Question about DX Lenses

Discussion in 'Digital Photography' started by Bodhi395, Oct 1, 2010.

  1. Bodhi395 macrumors 6502a

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    #1
    I'm a bit confused on one aspect of DX lenses. I know that if you put a 50mm full frame lens on a DX camera, the 50mm lenses will act like a much longer lens, maybe closer to a 75 or 80mm lens. However, is this the same if you put a 50mm DX specific lens on a DX camera. Will the 50mm DX lens act like a much longer lens too?
     
  2. Edge100 macrumors 68000

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    #2
    Yes.

    All lenses report their actual focal length, not the effective focal length. The focal length of a 50mm lens will always be 50mm, irrespective of sensor/film format; however, if you then crop that image by 1.5x (as you do on an APS-C/DX sensor), then you get a 75mm effective focal length.
     
  3. Bodhi395 thread starter macrumors 6502a

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    #3
    Thank you, that is what I was thinking.

    It leads me to the question though, why do companies make DX specific lenses? FX lenses will work the same on FX and DX cameras, so why not make all FX lenses?
     
  4. Cliff3 macrumors 65816

    Cliff3

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    #4
    In theory a DX lens can be made smaller, lighter and potentially less expensively because the lens only needs to cover the smaller area of a DX or APS-C sensor. The downside is that it either won't cover or will vignette badly on an FX sensor.

    Nikon's 17-55DX f2.8 is a bit of a theory buster. It's a real honker of a lens and is probably nearly as large and heavy as my Nikon 24-70 f2.8. The remaining DX lenses seem much smaller.
     
  5. Edge100 macrumors 68000

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    #5
    The "theory" is that these lenses can be made smaller and lighter, as Cliff3 mentioned.

    However, both Nikon and Canon make very few DX/EF-S lenses that I would call "excellent". With very few exceptions, the best lenses from these manufacturers are made for FF.
     
  6. JavierP macrumors regular

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    #6
    If there weren't dx lenses, you wouldn't be able to take wide angle shots with cropped sensor cams

    A Nikon 14-24mm f/2.8G ED (2000 U$S), just gets 21-36mm effective focal length with a D90, for example
     
  7. Bodhi395 thread starter macrumors 6502a

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    #7
    So I guess there is some technical reason you can't make a full frame lens that is really wide, to compensate for the DX cropping.
     
  8. JavierP macrumors regular

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    #8
    It seems that you are not getting it. Using easy numbers, a fx lens produces images that have an area of 2 cm^2 when projected on the back of the camera. Full frame sensors have an area of 2 cm^2, so they can see the whole wide angle image generated by the lens. Now you have a cropped sensor that has an area of 1 cm^2, you are going to lose the sides of the image. In order to get the whole image you need a dx lens that generates the same image but having 1 cm^2 area.

    If you can live with 24mm on a cropped sensor (35 mm full frame eq) being your widest, then buy all fx lenses.
     
  9. Bodhi395 thread starter macrumors 6502a

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    #9
    Ok, I think I get it now. So if you are using a full frame lens on a dx camera, you are not getting the entire scene the lens is seeing. However, a dx lens on a dx camera will get you the entire scene.

    So when people say full frame lenses work fine on DX cameras, that is not actually true, since you are losing parts of the scene.
     
  10. compuwar macrumors 601

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    #10
    Halfway there- you indeed get only a small part of the image circle of the lens on an DX camera if it's an FX lens.

    However, since the focal length is the focal length, FX lenses work fine on DX cameras since you're getting the full angle of view the DX sensor is capable of. You're not "losing" anything- a DX lens could have an image circle that covers say 1mm more than the DX sensor's long side- you wouldn't be "losing" anything- you're simply not using the entire image circle- if you have a DX lens that covers 1.2mm more than the DX sensor then it's still being used- it's not a "loss" against one that covers only 1mm wider.

    Let's say a 35mm FX lens has a coverage of 40mm and a DX lens has a coverage of 32mm- On an FX body, the first lens will cover the frame completely and the second will vignette in the corners. Both lenses will work on DX, though the FX lens will more than likely be brighter and sharper in the center, which is the only portion being used on the DX body. Ultimately, that usually means you gain a bit of IQ using an FX lens on a DX body (or cropping an FX lens on an FX body.) Very few optical designs show increasing sharpness and contrast as they move away from the center of the frame (and those that do generally degrade first, then improve and degrade again.)

    Paul
     
  11. JavierP macrumors regular

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    #11
    The losing parts only matter if you are shooting wide. Lets say that you want to conver the 10-200mm focal lengths, you are going to need 2 dx lenses.

    1) dx lens 11-16mm
    2) dx lens 16-85mm
    3) fx lens 80-200mm

    If you can live with a small gap (16-24mm) you will just need 1 dx lens

    1) dx lens 11-16
    2) fx lens 24-70
    3) fx lens 70-200
     
  12. Bodhi395 thread starter macrumors 6502a

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    #12
    Would it be possible to make an 11-16 FX lens?
     
  13. Full of Win macrumors 68030

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    #13
    Sure, fish eyes go to single digit focal lengths.
     
  14. El Cabong macrumors 6502a

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    #14
    Sigma has a 12-24mm FF (non-fisheye). Close. Maybe if they just tried harder.
     
  15. compuwar macrumors 601

    compuwar

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    #15
    What would be the point? 11mm AOV on DX is ~16.5mm, a range that's already covered on FX. A rectilinear 11mm lens would be large, protruding and expensive- likely there would be little market for it.

    Paul
     
  16. Bodhi395 thread starter macrumors 6502a

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    #16
    I guess I'm trying to figure out why there are DX lenses in the first place. Is the main reason they are cheaper, lighter, and smaller??

    Besides weight, size, and cost, it seems to me if they made FX lenses from 11mm all the way up to the max focal length, then all DX cameras could just use those.
     
  17. El Cabong macrumors 6502a

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    #17
    Yes.
     
  18. carlgo macrumors 68000

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    #18
    And then there are lenses made for rangefinders like Leica. The lenses are very small due to their closeness to the film/sensor plane. Also, they don't have auto focus or stabilization devices in the lens. And they are made of thin metal instead of thicker plastic/metal combinations. And I don't think NiCanon put much effort, if any, in making their lenses smaller.

    I have an old Leica CL and the lenses are just tiny, yet this is a FF camera.

    I am waiting for someone to make a Leica-like EVIL with dedicated little lenses, all affordable thank you.

    NiCanon don't know it yet, but the days of gigantic DSLRs is nearly over. Fewer and fewer people want to carry this stuff around anymore.
     
  19. Cliff3 macrumors 65816

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    #19
    It would seem that Olympus and Panasonic have already supplied you with a choice in the form of their micro 4/3rds cameras. The notion of affordability is subject to interpretation, and it is not a word I would use in the same sentence as 'Leica'.

    That depends on what you need to do with it.
     
  20. compuwar macrumors 601

    compuwar

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    #20
    Sales numbers and balance sheets would seem to support their position more than yours. That's not to say smaller cameras don't sell in larger volumes, but DSLR sales are at an all-time high and those numbers don't seem to be diminishing and per-unit sales prices seem to be going upwards...

    Paul
     

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