This is a common question we’re asked at TLS, and it’s quite a valid one. We’re aware of the implications of having the two gears close together. It can make it challenging when using the lenses on set with various motors and other accessories needed to be in such close proximity to the lens. However, there is good reason for this.

Take a look at this image here, it shows how close the iris and focus gears are on a lens.

This design feature is born out of necessity. Let’s take the Cooke Speed Panchro (CSP) rehousing as an example. They are a well-known lens within the cine world and the first full rehousing of a lens that TLS offered. The first item that needs to be considered is the position that the original optical unit sits, in relation to the film plane, when at infinity. The wider angled CSP’s sit relatively close to the film plane when at infinity. It is even possible to see on some of these lenses where the rear end has been angled to allow for the passing of the mirror on film cameras. This is the first hurdle to overcome when designing the rehousing, being able to fit all the new mechanics in when the optical unit needs to be within the same space as the PL mount.

Field of view

If we look at the example of the 18mm CSP, this is the lens with the largest field of view in the range. During the rehousing design, the aim is to be able to keep the optical unit internally focussed. Therefore, the outer casing of the rehousing should not ‘telescope’ as the focus is racked. With the optical unit at the infinity position, one can draw a line from the edge of the front element at the angle of the field of view. From this, it is possible to determine the minimum front diameter of the housing to allow for internal focussing and without the need for cut outs. While doing this process, the designer must also consider the travel needed for the optical unit to reach MOD too. Once these considerations have been accounted for, the overall length and front diameter of the lens can be achieved. In the case of the 18mm CSP, it means a short overall length (68mm from the PL flange) with a front diameter of 110mm. With such a limited amount of space to fit the internal mechanics and the outer casing needed room for both dual focus scales and iris scales, it limits the room available between the focus and iris gears.

Useability on set

Once the ‘worst case scenario’ lens of the set has been designed, the remaining set will keep the same spacing between the focus and iris gears. This is to make the lenses more user friendly when being used on a production. It allows for quicker changeovers of focal lengths, without having to readjust the motors and other accessories being used.

Strength and reliability

The way in which TLS rehouse their lenses uses a ‘main bearing’ to control the focus rotational movement. During the design process, it was calculated that the most appropriate position for this bearing was at the rear of the focus barrel. Therefore, the optimal position for the focus gear to be is as close as possible to the main bearing. This means there is less leverage for any error to occur. It also provides a more consistent and smoother feel to the focus movement when the lens is being operated by hand.

PL ears on camera

The reason for the position of the iris gear is mainly due to the physical restriction of the PL ears that are found on cameras. The PL ears are needed for the operator to get purchase on the lock-ring to be able to secure the lens onto the camera correctly. These PL ears protrude forward of the camera, therefore creating a physical restriction. Due to this restriction, the iris gear needs to be placed in front of the PL ears. Now we have a limiting factor pushing the focus gear more towards the rear of the lens as well as pushing the iris gear forward to a certain position. Therefore, in certain situations the iris and focus gears can be quite close together.

Not all rehousing's are the same

All the above does not mean that all lenses have these physical constraints though. If we look at the example of the Mamiya 645 lenses that TLS rehouse, these have a significantly larger space between the iris and focus gears. This is possible due to the optical design originating from medium format still photography. Therefore, the optical units sit much further away from the film plane than vintage cine prime lenses. This coupled with a narrower field of view from these lenses means that the gap between the two gears can be increased.

This image displays a TLS rehoused Mamiya 39mm, which displays the significantly larger space between the gears to that of other lenses.

At TLS, we always listen to our customers recommendations and suggestions. We work closely with industry professionals, including the end users of our products. If you have any thoughts on how we can improve any further or would like to discuss this topic further, please get in touch with us at sales@truelens.co.uk or calling our office on +44 (0)1455 848411.