When considering rehousing a lens, a phrase you will have come across is ‘floating elements’. But what does it mean? It sounds like something rather complicated. Perhaps the first thing to come to mind is ‘well how are elements floating within the mechanical system?’. In this article, we will explain what is meant by this term, as well as what it means for the performance of the lens.

To start off with, lets look at a more basic design for an optical lens. This is essentially a tube with multiple elements stacked inside. To focus the lens from infinity to MOD (minimum object distance) the entire tube of elements is moved in a linear motion forward and backwards in relation to the film plane. Generally, the tube of elements will move closer to the sensor/film plane to reach infinity. Therefore, by moving the elements further away from the sensor/film plane, it will focus of objects closer to the lens. A good example of this is the original range of Cooke Speed Pancho lenses from the 1930s-1960s. All these lenses are a ‘single lens head’ design as described above.

As optical designs advanced and end users expected better performance from their lenses, floating element designs started to enter the market. A floating element design is where there are one or more groups of elements that are moving at a different rate to another. The simplest version of a floating element design is where the rear element is in a fixed position and the remaining elements move together to focus the lens. This design is seen in the 55mm and 85mm Canon K-35 lenses. However, more complicated floating element designs can have two or three groups of elements that are all moving at different rates (and sometimes in different directions) to each other. An example of this can be seen in the 18mm T1.5 Canon K-35. Here the front element is fixed in relation to the sensor/film plane, however the rear and middle group move in opposite directions when the focus is racked.

The advantage of using a floating element design is to improve the optical performance of the system. Therefore, if a lens performs best when at infinity, it is possible to improve the sharpness of the corners and colour aberrations at close focus by using a floating element design.

Using a floating element design may improve the optical performance however, it does introduce a more complicated mechanical design. The mechanical designer now needs to develop a system that will move two groups of elements at different rates. Not only this, but the alignment between the two groups is critical to keep the optical performance of the system within specification throughout the focus movement. The physical constraints of these systems can also limit the MOD achievable of the optical system. This can be seen with the 24mm and 35mm Canon K-35 lenses. As these lenses are focussed from infinity to MOD, the rear cell moves at a quicker rate than the front cell, but in the same direction (away from the sensor/film plane). Therefore, there would come a point when the two optical groups would collide with each other. The point just before this happens, sets the limit for the MOD (12”/30cm in this case).

The animation above is showing the movement of the TLS 35mm Vega lens. Here we can see that the front cell remains in the same location in relation to the film plane and the mid and rear group moves towards the front group, at different rates, for MOD. The mid group moves 6.7mm and the rear group moves 7.8mm.

When rehousing existing lenses, floating element designs prove more difficult to perfect. Especially when using a non-linear cam system. It is imperative that the original cell positions are replicated with the new mechanics. Therefore, at any point along the non-linear cam, the two cells must be in the same position in relation to the original movement. Not only this, but the cells must be perfectly aligned throughout their movement. When rehousing, the mechanical design should include adjustments to allow for these issues to be properly corrected and aligned. In doing so, the original optical performance of the lens is kept intact, or in some cases improved during the rehousing process. Furthermore, the mechanical design should include a way to make sure that the glass cells never collide with each other. It should always be that the metal components would be the first contact, protecting the glass if the lens were to take a fall.

Now you have an overview to what a floating element design is, and how this can affect the rehousing process, you’ll understand why rehousing companies charge a premium compared to a simple optical design.

If you have any further questions, please do not hesitate to contact our customer service team via sales@truelens.co.uk  or calling our office on +44 (0)1455 848411.