS body to the lens and check whether the auto focus finds the same focus distance.
If you understand about differentiation, instead of the averages read how much the focus distance number changes with respect to the numbers in scaling of the Birger mount and use the ratio (change in focus distance)/(Erik's number) to approximate how much you need to adjust the collimation. This will converge very quickly to the right setting. The average will converge as well, it only takes a few more iteration cycles.
Hope this is helpful.
). But the brand new Tokina markings are way off. Like 1,8 feet instead of 3 feet. But it holds focus. On my Canon 17-35mm marks are good.
