Focus calculation questions
Posted 11 March 2008 - 03:15 PM
Please forgive my novice question, especially if it has already been asked. I've tried to go through as many posts as possible to see if I can find the answer and I haven't.
I work with a Bolex H16 Rex 4 with a Switar 18-86 mm Zoom lens. Having a reflex camera, I try to focus by looking through the viewfinder and adjusting the focus ring until the main subject of the frame appears to be in sharp focus. However, the results I've obtained doing this have been less than perfect. My question is:
How can I achieve sharp focus without relying solely on my less-than-perfect judgement through the reflex viewfinder? I read something about "tape focus" in a very old post on the board, how is that done?
Thank you in advance
Posted 11 March 2008 - 07:33 PM
1. Diopter - Check that you have the set the diopter (eyepiece focus) correctly for your eye. The diopter is the rotating focus ring on your eyepiece. If this is incorrectly set, then you won't be able to properly judge focus by eye. To make sure it's in focus, remove the lens from the camera and point the camera at bright light. This will illuminate the focusing screen, which will have crosshairs or framelines on it that you can focus on. Adjust the diopter until the markings are sharp. Then don't touch it! You can put some tape on it, so that it doesn't move. Readjust the diopter when your eyes get tired or at the beginning of every day, as your vision will change under these conditions. You can also adjust the diopter with the lens on as long as the image is way out of focus - in that case, zoom all the way in and defocus the lens first before messing with the diopter.
2. The camera's lens mount - If the camera's lens mount is out of spec, then your zoom lens will not stay in focus as you zoom in or out. If the mount is out of spec, then that means the distance between the mount and the focal plane is off and needs to be readjusted by a technician. This distance is called the flange focal distance (FFD) and is specific to the type of mount your camera has (I'm guessing c-mount in your case). You would have to send it to a technician for repair if this was the case. Fortunately, this is fairly easy to check. Working in a bright, well-lit environment, reattach the zoom lens to the camera body and set the camera on a tripod. Zoom all the way out and open the aperture all the way (wide open). Pick a stationary subject with a lot of fine detail, zoom in all the way, and focus on it. They slowly zoom out all the way. If the subject still looks sharp, then your FFD is fine. A "siemens star" focus chart can be very helpful for this procedure, though not necessary. You can print one out here: http://www.panavisio...tion/Charts.htm Once the FFD is properly set, you shouldn't have to readjust it again unless you drop your camera or subject it to some other violent trauma. With video cameras on the other hand, the FFD needs to be adjusted on a constant basis - this is known as "backfocus" adjustment and is done in the field.
*BTW, this is the procedure you should always use to focus a zoom lens - always zoom in and open the aperture to focus before zooming out to your desired focal length.
3. The lens mount - If the mount on the lens is out of spec, then the lens will stay in focus during zooming, but distance markings on the focus barrel will be inaccurate. If you only plan to focus by eye, this is not a big issue. But if you want to focus with a measuring tape, then you'd need to get the lens mount adjusted (collimated) by a technician. So first, let me explain: Your lens should have distance markings in feet or meters on the focus barrel. To tape focus, you would measure from the camera's focal plane to the subject and set the focus barrel of the lens to the distance that you measured. The focal plane (or film plane) is where the film runs through the gate, usually marked on the side of the camera with a white vertical line drawn through a circle. Given that the diopter, FFD, and lens mount are all properly set, eye focus and tape focus should match exactly. When this is the case, then you can rest assured that any further focus problems are strictly due to operator (or AC) error!
Posted 12 March 2008 - 09:36 AM
Once again, thanks for sharing the knowledge!
Posted 13 March 2008 - 01:03 AM
Anyway, good luck!