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confusion over focal length of lenses

Discussion in 'Adapted Lenses' started by grebeman, Nov 13, 2010.

  1. grebeman

    grebeman Mu-43 All-Pro

    Mar 13, 2010
    South Brent, south Devon (UK)
    Why is there so much fuss made about this aspect of m4/3 and this constant comparison to 35mm. Go back to basics. Take the diagonal of the negative, or sensor in this digital age and divide it by the focal length of the lens. The resulting number gives you a quotient (just a fancy way of saying a number) that gives you a measure of the angle of view of the lens. This is independent of the format of the camera.
    So a m4/3 sensor has a diagonal of just under 22mm, a 35mm negative has a diagonal of 43mm, a 21/4" square negative has a diagonal of 79mm, a 6x9 negative has a diagonal of 105mm, a 4x5" negative has a diagonal of 155mm. The standard lenses for these formats were always regarded as 50mm for 35mm, 80mm for 21/4" square, 105mm for 6x9 and 150mm for 4x5".
    When you divide the diagonal of the negative by the standard focal length for each format the result is approximately 1 for each of these. If you use a lens of shorter focal length the number obtained is greater than 1, if a longer focal length lens is used, the number is less than 1.
    Therefore on m4/3 the standard lens should be about 22mm focal length, lets call it 20mm and for simplicity and easy calculation lets call the sensor diagonal 20mm
    Fit a 50mm focal length lens and do the simple mathmatics and the result is 20/50 = 0.4, therefore on m4/3 a 50mm lens is a long focal length lens, on 35mm the result is 43/50 = 0.86, on 21/4" the result is 79/50 = 1.58. Exactly the same focal length lens, different results on each format, different perspective obtained.

    diagonal/focal length vs angle of view
    0.35 = 20 deg
    0.54 = 30 deg
    0.73 = 40 deg
    0.93 = 50 deg
    1.15 = 60deg
    1.40 = 70 deg
    1.68 = 80 deg
    2.00 = 90 deg
    2.38 = 100 deg
    2.86 = 110 deg

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  2. I personally have no problems with the traditional means of relating the field-of-view (FOV) or angle-of-view of a lens back to an equivalent focal length lens on a 35mm film frame. It has effectively become a base unit relative to the most widely used photographic format of the last century. Just as base units of mass, force, etc relate back to a measurement which may no longer mean anything to us, the same will most likely happen with lens classification as 35mm film becomes a distant memory.
  3. BillN

    BillN Mu-43 All-Pro

    Jan 19, 2010
    SW France

    Thanks for the posting - there are always quite a few postings all over the internet on this subject

    I wouldn't get too concerned the debate will go on forever

  4. grebeman

    grebeman Mu-43 All-Pro

    Mar 13, 2010
    South Brent, south Devon (UK)
    Yes I guess you're right there Bill, I should learn to relax :rofl:

  5. Ray Sachs

    Ray Sachs Super Moderator Subscribing Member

    Apr 17, 2010
    Near Philadephila
    If that system works for you, good enough. But I've been using 35mm equivalencies for so long, its the way my limited brain is wired. I know what 24mm or 35 or 50 or 200 means. I have no idea what 60 degrees means. I mean I KNOW what 60 degrees means, but I just don't think about it that way. I could learn, but I already had to learn to convert non-35mm format gear to 35mm equivalents and I'm over 50 now, so I don't want to learn anything else! :cool:  Or, at least, unless there's a really compelling reason to do so and I don't see one here.

    Don't worry, there are similar arguments in the cycling world over "gear inches" versus other methods of measuring gearing (some of which take the size of the tire and the length of the crank into consideration, and some ofwhich don't). And there are good arguments for moving on from gear inches, but I know what 27 gear inches means and what 50 and 108 gear inches means and feel like and I just don't see a compelling reason to rewire my increasingly difficult-to-rewire head to some other system.

    I'm sure there are similar arguments blazing in every technical pursuit in the world. Its a long and proud tradition!

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  6. Rudi

    Rudi Mu-43 Top Veteran

    Aug 16, 2010
    I'll have to agree with Ray here, Barrie. Your classification just seems too complicated to me (I know it's not, that's why I typed "seems"). Since I am so used to the 35mm system, it must by now be a part of my DNA, so a 35mm-equivalent FOV makes perfect sense to me without even having to think about it. Considering how many people use that convention, I'd hazard a guess that I am not alone! :smile:
  7. grebeman

    grebeman Mu-43 All-Pro

    Mar 13, 2010
    South Brent, south Devon (UK)
    I don't think in terms of angle of view, all you need is the ratio of the sensor diagonal to the focal length of the lens, then you know if your lens is wide angle or telephoto, and how much by how much that number deviates from 1.00. Perhaps my ideas are coloured by being more of a medium format convert, and thus 35mm sizes mean little to me.

  8. grebeman

    grebeman Mu-43 All-Pro

    Mar 13, 2010
    South Brent, south Devon (UK)
    To anyone who is interested my thoughts are based on fundamental photographic theory, the table of diagonal length to angle of view is taken from "The Manual of Photography", my edition being the 1975 reprint, this was formerly "The Ilford Manual of Photography" originally published in 1890.
    The ratio of diagonal length (of the sensor) to focal length of the lens is format independent.

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  9. NetizenSmith

    NetizenSmith Mu-43 Regular

    Oct 28, 2010
    I understand the 35mm film frame and therefore what to expect from particular focal lengths on that system. It's only natural that I will double the focal length in my head on a 4/3 mount to understand how much I'm going to get in the shot.
  10. Brianetta

    Brianetta Mu-43 Veteran

    Sep 5, 2010
    North East England
    Brian Ronald
    I think we should stop comparing everything to 35mm and accept that this is a micro four thirds forum.

    Personally, I'm unfamiliar with 35mm fields of view for given focal lengths. What really gets on my wick is when people start throwing in 35mm focal lengths (occasionally with the unqualified adjective "equivalent") without explaining why.

    My previous camera has a zoom lens that goes from 6.1 to 61mm, and that's recorded in its EXIF data. If I were to start converting all my focal lengths to whatever gave the same field on that, others would feel much as I do when they start doing so with 35mm.

    Grebeman: Rather than going with the diagonal, I'd rather just go with the horizontal. Then I can estimate the calculated field angle against the floor plane.
  11. usayit

    usayit Mu-43 Hall of Famer

    35mm has been around since the 1930's... You can't just expect that to change overnight. Besides... in the last 10-15 years, how many different sizes of sensors have been produced/sold. Measuring different formats against a well known "yardstick" makes it easier for all sorts of people to have discussions while staying on the same page. I have no doubt similar type of discussions were held when 135 started to overtake Medium Format in popularity.

    Somewhat related...

    The U.S. has been using the english system and has yet to move to the metric system for the same reason. Heck, even England has moved on.
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  12. photoSmart42

    photoSmart42 Mu-43 Top Veteran

    Feb 12, 2010
    San Diego, CA
    For folks who have used other formats besides m4/3, the simple 2x FOV relationship to 35mm/FF works fine. To those who haven't used anything besides 4/3 or m4/3, it's irrelevant - they're used to getting whatever they see from their lenses, and no reference point is needed. Having to memorize some giant table of fractional numbers would be overly complicated and unecessary IMO.

    Besides, where's all the fuss made over m4/3 and 35mm? The fuss is normally about aperture/DOF comparisons/equivalency), and less about FOV (which is easy to understand).
  13. Djarum

    Djarum Super Moderator Subscribing Member

    Dec 15, 2009
    Huntsville, AL, USA
    I'm not sure I worry too much about this. I use whatever focal length that gives me the FOV and perspective I want.
  14. Ray Sachs

    Ray Sachs Super Moderator Subscribing Member

    Apr 17, 2010
    Near Philadephila
    I have cameras that range all over the place. But to show you how much of a convention the 35mm equivalent is, the metadata information in my photo software (Aperture) includes a line for the actual focal length and a separate line for the focal length (35mm equivalent). For some cameras, this line is filled in, for some its left blank, so I guess its up to each manufacturer to decide whether to provide the information or not. But many DO and Apple though to include the space in their software. So its not just some arbitrary thing!

    And even in terms of today's cameras, a Full Frame DSLR, which is still sort of considered the gold standard, uses a sensor that is essentially the same size as a 35mm slide or negative. So it still has a basis in current equipment, just not as overwhelmingly prevalent as it was 30 years ago. Even then, I had a half frame Canon as a kid that used 35mm film but oriented each shot in portrait mode using half of a 'conventional' frame. And I had to convert that too (although being a fixed focal length single lens camera, it was not hard!). It wasn't tough then, its not tough now.

    I have no problem with you thinking about field of view and focal lengths however you'd like to. But I'd appreciate the same consideration - 35mm equivalents are, as someone else mentioned, in my bloodstream nearly down to the level of DNA. I can hear it a and know what it means without having to think about it beyond that. I appreciate that and will continue to use it, as I suspect most folks will for another 20-30 years. At some point it will be completely irrelevant and the generations of photographers who have it stored in their (our) bones will all be gone, and something else will probably rise up in its place. But not yet.

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  15. Narnian

    Narnian Nobody in particular ...

    Aug 6, 2010
    Richmond, VA
    Richard Elliott
    Actually I did the opposite of what Barrie (grebeman) did. I divided the focal length by the diagonal - the reason I did that was so that the larger numbers corresponded to the long lenses. I also prefer it because the number also corresponds to the magnification factor of the lens based on "normal". I did this back in the early '70s before I saw others that were more like Barrie's example. So don't hit me.

    			Lens Equiv	
    FOV	FL/Diag	Diag/FL	4/3	35mm
    116.2	0.311	3.214	7	14
    109.2	0.356	2.813	8	16
    102.7	0.400	2.500	9	18
    96.7	0.444	2.250	10	20
    86.3	0.533	1.875	12	24
    77.6	0.622	1.607	14	28
    67	0.756	1.324	17	34
    58.7	0.889	1.125	20	40
    53.1	1.000	1.000	22.5	45
    48.5	1.111	0.900	25	50
    25.4	2.222	0.450	50	100
    12.8	4.444	0.225	100	200
    6.4	8.889	0.113	200	400
    4.3	13.333	0.075	300	600
    3.2	17.778	0.056	400	800
    I always started from "normal" for whatever format I was using and then went from there.

    As for those who complain we are not using the metric system - we are using millimeters to refer to lenses, not inches. What more do you want? :wink:

    Edit: Here is the link to my spreadsheet I created this morning if anybody is interested. Haven't done arctangent functions in years!

  16. Brianetta

    Brianetta Mu-43 Veteran

    Sep 5, 2010
    North East England
    Brian Ronald
    All I'm saying is that (micro) four thirds should be that yardstick, on this forum.
  17. pdh

    pdh Mu-43 Top Veteran

    May 6, 2010
    and some people may just prefer to use 35mm as their yardstick (or their 0.9144metrestick perhaps) ... I'm not quite sure where the "should" comes into it on an open forum ?
  18. grebeman

    grebeman Mu-43 All-Pro

    Mar 13, 2010
    South Brent, south Devon (UK)
    I wonder just what sort of discussions took place following the introduction of 35mm format, no near instant forum for such discussions as we have today.
    I guess the moral is we all have our own way of visualising the results to be obtained from a lens of a certain focal length.
    The problem comes perhaps when a newbie wants to raise the subject and gets several seemingly confusing answers.
    Perhaps also the tendency to use zoom lenses has rather driven actual focal length into the background, although again I'm an odd ball in that I tend not to use zoom lenses.
    Anyway, stay calm gentleman and enjoy taking your photographs, now if only the weather would co-operate :smile:.

  19. carpandean

    carpandean Mu-43 Top Veteran

    Oct 29, 2010
    Western NY
    Like it or not, 35mm became the dominant film standard in the mid-late 20th century and that status was confirmed in the digital age when "full-frame" was used to describe DSLRs with an equivalent size sensor. As others have pointed out, there are far more sensor sizes now than there were film sizes (in common usage, anyway), so it is handy to have a single, normalizing standard. The obvious choice is 35mm/full-frame.

    It doesn't mean that everyone needs to think that way, especially on a forum dedicated to one particular format. However, realize that not everyone here is experienced in or wholly dedicated to :43:, so using that 35mm standard can help ensure that we're all on the same page. If you have a gut feel for what a 17mm or 140mm lens gives you in :43:, then just ignore the "(34mm or 280mm equivalent)" that is invariably placed after it.

    I do like the idea Narnian presented about using focal length divided by sensor diagonal so that longer lenses have a higher measure (loosely giving a magnification feel to it), but others might prefer the inverse, so that wider AOV lenses have a larger number. Either way, though, many people don't want to have to think about diving by the actual sensor diagonal or lens FL in the course of a conversation to figure out if a lens is wide or not. Like many here, if you tell me that you have a 24mm lens, I have a very instinctive feel for how that will work on a 35mm/FF camera; I have to tell myself it's a 48mm equivalent when on a :43: camera. Those coming into photography and :43: at the same time won't have such problems. As such, the "24mm (48mm equiv.)" description works because it helps both groups. I look at the equivalent focal length, while the other group looks at the actual focal length.
  20. Krang

    Krang Mu-43 Veteran

    Feb 19, 2010
    This is why I really don't understand all the fuss. We should just accept each others views :) 
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