If you’ve spent any amount of time working with lenses, you’ll have noticed the focus scale and the differences from lens to lens. On one end of the spectrum is the focus scale you find on a stills lens. These focus scales are generally very vague, just giving a few select distances with a large range where infinity may be. At the high end, you’ll find focus scales that have 26-30 individually marked distances, each with a precise line to indicate when that focus point is reached.

When dealing with cine lenses, you generally expect a scale with lines to the individual numbers to indicate when that distance is in focus. When this line is matched to the datum line, this indicates the focus point. There are several ways to achieve this, with pros and cons for each method.

Batch Produced Scales

Batch produced scales are ones that are manufactured with the focus marked already engraved onto them. Generally, these are seen on cine prime lenses such as the Zeiss CP.2/3’s, Ultra Primes, Master Primes, etc. Some rehousing companies also use this method. This is possible to achieve when the glass elements are being manufactured to a tightly controlled tolerance. Furthermore, the mechanical manufacturing and assembly of the lens need to be well controlled for this to work. If any of these components were outside of the tolerance, it would slightly change the parameters of the lens (such as focal length i.e. rather than being a 50mm, it might actually be a 49.9mm or 50.1mm). With a change such as this, it would alter the focus scale and render the batch produced scale useless.

Some manufacturers, such as Zeiss, use a clever system to overcome this. They batch produce a variety of focus scales for the same focal length and label them. You will notice on the scale you’ll find a letter code (such as AA or AB). This means that once the lens is manufactured and assembled, it can be measured for tolerance and the corresponding scale assembled onto the lens. This gives greater accuracy of the scale. An additional bonus of this system is that when ordering a replacement scale, you can quote the reference letters and receive an exact replica (or equivalent scale imperial or metric).

Batch produced scales will increase production of the lenses. Once the first scale(s) have been programmed into the machine, it will be able to keep reproducing the scale as many times as needed. Furthermore, there is less chance of any errors as there is less human input as they don’t have to line up each individual mark. However, there is more room for error in matching a batch produced scale to a manufactured lens. This could lead to a scale that is accurate for part of the scale (during close focus marks) but goes slightly out during the rest of the movement.

Individually marked scales

Individually marked scales are found on high end cine lenses and premium rehoused lenses. This method is generally used by Cooke. This means that once the lens is fully assembled and optically aligned, a technician will focus the lens at each point on the scale and mark the position. The position can either be marked by using an encoder and calibrating the points along the way, or by physically marking each point as it is done. This is far more time consuming than batch produced scales. However, it does mean greater accuracy as each mark has been tuned in by a skilled technician who can verify the scale is correct.

Individually marked focus scales are not only more time consuming to mark-up, but generally they are more time consuming to engrave as well. There is more setup time to start with. Furthermore, if there is any mistake at this point, either human or machine, this means the marking-up process needs to begin again. Using an encoder to set the focus points is a more efficient way of doing this process, however this comes at a large initial cost to set up the process. Putting the marks on by hand doesn’t take any longer to physically mark up the lens, but it does take slightly longer to engrave. This is because every point needs to be aligned by the engraving technician before the machine can make its mark.

One of the main benefits of individually marking a focus scale is when a non-linear cam is being used to drive the focus movement (you can read more about this process here - https://www.truelens.co.uk/what-is-a-cam-based-focus-system). This is because as the lens get closer to the infinity position, the fore/aft speed of the lens reduces dramatically. As an example, on a Canon K-35 85mm lens rehoused with a non-linear cam in the first 10 degrees of focus movement at the infinity end the optical unit moves 0.074mm (0.0029”). However, in the last 10 degrees of focus movement at the MOD end, the optical unit moves 0.876mm (0.0345”). What this means is that the focus marks are very hard to batch produce as the slightest of changed can equate to a large difference on the scale. 0.0127mm (0.0005”) difference would make 2 degrees of error on the scale. While this might not sound much, it equates to a large visual error as well as practical issues when using the lens on set.

There is also another advantage for individually marking lenses that are being rehoused too. Most vintage lenses and stills lenses were not manufactured to the exacting tolerances what machines can achieve today. Therefore, batch producing a scale for these is never going to produce matching scales to the optics. Couple this with a non-linear cam focus system and it would be very difficult to produce accurate marks on every lens produced.

Hopefully, this article has given an insight into the differences between the manufacturing process of focus scales and where/when each might be used. There is no best ‘one solution fits all’, but certain scenarios call for a specific approach.

If you would like more information on anything discussed above, please get in touch with us at sales@truelens.co.uk or calling our office on +44 (0)1455 848411.