Difference Between Auto Focus and Fixed Focus

Auto focus and fixed focus are two very important mechanisms discussed under photography. These two terms are usually misinterpreted, and an appropriate clarification is required of these two topics. This article will try to explain what auto focus and fixed focus are, their similarities and finally their differences.

Auto Focus

To understand the concept of autofocus, the concept of focus must be first understood. A focused image is the sharpest. In the sense of optics, the light coming from the “focused” point makes the image on the sensor, while light coming from an unfocused point will make the image either behind or in front of the sensor. DSLR cameras at the early age were manual focus ones. The focusing of a part of an image or the whole image was done manually by rotating the focusing ring on the lens tube. As the digital cameras began to emerge, the autofocusing systems were also developed. An autofocusing system is a system where the lenses are moved in order to sharpen a desired point or an area of the photograph. Auto focus is a very important feature in modern DSLR, point and shoot and even mobile phone cameras. A particularly significant effect of the focusing is the depth of field. It means how much of the photograph is focused in front of and behind the focused object. It must be also noted that every object on the same plane with the focused point from the camera will be also focused.

Fixed Focus

Fixed focus system is a lens system where the distances between lenses are constant. In other words, a fixed focus system has a fixed lens set. As mentioned earlier, the depth of field is a very significant effect of focusing. Imagine a point and shoot camera with a fixed focus. If the depth of field is very little, (i.e. the area behind and in front of the focused point is blurred), the camera would be useful only for a fixed length from the object. And both the background and the foreground cannot be focused simultaneously. The depth of field depends on several things. One is the aperture of the lens. If the aperture is large, the depth of the field will be small. Same goes to the zoom setting. But if the focus point is distant, the D.O.F. will be higher. Therefore, the fixed focus cameras are made focused to the infinity with small apertures and small zoom settings. This will allow the camera to focus almost all the objects in the field.

The phase “auto focus” is sometimes used in the context of “fixed focus”, since all the objects “automatically get focused” in a fixed focused camera. However, this is a misconception, and there is no automation or mechanical process involved in focusing the system.