The need for EXTREME speed: Rodenstock 50mm f/0.75 in m43 mount!

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The eBay reseller “jieying-usa” is reworking lenses so that they can fit on Micro Four Thirds cameras without the need of any adapter! And the Rodenstock 50mm f/0.75 is one of those lenses with m43 mount (Click here to check the acution on eBay). The lens focuses from 3ft to infinity. Wide open, images are very soft but unlike C-mount lenses the circle covers the full m43 size. You can see a few image samples on Flickr. The same reseller also makes the Kowa 55mm f/1.0 with m43 mount.

P.S.: A question to our readers. How many stops difference do you have between f/.075 and the f/0.95 from the Voigtländer?

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  • Chingachgook

    0.68 stops.

    • daxingwuxiang

      Rodenstock TV-Heligon 50/0.75

      [img]http://www.tricitychinese.ca/data/attachment/forum/201205/19/225040d7zd5dzr1s7ra7rj.jpg[/img]

  • Parci

    f0.7 is one stop from f1.0, so about 2/3 of a stop.

  • Andy

    Over a full stop actually.., .75 x 1.2 = .9 :)

  • peter

    [(ln 0,95/ln 0,7)/ ln (sqrt(2)]

  • peter

    ups… this is correct : [ln (0,95/0,75)/ ln (sqrt(2)]

  • Thom

    Will someone explain how an f/.75 lens is possible? I thought that an f/1.0 was a 1 to 1 in the amount of light let through. Does this mean that an f/.95 or .75 will give the film/sensor more light than is in the scene? That doesn’t seem possible. I’ve wondered this for sometime and have even asked some pro photographers, but I have never received an answer. I would like to understand what these numbers mean. Thanks.

    • mat

      My understanding: A 50mm lens with an aperture of 1.0 opens up 50mm at its maximum at the aperture blades. A 50/1.4 opens up 36mm; f2.8 would be 18mm (approx.), and so forth. You can see the difference when looking through the lens at maximum aperture. Therefore a 50mm/0.75 would open up about 66mm.

      The f-stop is the ratio of opening at the front of the lens to the focal length. 50 divided by 1 = 50; 50/2.8 = 18.

      The Voigtlander 25mm/0.95 would have a front opening (at the aperture blades, not necessarily the front element of the lens of 26mm.

      When you’re stopped down to, say, f22 the opening is very small, but gets smaller based on focal length. You can see this looking at any lens where you’re manually stopping down the lens A 50mm lens at f22 is still opened wider than a 28mm lens at f22 — 2.27mm versus 1.27mm.

      • mat
      • QKYC

        Actually, I think the F-stop is calculated as the focal length divided by the scaled aperture diameter at the focal length, not the actual aperture diameter opening at the lens. Think about it, the Tamron version of 300mm f/2.8 has a front filter opening of 112mm. At f/2.8, the apecture opening at to be 107mm. How do you fit the aperture blades in 5mm of space!? (okay, there are extra bits of lens barrel width, but still it’s a little too much stuff to fit in there)
        So, if the apecture opening is bigger than the scence, where is the light coming from? From outside of the scence?

        • Chris

          Often times the aperture blades sit in between lens elements more towards the middle or the back of the lens. At these areas, the physical size of the lens elements are usually smaller, allowing more space for the aperture blades.

  • lolicht

    a little bit less than 0,7 stops … (0,696)

  • Fimann

    Thom: f1.0 is just a masure that was decided by some human in the past. It s supposed so resemble the same light sensitivity as the human eye. So anything below 1.0 means only that the lens is more sensitive than the human eye.

  • Bu

    With that lens on it wouldn’t fit in my pocket any more :/

  • hd72

    Those photos on Flickr are mostly extreme closeups, but the m43 modified version sold on eBay has a minimum focus distance of 3 feet… Flickr also has a photo of the lens, and it looks nothing like the one that was modded. I guess they produced different versions of the 50mm f/0.75, so results will greatly vary.

    The Flickr photos are definitely gorgeous in an artistic way — aside from that spotty tomato. :)

  • A lens’ transmission is proportional to the area of its aperture, so if you set f equal to 1, divide by the f-number, and multiply by pi, you’ll find that the f/0.75 lens should transmit 26.6 percent more light than the f/0.95 lens.

    That’s assuming all other things are equal, which they almost never are. in this case, I suspect that the Voigtlander lens’ more modern design and coatings make their actual light transmission almost exactly equal. If somebody would like to send me both of them, I’ll be happy to perform some tests!

  • Esa Tuunanen

    How such simple thing can produce so much misconceptions?

    Instead of aperture F-number should be called as focal ratio because that’s what it is: Focal length divided by size of lens (or mirror) used for gathering light.
    In case of optics with variable aperture (vs. fixed aperture telescopes) like photographics lenses diaphragm is deeper inside the lens where it simply blocks light coming from outer area of lens letting in only light coming from center area of lens effectively controlling size of light gathering area and hence focal ratio.

    • The aperture is the “hole” in the lens through which light energy passes, and the f-number is simply a way of measuring that “hole” so that lenses of different focal lengths can be compared in terms of equivalent exposure. Would that make it more clear?

  • llamaresurgency

    There isn’t much point to buying a f-less-than-1 for a Micro 4/3rds camera, because the 2x multiplier will render the lens to have much less depth of field. It’s more like a 100mm f/1.4, but with much worse quality. And the quality of this one looks terrible.

    While I haven’t received mine just yet, a Lensbaby tilt transformer is probably a much better way to get razor thin depth of field, if you’re into that. Otherwise, pick up an SLR and a 85mm f/1.8 or so (cheap), or a f/1.2 (not).

    Also for those unfamiliar with lenses this fast, it’s often impossible to use them in medium lighting conditions wide open, because the camera shutter simply doesn’t operate fast enough. So I really don’t see a point getting much below f/1.2, unless you have a lens that produces something legendarily wonderful like a Noctilux. Which this won’t.

    • ljmac

      What a ridiculous comment! Shallow DoF isn’t the only reason for a fast lens – the main one is brightness. An F0.75 will lens will have the same perceived brightness (i.e. light intensity, which is what determines exposure) on 4/3rds as it will on full frame. And even if you DO want shallow DoF, a larger aperture lens will give you shallower DoF on any format. A 4/3rds user doesn’t care if the lens will give him shallower DoF on full frame – he only cares that it will give him shallower DoF relative to slower lenses on HIS camera.

    • hd72

      I agree with ljmac – these comments are absurd.

      There’s no point in this lens when someone can just buy a 5DMarkII instead??!! Of course a m43 camera will give more depth of field than a full frame camera – in all situations. This lens will give you a much shallower depth of field – relative to all your other m43 lenses. It doesn’t matter how it compares to a completely different system.

      Also, if you plan to use a bright lens, wide open, in bright light… that’s why they make neutral density filters. They’re not just for blurring waterfalls – portrait photographers use them to keep a shallow depth of field in well-lit scenes. And you can pick one up for much cheaper than a full-frame camera!

  • Max

    Yes, but the universal F-stop is important in the old days of using a light meter, that you’d adjust the dial to show you the proper f-stop and shutter speed. So the f-stop light transmission must be independent of which focal length lens you use. So a properly exposed negative using a 28mm f2.8, would give the same correct exposure with a 200mm f2.8.

    But today with in camera meters the actual number is meaningless. Just an artifact of the 20th century.

    On a side note to Esa Tuunanen, do you have a calculation for minus diopter filters? say a -4 diopter alters the focal distance by X? I know people use + diopters for shortening the focal distance in front of the lens for Macro. But I would like to try a – diopter behind the lens to lengthen the focal length to adapt a lens to m43. I’ve search on the web but no specific measurements. Like -4 diopter would increase the focal distance by 2mm?

    I have a nice vignetting c-mount lens I’d like to modify. Like give it some reading glasses. I think it should work:-)

  • Julius

    0,95/0,75 = 1,266
    1,266/1,4142 = 0,8956
    A little less than one stop…
    (1,4142 is square root of 2)

    • Voldenuit

      That’s not how you calculate stops. peter has the correct formula (in his second post).

  • Joost

    I already provided this information on this website before but I will do it again:

    This modified lens is not a f0.75 lens anymore, because it is modified using a negative lens (think teleconvertor), to make it able to focuss to infinity.

    The examples you see on flicker are 0.75 but they are extreme closeups because without an extra lens it is (almost?) impossible to make these lenses focus further away.

    • Vlado

      why should a negative lens change F-number …. the entrice pupil and focal length stay some (if the optic is made correctly) what does change is the T-number …

      • Joost

        Because the focal length does change in this case:”After rework image little large than 50mm”

    • hd72

      Since we’re repeating ourselves… This is apparently the lens that took most of those photos on Flickr:
      http://www.flickr.com/photos/odderikgarcia/2378068456/
      They could both have the same focusing distance — i can imagine that there’s not much need for any x-ray lens to focus to infinity LOL

  • Vlado

    “P.S.: A question to our readers. How many stops difference do you have between f/.075 and the f/0.95 from the Voigtländer?”

    you sure you asked f/.075? not f.75 ;)

    for f.75 it’s 0.6824 …

  • It is 0.6821 stops. Check it on this page, You can calculate anything here.
    http://imaginatorium.org/stuff/stops.htm

  • qbic

    lol quality

  • marauder3c

    If you’re looking at super-fast lenses for low light, don’t forget that as your lens gets longer your ability to hand-hold goes down. Comparing this to the 25/0.95, you might gain some speed from the larger aperture, but lose a full stop in your ability to hold the lens steady (rule of thumb suggests 25mm use 1/50s and 50mm use 1/100s on m4/3).

  • Strong_V

    haha, great mathematicians …
    count has an accuracy of 10 decimal places …
    the right answer – 2 / 3 stop :P

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