Sunday, 24 October 2010

Focal length and focus distance

You'll notice that the focal length of most lenses is specified at infinity focus. It turns out that some lens designs imply a change of focal length, as the focus distance changes.

Here's a comparison of two quite different lenses. The Nikkor 200mm f/4 AIS is a traditional fixed focal tele lens. Compact, light, and reasonably fast, it is a classic lens. It is also rather simple from a mechanical point of view. If features "only" five lens elements.

The other lens is the Panasonic Lumix G 45-200mm f/4-5.6. Since it is a zoom lens, and it also has special lens groups for the OIS, it features a whopping 16 lens elements.

They are both shown here, the Nikkor 200mm lens with the Nikon-M43 adapter mounted. The 45-200mm lens is shown zoomed to maximum tele, to be comparable with the Nikon lens:


They are also quite different when it comes to focusing. The Nikkor has a traditional focus mechanism, which simply moves the entire lens assembly forward when going from infinity to close distance focus. In the picture below, it is shown focusing at infinity (left) and just below 2 meters (right).


The Panasonic Lumix G 45-200mm f/4-5.6, on the other hand, features internal focusing. This is very practical, since it means that the length of the lens stays the same regardless of the focus.

Also, the internal lenses moving around are much smaller, and faster to move about. This makes the focus faster, and requires less juice from the batteries. And the 45-200mm does focus very fast indeed!

The negative side of internal focusing, is that it can affect the focal length when changing focus. Let's compare the two lenses when focused far and near. The upper and lower images are taken at exactly the same place, but with the focus placed at the parked car (upper), and the foreground flower (lower).


What we see here, is that when focused far away, the field of view of the lenses are mostly the same. The Lumix has a slightly wider field of view, but there is hardly any significant difference. When focused at 2 meter distance, however, the field of view is significantly wider with the Lumix lens.

Mostly, this is not any problem at all. When recording video, however, it can be annoying if the field of view changes significantly during focus. This feature is usually referred to as "focus breathing", as the objects recorded will pulse in size as the focus moves back and forth.

With contrast detection autofocus (CDAF), this can in fact be a big problem during video recording, as the camera must jog the lens focus back and forth to verify that the focus is correct. You can see these focus movements in a video recorded using the Lumix 20mm f/1.7 pancake lens. The Lumix 20mm f/1.7 lens features a traditional moving lens assembly, and does not suffer from focus breathing. This is good, since the continuous autofocus operation is not very visible in the video, but on the other hand, the lens is probably not as solid and weather resistant with this construction.

I first noticed this change of focal length when examining the bokeh of the 45-200mm at 200mm, compared with the Nikkor 200mm lens.

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