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21mm vs 24mm

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  • 21mm vs 24mm

    Hello -
    I'm sorry if this has been already discussed. I searched the forums but didn't find a real answer to this question, although I might have missed it as it is hard to find oneself in these constant crop factor debates
    I have seen almost everyone talk about the different 24mm lenses they will be using as their "normal" lens on the BMC. However, only a very few have mentioned 21mm lenses, which would most likely resemble a 50mm lens on the BMC more than any 24mm (≈48mm vs ≈55mm). There aren't many options, but both Zeiss lenses seem really interesting. The Zeiss Distagon ZE T2.8 for the lower budgets and the Zeiss CP.2 T2.9 for the higher end productions. Am I completely wrong to think this? If so, please explain. And if, despite this, you still decide to go for a 24mm, are there any other reasons besides the economical one?

    Edit: I'm sorry I didn't specify but I am obviously referring to the EF mount version of the BMC and the ZE mount for both Zeiss lenses. MFT lens options are obviously also welcome.

  • #2
    One reason might be to get a fast f/1.4 lens. That's two stops faster than the Zeiss. Big difference.


    • #3
      You can look at a "normal" lens in two different ways, IMO:

      1) A lens that comes close to the look of a 50mm on full-frame, or 2) the length of the diagonal of the sensor.

      The diagonal of the sensor on the BMC is 18.13mm.

      If you go with the diagonal of the sensor for your "normal" lens, the best option, IMO, would be the Voigtlander 17.5mm f/0.95 MFT mount.

      This might be considered a "wide-normal" lens if you're looking for something that resembles a 50mm in 35mm photography, or it might be exactly what you're looking for.

      If you want something longer than that, you might fancy the Voigtlander 25mm f/0.95.

      I personally think the MFT lens mount is superior, but if you want to stick with the EF mount and don't care about anything larger than f/2.8, you really need to try both of those Zeiss lenses.

      On FF 35mm, some people prefer slightly wider than 50mm for their "normal" lens.


      • #4
        Agreed. 24mm is already seriously wide for the FF sensor it's designed for, 21 is getting into ultrawide territory, yet we're using them as std lenses with a much smaller covering circle than they were designed for.

        A 24 mm will have a few fewer compromises than 21 and will be somewhat less retrofocus, a bad thing generaly in lens design, so can be made a tad faster, say f1.4 rather than f/2.8. I know Sigma do a 20 f/1.8 but that lens is so flawed that I really wouldn't consider it, as well as being big and heavy.

        This is a very good reason for going m4/3 since the covering circle can be smaller (Fewer compromises) and the flange-focal distance is shorter, hence a 24mm can be much less retrofocus, about as much, say as a 50mm on 135 format.

        Edit, Robert J got in while I was typing.