The Pentacon Six System
by TRA

Format, Focal Length & Focus


What are the advantages of larger-format cameras?

Elsewhere (here), I have referred to higher image resolution.  This will be obvious, although as digital technology develops, the gap between the best digital cameras and medium format film cameras decreases.

But perhaps there is a more important answer.  One that cannot be overcome by advances in technology:
the shallower depth of field of lenses that are normally used on medium format cameras.
This means a shallower in-focus zone within the image, with subjects that are nearer to the camera or farther away from it being increasingly fuzzy.

You might say, “Who wants that?  Isn’t it better to have everything in sharp focus?”

The answer is, “Perhaps.”  It depends on the type of photography and the subject matter.  For instance, portrait photography is one of the areas where it is often desirable to limit the depth of field, in order to direct the viewer’s attention to a particular part of the image, usually the subject’s eyes.


This outdoor portrait, apparently for fashion advertising, would be spoilt if the background were in sharp focus.
In “PENTACON six”
Camera advertising brochure published by VEB Pentacon Dresden in 1967, p. 7
(Subject and photographer not named)
  

This image has been slightly cropped to give a vertical picture, as is suitable for many portraits.
A sharp background would detract greatly from the impact of the picture.

Schauspielerin Heidemarie Wenzel by Manfred Uhlenhut
(Actress Heidemarie Wenzel)
in “Pentaconsix Praxis” by W Gerhard Heyde, 1st edition, 1974, p. 95
Unfortunately, technical details of lenses and apertures used are not given.
I would suspect that the 80mm Biometar was used for the image on the left and the 120mm Biometar or the 180mm Sonnar for the image on the right,
probably at or very near maximum aperture (f/2.8), regardless which lens was used.

Of course, many people just want sharp photos.  They want everything in the photo to be sharp.

But if we look at professionally-taken portraits, for instance, we are likely to see that the background is deliberately out of focus.   In fact, on the TV and in the cinema we may observe scenes of all sorts where the focus moves backwards or forwards from one thing that is in the frame to another – without changing the composition.

To give a different example, if you see in a street a tree with beautiful blossom on it, you may wish to take a picture of it.  But the picture would probably be better if the building behind the tree were more or less out of focus.

Depth of field

The zone of sharp focus is usually called “depth of field”.

Depth of field depends on three things:

  • the focal length of the lens,
  • its aperture
  • the distance from the subject. 

It doesn’t change with the size of the film or the sensor.  (However, as the images from smaller film and sensor sizes are frequently enlarged more for viewing than the images from larger film/sensor sizes, it can appear that the depth of field has changed.  In reality, all that has happened is that any out-of-focus areas of the image become more obvious if they are enlarged more.)

The three rules are:

  1. The longer the focal length of the lens, the shallower the depth of field.
  2. The wider (bigger) the aperture of the lens, the shallower the depth of field.
  3. The closer the camera is to the subject, the shallower the depth of field.

Therefore, if the photographer wishes to control the areas that are in focus in his/her images,

  • the longer the focal length, the better
  • and the wider maximum aperture available on the lens, the better.

The distance of the camera from the subject obviously depends on composition and other related factors.







In the image to the right, the very shallow zone of sharp focus has been carefully targeted on the subject, which is framed by out-of-focus components in front of it.  The out-of-focus components are sufficiently recongnisable to give a context, while directing the viewer
s attention to the main subject, the young lady working with the microscope and her colleague or teacher.  The out-of-focus components thus contribute to the image, instead of confusing the viewer as to what the subject is meant to be.
  

“Biologiestudenten” by Heinz Dargelis (Biology students)
in “Pentaconsix Praxis” by W Gerhard Heyde, 1st edition, 1974, p. 133

The photographer can increase the depth of field by using a smaller aperture on the lens.  But it is, by definition, not possible to reduce the depth of field by using an aperture that is larger than the maximum aperture of the lens!

Here are the standard focal lengths that are common for various formats, plus the focal lengths that are desirable for portraits with each of them:

Format

Standard focal length

A good portrait focal length

Samsung S4
(5.08 × 3.81mm sensor)

4.2mm

(in 35mm: 31.0 mm)

12-13.5mm

(in 35mm: 90-100mm)

110 film (approx 17 × 13mm)

24mm

45-50mm

APS-C format camera
(30.2 × 16.7mm)

Approx 32mm

57-64mm

35mm camera
(36mm × 24mm)

50mm

90-100mm

6 × 6 medium format camera (54 × 54mm)

80mm

150-180mm

4 × 5" (approx 127 × 102mm)

150mm

300mm

 

This information is partly derived from a Wikipedia article, here: https://en.wikipedia.org/wiki/35_mm_equivalent_focal_length

As well as using lenses of longer focal lengths on cameras of the format for which they were intended, it is in theory also possible to use them on cameras of any smaller format, assuming that the smaller format camera permits the lens to be changed, and subject to the availability of a suitable adapter.

Example

The 110 format was introduced in 1972, apparently with the aim of producing much smaller cameras.  The Pentax 110 was the only 110-format camera that accepted interchangeable lenses.  Its standard lens had a focal length of 24mm.

Regardless what camera it is used on, a lens will always project an image of the same size of whatever is before it.  The angle of view of the lens won
t change.  But – to give an extreme case – if a 150mm lens, considered a standard focal length on a 5×4" camera, is used on a 110 film format camera, the area of view recorded on the film will be nowhere near the total image projected by the lens.  In fact, it will show an area equivalent to what one would obtain if using a 300mm lens on a 35mm (“full frame”) camera.  This may be great for photographing distant wildlife, but it will be no good for portrait photography, as you will need a studio the size of an aircraft hangar in order to get far enough away from your subject.  And then you will need a loud hailer or a telephone in order to communicate with him or her!

However, the optical characteristics of the lens will not have changed.

Note that with digital cameras, we must ignore the effect of “digital zoom”, since this merely takes a smaller section of the image and digitally enlarges it to fill the frame.  It cannot change the optical characteristics of the lens that the camera has.

How can we obtain that control over focus?

Depth of field is calculated by lens manufacturers on the basis of a series of assumptions, including how much they think that the image is likely to be enlarged.  The key idea is that they measure or calculate the size of what they call a “circle of confusion”, which is the size that a point or a dot is reproduced if it is out of focus.  (The more out-of-focus it is, the larger it will appear to be.)  The in-focus zone of a lens of a given focal length does not change with the format.  However, as smaller-format images may be enlarged more than larger-format images, for practical purposes, different sizes of circles of confusion are considered acceptable for different formats.  Smaller circles of confusion are considered necessary for the smaller formats.

Here is a summary of the depth of field for portrait lenses on various formats, based on a camera-to-subject distance of 2.5 meters, with the lens focussed at 2.5 metres and using the data calculated by http://www.dofmaster.com/dofjs.html.  Conversions from metric to imperial are rounded and are taken from http://www.onlineconversion.com/length_common.htm.

Format

Portrait focal length

Aperture

Near limit

Far limit

Total depth of field

Approx Imperial

equivalent

4 × 5"

300 mm

f/4

2.48 m

2.52 m

0.05 m

2"

6 × 6 cm

180 mm

f/2.8

2.48 m

2.52 m

0.05 m

2"

35 mm

90 mm

f/2.8

2.44 m

2.56 m

0.13 m

5"

 

APS

50 mm

f/4

2.28 m

2.77 m

0.49 m

19¼"

50 mm

f/5.6

2.2 m

2.9 m

0.71 m

28"


5.08 × 3.81 mm

(mobile phone)

12 mm

f/2.8

1.13 m

Infinity

Infinite

Infinite

12 mm

f/8

0.56 m

Infinity

Infinite

Infinite

4.3 mm

f/2.8

0.24 m

Infinity

Infinite

Infinite

In fact, you are not likely to find a mobile phone with a 12mm lens.  It is likely to have a lens of approximately 4.3 mm.

This shows us that with a portrait lens on a 4 × 5" or 6 × 6 cm camera, focussing at 2.5 metres and using the lens at maximum aperture, we will have a sharp zone that is just 2" or 5 cm deep.  Of course, we can increase the depth of that sharp zone by stopping down the lens to a smaller aperture.

However, on an APS-format camera (which includes many digital cameras) with a 50mm portrait lens at aperture f/5.6, the depth the zone of sharp focus will be 28" or 71 cm.

A mobile phone will normally use face-recognition technology in order to focus on that subject.  However, the area of sharpness when the lens is focussed on a subject 2.5 m away will extend from 24 cm (about 9½") in front of the phone to infinity.  In other words, everything will be in focus.  For many subjects, we may want that, but for others, it is undesirable.

With a mobile phone, how can we reduce the area of sharpness and control it?

We can’t.

  • Not with a mobile phone.
  • Not with the simplest digital cameras with their small sensors and lenses that only have a digital (not optical) zoom.
  • Hardly at all with the majority of more sophisticated digital cameras with sensors that are more or less the APS size.  Even if they have genuine optical zoom lenses.  Even if they can accept other lenses.

As can be seen from above, a 50mm lens operating at f/5.6 has a depth of field of nearly three-quarters of a meter, nearly 2½ feet.  That is wider than the distance from one shoulder to the other on the majority of adults.  I can’t see how it is possible with such a lens to limit the depth of field, if desired, to the person’s eyes, for instance.  You may of course be able to move the person away from the background, so that the background is less sharp and more or less out of focus.

Being able to control depth of field starts with cameras that have a format of 35mm (i.e., a film or sensor size of 24 × 36 mm, now commonly called “full frame”), and even with them it only really becomes noticeable if you meet three criteria:
  • use lenses of longer-than-normal focal length;
  • these lenses must have a larger-than-average maximum aperture for their focal length
  • it must be possible to focus the lens manually.
In some leading digital systems, such lenses do exist.  They are likely to cost in the region of £1,000 – £1,500 each, sometimes, much more than this.

So for many people, controlling depth of field only begins to be viable with medium format film cameras, and it is surprising to observe from the above table that on many occasions you can achieve the same degree of control with a 6 × 6 cm camera to that which is achievable with the much larger 4 × 5" (or 9 × 12 cm) cameras.  This is because the lenses for these larger cameras generally have a smaller maximum aperture.

And of course, as stated higher up, you can increase the depth of field by using a smaller aperture.  However, if your imaging device has a lens with a very short focal length and perhaps a not very large aperture, there is nothing you can do to reduce the depth of field.

For more on this subject, see the pages on this website on

In April 2017, I have now added examples of depth of field with the 80mm Biometar lens in macro photography.  See them here.

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© TRA First published: August 2016  Revised: April 2017