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Always Focus on the Lens

Behind the Lens speaks to Van Diemen about the risks of removing lens coatings and the benefits of choosing T stops or F stops.

Van Diemen designs and manufactures Motion Picture Lenses using its patented Wireform™ lens drive technology, offering variable-pitch close focus movements. Its well-equipped Service Centre and highly skilled engineers offer optical support for maintenance and repair as well as bespoke engineering.

Why don’t all prime lenses have focus and Iris gears in the same position for all focal lengths they would be so much easier to use on set.
This is not always possible as different focal length lenses move different distances to focus from Infinity to close up. A wide angle lens moves a few millimetres to achieve this a 10omm or longer focal length lens will move much further to go from Infinity to close focus. This fact makes it difficult to locate the focus rings in the same place relative to the film plane on all focal lengths whilst retaining a compact form and good close focus specification.

What limitations are there on choice of donor lenses for conversion to PL mount?
There are surprisingly few constraints. All 35mm SLR photographic lenses are potentially suitable for conversion. A small number of such lenses have physical glass dimensions in the area of the PL mount which make it impossible to mount the lens in PL, this would probably not be problematic with the new LPL mount. A second consideration is the position of the sensor and any architecture surrounding it. Some photographic lenses get very close to the film plane (sensor)and may foul filter or other structure, if using a film camera there is also the mirror shutter to consider. Inexpensive test pieces can be made to avoid the risk of damage to lens or camera.

Why can’t the back focus be adjusted by shimming on some anamorphic lenses?
The relationship between the prime lens and the anamorphic glass during focusing is critical both the anamorphic and the prime lens move at the same time but at different pitches in some anamorphic lenses, adding or removing shims without adjusting the internal settings will severely affect the lens performance. Any back focus adjustments have to be made in conjunction with a repositioning of the optics internally not a job for the feint hearted.

What are the risks of removing lens coatings?
The T stop may change for the worse as more light will be reflected away from the sensor. This may depend on how many elements have had their coatings removed.
It may happen that a powerful hot spot occurs which can render the image unusable.
Poorly executed coating removal can modify the lens curve and produce some unpleasant distortions. If it decided to go ahead with the removal process it should always be done using polishing tools of the correct curvature.

Are 3D printed parts acceptable in lens construction?
A.3D printers have developed very quickly into effective tools for manufacturing some lens components and have indeed provided some significant benefits over machined aluminium or acetal parts.
The use of a printed Iris drive cone which bears little load when in use has the side advantage of deadening sound unlike aluminium which acts like a bell. The printed component is also ideal for a one only component which can be made quickly and economically. This tool has generally reduced the time required to build a new lens conversion.
The printer is also used to produce test pieces, before manufacturing expensive metal components, this can and has avoided some expensive mistakes.

T stops or F stops?
In the days of film emulsion it was very important to know the T stop characteristic of your lens because the exposure of was critical to the quality of the end product in correct exposure being difficult to correct. Today this is less of a problem as there is an immediate visible view of the shot you are working on and it can be adjusted live. The choice of stop T or F we find is down to client preference. The purist would choose T stops because it is an accurate reflection of what light level you are getting however the amount of change in illumination for 1 so is the same in T or F. We now always scale the lenses to the client’s choice.

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