Dumb question of the day - FX vs. DX lenses

Discussion in 'Nikon' started by wade_thompson, Sep 20, 2010.

  1. ok, I'm opening myself up for total ridicule here.. LOL... But I am going to ask a very basic question about FX vs. DX.. but not about the camera... I understand the difference there....but my question has to do with FX vs. DX lenses.
    I think I have 3 lenses that can go either way. Nikon 80-200mm f2.8, Nikon 50mm f1.4, Tamron 28-75 F2.8
    But I have 2 that are only DX: Nikon 18-200mm f3.5-5.6, Tamron 10-20mm f3.5-5.6
    My question is this:
    What physical property in a lens makes a DX lens not able to be used on a FX insofar as filling the image sensor?
    1) Is it that the max aperture has to be 2.8 or better for it to be a FX compatible lens?
    OR 2) Is it that the lens curvature has to bend the light less so that it goes to the FX sensor width?
    Just curious.
    Thanks in advance.
     
  2. A DX lens is designed to project a smaller image circle that will cover a DX sensor but not an FX sensor. This allows the lens to be smaller overall and lighter in weight. It has nothing to do with the maximum aperture.
     
  3. sometimes i like to use my dx lenses on full frame fx, some of them luckily work with slight vignetting. the 35mm 1.8 dx works with slight vignetting wide open, the 18-55 dx kit lens works at 24-55 mm, the 55-200 vr works with vignetting and the tokina 11-16 works at 16mm with no vignetting. sometimes its a cheap hack-way to abuse ur old DX lenses on full frame if u want to have a little cheap fun :)
     
  4. but how much does the max aperture fit into the equation... I can see where the glass being narrow at the base of the lens could affect the size of the image too
     
  5. Wade, Image circle is the only difference. f2.8 is always f2.8. Doesn't change with sensor size.
    There are a few DX lenses that you can get away with covering the frame on FX, but the corners may or may not be acceptable.
     
  6. Picture the FX sensor (which is the same size as a 35mm film frame, which is 36mm x 24mm) vs. the DX sensor (24mm x 16mm). The FX sensor is larger that the DX. Now picture the lens as a device that projects an image of what's in front of it, to the sensor behind it.
    The lens elements are circular so the image being projected is a circle. Since the FX sensor is larger than DX the circle that covers it has to be larger - the dimensions of the FX rectangle are 1.5x as large as the DX rectangle, so the diameter of the FX circle is 1.5x larger than the DX. The FX circle can cover a DX sensor because the DX sensor is really like cutting a smaller rectangle out of the FX sensor, but it doesn't work the other way around.
    When you use a DX lens on an FX camera, depending on the lens and how it was made, you might get a complete image (the lens might be making a larger circle than it really has to) or you might actually see the outline of the circle that's too small to cover the sensor. If you look at this thread: <link> you can see examples. Tony use the 11-16mm Tokina DX lens on his F5 film camera (the effect is the same as with FX digital) and the lens mostly covered the frame at 16mm but when he zoomed out to 11mm the circle was smaller and you can clearly see what happens. (It's pretty common in a DX zoom that the circle is larger when zoomed in, it's just how the optics work out and you'd never realize it because it doesn't have any noticeable effect when used on a DX camera.)
     
  7. 1) Max Aperture has nothing to do with whether or not it's a dx / fx lens.
    2) the Dx lens is designed to provide optimum coverage on a smaller sensor; The Fx lens is designed to provide optimum coverage on a Full Frame (35mm) sensor.
    - Dx lenses can be used on Nikon Fx bodies - the camera will detect the Dx lens and switch to "Crop" mode - which basically makes the 12.4 mp camera into a 6.4 mp Dx body.
    - Fx lenses can be used just find on Dx bodies. There is no crop mode on the Dx bodies. You will get a narrower field of view - "Crop Factor" but there is no vignetting.
    Dave
     
  8. ok, so if it's not aperture, then it's got to be the curvature of the lens projects & focuses into a larger space at the same distance from the mount... so, it's only less curvature of the lens itself that makes it an FX lens, correct? that is what I am asking , what is the mechanical mechanism that makes an FX lens different than a DX lens.
     
  9. ShunCheung

    ShunCheung Administrator

    - Dx lenses can be used on Nikon Fx bodies - the camera will detect the Dx lens and switch to "Crop" mode - which basically makes the 12.4 mp camera into a 6.4 mp Dx body.​
    To be more precise, currently on all Nikon FX-format bodies, there is an auto DX crop option: when the FX body detects that a DX lens is mounted, it will only use the DX area on its sensor to capture the image. However, that option can be switch on and off; you can always choose to have the camera capture either an FX image or a DX image, regardless of the type of lens mounted. (In other words, there are 3 options: always FX, always DX, and auto DX crop.)
    Nikon's FX format is roughly 24x36mm while the DX format is 16x24mm (the actual sensor size is a bit smaller than those). Therefore, FX is roughly 2.25 times the DX area. A 12MP FX DSLR will capture a 5.3MP image in the DX mode.
     
  10. ok, I found this in an image search.. .definitely it's the lens curvature...

    Attached image in violcation of photo.net's copyright requirement removed.
     
  11. Therefore, FX is roughly 2.25 times the DX area.​
    Right, because the crop factor (which is linear) is 1.5, and 1.5 x 1.5 = 2.25.
    It would take someone more technically knowledgeable about lens design than I to answer Wade's question, but I think the diameter of the elements is the main reason. If you take a simple thin lens of a given curvature that is, say, 2" across, it will project an image of a certain size at a certain distance (focal length). If you then trim the lens down to be only 1" across (preserving the center portion), I think it will continue to project the same image, but less of it -- by cropping the lens, you crop the image it projects. This gives you a physically smaller, lighter-weight piece of glass that projects a smaller image. I am certainly open to correction on this, but I think this is the core difference between FX and DX lenses -- not the lens curvature per se, but its diameter.
     
  12. Wade, where did you find that image, and what was it trying to illustrate? One thing I notice is that the lenses shown in the two drawings have completely different optical formulae, so I'm not sure what it is safe to infer from that picture.
     
  13. it was just a basic google search so the drawing may be suspect.
     
  14. ok, so if it's not aperture, then it's got to be the curvature of the lens projects & focuses into a larger space at the same distance from the mount... so, it's only less curvature of the lens itself that makes it an FX lens, correct?​
    No. The curvature in and of itself has nothing to do with it. That's why FX lenses work fine as-is on DX cameras.
    Lenses are designed to get the necessary image circle at the necessary aperture. Where an optimist's glass is half full and a pessimist's is half-empty, an engineer would see twice as much glass as needed. For DX, that usually results in a different design that's smaller, lighter, and less expensive.
    that is what I am asking , what is the mechanical mechanism that makes an FX lens different than a DX lens.​
    Design can certainly affect lens element curvature, but there's no inherent difference between FX and DX other than the image circle size. Any curvature difference results from DX 'corner cutting' design headroom that's available to the lens designer in order to make the DX lens smaller and less expensive compared to FX lenses of the same focal length.
    It's not about bending light, it's about 'lopping off' what's not necessary.
     
  15. ok, so then now I'm back to square one. In other words: CONFUSED!!! lol!
     
  16. ShunCheung

    ShunCheung Administrator

    Wade, maybe this old thread will help: http://www.photo.net/nikon-camera-forum/00Pt3E
    I would like to use the opportunity to remind everybody that per photo.net Community Guidelines, #3: http://www.photo.net/info/guidelines/
    Any image you post should be your own work. If you want to reference an image shot by someone else, please link to it rather than post the image itself.​
    That includes images you "hot link" into forum posts; i.e. images that appear in line in our forum threads.
     
  17. ok, so then now I'm back to square one. In other words: CONFUSED!!! lol!
    It's just a smaller image circle. Don't let the extra FUD that some add to the whole issue dismay you. The link Alan shared in the second response is what you need to know.
     
  18. [​IMG]the one guy that did the pine tree drawing with the 4 combinations of DX/FX lens vs. DX/FX sensor shows a different angle the light is bending based on the lens chosen which was the first assumption that I was told was not true. So... it looks like to me, the angle the image comes in on a FX lens HAS to be different than a DX camera! or do I just have a mental block on this?
     
  19. Yes, you are correct. The angle of view on DX is smaller. The image is cropped. Your diagram doesn't quite show this, because you don't show the same focal length for both.
     
  20. or do I just have a mental block on this?​
    Maybe. It seemed like you might be thinking, for example, that a 35mm DX lens would put the same overall image onto a DX sensor that a 35mm FX lens does onto an FX sensor, and that's not the case.
    If the focal lengths are the same and the focus distance is the same, objects in an image that fall on both sensors would be the exact same physical size. Less total image 'real estate' will fall on the DX sensor since it's physically smaller, and it 'sees' less effective angle of view. The total image 'real estate' that doesn't fall on the smaller DX sensor can therefore be done away with. That's all a DX lens does.
    the angle the image comes in on a FX lens HAS to be different than a DX camera!​
    Sort of. Total angle of view, yes; but if the focal length and distance are the same, objects in the image falling on either sensor are the same physical size. In the bottom illustration you need to make the object part (left end) of the 'sideways hourglass' smaller, but keep the angles the same (in degrees) and the crossing point at the same place as the top 'hourglass'. The DX image circle ends up being smaller on both ends by the same percentage, while the physical objects in an image remain the same size.
     
  21. OK I get the bit about reduced Mp count, but will it still give the same focal range - on a DX the range of the kit 18-55 is equivalent to approx 28-80 in traditional focal lengths (with 35mm being "normal" eye view instead of 5omm).
    What happens when I put that lens on a D700 (until I can afford an FX lens to go with the body :) ) - does it give genuine 18-55 or still mimic 28-80?
     
  22. Brian,
    here's a decent explanation of crop factor.
    Here's another one, which won't embed for some reason, found by searching "crop factor explained" in the search box at the top of this page. http://www.photo.net/nikon-camera-forum/00VXrv
     
  23. It never "mimics" anything other than itself. It is 18-55 no matter what camera you put it on. With a DX lens on an FX camera, the smaller image circle projected by the DX lens might give blackened corners. Image magnification remains the same.
    The utter confusion caused by the alleged "crop factors" should be reason enough to stop using that nonsensical concept.
     

Share This Page