Over the last week I have been doing a lot of flower pictures with my macro lens and extension tubes. With the combination of a macro lens and extension tubes I was regularly at magnifications greater then 1:1. When shooting at higher magnifications autofocus becomes less and less effective. Given all this, I thought it would be a good time to talk about manual focusing.
The ineffectiveness of autofocus at higher magnifications occurs for a couple of reasons. The millimeters depth of field doesn’t give the autofocus much to work with. When the lens does not catch the subject while trying to focus it will track through the entire focal range as it searches. The autofocus system in cameras works on contrast to achieve focus. When there is minimal contrast the camera will struggle.
Depending on the subject, the contrast needed for focus can be in short supply. Parts of your subject can easily blend together. Sometimes you will only be able to get a portion of the part you are working with in focus. A lack of color variation will also affect contrast for focusing.
Manual focusing is a skill; it requires a sweet touch to get it right. Like any skill it will take time and practice to develop. Many people have not manually focused a camera, ever. It can be tough to know if you have razor sharp focus. Can you tell the difference between soft focus and sharp focus when looking through the camera?
The same factors affecting the autofocus system will also challenge the manual focuser. When colors run together you will have to train your eye to know when you have what you want in focus. When you have a DoF of Millimeters you will need to have a soft touch with the focusing ring to get that perfect spot in focus. Learn how to move the focusing ring ever so slightly to get the perfect shot. The tiniest movements will be needed. I use my thumb and pointer finger on the bottom of the ring to achieve these movements. Do not hold the ring tightly. Rather let your fingers almost glide over the surface.
Manual focusing is a skill that can be improved with practice. Try the next time you are shooting. Start with anything you are photographing to get a feel of the focusing ring. Many lenses have full time override. With these lenses you can manually focus even when the camera is in the autofocus mode. See if your lens is one of them. If not put your settings to manual focus. The more time you spend manually focusing the more comfortable you will be with it.