Johannes Vermeer The Geographer 1668. Oil on canvas. 53 x 46.6 cm (20.8 x 18.3 in. Städel Museum, Frankfurt
If you were the captain of a Dutch East India Company (VOC) ship sailing on stormy seas, accurate maps were essential to survival and arrival at your destination. Often, it was destination unknown. Much of the world was still uncharted. The Northwest passage to spice islands was a dream and a blank. Charts were often inaccurate. One false waypoint, and your ship might run aground or be shredded by hidden rocks and reefs.
Frederik De Wit, 1660 map. Nova Totius Terrarum Orbis Tabula. Amsterdam: Frederick de Wit in de Calverstraet in de Witte Paskaert. Library of Congress.
Geographers and map-makers in the Dutch Golden Age worked at an intersection of art, science, commerce, engraving and publishing. They were in great demand.
These vintage maps, like vintage lenses today, required careful updates as new routes were discovered and new waypoints found.
Another industry flourished in the Netherlands of the 17th century— optics, grinding optical elements and manufacturing lenses.
Although vintage cine lenses are decidedly not four centuries old, the two disciplines of lenses and mapping converge today, as Howard Preston explains Preston Cinema Systems’ new All-Points Lens Mapping.
Read on.
Preston HU4 Focus-Iris-Zoom Hand Unit
Preston All-Points Lens Mapping
by Howard Preston, President of Preston Cinema Systems
1. Introducing All-Points Lens Mapping
Accurately setting the focus distance of a lens should be simple. The reality is more complicated.
Unless the desired focus distance is one engraved on the focus barrel, the distance setting must be estimated. This estimation, called interpolation, can be accomplished if the focus calibrations fall on a simple smooth curve when plotted against focus ring rotation.
This focus curve is key to accurately setting focus. Preston HU3 and HU4 hand units calculate the focus curve for each lens to enable interpolation between the calibrations on the focus ring.
If the lens focus calibrations follow a smooth curve the standard 10-point focus calibrations allow for high accuracy focus mapping. If that’s not the case, new All-Points Focus Calibration, only available with the HU4, should be used. Two examples illustrate each case.
2. Contemporary Cinema Lens
The graph below shows the red focus curve representing the focus calibrations of a contemporary cinema lens. The distance calibrations are shown as blue dots on the graph.
For this example, the blue dots lie on the red dashed line with an accuracy of better than 0.1%. The focus curve gives the correct rotation angle not only for the calibration distances, but for all of the distances in-between.
When the focus curve fits the lens calibrations with high accuracy, lens mapping with the standard 10-points calibration will provide matching accuracy—so when a distance is set on the hand unit focus knob, it will exactly match that on the lens focus ring.
The majority of modern, high end cine lenses have focus accuracy similar to this example and allow highly accurate focus mapping with the standard 10-points calibration.
Contemporary Cinema Lens
Focus Ring Rotation (Encoder counts) vs Inverse Distance
Vintage Lens – Focus Ring Rotation (Encoder Counts) vs Inverse Distance
3. Vintage Cinema Lens
In contrast to the contemporary cinema lens, the graph above shows data from a vintage cinema lens.
The Focus Calibration points in the graph bounce below and above the red focus curve; many dots don’t even contact the focus curve. Those dots that don’t touch the focus curve represent an error of at least 1.5%, which is about 7 times worse than the maximum acceptable error of 0.2%.
It’s not necessarily that the vintage lens is optically deficient. Wear and tear may have degraded focus cams and cam followers or helical threads. Shipping and dropping may have loosened an element.
Howard Preston with a vintage Debrie mechanical remote follow focus hand unit in La Cinémathèque Française equipment collection.
4. HU4 firmware update HU4-2.000
To accommodate lenses such as the vintage lens described on the previous pages, the Preston HU4 firmware update HU4- 2.000 adds All-Point Focus Calibration.
With All-Points Calibration, all marked focus distances on the lens ring are used for calibration so that all focus distances onbthe lens are exactly matched on the HU4 display and focus ring.
Even with All-Points Calibration, distances between the focus calibrations must still be interpolated and focus accuracy will only be as good as the combined accuracy of lens calibration and interpolation.
Remember, the focus chart is your friend.
5. Focus Calibration Page on Preston HU4
5a. The Focus Calibration page on the HU4 running on the new firmware now gives the user the choice of 10-points Standard Calibration (above, left) or All-Points.
5b. When All-Points is chosen (above, center), the scroll wheel is used to enter the number of distances engraved on the focus ring. Do not include infinity in the count.
5c. Lenses calibrated with All-Points are identified with an underlined F (above, right).
6. New Preston Firmware Updates
The new firmware updates supporting All-Points Focus Calibration are:
- MDR3 1.148
- MDR4 1.065
- DXL 1.029
- HU4 2.000
If the HU4 is using a lens record created with All-Points Focus Calibration with an MDR running a previous firmware version, the lens can be controlled normally, but LR2 and video overlay functionality will not be supported.
