One of the cheapest and most versatile lens in any manufacturer’s repertoire is the 50mm lens. Look around, almost every lens buying guide will recommend you to get a ‘nifty fifty’.
The f/1.4 and the f/1.2L have similar sized focusing rings, while the f/1.8 has a thin flimsy ring also known popularly as ‘a joke’.
The major draw for all these lenses is their aperture. Assuming an APS-C sensor focusing at 10 feet;
- At f/1.8, the depth of field (DOF) is about 0.8 feet (10 inches).
- At f/1.4 the DOF range is 0.65 feet (8 inches)
- At f/1.2 the DOF range is 0.54 (6.5 inches).
If you’re shooting video at these f-stops, and your subject leans, turns or moves, you’ll have a hell of a time using the focus rings on these lenses to follow them around. On a large display or cinema screen, any focusing mistake can’t be hidden.
So, what’s the point of a cool 50mm f/1.8 lens if you can’t shoot at f/1.8?
One way to get around this problem is by using auto focus (AF). However, filmmaking is not always about documentation. You might want to rack focus (move focus from one point to another) at any time for various reasons.
In the professional video world, you need a system that is consistent enough to deliver at every turn. The bigger the production, the greater the cost of each take. How many takes can you screw up before you are kicked out?
Traditional high-end lenses made for film cameras have good focusing rings. Older manual still camera lenses also have good focusing rings (there was a time when auto focus didn’t exist and people still got the shot). Newer lenses, especially the smaller semi-plastic variety, have the worst focusing rings seen in history.
If you are okay using autofocus, fine. If you are zen master who can pull focus day in and day out on a 50mm f/1.8 plastic lens at f/1.8, respect.
For everyone else, there’s the follow focus system:
Simple two-finger focusing (your hands only) gives two points of contact on the lens.
A follow focus system has the same two points of contact (there still is a ring to turn), plus at least two more – the contact of the mechanism on the lens, and the gear that allows this system to work.
This doubles the chances of instability. The only way this can be overcome is by using a precise mechanism that is also rugged enough to work smoothly, without error, for years.
Want a cheap follow focus system? You’re better off without one.
Rule of thumb: Never, ever settle for a cheap follow focus system that does not guarantee perfection over years of daily use.
A good focusing system improves the throw on the ring. E.g., if it takes a 3º turn to rack focus over 1 feet, a follow focus system can get you over 1 feet with, say, a 30º turn. This allows for precise movements. The finer this precision the better. The best systems also provide choices of ‘gears’ so you can control the amount of ‘throw’.