chuck colburn
02-18-2007, 06:27 PM
For those who are thinking about using a still camera zoom lens and who have a digital camera to mount it on, here are a couple different test targets.
Make copies as large as you can and mount them on a wall or test stand. A couple of photo floods, one on either side at 45 degrees to the target, (like on a copy stand) makes for the best lighting. Center the cross hairs in the camera eyepiece to the center of the target you are using. Get in as close as pratical on the longest focal length so the target stays as large as possible when you zoom wide. The first target is best for testing for the presence of astigmatisim and the second is good for checking tracking and focus shift as well as back focus at the shortest focal length. It will be easier to detect these errors if you set the camera up for black and white. And if you camera has a video out, it's easier to watch on a decent monitor then the LCD or through the eyepiece.
http://www.sinepatterns.com/images/Sector%20Star.gif
http://en.wikipedia.org/wiki/Image:Siemens_star.svg
Other tests such as curveture of field, rectilinear correction and chromatic errors require different targets. But if the zoom has bad focus shift or extreme tracking error it's a mote point unless it's a new lens and it will be repaired under warrenty, as the shop hours for repair will be prohibitive. Front and back focus errors can be corrected at the Ye Olde Optical Repair Shoppe. Check carefully before buying used optics especially zooms.
Chuck
Make copies as large as you can and mount them on a wall or test stand. A couple of photo floods, one on either side at 45 degrees to the target, (like on a copy stand) makes for the best lighting. Center the cross hairs in the camera eyepiece to the center of the target you are using. Get in as close as pratical on the longest focal length so the target stays as large as possible when you zoom wide. The first target is best for testing for the presence of astigmatisim and the second is good for checking tracking and focus shift as well as back focus at the shortest focal length. It will be easier to detect these errors if you set the camera up for black and white. And if you camera has a video out, it's easier to watch on a decent monitor then the LCD or through the eyepiece.
http://www.sinepatterns.com/images/Sector%20Star.gif
http://en.wikipedia.org/wiki/Image:Siemens_star.svg
Other tests such as curveture of field, rectilinear correction and chromatic errors require different targets. But if the zoom has bad focus shift or extreme tracking error it's a mote point unless it's a new lens and it will be repaired under warrenty, as the shop hours for repair will be prohibitive. Front and back focus errors can be corrected at the Ye Olde Optical Repair Shoppe. Check carefully before buying used optics especially zooms.
Chuck