The Pentacon Six System
by TRA

What image do I get when I use a Pentacon Six lens
on my 35mm camera
or on my digital camera?

It can be difficult to understand in advance what the effect is going to be when a Pentacon Six lens is used on a 35mm camera, or even on a digital camera with a sensor that is smaller than the "full frame" 35mm size.  Here is a typical question, received in March 2016 from a visitor to this website:

Q: Hello
Thanks for providing this great resource with your website! I have recently bought two Zeiss lenses with a Pentacon Six mount: a Flektogon 50mm/f4 and a Biometar 80mm/f2.8 (both as "zebra" versions) and mounted them via a "lensless", plain "mechanical" adapter to my Nikon D810. What a joy to use them!

What I did not understand from the information on your website: what exactly is the equivalent focal length when mounted on a full frame camera?  I had the impression from your list on that they would have a different angle of view, when mounted on a 35mm camera.  Optically, I do not get a wider or narrower angle of view - the Flektogon 50mm for example shows the exakt same field of view as my normal 50mm Nikon lens. Where is my error of thinking?

Furthermore, Id like to ask: the lens's calculation is done to properly fill the medium format. Mounting it on a full frame camera - will it produce better results (better corner sharpness, less vignetting), because only the central part of the lenses "projection" is used by the sensor?

Thanks for your help
Kind regards

Here is the reply that I sent him.  I have added titles for the purpose of this web page.

Hello J

Thank you for your e-mail.

Angle of view and how much of the scene is covered

The purpose of my page of 35mm and medium format equivalent focal lengths is to help people who are familiar with what they see through a given lens on a 35mm camera to find out what lens they will need in order to see (approximately) the same amount of the scene before them on a medium format camera such as the Pentacon Six.

As you have discovered, a lens with a focal length of 50mm still has the same focal length, regardless what camera it is mounted on: its optical characteristics obviously don't change.

The only thing that changes is the amount of the scene before it that is included on the film (or sensor). So to see on a 35mm camera the same the amount of the scene that Flektogon records on the 66 frame, you would need a lens with a focal length of a little under 28mm on the 35mm camera.  However, if you mount the 50mm Pentacon Six Flektogon on the 35mm camera, it is still a 50mm lens, so it will show the same amount of the scene as a 50mm lens that had been designed for the 35mm camera in the first place. In fact, it has the inconvenience of being larger and heavier than a 50mm lens that had been designed to cover only the 2436mm 35mm "full frame" format.

In summary, the focal length doesn't change, only the amount of the scene that is in front of it that is reproduced on the film, which is determined by the size of the film:
smaller film area = smaller amount of original scene reproduced.
So when the 50mm Flektogon is used on a 35mm-format camera, the image on the film (or sensor) will the same same size and coverage as an image shot with a 50mm lens that had been designed for the 35mm camera.

The question is a different one:
If I know what the 50mm Flektogon shows me on my Pentacon Six,
what lens do I need on my 35mm camera, in order to see the same amount of the scene?
Answer: a lens slightly wider than 28mm focal length
(as indicated on the chart referred to in the link above).

we could ask a question in the opposite direction:
I have a 28mm lens on my 35mm (full-frame) camera.
What lens would give the same amount of the scene on my Pentacon Six?
Answer: a 50mm lens such as the Flektogon.

Image quality

Now to your other question, which concerns the resolution of the image.  With all lenses, resolution reduces as one moves further away from the centre of the image.  In fact, all lenses project an image that is, in absolute terms, larger than the required format. However, resolution and brightness both reduce as one gets further away from the centre of the image, so they are designed to give adequate resolution and (supposedly) no obvious reduction of brightness (called "vignetting") within the intended image area.

So the Flektogon should give a sharper and more evenly-illuminated image on a 35mm camera than a 50mm lens designed for a 35mm camera.  Of course, this all depends on the quality of the lens designed for the 35mm camera.

You should of course get NO VIGNETTING AT ALL when using the 50mm Flektogon on a 35mm camera, so it will be better than many 50mm lenses that are designed for 35mm cameras (where there often is visible vignetting).

As regards resolution, as images shot with a medium format camera need to be enlarged a lot less than images shot with a 35mm camera, some very expensive lenses designed for a 35mm camera may well in theory have a higher resolution than the 50mm Flektogon, although I don't have any figures for this.  However, because of the greater degree of enlargement of the image recorded on the film that is required with a 35mm camera, the resolution of the resultant image even if shot with a super-high-resolution lens on that 35mm camera will not be higher than the resolution of an equivalent image that was shot with the Flektogon on a Pentacon Six.

More important, since the size of the film surface is much smaller in the 35mm camera, factors such as film grain are likely to make the image from the 35mm camera have MUCH LOWER RESOLUTION than the resultant image from the Pentacon Six, because the chief limiting factor with equipment of this quality is not the lens but the resolution and grain structure of the film itself.  The gradation, for instance of skin tones, is frequently also far smoother on a 66 image than on a 35mm image.

The adapter used

Finally, when mounting the Flektogon or any other medium format lens on a 35mm camera (or smaller, such as a digital camera with an APS-C-sized sensor), it is essential to ensure that the adapter is absolutely matt-black inside, with no reflective surfaces. The reason for this is that the medium format lens covers a wider area of the original scene and transmits a much larger overall image than a lens that was designed for use on the 35mm camera.  None of the light from that "wasted" image must be allowed to "bounce around" or reflect within the adapter, since if it reaches the film, it will reduce the contrast and quality of the result.

This problem is almost unheard-of, but, many years ago, I did have a poorly-made adapter that was not matt black inside, and when I tried out a 50mm Flektogon on a 35mm Minolta SR-T 101, reflections occurring within the adapter did reduce the contrast of the image in the camera.

The main use for medium format lenses on 35mm cameras

In my option, there are two the principal uses for medium-format to 35mm adapters

  • to enable photographers to use the LONGER medium format lenses on their 35mm camera, and in these circumstances the result is usually extremely satisfactory
  • to enable photographers to take advantage of the larger size of the image projected by the medium format lens, in order to use a tilt or shift adapter to achieve other effects in their image.  (See introduction to shift and tilt lenses, starting here.)

A problem with some digital sensors

Another problem can occur when using with digital cameras wide-angle lenses that have been designed for film cameras, due to the design of sensor protector in some digital cameras, which may have a honeycomb-like structure (like a series of pencils bundled together and stood on end).  This requires the image-forming cone of light that enters the image area to be almost the shape of a canned drink, i.e., with nearly parallel sides.  Wide-angle lenses by definition have to bring in the rays of light that form the image at a much greater angle, a real cone-shape. The result can be some vignetting in full-frame digital cameras when using lenses that have been designed for full-frame film cameras, even when those lenses do not vignette when used with film.

I hope that this is helpful.

With best wishes

"Mr Pentacon Six"

Crop factor

A related question that I have also just received is this one:

Q: Hallo,
Thank you very much for bringing to your website so much valuable info regarding Pentacon-Six System.
My idea is to use (with aproppriate adapter) one of those P-Six lenses on my Nikon D300 (APS-C Matrix).  I am wondering what the 'crop factor' would be when using P-Six lens on DX camera.
It is 1,52 for FX lenses when using them on DX. The middle format lenses which are P-Six fit bigger matrix format than FX.
Could you please give me a hint ?

Kind Regards,

A: Hello M

Thank you for your e-mail.  As you will see from the above explanation, a 50mm Pentacon Six lens is still a 50mm lens when it is mounted on a 35mm full-frame camera, and of course it is still a 50mm lens when it is mounted on a camera with an APS-C-sized sensor or negative.

The idea that manufacturers had when introducing the concept of "crop factor" was to tell potential customers who were familiar with full frame 35mm cameras what the image would be like if lenses from those cameras were used on their new camera that had a much smaller frame size or sensor.

Since the smaller frame or sensor is cutting off, or cropping, the edges of the image, you will only see a smaller portion of the scene being photographed.

If you have a mental image of the coverage of a lens with a focal length of 50mm on a 35mm camera, if you put that lens on your APS-C camera, it will only record the central area of the image.

How much will it record?
The "crop factor" tells you.  In this example if we multiply the 50mm by 1.52, we get the number 76.
So the image that your APS-C film or sensor will record will be the same as what you would have got on a full-frame 35mm camera if you had used a 76mm lens.

Manufacturers have (with few and very minor exceptions) never tried to explain a "medium-format to APS-C crop factor", because there was not amongst most consumers a public that was familiar with the coverage of different focal lengths on medium format cameras.

So a 50mm lens is a 50mm lens.
On a medium format camera it will show a certain amount of the scene before it.
On a 35mm "full-frame" camera it will show a lot less (we might say a little more than half the width, and a little less than half the height).
On an APS-C camera it will show less still.

I hope that this will make the whole matter a little easier to understand.

"Mr Pentacon Six"

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TRA First published: March 2016