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Testing Lenses Thread.


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#1 ryan knight

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Posted 01 December 2009 - 03:27 PM

can anyone point me to or link me to a good thread on testing film lenses for resolution?

not about a particular lens set, but just how the overall process is executed.

thanks,

rk.
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#2 Tom Jensen

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Posted 01 December 2009 - 04:05 PM

There are only a few things you need to know when testing a lens.
1) Physically look at the lens to see if it is scratched, if it is dented or if the gears are chipped or mangled. Make sure the elements are clean.
2) Shake it and listen for loose elements. If you hear a thin, tinny sound, it is probably the iris leaves.
3) Turn the focus all the way to infinity and back to see if it is smooth and it doesn't bind. Do the same for the iris but look inside to see if the leaves are opening and closing.
4) Place it on the camera and make sure it fits smoothly. If not is is usually a dirty lens mount or camera mount.
5) Hook up the follow focus and make sure it moves smoothly and does not bind.
6) Use a focus chart and check the focus at different footage. Fill the frame with the focus chart. Make sure you the marks on the lens match your taped off distances. Focus two different ways. Try to eyeball focus through the lens making smaller and smaller focus moves until you dial it in. Then set the exact focus and move it towards infinity and do it again moving the focus closer. Make not if it gets sharper as you move away from the set focus distance. Make sure your diopter is sharp.
7) On all lenses look for any breathing or focus shifts, color aberration or any thing that looks odd to the eye. You will see the color aberration on the chart as you shift focus. It will turn from red to green usually.
8) For zooms do all of the above and more. Make sure the lens zooms out smoothly. Zoom all the way in and make sure that the zoom stays focused all the way through the zoom range as you zoom out.
9) Zoom in on a chart and line up on one quadrant of the cross hairs on a right angle figure on the chart and zoom out slowly. The cross hairs should not move off the right angle. This test is designed to see any deviation or shift up or down when you zoom out.
10) Take all lenses outside and see if they focus on infinity.
11) Make sure your bellows and Matte Box fits as well as follow focus units, speed cranks and whips.

Now there are two other things that are very important but they will be done by the lens tech.
1) The lens is placed on a collimator to make sure the lit focuses on the exact plane as the film. If not, the mount will be shimmed.
2) The lens should be projected. The projector has a chart inside that is focused on a flat surface, the wall. The distances on the lens should match the projector. You are looking for resolution from top to bottom and side to side. Wide angle lenses are the most critical.

DP's will shoot lens tests at different stops. They are usually looking to see if the set matches, they looking at the overall contrast, the effect of shooting wide open, skintones, color reproduction and the look of the lens.

If you have never seen a lens collimated or projected, ask your rental house lens tech if he will show you the process.

That's all I can think of without looking at my notes so if anyone has anything to add, feel free.

Also, if a lens is bad, send it back.

Edited by Tom Jensen, 01 December 2009 - 04:06 PM.

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