Using the lens focus ring to focus is the most basic and economical way to pull focus. It’s a good way to jump into the world of manual focus and also offers the simplest, smallest and lightest setup. In many situations, however, there will be a need for a better and more refined way of controlling focus. Turning the focus ring by hand can introduce vibration into the image, and when the camera is mounted on a tripod, it becomes an even greater challenge to pull focus. Additionally, turning the focus ring perpendicular to the lens is not as comfortable as turning it parallel to the lens—the latter provides better leverage and is generally more comfortable.
A follow focus system has a knob that, when turned, engages a gearbox that transfers the turning force to an external gear, which then engages the lens focus ring. Most also offer the ability to mark focus points onto a disc that can be used to hit specific focus points that need to be repeated during a take.
DSLR lenses have a much shorter throw than cinema lenses, and it's almost impossible to pull cinema lenses by hand because they need to rotate about two turns to get from end to end; a mechanical follow focus device resolves this.
If a plan calls for working with a 1st AC (focus puller), having a follow focus device is very important to allow the assistant to pull focus without getting in the way of the camera operator or cause vibrations in the shot by turning the lens directly.
Size and Weight
A follow focus needs to be durable enough to endure years of production use and be comfortable to control. On the other hand, it needs to be small enough to work with small still lenses (which don't have a lot of mounting space), as well as fit into the overall small package of an HDSLR. Some have been designed to work for large cinema lenses and are therefore less suitable for this application. An ideal balance will depend on personal preference and the kinds of shooting for which it will be used.
Generally, follow focus units mount either by sliding onto the rods, or by clamping onto the rods from the side. The benefit of clamping onto the rods is that other accessories, such as the matte box, don’t need to be removed in order to mount or dismount the follow focus. This can be a real time-saver on set, but is not crucial. Most follow focus units are designed only to work with either the 15mm or 19mm rod standards, but some offer adapters that can be added. HDSLRs will almost always be mounted to the 15mm lightweight standard.
Marking discs are removable plastic rings that mount onto the follow focus knob, or handwheel. They are used to mark (with a dry-erase marker) specific focus points for planned shots. These marks are used in conjunction with other marks set on the floor with tape, or in reference to other premeasured objects in a scene. The focus puller adjusts the follow focus as the subject moves and "hits" the predefined marks.
Another option is to put marks on the lens barrel itself with a small piece of tape wrapped around the barrel. This, however, will work only on lenses that have hard stops in the focus ring. Another good feature to look for is a marking disc that is viewable from different angles—not just head-on. The marking disc must also lock into only one position on the handwheel so that when the disc is removed and remounted, the marks will still align properly in relation to the lens.
Focus Indicator Arm
Some follow focus units have a small arm that helps define a starting position for the focus marks. It can be used to ensure the marking disc aligns properly, even if there isn't a locking mechanism on the marking disc.
Play is when the knob is turned slightly, but the gear doesn't react immediately. This can either be due to design and quality, or because of wear and tear. Most units have some play, but the higher-quality ones will have much less, to the point of being a non-issue. Some units even have a feature to adjust the play in the gears to the desired level.
Drag is the amount of friction that exists when turning the handwheel. Depending on preference and the kind of lens being used, more or less drag may be desired. With many still lenses, more drag is preferable because the focus rings are generally looser to begin with. The exception is manual still lenses, which have more inherent drag than modern autofocus lenses. A good follow focus will have the ability to adjust the amount of drag so that it can be customized for particular setups.
Depending on the design of the gear transmission inside the follow focus, there may be a small amount of backlash. Backlash can occur when the follow focus is turned in one direction and then suddenly stopped, or turned in the reverse direction, and the handwheel will snap back. This can throw the focus off when working in very shallow depth-of-field and focusing at close distances.
There are situations in which it will be troublesome to get the follow focus's drive gear to reach and engage the lens gear properly, either because of different lens diameters, different size add-on lens gears or because of camera base-to-lens height. For HDSLR applications, the more ways a follow focus can be adjusted, the better. Although all components (e.g. camera, lenses, lens gears) should be considered for the amount of adjustability needed, the more the follow focus can adjust, the easier it will be to integrate with new pieces of equipment in the future. The two basic methods of adjustment are height and side-to-side (i.e. perpendicular to the lens).
Another equally important quality is the ability to make quick and simple adjustments. The follow focus should be designed to easily mount to the rig and adjust simply.
Flippable Drive Gear
The ability to flip the drive gear from one side of the follow focus to the other is essential for working with short lenses, like a Canon 50mm 1.4 USM. With such lenses, the drive gear needs to be on the side closest to the camera, while longer lenses may require the opposite, or the follow focus would hit the matte box. Additionally, a good design will allow the drive gear to be flipped without tools and be quick to switch sides.
Most focus rings on still lenses rotate in the same direction as cinema lenses, making them compatible with standard follow focus units. Some lenses, such as the Nikon series, turn in the opposite direction. Nikon manual lenses are used by many HDSLR users (even Canon users) for their manual focus control, sharpness and color. Therefore, it’s important for a follow focus to be able to reverse the direction of the drive gear for experienced users who are used to turning in the "right" direction, and for those who have various lenses that turn in different directions.
There are different standards of gear pitches for different types of lenses. The 0.8 film pitch is the cinema standard (.5 and .6 are for broadcast lenses), but some follow focuses can be adapted to other standards and sizes. Within the same pitch standard, some companies offer different size gears that vary the amount of focus throw—smaller gears provide larger throws and larger gears decrease it. Depending on the lens and the lens gear attachment being used, having the option to change the size of the drive gear is important for getting more precision focus with a particular lens.
For lenses that don't have hard stops (i.e. most autofocus lenses), there is no practical way of pulling repeatable focus marks because the focus rings on these lenses continue to turn past their focus limits (which takes the marks on the marking disc out of alignment). Some follow focus units can apply a hard stop to both ends of the focus range in order to limit the focus range to that of the lens. Ideally, this control should be on the follow focus itself. If, however, the chosen follow focus does not have this option, there are lens gears that have the ability to apply hard stops to the lens' range that should be used.
The handwheel should provide a comfortable and secure grip. Larger handwheels also increase the focus throw of the lens, and vice-versa. Some models include a second handwheel that goes on the right side of the camera. This offers more options for the 1st AC to be positioned, but is not essential for most HDSLR productions.
Most follow focus units can accept additional focus control attachments, such as a whip, and crank into a standard port in the handwheel. The size of the port needs to be industry standard in order for it to be compatible with the range of attachments from other companies.
Still lenses were not designed to be controlled by a follow focus or motor. Therefore, they require an additional gear ring to be attached to the lens's focus ring. Typically, this will be a standard 0.8 film pitch lens gear so that it is compatible with a follow focus of the same pitch.
Because of the varying characteristics of lenses and follow focus units (i.e. size, focus throw, drive gears, etc.), it may be a good idea to own several different sizes of the same pitch to use with different lenses. Ideally, each lens you use should have a lens gear.
Focus whips attach to the focus handwheel of a follow focus unit to extend the reach of the focus puller. This allows the operator to stand at a distance or toward the back of the camera for more flexibility. A whip also helps isolate vibration caused by turning the focus handwheel. For fast-moving handheld situations where there is a dedicated focus puller (i.e. 1st AC), a whip allows for some slack between the operator and focus puller. Smaller whips can also be used by operators pulling their own focus for greater flexibility and comfort.
A focus crank attaches to the handwheel of a follow focus to increase leverage when pulling focus. Cranks are usually used to rack focus over larger focus distances without having to twist the hand in an awkward manner.
The crank's handle can also be used to "feel" where the focus is positioned, so there's less of a need to keep glancing at the marking ring or lens.
There are many situations in which the focus puller cannot access the lens directly. When the camera is on a stabilizer, car mount, dolly, jib, or even in some handheld situations, it would be impractical or impossible to pull focus using a manual follow focus system. In these situations, a remote follow focus is needed.
The basic idea is that an external drive motor is mounted onto the rod system, which has a gear that engages the lens gear. The motor is connected to a control box that is also the receiver, which is mounted on the camera.
The control box is connected to a battery or to a power tap on the camera (which HDSLRs do not have) and the motor draws power through the receiver. The last piece of the puzzle is a remote control that sends commands (wired or wireless) to the control box when the handwheel is turned.
On some units, specific focus points can be programmed (or "remembered") while the shot is being rehearsed and then activated during recording. Most high-end productions will rent expensive systems for the shoot, but more cost-effective solutions are available for purchase.