There are different methods to test a lens. One of the simplest and most effective is to reverse the “normal” process: the camera lens becomes a projector lens. Lens test projectors have been around for quite some time. But here’s something new.
The Portable Lens Checker (PLC) from DENZ is a lightweight, small and versatile test projector for all lenses with a PL-mount—for sensor sizes up to Full Format (24x36mm). It can be used in a rental house lens, service facility, or on location to test lenses on any movie set.
The projector consists of the PL lens mount, a built-in test slide/test target, a focus mechanism and a lighting unit. The cylindrical projector housing can be rotated in its holder and can be mounted on a stand, tripod or table. Corresponding support systems with 15 or 19mm tubes for heavy lenses are available from DENZ.
How the PLC works
The distance between the test target and the flange surface of the PL mount is exactly 52.00 mm. This is the PL lens mount standard. On the PLC, this flange focal depth (FFD), also called back-focus, can be adjusted ± 0.7 millimeters to an accuracy of 1/100 of a millimeter (10 microns). There are cutouts on both sides of the projector housing, and a knurled silver and blue adjustment wheel is rotated inside. A scale and index line at the top of the projector is calibrated in hundredths of a millimeter, and each index mark shows 10/100 (1/10th) of a millimeter.
The test target is illuminated with an LED light source. LED light is often referred to as a “cool light source,” because little heat is radiated forward by the light. Nevertheless, an LED element can get very hot, and if the cooling is not done properly, the life and performance of the light source is affected. The PLC projector has a built-in fan that runs continuously.
Through an ingenious labyrinth system, the air is sucked sideways behind the projector bracket through many small ducts and directed into the interior of the projector and blown out by the fan at the rear. No dust can come in contact with the test slide by the cooling system. The lens mount and the focus mechanism hardly get warm even with hours of use. This is particularly important with regard to thermal accuracy in a test device.
How to use the PLC
The PLC can be mounted on a tripod and leveled with its bubble level at the top, perpendicular to the projection screen or wall. Plug in the power cord, and turn the projector on. The on / off switch is located on the rear left side of the housing. The light source can be adjusted to three brightness settings by simply pressing the button. The PLC switches off by pressing and holding the button down.
On the left side of the PLC, there is a fold-out bracket at the zero position of the test target—the image plane. This is where the recommended Leica Disto D110 (E7100i in the US) distance measuring device can be used and the distance to the projection screen is measured. Peter Denz added the retractable bracket after NAB in response to suggestions from some customers, especially because the distance measuring device also has to work with lenses that have a large front diameter. The photos below are prototypes.
If the measured distance is the same as the lens focus scale, then the test target on the projection screen should be sharp. You can find the best sharpness by adjusting the projector back-focus ring. If the scale shows a deviation, then either the shimming of the lens is not correct or the focus scale of the lens is inaccurate. Improperly calibrated lens scales are more common than one would think.
Checking Zoom Lenses
The PLC can be used to check zoom lenses throughout their entire range.
- The scale on the PLC is set to zero.
- Zoom in to full telephoto. The test target will now appear quite small (see above).
- Set the best focus on the lens. The projector’s light intensity may be reduced by briefly pressing the on / off switch to so the contrast of the test target is not washed out.
- Next, zoom out to the wide angle position, which makes the projected test target appear large. The light intensity may be increased.
- Now turn the flange focal depth ring of the projector until the sharpness of the test target is optimal. The deviation from the zero position can be found on the scale.
- This procedure is repeated: go back to the telephoto setting, check sharpness by rotating the lens focus ring, zoom out to the wide angle position, and check sharpness by turning the projector’s flange depth ring. The value now shown on the scale of the projector should be zero. If not, it indicates that there’s a deviation of the lens focus scale, shimming of the lens may be required, or the optical elements are out of adjustment (maybe the lens was dropped or shaken during shipment). If so, the lens should be checked on a collimator.
Full Frame Test Target
Think of the test target as a high-precision slide in a projector. The PLC test target was developed by DENZ especially for this projector—covering formats up to Full Frame 24x36mm. For this purpose, an oversized original was created, which was then transferred to a silver-coated glass plate using the same reduction technique as in semiconductor manufacturing. Resolution with up to 200 line pairs per millimeter has be achieved. It took a DENZ employee 14 days just to design the template. The test slide has a protective layer; nevertheless it is delicate and should be cared for the way you would treat a sensor. If no lens is used, the protective cap should always be locked in the PL mount of the projector.
The test target has two circles that indicate whether the image circle of the lens covers Full Frame format or Super35. With prime lenses, interesting things can happen: some S35 lenses, especially those with longer focal lengths, cover much larger areas. Six formats, from one inch to Full Frame, are shown on the test target.
|Micro Four Thirds||17.3×13.0|
|Full Frame FF||36.0×24.0|
Several Siemens Stars help in the assessment of the resolution over the entire image circle. Each sensor format you are testing has Siemens Stars in the corners. The Siemens Star consists of six circular rings with different line frequencies of 15/25/35/50/70/100/140 LP / mm. For 2x squeeze anamorphic lenses, 8 squeezed stars are included. They should appear round when projected. Furthermore, 16 frequency raster groups with 8 double squares (meridional and sagittal) are distributed over the test pattern in grades 12/18/25/35/50/70/100/140/200 LP / mm.
What else the PLC can show
With the grid raster groups one can examine the resolution. Although this assessment always depends on subjective viewing, one can see how good the optical performance is in the image center and in the peripheral areas. A sharpness profile can be determined for image field curvature. You can see distortion, aberration, ghosting, flare, and breathing. You can see if a zoom tracks straight or if the image moves off axis, and you can see if the axis remains steady while focusing.
You can check the scales of the lens for their correct index marks and assess the image field shading (darkening toward the edges).
How is the projector calibrated?
Each PLC must be calibrated at least once, which is usually done before delivery by the manufacturer. With the PLC, the default flange depth of 52.00mm is exactly set to the 0 mark of the scale, which does not change as long as the device is treated with care. If you send your projector out with the cameras on a production and are not sure how much bouncing around it endures day to day, DENZ supplies a depth gauge, a PL insert and a basic collar for zeroing the dial. The projector is best positioned vertically, the PL insert is locked in the mount and the gauge is pushed carefully into the device through the insert. The sapphire tip of the dial gauge is softer than the glass of the test target and should not scratch it. The flange depth should be exactly 52mm + 10mm of the PL insert.
The lens mount on the PLC is not interchangeable (yet). DENZ will also supply a PLC with an 18mm FFD Full Frame E-mount soon, using the improved version similar to the one Sony has on their FS7 II. From there, almost all other lenses can also be attached and tested using commercially available adapters (e.g. PL to E, EF to E, Leica M to E, XPL to E, PV to E, Leica S to E, etc.)
Translated, edited and added to original text from Denz.