One would easily think that focusing with a telephoto lens manually would be a peace of cake; but the reality is that there a ton of complicated physics happening. In fact, it’s both exhausting and requires the photographer to be incredibly deliberate. Perplexed at how this is so difficult? Well let’s break it down for you.
The Problem with Manual Focus is You
When you manually focus a lens, what you’re doing is using your fingers and hand to turn a ring on the lens–therefore moving the lens. I don’t care how good of a grip you have on your camera, you’re not keeping it stable when manually focusing. With wider angle lenses this is less of a problem due to how depth of field works. A 24mm f1.4 lens focused five feet away set to f2.8 will have more of the scene in focus than a 135mm f1.8 lens set to f2.8 and focused to five feet away. With the longer lens, just a sliver will be in focus. When a sliver of the scene is in focus and you’re moving the lens about, the focus plane gets thrown off–which can result in a person’s eye not being in focus when you’re trying to get it for a portrait.
The Solution isn’t Image Stabilization
The solution is to stabilize the camera. You can’t stabilize the lens enough; and to that end you need a tripod. It needs to be set down so that it doesn’t move. With the stability that a tripod gives, you can focus the lens and more or less just move the ring which will in turn change the focus. Not only does a tripod help with that but it also gives a photographer a better compositional tool to work with. When you have to move and adjust carefully, you really change the way that you compose and end up spending more time on ensuring that you can get the image than shooting away mindlessly.