Controlling Focus with the Pentacon Six
One of the great advantages of using a Medium Format camera is the control of focus that it gives to the user. With a Medium Format camera like the Pentacon Six you can control the sharp and out-of-focus areas of the image for pictorial purposes. At the two extremes are:
Depth of Field
When the image in a camera is focussed, a point or plane at a certain distance from the camera is selected to be the sharpest point. There will be an area in front of and behind this point which will be in acceptably sharp focus. This area is called the “depth of field”. (Often abbreviated to “DOF” in lens tables.) Approximately one third of the depth of field is in front of the point of focus, and two-thirds is behind it.
Obviously, the image doesn’t suddenly change from sharp to unsharp; there is a gradual transition, with the image becoming increasing unsharp the further the object is from the plane of sharp focus. Using a mathematical formula, lens designers decide how far away from the plane of sharpest focus can be still considered to be acceptably sharp, and lenses of professional quality have usually indicated this with “depth of field” indications on the barrel of the lens. The great thing about this is that the photographer can then choose what (s)he wants to have in focus or out of focus in the image!
Depth of field is determined by:
Depth of field is shallower with wide apertures (for instance, f/2.8) and deeper with small apertures (for instance, f/16).
The depth of field is greater (deeper) the further the subject is from the camera. With subjects that are very close to the camera, the depth of field is small (shallow). In macro photography, the depth of field may only be a few millimeters.
Sometimes it is impossible to obtain the desired depth
field with a given lens, because using an extremely small lens aperture
may result in the need for impractically-long shutter speeds. In
such cases, it is possible to increase depth of field by using a tilt
When using differential focussing, the photographer chooses to throw certain parts of the image deliberately out of focus, to concentrate the attention of the viewer on a selected part of the image. This is commonly done with portraits, where the main focus should be on the eyes, with a gradual decrease in sharpness in front of and behind the plane of sharpest focus.
To achieve maximum differential focus, it is necessary to use:
In order to use these wide apertures, you may need to use a fast shutter speed to avoid over exposure. The exact speed chosen will depend on the “speed” – i.e., the sensitivity – of the film and the prevailing lighting conditions. To be able to use very wide apertures it can be useful to have a camera with a top speed of 1/1000 sec, such as the Pentacon Six, unlike those with a top speed of only 1/500 sec, such as most Hasselblads (the 500 C/M, for instance), the Norita 66, etc.
The word used to describe the out-of-focus areas of the image is “bokeh”, which is apparently a word of Japanese origin. The aim in most cases is to obtain out-of-focus areas that do not draw attention from the main subject of the image, and this is generally achieved by lenses which have a large number of aperture blades, as these produce a round aperture, as opposed to those lenses with 5 or 6 blades, which produce a pentagon or hexagon shape when stopped down.
In the opinion of many people, the worst bokeh is produced by mirror lenses, which tend to render out-of-focus highlights as donut shapes. However, this is a question of style and fashion, and a few years ago such shapes were all the rage. The fashion will undoubtedly return. In any case, you do not have to be governed by fashion!
This maximimises the depth of field of the image, rendering most or even all of the image in sharp focus, both those components that are close to the camera and those that are far away. This is frequently desirable with landscapes, where the image at the virtual “infinity” distance is sharp, and so is perhaps a foreground of branches or leaves that may be framing the image.
To achieve maximum hyperfocal focus, it is necessary to use:
The following two pictures of the 80mm Biometar lens on the Pentacon Six help to explain how to achieve hyperfocal focus.
The ring nearest the camera is the aperture control ring,
with the aperture numbers engraved on it. Set the chosen aperture
against the red index mark just in front of the aperture ring.
(The right 2.8 is in red, as this is the index mark that you need to use, instead of the red line, if shooting on Infra-Red film.)
Either side of the red index mark, the aperture values are engraved on the fixed ring.
These tell you the available depth of field.
If the main subject is at “infinity” I could focus at infinity (the oo symbol), but this would “waste” some of the available depth of field.
Here the lens aperture is set at f/2.8, the maximum aperture.
I have moved the infinity mark opposite the right-hand 2.8 mark.
By looking at the left-hand 2.8 mark, I can see that everything will be acceptably sharp from 15 metres to infinity.
Here the lens aperture is set at f/16, which is very small, although not the smallest on this lens.
I have moved the infinity mark opposite the right-hand 16 mark.
By looking at the left-hand 16 mark, I can see that everything will be acceptably sharp from 3 metres to infinity.
1) To use the smaller apertures, you may need to use a slower shutter speed – again depending on the film speed and the light available. In some cases, use of a tripod will be advisable (generally for shutter speeds longer than 1/125 sec if using the 80mm Biometar).
2) It is generally reported that the depth-of-field scales
on the “Soviet” lenses (Arsenal factory, etc) are over-optimistic, so the
zone of acceptable focus may be less than expected. You can compensate
for this in the following way:
If you set the infinity mark to f/16, for example, set the actual aperture to f/22 (and adjust the shutter speed accordingly).
Choosing which parts of the image to have in focus or out of focus requires more thinking than using a “point-and-shoot” camera or a camera with an auto-focus lens. But it takes the control away from the camera and puts the photographer in control of the image! Your own experience will soon tell you the settings that give you the results that you find most acceptable for each situation.
See also the explanation here.
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© TRA June 2009, March 2012