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I need some help understanding the Speedbooster for M43
  • I have a Panasonic GH3 and am considering the Mitakon Lens Turbo for M43 to Canon EOS EF. I understand this adapter gives an extra stop of light and allows me to use 35mm glass. My question is, how do I determine what the f-stop will be and focal length of a full frame lens once equipped with the lens turbo? For example, if I attach a Canon 50mm f1.4, would the focal length still be 50mm and the f-stop still 1.4 - or would it be less?

    I appreciate your help,

  • 12 Replies sorted by
  • When you attach a Speedbooster to any lens, it gains a stop of light gathering capability, and effectively shortens the focal length by 0.71x. So your Canon 50mm F1.4 will behave roughly like a 35mm f/1. On a GH3, to calculate full frame equivalence you need to multiply the focal length of a lens by 2, so 2 x 35mm = 70mm.

    Here is where it gets complicated: you have the light gathering ability of an f/1 lens, yes, but not the depth of field. You still have the depth of field of a 50mm f/1.4 lens that was cropped down to 70mm equivalence, or expressed another way, a 70mm f/2.8 lens.

    So to summarize, your 50mm f/1.4 lens with a Speedbooster on a GH3 will look similar to how a 70mm f/2.8 lens looks on a full frame camera, except that you have the light-gathering capability of an f/1 lens.

  • Thank you, Sangye, that helps a lot. Where I am still uncertain is the depth of field. So, I will be able to go down to f1.0 (in terms of light-gathering in low light and such) but the max DOF I will achieve is f2.8?

  • What your saying is that, in order to have an actual depth of field of f1.4 with the speedbooster - I would need to have, say, a 50mm f.07?

  • Let me explain it simpler.

    1. You have Nikon 50mm F1.4 FF lens
    2. Make photo on Nikon FF body. Store it.
    3. You add Lens Turbo adapter.
    4. You get APS-C body, say Sony NEX. And attach adapter and lens to it.
    5. You make photo on this NEX body.
    6. If you compare EV values of photos they will be almost equal.
      Field of view will be equal. DOF will be also equal if you look at photo at same size.

    All adapter is doing is focusing larger image circle of lens to smaller area ( 0.71x time smaller).

    FF will still have better dynamic range (and noise), of course.

  • Now I am really confused. You are saying that, essentially, the 50mm f1.4 would effectively preform exactly like a 50mm f1.4 on my M43 camera with the lens turbo adapter - the only difference is maybe dynamic range.

  • You are saying that, essentially, the 50mm f1.4 would effectively preform exactly like a 50mm f1.4 on my M43 camera with the lens turbo adapter - the only difference is maybe dynamic range.

    No, read CAREFULLY - "APS-C body".

    I just made it as simple as possible. m43 sensor is smaller, so it is more complicated (you have image crop).

  • @Vitaliy_Kiselev -- EV values will not be equal, provided that it was shot at the same shutter speed, f-stop, and ISO. The APS-C body will be 1 stop brighter with the Speedbooster refocusing the image circle onto a smaller sensor. Of course ISO performance on smaller sensors generally isn't as good as on full frame sensors, so the image may be a little noisier depending on complex things like pixel pitch and sensor technology, but it will be brighter.

  • @Sangye

    Nope.

    Suppose we have two 20mp sensors, FF and APS-C.
    Just pixels on FF one are larger, of course.
    In the case of using same lens (and lens turbo on APS-C) we have same amount of photons hitting same pixel per same amount of time. In reality Lens turbo lenses also are not perfectly transparent, so, APS-C will be slightly less, but we can ignore it here for simplicity.
    Of course, total photons that hit sensor are also about the same. It is whole purpose of such adapters.

    So, with same sensor gain values we will get same EV values.

    If you mount FF lens on APS-C body using adapter without lenses all you have is central portion, hence, less photons hitting sensors per same amount of time.

    Thing that adapter can't change is actual pixel capacity that defines dynamic range, for FF sensors it will be better.

    The APS-C body will be 1 stop brighter with the Speedbooster refocusing the image circle onto a smaller sensor.

    APS-C body will be 1 stop brighter than SAME APS-C body with same lens mounted via simple adapter. Period.

  • @Vitaliy_Kiselev -- I always thought that ISO was standardized across differing sensor technologies? That a photo taken, for example, at F/2.8, 1/100s, and ISO 400 will have the same EV whether it's taken on a 20MP full frame camera or a 20MP APS-C camera? I understand what you are saying about total number of photons hitting the same number of pixel sites, and I know that the gain values will vary to achieve the same ISO value, but once the ISO, shutter speed, and f-stop match, the EV has to match too, right?

    If that is true, then using a focal reducer on the APS-C body to make the image circle smaller and brighter than on the Full Frame camera should have the effect of boosting the EV by one stop. Of course you are still getting the same number of photons and you aren't magically creating light, and the only reason that it looks brighter is for ISO 400 on 20MP APS-C to match ISO 400 on 20MP full frame, the APS-C body needs to increase the gain...

    Please correct me if I'm misunderstanding how ISO works and how it relates to actual sensor gain values.

  • If that is true, then using a focal reducer on the APS-C body to make the image circle smaller and brighter than on the Full Frame camera should have the effect of boosting the EV by one stop.

    Very simplified view is to understand ISO as standardized (vaguely) gain values.

    As for things I am telling, you can make your own experiment.

  • The math can get confusing, so start with this: these focal reducers usually have a focal reduction factor of 0.71x.

    To determine the focal length: Multiply the lens' focal length (e.g. 50mm) by the reduction factor (e.g. 0.71x):

    50 * 0.71 = 35.5mm

    To determine the light gathering ability: Multiply the lens' aperture (e.g. f/1.4) by the focal reduction factor:

    1.4 * 0.71 = 0.994 (round this to f/1.0)

    Depth of field does not change, but as the field of view widens, the image can have the appearance of a shallower depth of field.

    Once you use a focal reducer, it makes more sense.

  • @sangye, you are right. Same exposure (f-number and shutter speed) and same ISO setting means the same exposure value in the exposure meter, regardless of the sensor size.

    If you cut a frame of ISO 100 film in half, it is still ISO 100. The film doesn't care how big it is.

    Larger formats tend to have slower lenses, and thus need to use higher ISO settings to be equivalent.