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Focal lengthes in e mounts

Discussion in 'Sony Alpha E-Mount Lenses' started by alaios, Dec 4, 2013.

  1. alaios

    alaios TalkEmount All-Pro

    Jan 11, 2013
    Düsseldorf
    Alex
    Hi all,
    something that sometimes gets confusing me.

    When I but a legacy lens, for example some 1980s Minolta. The real focal length is what is printed on the lens multiplied by 1.5

    but when I buy a native e-mount lens, like the Sigma 30mm the focal length is indeed what is written over the lens with no multiplication.

    Is that right ?

    Regards
    Alex
     
  2. veljko

    veljko New to TalkEmount

    4
    Dec 4, 2013
    Any e-mount lens or any other lens should be multiplied by 1.5 crop factor (in NEX cameras case), to calculate full frame equivalent. Having said that, the Sigma 30mm is actually 45mm lens considering crop factor and magnification of the frame.
    veljko
     
  3. quezra

    quezra TalkEmount Top Veteran

    916
    Aug 22, 2012
    You're wrong that there's no multiplication on native E-mount lenses - it is still there.

    Generally, in the interchangeable lens world, lenses are always identified by their true focal length in mm (manufacturers aren't always perfectly accurate here though!). Comparing across different systems, you generally need to multiply by the crop factor to get an idea of their relative field of view. So a m4/3 25mm lens is a 50mm lens in FF equivalent, the Zeiss 32mm is closest to this on APS-C NEX, etc. Old legacy lenses are almost all full frame, but the effect you see on a NEX camera will be affected by the crop factor. But put it on an A7 and you get back the original length.

    It's mostly in the compact market where they ignore the lens's true focal length and give only the 35mm equivalent of the focal length - e.g. the RX10 has a "24-200mm" lens but actually it's a 8.8-73.3mm lens... it's just that the crop factor of all the different compact sensors is a nightmare to calculate!
     
  4. davect01

    davect01 Super Moderator

    Aug 20, 2011
    Fountain Hills, AZ
    Dave
    It is a bit confusing.

    I myself don't really care what the full frame equivalent is, since I don't own a full frame, nor compare my images to full frame images. I know this is important to some, just not me.

    I do care a bit when comparing legacy versus native emounts. Simply remeber that when you add an adapter you are going to be closer. The math is a bit fuzzy depending on what adapter you buy, if the lens was a full frame or not, etc.
     
  5. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    First off, the focal length of a lens stays the same no matter what camera you mount it on. If you use an old Pentax 30mm f/2.8 (designed for 35mm film cameras), it behaves exactly the same as your Sigma 30mm when you mount it on a NEX camera: the field of view is the same, you'll see the same image. The Sony 50mm f1/.8 shows the same image, covers the same area as an old Minolta 50mm lens.

    The problem with all this crop factor crap is that people wanted to know what a 30mm lens would be like compared to the focal length on a 35mm/"Full-frame" camera, because in earlier days almost everybody used 35mm cameras and most people had a feel for what a lens would do on such a camera. The answer here is that 30mm on a NEX camera has the same field of view as 45mm on a Minolta X-700.

    You can and should ignore this crop crap. A 50mm focal length is a 50mm focal length no matter what, native E lens or legacy lens.
     
    • Like Like x 1
  6. Gandalf

    Gandalf TalkEmount Regular

    59
    Sep 5, 2013
    Nope. The focal length of any lens remains what is written on the lens, no matter what camera you put it on.

    All that changes is the field of view, and that is because the format of the camera is different. (i.e., a smaller sensor sees less of the image that is projected by the lens, so the field of view is less.)


    If you are not changing back and forth between full frame and APS-C, or between APS-C and 4x5, or whatever, the best advice is surely to stop worrying about the differences and just get comfortable with what each lens does on your camera...
     
  7. davect01

    davect01 Super Moderator

    Aug 20, 2011
    Fountain Hills, AZ
    Dave
    I would agree as I am a one system guy. I really don't care how my lens compares as placed on a FF.

    It does however matter when comparing E-Mount versus Legacy mounts.

    An E-mount 30mm, (such as my Sigma), will not have the same "field of view" as a legacy 30mm + adapter will.
     
  8. WoodWorks

    WoodWorks Super Moderator

    Dec 12, 2012
    Ashland, OR, USA
    David
    That's not correct, Dave. Unless A) you're using a "turbo booster" style adapter, or B) your 30mm legacy lens isn't actually a 30mm, then an e-mount 30mm and a legacy 30mm plus "dumb" adapter will have exactly the same field of view. A simple adapter with no glass doesn't have any effect on the field of view like, say, a teleconverter would.
     
    • Like Like x 1
  9. alaios

    alaios TalkEmount All-Pro

    Jan 11, 2013
    Düsseldorf
    Alex
    Hi,
    that what I also thought and asked here. The main motivation for starting this post was my feelind that the 19mm sigma lens (around 30mm on full frame) adds a bit of distortion around the frame edges (making lines appear longer).

    R
    A
     
  10. Gandalf

    Gandalf TalkEmount Regular

    59
    Sep 5, 2013
    Sorry, that is incorrect. The field of view depends on only two things: the format of the camera (i.e., sensor size) and the focal length of the lens. A 30mm lens will still be a 30mm lens regardless of what mount it is in, or what camera it is adapted to (with the obvious caveat that we are talking about direct, "glassless" adapters.) And any 30mm lens, on the same camera, will have the same field of view.
     
    • Like Like x 1
  11. davect01

    davect01 Super Moderator

    Aug 20, 2011
    Fountain Hills, AZ
    Dave
    I have the e-mount 30mm Sigma and a legacy 28-80 mm Sigma and I can tell you the legacy lens is a lot closer.

    I will take a few shots to show the difference.
     
  12. WoodWorks

    WoodWorks Super Moderator

    Dec 12, 2012
    Ashland, OR, USA
    David
    What do you mean by "a lot closer" Dave? Unless your Sigma legacy zoom is seriously out of whack, when it is set to 30mm, it should exactly match the field of view of your 30mm prime. A glassless adapter, as Gandalf has stated, will have no effect on its field of view.

    But I look forward to seeing your samples. No doubt there's a logical explanation for the difference you see.
     
  13. eno789

    eno789 TalkEmount Top Veteran

    720
    Jan 1, 2012
    NoCal, USA
    Brian
    You can only have wider field of view (angle), or narrower field of view.

    With the same focal length, some lens can focus closer than other. That's a separate attribute of the lens. For example, there can be 50mm normal lens with MFD (minimum focus distance) of about 50cm, there can be 50mm macro lens with MFD of 20cm.

    When people say effective focal length, it's just a short way of saying the focal length on 35mm film (or full frame sensor) that gives you the same field of view.
     
  14. veljko

    veljko New to TalkEmount

    4
    Dec 4, 2013
    Maybe it's worth to say here, some cheap adapters were incorectly made in sense of distance which have to cover from sensor to lens. In practice, every NEX adapter should be 25mm deep from e-mount to adapted mount. If it is longer, the crop factor will be bigger and opposite. For example I possess M42 to e-mount adapter which is 2mm longer than it should be. This fact couses that my 50mm CZ tessar is practically 55mm lens, or 80mm instead of 75mm considering crop factor. Sorry for my sometimes incorrect english.
    V.
     
  15. veljko

    veljko New to TalkEmount

    4
    Dec 4, 2013
    Correction, not every adapter for NEX should be 25mm deep from e-mount to adapted mount, I didn't mean that, it depends of flange focal distance also known as the flange-to-film/sensor distance and it is different for different cameras and manufacturers. But what is important here, it must be correctly calculated in order to retain original focal lenth of the lens. Regards
    V.
     
  16. Gandalf

    Gandalf TalkEmount Regular

    59
    Sep 5, 2013
    Flange focal distance enables the lens to focus, it does not and can not affect the focal length.

    Nothing in the adapter can change the focal length of a lens. If the adapter is too short, turning the focusing ring of the lens will move the lens toward or away from the sensor; when the lens is focused at infinity it will be the exact same distance from the sensor as it was on its original camera, or any other camera using any other adapter.

    In fact, if your adapter was 2mm too long, it is doubtful that you would be able to reach infinity focus since very few lenses have 2mm of overshoot in the focus mechanism.

    ----------
    Something that I think has not been mentioned, is that the focal lenght of many lenses is not exactly what is marked on the lens. Fifty millimeter lenses from various manufactureres might range from 48mm to 52mm or even more. But, whatever the actual focal length, it does not change via a simple adapter.
     
  17. nex5r

    nex5r TalkEmount Rookie

    15
    May 30, 2013
    I compared my nex-5r with my new nex-7 and with the same lens there are differneces in focal length... the 7 seems to be closer to the subject. Is there a crop factor difference bc of the difference in megapixels?
     
  18. NickCyprus

    NickCyprus Super Moderator

    Oct 11, 2012
    Cyprus
    Nick
    Logically there shouldn't be any difference.
    Really doesn't make any sense but I'm no expert so...
     
  19. nex5r

    nex5r TalkEmount Rookie

    15
    May 30, 2013
    I will shoot some pix a little bit later and post them as comparison both with the 18-55 kit-lens...


    Sent from my GT-I9505 using TalkEmount mobile app
     
  20. nex5r

    nex5r TalkEmount Rookie

    15
    May 30, 2013
    OK... They don't are different if they are both with the kit-lens, but the kit-lens at 35mm is not the same as the 35mm f1.8!

    Sent from my Nexus 4 using TalkEmount mobile app