Mods & DIY

Modding a 28 mm Canon Demi C lens for M39 mount – Guest post by Tobias Eriksson

Following on from my Canon Demi C review, I wanted to write something about the orphaned lens that came about from me only shooting it with the 50mm. Having that nice lens gathering dust in a box is no fun. And me getting my first interchangeable lens digital camera – the Fujifilm X-Pro1: My brain wants to try things out. With the X-Pro being digital results are instant when it comes to lenses and visual experimentation. I end up holding my unused Canon Demi C 28 mm lens in front of the image sensor. Getting an approximation of the correct distance from lens to film plane/sensor had me looking around for something to mount the lens on.

Modding

I’d glimpsed people’s (and companies’) body cap lenses on blogs so that seemed the approach to take in this case. The lens is only 47mm wide at the outer edge of the housing. And the distance to the film plane seemed to be within easy adjustment range.

So I made a hole in my only M39 body cap and adjusted the lens position after some fiddling (No: Carving). Then attached it with Sugru rubber clay and textile glue after adjusting for infinity and 1m focus.

As such, I am now in possession of a 28 mm half frame lens which will never give me a 42mm equivalent focal length on my X-Pro’s half frame sensor.

Canon Demi C with 50 mm lens and 28 mm (modded) lens.

Aperture

There are no aperture setting options for the Canon lens since they are placed inside the lens mount on the Canon body. So the Canon is wide open, constantly at f2,8.

Focusing

The 28 mm Canon has a very short throw but feels very good, despite the zone focus “click stops”. I prefer a short throw over long ones. I’ve used lenses with 300-360 degrees’ throw on the focus ring and it’s very hard for me to get used to it. More – it’s annoying. But this lens is somewhere in Olympus XA and Olympus 35 RC territory when it comes to focus throw, which is very good.

Testing

Even in these dim light conditions the lens is easy to focus. It sharply renders details in the wood grain. However the images get blurry toward the edge of the picture. There’s a very visible limit to its field of focus. However the focus in the centre of the image is very much “good enough” for me to want to utilize the lens in these light conditions in future.

The blurred edges around the image has gotten me puzzled. Shouldn’t it be in focus in the whole frame since the Fuji sensor and the half-frame image surface both show the same size (42 mm) image? This is a question for you science people out there to answer in the comments section below.

Note the very obvious blurry edges.

Top corner of above picture – see blurred area along left side and in corner.

On the outdoor pictures the problem of the lens non-focus zone on the right and left edges and also corners becomes very visible. I don’t mind it per se, considering the nature of the pictures I aim to take with it (family outings etc), but it is something to consider every time when composing the pictures. Maybe that’s a bit much to ask…

The fuzzy focus adds character to some pictures…

… while some compositions suffer severely from the blurry edges.

The lens renders very sharp in the middle regions – much more so than my Industar 69 which is a bit soft in colours and general focusing. Both are very good lenses for my retro-nostalgic preferences in picture making, though.

Note that there seems to be less blur around the edges in this picture.

Body cap lens

Conclusion

This modded experimental lens is – despite its limitations – a fun lens to use for my family occasions and excursions, where the X-Pro comes most to use, at the moment. Being strapped on cash, this is a “wide angle” solution I can live with. Also it feeds my unstoppable urge for shallow depth of field in images because of the permanent f2,8 aperture.

Thank you for bearing with me on this stupid journey of not discovering much we didn’t already know. Still, I hope you’ve learned something. Please share your thoughts in the comments section below.

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19 Comments

  • Avatar
    Reply
    Hamish Gill
    October 29, 2017 at 3:32 pm

    Good work! I do like this sort of thing!
    It’s blurry toward the edges, simply because of how differently camera sensors react to the angle of light compared to film – that’s as I understand it anyway

  • Avatar
    Reply
    Tobias Eriksson
    October 29, 2017 at 4:22 pm

    Thanks!
    Well, that explains things. And I’ve come to understand that a lot of lenses are not performing at their best wide open.

  • Avatar
    Reply
    Stefan
    October 29, 2017 at 5:43 pm

    I think the filmplane in the demi is curved. The whole angle of light thing should give fall off I think.

    • Avatar
      Reply
      Tobias Eriksson
      October 29, 2017 at 7:29 pm

      That explains a lot!

      • Avatar
        Reply
        Hamish Gill
        October 29, 2017 at 7:41 pm

        Is it curved – when you open the back you should be able to see

        • Avatar
          Reply
          Terry B
          October 30, 2017 at 2:36 pm

          Hamish, very unlikely. The fall-off will be down to the usual performance of digital sensors with film-era lenses, as you say, plus any inherent drop off in edge performance of 99.9% of lenses, even Leica!

          • Avatar
            Hamish Gill
            October 30, 2017 at 9:24 pm

            That was my gut response too

    • Avatar
      Reply
      Terry B
      October 30, 2017 at 2:40 pm

      Highly unlikely, unless you have empirical evidence to the contrary.

  • Avatar
    Reply
    Richard Patterson
    October 29, 2017 at 6:33 pm

    Hi Tobias,
    Interesting write up.

    The Fuji X Pro 1 has an APS (C) size sensor (not half frame) which is a crop factor of 1.4. Hence a 28mm on the Fuji gives a field of view equivalent to 42 mm on full frame as you say (1.4 x 28 = 42).

    I’m not familiar with the Cannon Demi, but if it is half frame the lens will be designed to provide a half frame image circle, not APS(C). You are lucky you only got blurred edges not vignetting. If you try the lens on a micro four thirds body it should work properly as the m43 sensor is almost the same size as half frame. Same logic applies to the Industar 69, it will not work on the Fuji XPro.

    Hope this is clear!

    Regards

    Richard

    • Avatar
      Reply
      Hamish Gill
      October 29, 2017 at 6:42 pm

      Hi Richard,
      The Xpro1 sensor is 23.6mm × 15.6mm, half frame is 24mm x 18mm – aps-c is a 1.5-1.6x crop depending on manufacturer
      The Industar 69 works fine (ish) on aps-c too – here is a very old (cringeworthy) blog post from when I modded one to mount on a epson rd1 which has a 23.7 x 15.5mm sensor. It vignettes, but not to black.
      Cheers,
      Hamish

      • Avatar
        Reply
        Matt Williams
        October 30, 2017 at 5:21 am

        It can be a bit confusing when you first think about it because in your head you think “Full frame vs APS-C = 1.5x crop” and “half frame would be 2x crop”. It’s not illogical thinking, but it is incorrect. Simply because it is half frame, but only half in one of the two dimensions. APS-C crops in on the central portion of the sensor, which is harder to compare in your mind.

        But, if you think of a rectangle, split it into four equal sections. Two of those would be “half frame.” One quadrant would be Micro 4/3. Two of those would also be APS-C.

        Or, if it’s easier – Micro 4/3 (2x crop) is 0.5x on the horizontal and 0.5x on the vertical. APS-C (1.5x crop) would therefore be…. 0.5x (half) on the horizontal and merely 1x (no crop) on the vertical (and 0.5x + 1x = 1.5x).

        I was a math/science nerd in high school, so it’s easy for me to approach it in such terms. Maybe would be for others too.

        Hamish is absolutely correct about different sensors reacting differently to how light hits the sensor at various angles. For example, the Leica M8/M9 are notably better in the edges of the frame with certain M-mount lenses than, for example, a Sony full frame camera. Sensors are optimized for their native lenses, so some non-native lenses can react wildly different from sensor to sensor.

    • Avatar
      Reply
      George Appletree
      October 29, 2017 at 9:07 pm

      Xpro1 factor is 1.5

      • Avatar
        Reply
        Terry B
        October 30, 2017 at 3:06 pm

        Matt. Also Leica sensors use angled microprisms.

  • Avatar
    Reply
    Hamish Gill
    October 29, 2017 at 7:12 pm

    Are we talking at crossed purposes? – https://www.fujifilm.eu/uk/products/digital-cameras/model/x-pro1/specifications-4484
    “Image sensor 23.6 mm x 15.6 mm (APS-C) X-Trans CMOS with primary color filter”

    Full frame is 24 x 36mm, half frame is 24 x 18mm – exactly half, but the frame is vertical

  • Avatar
    Reply
    Richard Patterson
    October 29, 2017 at 7:27 pm

    You’re quite right!

    PS I think I read your post about modding the Industar 69 before I modified mine. Really pleased with the results.

    • Avatar
      Reply
      Hamish Gill
      October 29, 2017 at 7:42 pm

      Brilliant – which camera do you use it on?
      I’d do it again, but these days the lenses cost a fortune compared to what they did back then…

      • Avatar
        Reply
        Daniel R Westergren
        August 15, 2018 at 6:37 pm

        Ha, That’s how I found your wonderful website as well. Use the Industar 69 on fujifilm x-pro1, and x-T2. Works great, but at f5.6 it’s too good, no personality, may as well use an autofocus lens. At f2.8 it’s nice, but don’t be tricked into putting the main subject anywhere but in the center if you need it to be sharp.

  • Avatar
    Reply
    Richard Patterson
    October 29, 2017 at 8:54 pm

    I use it on an Olympus Epl-3. The unmodified lens was £5.50 + £6.00 postage from Belarus! I’ll email you a sample pic.

  • Reply
    Modding a Halina Anastigmat f/3.5 45mm - by Tobias Eriksson - 35mmc
    July 23, 2018 at 3:01 pm

    […] (yes – retro!) look of the pictures produced by both my Industar 69 and my modded Canon Demi 28 mm lens, on my Fujifilm X-Pro, so was keen to see what soft and perhaps swirly impressions I could get […]

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