DXmm? Really Dumb Question ...

Discussion in 'Lens Lust' started by Minuteman3, Jul 8, 2007.

  1. This has been something I have been too embarrassed to ask, but finally decided "what the heck?" (Perhaps those nice glasses of wine helped ...)

    I understand with Nikon's digital sensor, that there is a conversion factor for lenses vis-a-vis 35mm cameras. On a film camera, a 50mm is a 50mm. On my D80, the 50mm is essentially a 75mm. Etc. Got it.

    But ... are the DX lenses the same? Or are they already "converted" to the digital format? In other words, would my 18-200 be a ~12-135 if it was on a 35mm body? (And, yes, I understand that DX lenses will not work properly on a older film body...) Or, is the 18-200 designation what it would be on a 35mm?

    Thanks for finally clearing up this question for me ...

    Cheers!!

    Ken
     
  2. ultimind

    ultimind

    990
    May 13, 2007
    Cleveland, OH
    DX lenses are designed to only cover the 1.5x cropped sensor. If you put a DX lens on a 35mm body, it will vignette on the edges because the lens is only designed to cover the smaller area of the DX sized sensor. It allows Nikon to produce cheaper and less complex lenses because the area that is required to cover is so much smaller.

    Hope this helps!
     
  3. ultimind

    ultimind

    990
    May 13, 2007
    Cleveland, OH
    Look at the price of any of the older non-DX Nikon zooms that start at 17 or 18mm. They're outragously expensive compared to the equivalent lens in DX. Because 17mm on a 35mm camera is redonkulous wide! 17mm on a DX sensor is only about 26mm. The precision required on full-frame lenses that go that wide is sooooo much more than if you only have to cover the smaller crop of a DX sensor.

    I'm beginning to be weary of DX lenses incase Nikon starts to change their mind about DX cropped sensors. If Nikon produces a full frame DSLR, we're screwed as far as our DX lens purchases go.
     
  4. Thanks, and yes I understand this part ... they (DX) won't work properly on a 35mm film camera. But, my question deals with the "mm" involved in the conversion.

    Perhaps this might make it clearer:

    35mm w/100mm AI/AIs lens = 100mm lens
    D80 w/100mm AI/AIs lens = 150mm lens equivalent

    D80 with 100mm DX = ?? mm on a 35mm camera? 75mm or 100mm?

    Back to the wine ...

    Cheers!!

    Ken
     
  5. ultimind

    ultimind

    990
    May 13, 2007
    Cleveland, OH
    My bad... DX millemeter ratings are actually 35mm measurements.

    100mm DX lens is 100mm on 35mm. BUT the image area it covers is less than a full 36x24mm, so it will vignette.
     
  6. Chris101

    Chris101

    Feb 2, 2005
    Arizona
    Hi Ken,

    DX lenses are designated with their actual focal length. As far as I know, only p&s digicams 'pre-convert' focal length on their lens labels, and then only some of them. Usually you will see the word (or abbrev) 'equivalent' when the 35mm equivalent focal length is stated.

    Since the DX format is very common, it pays to get used to thinking in terms of the angle of view on a DX format camera. For example, a 30 or 35mm lens is a normal lens, and a 12 or 14mm lens is a superwide.

    I doubt that Nikon would obsolete their DX lenses. Even if they do make the mythical 24x36mm sensor some day, it stands to reason that they will have a DX crop mode, much like they have a 2x crop in the D2x camera. Oh, and some DX lenses DO cover a 35mm sized format. The 12-24DX Nikkor, for example, works on my film Nikons from 18mm to 24mm, and costs less than getting 18, 20 and 24mm lenses.

    What Ultimind says is true though. The 20mm f/2.8 IS somewhat shrper in the corners of the film format, but only slightly. The 12-24DX beats the 20mm at 20mm inside the DX area, but again, only by a tiny margin.
     
  7. ultimind

    ultimind

    990
    May 13, 2007
    Cleveland, OH
    Not just in sharpness but in the complexity of let's say a 17-35 F2.8 AF-S lens. It's a very complex optic and requires a very high amount of precision to cover 17mm + zoom in full frame. This is what I have always been lead to believe atleast... Perhaps someone with more technical experience can shed some more light on this subject.
     
  8. agw0

    agw0

    475
    Oct 28, 2006
    Munich, Germany
    Hi Ken,

    I think there is more confusion about this than necessary, probably because people often refer to this in a misunderstandable way.

    First, the "mm" refered to on the lens is the focal lenght, a physical design parameter of the lens, that is independent of the camera attached to it. So, a 100mm lens is 100mm lens, regadless.

    Second, as mentioned elsewhere, different lenses may be designed to cover different (film or sensor) formats. E.g. a DX lens is designed so that it's image circle (just) contains the 23.6mm*15.8mm of the Nikon DX sensor, a "normal" 35mm film lens is designed so that it's image circle covers the 36mm*24mm frame of the 35mm film, and a large format lens may be designed to cover 4"*5" or even 8"*10" film (usually even larger to allow for movements of the lens parallel to the film plane).
    This coverage, of course, is also independend of the attached camera / sensor format. If you attach a 35mm film lens to a 4"*5" camera, all you will record is a small circular image.

    Third, the field of view of a lens of given focal length depends on the size of the recording sensor (or film frame). So, a 100mm lens has a different field of view on a DX camera than on a 35mm film camera than on a 4"*5" camera (if it would cover that frame). The field of view can be computed as 2*arctan(L/2*f), with f the focal lengh, and L the linear dimension of the sensor or film frame. Therefore, the 100mm lens (coverage of the respective format assumed) has (in landscape orientation) a horizontal field of view of 13.5 degrees on the DX sensor, 20.4 degrees on 35mm film, and 64.8 degrees on 4"*5" film.

    It is this relative difference in field of view that is often refered to as "crop factor" when comparing DX and 35mm format (which, since most lenses used on either actually can be used on both actually are quite comparable). It results from the difference in linear dimension L in above calculation, which differs by a factor of about 1.5 between the two formats. Calculating the angle of view, L beeing 1.5 times smaller corresponds to a 1.5 times larger f. So, a lens of a given focal length f has on DX a field of view equal to that of a lens of focal length 1.5*f on 35mm film.

    As mentioned elsewhere, DX lenses are designated with their true focal length (not their 35mm equivalent). So, a 100mm DX lens on D80 has the same field of view as a 150mm lens on 35mm film.
     
  9. weiran

    weiran

    966
    Jan 2, 2007
    Nottingham, UK
    Arno has it spot on: focal length is independent of the film format, but the final "angle-of-view" or equivalent focal length depends on the film format. For example 50mm is the standard length on 35mm, but on a large format it might be 80mm, and on DX crop cameras its 30mm.
     
  10. Larlec

    Larlec

    30
    Jun 18, 2007
    northern Colorado
    Just a quick related question from another rookie, what happens with the DX lenses if Nikon produces a "full frame" digital camera? will they still be completely usable?

    The only stupid question is the one left unasked.
     
  11. Generally speaking, the answer is no, DX lenses are designed to work on the physically smaller 1.5x APS sensor. Using the lens on a full sized 1.0x sensor would vignette. It might work on a 1.3x, but it all depends on the circumstances. Whether Nikon has painted itself into the proverbial corner with its decision to develop a whole series of DX lenses around the APS sized sensor is an interesting question. Everything should work out fine IF Nikon can figure out how to reduce the noise levels in the APS sized sensors to compete with Canons full frame sensors. (the smaller the pixel gets the more they are susceptable to noise). On the other hand Nikon might just be building the DX line as a full featured mid-range system for the amateur market which doesn't need low noise sensors. How this plays out will be interesting to see.

    BTW I found THIS a great help for answering the crop factor/magnification question which, as has been explained, is commonly but incorrectly confused with focal lengths.

    Cheers!
    Gary
     
  12. Larlec

    Larlec

    30
    Jun 18, 2007
    northern Colorado
    Thanks, Gary, that's kind of what I had figured. The next few years should tell a lot. The rapidity of the technological change is just mind blowing.
     
  13. weiran

    weiran

    966
    Jan 2, 2007
    Nottingham, UK
    DX lenses can only be used on cameras with DX sensors or smaller, so no most DX lenses can't be used on a full frame sensor without heavy vignetting. Although there are lenses such as the Nikon and Tokina 12-24mm which are usable on FF from 18mm onwards as the image circle produced is big enough for FF. Most DX lenses I've tried don't work this way though.
     
  14. slappomatt

    slappomatt

    811
    May 13, 2006
    San Diego CA
    I beleive nikon made some film APS SLR's before. dont remember what they where called but they had their own lens too. and they got left behind after that format faded. they wont even work on similar sized digital sensors. so the possiblity of DX lens becoming obsolite is very real.

    that being said I own several DX lens and dont worry about it that much. there are alot of DX bodies out there that would keep me happy for along time. I'm not all that worried about FF.

    Even IF they do come out with one. I wont be able to afford it for a long time. and by the time they came out with a affordable FF camera it would still be out of my price range (even used.)for several years. so a generally affordable (even used) FF camera is probably a decade away or more. IMHO
     
  15. weiran

    weiran

    966
    Jan 2, 2007
    Nottingham, UK
    I think Nikon's APS line was called Pronea, and it died out because no-one bought one! DX is very popular and wont be going away for a long time, as even if Nikon make a FF sensor it wont be affordable for the low end (D40) DSLRs for a long time.
     
  16. Chris101

    Chris101

    Feb 2, 2005
    Arizona
    Lots of pros and other shooters are quite happy with their dx formatted sensors. Your statement sounds like marketing lingo.
     
  17. True enough Chris, although even as a casual amateur I really wish I had Canon's 5D sensor in my D200 when shooting moose a week ago! A dozen otherwise fabulous shots were blurred due to low lighting conditions and a slow lens. Sure, IF I had the proper lens+tripod at the time I could have saved some shots but when does that ever happen?! ;o) A "faster" (low noise) sensor might would have allowed me to set the ISO to stupid high numbers and possibly recovered a shot or two. On the other hand, even if Nikon had such a sensor I probably couldn't afford it!

    Gary
     
  18. Larry, I think you'll get dark corners on a FF sensor. The image circle of DX lenses is sized for the digital sensor, which is smaller than 35mm.
     
  19. Folks

    Thanks so very much!! Learned a lot from this thread.

    My main "issue" is all those years of shooting 35mm, I sort of have certain things lodeged in my old brain, i.e. 50mm is "normal", 20mm is "pretty wide", 85mm-105mm is good for portraits, etc.

    I sometimes get a bit confused when I think about the DX numbers ... but then I look through the viewfinder and it doesn't make any difference!!

    As to the lifespan of DX lenses, there are SO MANY D-XX's out there, that I would doubt they would be "obsolete" any time soon. Unless, of course, Nikon came up with a full-frame sensor that was VASTLY superior to the DX size. What I'm seeing with my D80 would indicates to me that it's hard to think of a VAST improvment, although I guess from reading this and other topics, high ISO noise is an issue for some folks. Personally, I have not had a problem with that and my DX results are stunning!!

    Thanks again.

    Cheers!!

    Ken
     
  20. Great question. I am learning a lot.

    Thanks,

    Mike
     
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