DX vs FX focal lengths?

Discussion in 'Nikon Cameras' started by darkblue-x, Oct 23, 2017.

  1. john.margetts

    john.margetts No longer a newbie, moving up!

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    I'm not surprised you get confused - most of the explanations are awful. Even where they are right they are very badly expressed.


     
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  2. john.margetts

    john.margetts No longer a newbie, moving up!

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    Accurate and well expressed.
     
  3. Tomasko

    Tomasko No longer a newbie, moving up!

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    GOOD LORD!

    Do you seriously think he needs to know about medium or large formats? He obviously has problems with the most basic Math. I'm talking about the most common commercially available gear that would be relevant for his needs using a crop/ff DSLR.
     
  4. Tomasko

    Tomasko No longer a newbie, moving up!

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    What are you talking about? 35mm isn't DX, it's FX.
    I don't know what you mean by "dedicated dx". Focal length is focal length, doesn't matter what mount you put it on. If you're converting from DX (CROP) to a FX (FULLFRAME), you're multiplying. If you're converting back, from FX (FULLFRAME) to DX (CROP), you divide. Both operations involve whatever focal length you're interested in AND the crop factor you're converting to/from. Focal length after both operations stays the same, only FOV changes.
     
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  5. john.margetts

    john.margetts No longer a newbie, moving up!

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    Any photographer only needs to know about the formats he actually uses. What you wrote about focal lengths being written in respect to 35 mm/fullframe was complete rubbish. The Op is confused, not because of his poor maths skills, but because of the poor explaining skills of a majority of posters in this thread.
     
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  6. WayneF

    WayneF No longer a newbie, moving up!

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    No. Maybe you just meant the Equivalent focal length is always with respect to full frame, but the marked focal length of the lens is Not about full frame sensors. Not about any sensor. The lens is always marked to show what the lens actually does, how it actually focuses. If any 24 mm FL lens is focused at infinity, the focused point is 24 mm behind the node in the lens, which is where any sensor has to be. Sensor size does Not affect focal length. Focal length is "distance to sensor", but the marked Focal Length is this focused point at infinity (focus at infinity is what is marked). DX and FX can use the same lens only because the Nikon F mount is the same distance from sensor for every lens, in every case. The sensor size just frames that image, crops it so to speak, crops size which is also Field of View, so that the Field of View seen by the sensor varies with sensor size. But the focal length is always as marked. Lens FL does Not change by merely attaching a different sensor.

    A smaller sensor (perhaps like in a little compact camera) will use a much shorter lens (maybe 5 mm FL for a much wider view on this tiny cropped area), so that its small sensor will still show a normal field of view. Because we expect normal view photographs from it. So its lens spec may say like 5 mm. It probably also says "35 mm film Equivalent" is maybe 24 mm. That's a very convenient comparison if you have 35 mm film experience (popular since about 1930). It simply means a 24 mm lens on a 35 mm film camera would see the same Field of View, but it does not change this lens which is still 5 mm FL. This sensor size is further represented by saying the sensor Crop Factor is 24/5 = 4.8x Crop factor (in this hypothetical 5mm / 24 mm case).
     
    Last edited: Nov 9, 2017
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  7. astroNikon

    astroNikon 'ya all Bananas I tell 'ya Supporting Member

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    FYI .. trying to make it easy to understand the basic concept ....

    FX EQUALS 35mm. Both are 1.0x for calculation for what you see on the lens.

    with the lens IGNORE all indicators of DX or FX

    The Field of View (which is NOT the lens) is totally based on SENSOR SIZE assuming all lens/camera are the same.

    So a 50mm lens .. not matter what .. is a 50mm FOV on a FX/35mm film camera
    on a APS-C Nikon (yes, Canon is slightly different and different size sensors are even more different) it's like a 75mm FOV lens.

    For a 50mm lens ...
    on DX is 50 x 1.5 = 75 FOV to get back to FX .. which is 75 x (1/1.5) = 50 for FX FOV
     
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  8. petrochemist

    petrochemist No longer a newbie, moving up!

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    There are a some fixed lens cameras that refer to their lenses using 35mm equivalent focal lengths. Apart from that the only place I've seen equivalent focal lengths used for describing lenses is in e-bay listings where someone is trying to gild a pile of s***e.

    If you only use one format equivalence is pretty pointless - only being relevant for using old guidelines on shutter speeds (for things like hand-holding, star trails etc.).
    When I used my 120mm medium format lens on my crop DSLR it gave the same view/DOF/shutter speed as a native 120mm lens would on that camera at the same aperture. The total distance (lens surface to sensor) needed to focus the image was also the same as it would have been on the medium format camera at the same object distance...
     
  9. Braineack

    Braineack Been spending a lot of time on here!

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    punctuation matters. There's a period. That means end of sentence.

    it reads: This lens is a F-Mount Lens/DX Format. PERIOD, STOP. This lens would be like a 16.5-30mm on an FX.

    All Nikon DSLR lenses are F-mount. This is a 11-20mm DX lens. It would perform like a 16.5-30mm on an FX.


    again:

    11 * 1.5 = 16.5mm
    20 * 1.5 = 30mm



    this is why I said stop worrying about it. it's causing confusing you don't need to worry about. Don't buy DX only format lenses if you plan on ever going FX.
     
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  10. darkblue-x

    darkblue-x TPF Noob!

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    See the biggest problem I have here is the fact that you insist on underhandedly calling me a dumbass. The belittlement isn't appropriate as much as you find this maelstrom frustrating.
    The fact that you don't seem to understand what it is that I am asking may pose the problem. You cant expect a layman seeking education to magically give you a camera-club jargon definition of what they are wondering.

    I want to window shop for when I buy FX. may be this Black Friday coming or more likely next year.

    SO...when I jam a my 16-85mm DX nikkor at the 16mm focal length, I see a certain angle of view in the viewfinder.
    What I understand and that I am being told is that in order to see the same angle of view on a FX camera with an FX lens, that that focal length would be 24mm.
    Now is this correct or are we still going back to square one?

    Refer to the following information:
    Crop Sensor (APS-C) Cameras and Lens Confusion

    Under the:
    Wideangle Lenses

    Section they advise what the focal length equivalents are between the two types, first the FX and then the APS-C.

    Ysarex understood what I was saying and kept it simple.
    31.3mm crop yields the same angle of view as 50mm fx lens on fx.

    where I use 35mm prime on dx for portraits, I would be mistaken buying a 35mm fx lens for fx, it wont be the same angle of view. I would need to buy a 50mm lens to get the closest equivalent, or even better tamron makes a 45mm fx lens that seems to mimic eye view.
     
    Last edited: Nov 9, 2017
  11. Alan92RTTT

    Alan92RTTT No longer a newbie, moving up!

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    You should have put that in your initial post ;)

    Knowing this, 1.5 is the magic number.

    If you have a lens you like on your D7100 take its MM and multiply it by 1.5. A lens with that result on a FX camera will result in the same field of view. Its really that simple.
     
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  12. darkblue-x

    darkblue-x TPF Noob!

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    Yeah definitely no problem there.
    Some have told me to start investing in FX lenses now if I know I will make the switch. That's where it gets tricky...if I want a angle of view on FF camera of 39 degrees I need to buy a 50mm FX lens. but then that wouldnt give me the same angle of view should I jam it on my APS-C in the interim.
    That's the trickiest part.
    Being said. F all that noise. I'll chill for now and in the future ill know to buy the 50mm fx lens on FF to get the same angle of view as I do with dx lens 35mm on APS-C camera.
     

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