I'm asking... why manufacturers, sometimes, don't include aperture control ring?
The basic answer is because an aperture ring on the lens is not needed to control the aperture of the lens with modern cameras.
Early old mechanical cameras with no electronics could not select and control the aperture, so the photographer had to do it. The simplest designs linked the aperture control on the lens directly to the blades in the aperture diaphragm. As cameras became more sophisticated and started including through-the-lens composing and focusing, built in light meters, and 'brains' that could compute exposure automatically, various mechanical linkages between the camera and lens were devised to allow the camera to read the aperture setting of the lens and (sometimes - depending on the camera's capabilities and the 'mode' selected) control the lens' aperture.
Most camera systems today use digital electronic communications between the camera and lens. There is a small motor in the lens that moves the blades in the aperture diaphragm and the motor responds to an electrical signal from the camera body that tells it when and how far to move. The aperture setting can be selected automatically by the camera or it can be selected by the user using controls on the camera body that communicate this setting to the lens at the appropriate time.
Even the few camera systems that still use a mechanical connection to move the aperture blades have electronic communication between the (current) lenses and (current) cameras that includes data about the lens' maximum aperture and the current position of the aperture ring. If the aperture ring on the lens is set to an 'automatic' setting (sometimes locking the aperture ring at the narrowest aperture is also the 'auto' setting) the camera can control the aperture via the mechanical linkage.
There are advantages to letting the camera tell the lens when to stop down and what aperture to stop down to over setting the aperture via a direct mechanical ring on the lens that controls the aperture position with no connection (either electronic or mechanical) to the camera:
- Metering and focusing can be done with the lens wide open. This is particularly an advantage with autofocus, where a wider aperture allows faster and more accurate performance of an AF system. Even with manual focusing, the viewfinder is brighter and the depth of field is narrower when a lens is wide open. This allows easier and precise manual focusing.
- The camera can select and automatically change the aperture based on selected parameters. In Shutter priority mode or Program mode the user selects the shutter time and the camera selects an aperture value that results in correct exposure as calculated by the camera's light meter. Even in Aperture priority mode or Manual exposure mode it is usually faster to move a control dial to change the Av than to move an aperture ring on a lens.
There are further advantages to the camera controlling the aperture electronically rather than mechanically:
- More accurate and consistent exposure. The mechanical connections between cameras and lenses that were common when lenses had aperture rings are subject to several issues that can result in inconsistent exposure from one frame to the next or even inaccurate exposure for every frame. Mechanical linkages wear as they are used and need periodic calibration (both on the camera side and the lens side) to insure they are moving the aperture blades to the correct position. The levers in some cameras are prone to bending if a lens is incorrectly attached, resulting in incorrect aperture position for all succeeding shots made with any lens.
- Faster performance. High frame rates in 'burst mode' can sometimes result in the camera going faster than a mechanical aperture linkage can keep up, resulting in exposure beginning before the aperture is fully stopped down, particularly when narrower apertures are selected. Colder ambient temperatures can also slow a mechanical connection to the point even a single frame may not be stopped down properly when the exposure is made. Electronically controlled apertures can be stopped down much faster. The camera can also be designed to confirm the position of the aperture before releasing the shutter. In addition to less lag due to mechanical linkages, the motor for each lens can be matched to the size of the diaphragm to which it is directly attached in each lens.
There are a few modern cameras, such as the Fuji X series, that put the control for the aperture on a ring on the lens. But in such cases the ring on the lens is often a fly-by-wire switch that communicates its position to the camera which then sends an electronic signal to the aperture diaphragm.