Medium Format Lenses with the Pentacon Six Mount
A comparative test
by TRA

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:

This control is achieved by varying the depth of field.

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:

Focal length
Depth of field is shallower with longer lenses (for instance, 300mm) and deeper with shorter or wide-angle lenses (for instance, 50mm).

Aperture
Depth of field is shallower with wide apertures (for instance, f/2.8) and deeper with small apertures (for instance, f/16).

Distance
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 of 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 lens. 


Differential Focussing 

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:

To control the area of sharpest focus, it is possible to refer to the markings on the lens barrel, but the best way of ensuring precise focus is by observing the image on the ground glass screen in the viewfinder, possibly with the help of a magnifier head instead of the prism, or the prism magnifying attachment or “Focussing Telescope” on the back of the prism.

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.

Compare the depth of field with these two images


Pentacon Six with Kodak 160 ISO negative film
120mm Arsenal Vega 28 lens, closest focus, 1/60 f/16
Scanned with Epson V750 Perfection PRO with VueScan v.9.5.17.
[C337-6]


Pentacon Six with Kodak 160 ISO negative film
120mm Arsenal Vega 28 lens, closest focus, 1/1000 f/4
Scanned with Minolta Dimage Scan Multi PRO with Minolta driver
[C337-7]

Note that these two negatives have been scanned with different scanners and different software (and, incidentally, more than ten years apart, which shows that film is a good storage medium).  The different scanners and software have resulted in slight differences of colour.  The negative holders for each scanner were also different, resulting in a slight cropping of the bottom of the left-hand image.  (If having an exact match had been essential, I would have re-scanned the second image with the Epson scanner and VueScan software.)

Two factors reduce the depth of field with both of these images:

The image on the left was taken first. I aimed for the greatest depth of field possible.  I was using the camera hand-held and 1/60 sec is slower than would normally be recommended with a 120mm lens, but with a good posture and hold it is possible to achieve sharp pictures at this speed hand-held with the Pentacon Six (unlike with the Pentax 6×7!).  To get accurate exposure at this speed with the prevailing light, I was only able to stop down to f/16, even though the minimum aperture on this lens is f/22.  If I had used the minimum aperture of f/22 at 1/60, the image would have been under-exposed (by "one stop").  However, I was not sure about the image: perhaps there was too much background detail, distracting the viewer's attention from the the flowers in the foreground.  So I decided to go to the opposite extreme.

The image on the right was taken a few seconds later.  I knelt on the ground in order to get as low as possible and chose the fastest shutter speed, 1/1000 sec, which enabled me to open up the lens to f/4.  If I had used the maximum aperture of f/2.8 at 1/1000, the image would have been over-exposed (by "one stop").  Of course, if I had had a neutral density filter (or a polarizing filter) with me, I could have put that on the lens and opened up to f/2.8 -- but one rarely carries all possible accessories.  Not all of the foreground flowers are at the same distance from the lens, so some of them are less sharp.  Nevertheless, the background is thrown fully out of focus, which I consider produces a much more pleasing picture.  With a slight crop of the left and right-hand edges of the image (especially the left), I have made a vertical print about 40 cm tall that my wife and I enjoy having on a wall in our home.

Perhaps the second image shows that images with out-of-focus areas can be pleasing.

You can learn more about the effects of different formats and lenses on depth of field here.

Bokeh

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!

Hyperfocal Focussing

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:

To control the hyperfocal distances, it is necessary to refer to the markings on the lens barrel, although stopping down the lens can give some indication in the viewfinder of the depth of field (at the cost of temporarily darkening the image in the viewfinder).

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.
[C463-14.jpg]


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.
[C463-13.jpg]

Notes
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).

Conclusion

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. Revised August 2016