The Pentacon Six System
by TRA

80mm Biometer Bokeh & Resolution


Just how good is the 80mm Biometar lens?

I recently received an enquiry from someone who was unhappy with the quality of the images produced by his Biometar lens.  This surprised me, as I have used many of these lenses over the years, and have never been disappointed with the results from any of them.  So why might someone get poor results with this lens?  There can be many reasons, all of which can apply to any lens with any camera from any manufacturer.

For instance:
An unsharp image may be caused by using too slow a shutter speed or by a focussing error.  Experience can help one see the difference between camera and/or subject movement (caused by too slow a shutter speed) and a focussing error.

What can cause a focussing error?

Most of these causes are unusual or even totally unheard of with the Pentacon Six, but they are theoretically possible:

  • Focussing screen at the wrong height - only likely if it has been installed by a user who does not know what they are doing.  (I do know of one person who installed a screen upside-down - this will produce a focussing error that will be obvious at larger apertures.)
  • Mirror out of alignment.  I have only ever heard of this with Kiev cameras (and only ones that were faulty!).
  • Damage to film pressure plate - this will be obvious if you just look at it.
  • The lens has been disassembled and reassembled incorrectly by a previous owner, not unheard-of, although I personally have never met a case with the Biometar.
  • The lens has been dropped and an element has moved from the correct position - but if this has happened, there is likely to be visible distortion of the barrel of the lens.
  • Finally, let's be frank, it could be the photographer's eyesight!
So I have looked out a couple of pictures that I have taken at wide apertures with this lens.  Why at large apertures? 
  • Firstly, if there is a focussing error, it is more likely to be visible when the lens is used at a wide aperture. 
  • Secondly, if a lens is giving sharp results at wide apertures, it will give sharp results at other apertures. 
Bokeh

At the same time, we can look at the bokeh of this lens - how it renders out-of-focus highlights.

I will start with an image shot in 1982 on out-of-date Agfa CT18 reversal film.  The camera was my first Pentacon Six (non-TL), which must have been manufactured in 1966 or 1967 (see here).  It is reasonable to assume that the Biometar was from about the same year.  It had the leather trim to the focussing ring and was single coated.  This was a hand-held shot, the shutter speed was 1/500 sec and the aperture was f/3.5 (half a stop down from the maximum aperture of f/2.8).


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I composed this image for use in a widescreen mount (24mm high 56mm wide), so the rest of the image is no longer available, but this image does show the whole width of the frame, reproduced here at low resolution for web viewing.

Here are some higher-resolution selections from the same image.
 

Scanning images always results in softening of the image, but no image sharpening has been applied here.
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Here we are looking at 25.3% of the width of the total image.

I have chosen this crop to show the bokeh.

For a portrait it is often desirable to have an extremely shallow plane of sharp focus, with everything in front of or behind that plane not only out of focus, but with a rendering of the out-of-focus highlights that does not distract the viewer's attention from where the photographer intended him or her to look.

The 180mm Sonnar (see below) is usually recommended as the best portrait lens for the Pentacon Six.  Its maximum aperture of f/2.8 is extremely large for a lens of this focal length with coverage for medium format.

The image that we are looking at here was not composed as a head-and-shoulders portrait.  However, when we crop the original image down tremendously to a tiny portion of the original frame (approximately one-sixteenth of the surface area), the results are still outstandingly good, in my opinion.

Incidentally, this picture was taken with the prevailing daylight, without the use of flash or reflectors of any sort to reduce or control the shadows.  It demonstrates that when the lighting conditions are right, extremely pleasing results can be obtained without the need to carry large amounts of accessories.
 


 

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Here - as is often done on the web! - we have enlarged a small portion of the image far more than we would in real life.  We would not view an image enlarged this much from anything like close as computer users sit to their screen.

At this degree of enlargement, it becomes clear that the point of sharpest focus is not - as it should have been! - on the eyes.  Even so, the image is acceptably sharp.

I have slightly over-exposed the highlights, losing a little bit of highlight detail.  Slide or reversal film does of course have very little exposure lattitude, and I prefer to shoot negative film most of the time for this reason.


 

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A very small amount of sharpening has been applied to this scan and the one above, of the eyes, to restore some of the original sharpness that I can see in the slide.

This section of the image starts under 20% in from the left-hand edge of the frame.

The resolution of the 80mm Biometar can here be seen to be outstanding, especially considering that this shows its performance only half a stop down from full aperture..


 
In the summer of 2012 I took a picture of a plaque outside one of the buildings of St Andrews University.  I was using one of the last Pentacon Six TL cameras manufactured and a multi-coated 80mm Biometar from the batch that was re-calculated in October 1979 and manufactured in March 1989.

I deliberately wanted to throw the background out of focus, so I used the fastest shutter speed (1/1000) and the largest aperture that gave me the correct exposure.  I was shooting with Fuji 160S film and - unusually for me - I didn't record the aperture, but the day was overcast and it is likely to have been between f/2.8 and f/4, the same as in the above photo.

Pictorially, this is not a great photo, but it is an excellent test of lens resolution.  This looks technically excellent, and there is also no noticeable colour fringeing and no rectilinear distortion (whether barrel or pin-cushion).

It also shows the lens bokeh at a wide aperture.

It was scanned following my usual procedure, on my Minolta Dimage Scan Multi Pro at 4800 dpi, which produced a file that at 300 pixels/inch would have printed at a size of approximately 89 cm 89 cm.  Naturally, the copy that I reproduce here is greatly-reduced and saved in jpeg format with a substantial compression.  In spite of this, it is possible to see by the narrow depth of field and the out-of-focus highlights in the background that a very wide aperture was used.

A small amount of sharpening was applied in the computer to compensate for the softening that is introduced by the scanning process.  The image as reproduced on my monitor here is still not as sharp as the lab print that I have.  Nevertheless, it does perhaps enable visitors to this website to get an impression of the quality of this lens.



Outside St. Salvator's College, St Andrews
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These pictures were taken decades apart, using Pentacon Six camera bodies and Zeiss Biometars that were produced decades apart.  They were shot as photographs, not as "tests".  Nevertheless, in spite of other differences (for instance, the use of different types of film), both pictures show that the 80mm Biometar is a superbly sharp lens that also produces pleasing bokeh at wide apertures.
 
 

The 180mm Sonnar at full aperture
on Fuji 160 NS negative film
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For the purposes of comparison, here is a small section of an informal family picture taken with the 180mm Sonnar at maximum aperture.  It covers approximately one third of the width of the frame.

It is clear that the focus was on the far eye, and at this great degree of enlargement this picture shows the extremely shallow depth of field with this lens when used at f/2.8.  In practice one would be more likely to stop down to at least f/4 and more probably a minimum of f/5.6, in order to ensure that both eyes were equally in focus.

Hower, this was taken with low ambient light with a shutter speed of 1/125 sec, slower than would be recommended for a 180mm lens.  Even so, I knew at the time that the shadows would be under-exposed by 1 - 2 stops but wanted to get the shot anyway.  At normal degrees of enlargement, the result with the whole of the frame is a perfectly acceptable - even quite pleasing - picture.

Resolution

In 1977 I bought my first Pentacon Six.  It was not the "TL" version and so had probably been manufactured in 1966.  It had an early, single-coated version of the 80mm Biometar lens and for the first eight or nine months I only had the waist-level finder and the standard focussing screen.

Here is one of the first colour shots that I took, hand-held.  It may not be "great art", but it was a great test of the resolution of the lens.

Film: Kodacolor II (100 ASA/21DIN).  Exposure: 1/125 f/9.5, hand-held.

Of course, to make this image a reasonable size for downloading and displaying on a computer monitor, I have had to reduce the resolution massively.  I scanned this negative today on an Epson Perfection V750 PRO using the SilverFast SE Plus software that was supplied with the scanner.  I scanned at 300 pixels per inch (much less than the 4800 ppi maximum offered by the scanner and the software).  The resultant file was about 330 MB in size, with a width of 10,585 pixels, which at 300 pixels per inch would have produced a print 89.62 cm wide.  (This is slightly more than 35" wide, or fractionally under three feet wide.)  As reproduced for this web page, the image is only 709 pixels wide.  I have also used jpeg compression to reduce the file size substantially.

Below it I reproduce the bottom right-hand corner of the same image, taken from the same scan.  At the size scanned, this section of the picture is 3,152 pixels wide, equivalent at 300 ppi to 26.69 cm wide.  I have also reduced it to 709 pixels wide and compressed the jpeg substantially.









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The roughness is in the faces of the bricks, not the resolution of the lens or the film, as can be appreciated by the sharp blades of straw.  Any chromatic aberrations (such as colour fringeing) would also have been clearly visible on the straw, if such had been present.

The result is truly outstanding.


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The bottom right-hand corner of the above image.

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

To go back to the introduction to the 80mm lenses, click here.

To return to the introduction to the lens data, click here.

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TRA  April 2013, latest revision, August 2016