How to choose your focus or zoom ring

On interchangeable lens system, one may want to manually control both zoom and focus underwater. Unless using an expensive port with a knob, there may often be a difficult choice to do between controlling zoom or focus from the camera housing. The market and available technologies do not ease this choice. 

What is "zoom" ? What is "focus"  ? 

Both zoom and focus control the way the image is projected upon the surface of the image sensor.

• Focus adjusts the location of the focal plan, i.e., the location relative to the sensor where the subject of the picture will be sharp (accordingly to the light path followed through all the optical elements of the lens). Adjusting focus allows for example to have a sharp subject in the background while the foreground is soft, the opposite being also possible.  
  
  
• The zoom adjusts the angle of view. This means that in a "wide" or "large" zoom, one might capture a scenery while in telephoto or narrow mode for the same scene, the camera will capture a detail of the scenary (e.g. an animal) which will be magnified for the viewer. 

What is a ring and what is its purpose ? 

A ring is "simply" a toothed gear fitted on the lens and coupled to some gears in the housing or port. Those gears ultimately are coupled to an external knob which allows the diver to operate the gears and control the lens. Those rings are complex to develop due to the diversity of housings, cameras. 

 
An EOCEAN focus ring for Ikelite housing and Lumix DMC-LX100 camera.

Mechanical or electronic control ?   

Depending on the camera or lens, zoom and focus can be both controlled electronically (using small motors) or be totally mechanical. It is in appearance only some technical informations but this is actually important in regards on how the lens can be adjusted underwater.  

In fact, a motorless lens will always require one focus ring and one zoom ring in order to be fully usable underwater. Some lenses of that type are excellent and some of them are even inexpensive... on land. Underwater, it may cost more as it is necessary to have both rings and a dedicated knob on the port. 

Meike 35mm F/1.7 MFT: an inexpensive lens without electronic parts. 

The consumer market is full of lenses with electronic focus, the most known incarnation is the implementation of autofocus. In video, autofocus can be an issue is bad visual conditions (e.g. low light) therefore manual focus is sometimes a better solution. 

With electronic systems, the focus ring is not a truly mechanical ring but a rotary sensor sending infos to the camera body to adjust focus on the lens. This is called "focus by wire".  

Leica 12-60 f/2.8 : mechanical zoom with electronic focus.  

Regarding the zoom, there are different flavor of control. Some are motorless and in that case, a zoom ring is necessary. Others are motor driven and in others, this motor-driven feature can be disabled. Motor-driven zoom can be remotely controlled on the camera using custom function buttons (e.g. Fn) assigned to zoom. It depends on the body camera so a good look at the manual is certainly usefull. 

Olympus Zuiko 12-50 : Motor-driven zoom that can be disabled and an electronic focus. 

The best of both worlds 

It all depends on the specifications of the lens, the camera body and the housing but we may consider the following cases to benefit from the best compromise from your equipment. 

Housing and Port having both a knob (2 knobs in total) 

In that case it is possible to use any lens as long as a focus and a zoom ring exist for this lens.  

Housing and Port to share one knob.

This is the most common situation. The choice can be difficult to make. The manufacturer trends is to give priority to the zoom which allows the lens to be polyvalent underwater. Depending on the use and type of lens, the zoom lens is sometimes not the most useful one. 

- if your lens has no motor, it may be difficult to use it underwater. A zoom ring is not necessarily the best choice underwater has its use is limited underwater and you still need to focus. In that case, it is preferable to have a housing and a port having both a knob. 

- if your lens has no zoom (this is the case of lenses with set focal lens such as macro lens and certain wide angle lens) , a focus ring is naturally the only available option. 

- if zoom is mechanical but focus allows autofocus, a zoom ring is the only solution to access zoom. It is advisable to let the autofocus button on if it exists on the lens. In case of issues with autofocus, it's still possible to disable it on the camera body and to do a temporary focus using a custom button assigned to the AF-Lock function. 

- if your zoom is motor driven,  depending on how precisely you expect your zoom to behave, if manual focus matters, it may be useful to prefer the focus ring and set the electric zoom to custom buttons that will be used to zoom in or out. The knob on the housing will be used for manual focus.  

The EOCEAN ring for the Lumix Vario PZ14-42 lens allows to be set for zoom or focus thanks to the motor driven zoom and focus of this lens.

E-OCEAN offers a growing choice of focus and zoom rings for different lenses. Some rings are polyvalent in a sense they can be configured to act as zoom or focus ring or electric/mechanical zoom. E-OCEAN can also make specific rings on request.