35mm Equivalent on Crop Sensor - DX vs FX Lenses

Discussion in 'Photographic Discussions' started by AlexGavillan, Aug 2, 2018.

  1. AlexGavillan

    AlexGavillan TPF Noob!

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    OK so, I think I know the answer to this question but I just need someone to clarify. When looking at lenses (Nikkor to be specific) I know that there are FX and DX lenses. The DX lenses will have the mm as to how it relates to 35mm film, got it! But here is where I get confused and what I think they (The manufacture means)

    I have the 1.8 AF-S Nikkor 35mm DX lens. I know that this gives me about 50mm since I have the 1.5 crop factor BUT on the lens itself (this is the DX version) it states 35mm. Here is my guess and please excuse the non technical terms as I don't think i know them.

    My guess is that since the sensor on a full frame camera is bigger the opening on the lens itself (the part that connects to the front of the camera) is larger to accommodate for the full frame sensor, but the DX one, since its a crop sensor has the opening that works best for a crop sensor camera.

    I guess I get confused because I would just assume that on a DX lens the manufacture would put "Oh you want a 50mm on your crop sensor well actually put 50mm on that type of lens VS just putting the 35mm equivalent." Example, if I want a 105mm lens that's DX there is still a 1.5 crop factor which really gives me the focal length of a 155mm.

    I hope this makes sense and someone can just say, "Yup, that's what that means." haha

    Thanks,

    Alex


     
  2. john.margetts

    john.margetts No longer a newbie, moving up!

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    Your lens is 35 mm on any camera it is attached to and even when it is not attached to anything. Putting the lens on a DX camera does not and cannot alter the focal length.

    The opening is the aperture and is entirely separate from the focal length and also does not change depending on FX or DX.

    Unless you have both an FX and a DX camera, you should ignore the crop factor and just learn how your particular lenses work on your particular camera body.

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  3. BrentC

    BrentC Been spending a lot of time on here!

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    Focal length of a lens never changes no matter what sensor camera you have it on. Your field of view changes depending on the sensor. So when we say a 35mm lens is equivalent to 50mm on a crop sensor what we are really saying is that the FoV of a 35mm lens on a crop sensor is equivalent to the FoV on a 50mm lens on FF.
     
  4. AlexGavillan

    AlexGavillan TPF Noob!

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    Thanks guys .I used the wrong term when I said focal length. I know that that doesn't change. But I don't think my question is being understood .

    This one might be easier. Why is there a DX and FX lens types? It's not all the same in that regard, what's the technical difference (if any) realted to that.
     
    Last edited: Aug 2, 2018
  5. ceemac

    ceemac No longer a newbie, moving up!

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    A camera with a full frame sensor requires a wider (diameter) lens so that the entire sensor is covered by the light from that lens. A lens for a crop sensor camera won't let in enough light to use the entire sensor on a full frame camera. A full frame lens will work fine on a crop sensor camera. It's just bigger, heavier and more expensive.
     
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  6. AlexGavillan

    AlexGavillan TPF Noob!

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    Perfect OK so that really helps. Then my next question (and this is probably just an industry standard thing) Since I am dealing with a Crop sensor DSLR, ANY lens I put on it I have to multiply the focal length times 1.5, even if its a DX specific lens. Would it not make sense that if a lens is DX format the manufacture puts the (cropped) focal length? I.E if its a 50mm DX put 50mm on the lens vs the 35mm equivalent..

    So what I can conclude, which affirms my thoughts, DX lenses have a smaller diameter so that it properly covers the sensor of a crop sensor camera. I understand that an FX lens will work on a crop sensor body but not the other way around, or just not as well, probably vignetting on the corners or something like that, or nothing at all.
     
  7. snowbear

    snowbear fuzzy-wuzzy Supporting Member

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    I regularly use my 18-200mm DX lens on the FX D750 body. However, the camera will default to going into "crop mode" meaning only the part of the sensor matching DX will be used. This can be changed so it doesn't automatically crop, which will give you the mentioned vignetting. If I get a chance this weekend, I'll post some examples.
     
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  8. john.margetts

    john.margetts No longer a newbie, moving up!

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    No! No! No! You do not have to multiply the focal length and you should not. Use your 35 mm lens on your DX camera and know that it is 35 mm. Know how it behaves on your camera. You have no need to worry about how it behaves on a camera you do not have.



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  9. AlexGavillan

    AlexGavillan TPF Noob!

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    Thanks John. But you do see what I'm saying right? I cant buy a 105mm lens and expect it to be 105mm on my D3400 right? There is a crop factor Maybe I'm not having that light bulb go off haha, need that ah ha moment...

    This for example:
    The DX sensor makes possible the production of lighter, smaller cameras, but because it covers a smaller portion of the image projected by the lens, a 1.5x crop factor (so called because the smaller sensor crops the image compared to an image from a 35mm film frame) is introduced. This means, for example, a 24mm lens on a DX sensor camera will provide an approximate 36mm view.

    Reference: FX & DX Format Lenses Explained | Learn About FX Lenses & DX Cameras from Nikon
     
    Last edited: Aug 2, 2018
  10. snowbear

    snowbear fuzzy-wuzzy Supporting Member

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    yes - the 1.5 is the equivalent view angle. Unfortunately a lot of confusion has been created with the terminology used.
     
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  11. 480sparky

    480sparky Chief Free Electron Relocator Supporting Member

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    And this is why I wish the 'crop factor conversion' idea would disappear from the face of the earth. Erase it. Delete it. Send it to the Trash Bin. Eradicate it. As if it never existed.

    What I find amazing is back in my film days, there was no such thing as a 'conversion factor' to compare lenses between 135, 120 and 4x5 formats. I never heard of a number to multiply (or divide by) when changing from 35mm to 6x4.5 format, or to 6x7 format, or to 4x5 format. I never had to 'convert' the 80mm lens of my Mamiya 645 to 'the equivelant of __mm on my 35mm camera". Nor convert the 150mm on my 4x5 to 'the equivelant of __mm on my RB67 camera".

    I understand why the manufacturers created the 'conversion' factor. It was a marketing tool to help sell fledgling digital SLRs to an uneducated populace. The populace that had grown up with one format: 35mm. Everyone and their uncle owned a 35mm film camera. Most carried a 50mm 'standard' lens. Many purchased 28mm wide-angles and 135mm telephotos. So that was 'the gold standard' back then.

    Then along came digital. But the sensors were smaller than a 35mm frame. So the manufacturers needed an easy way for those transitioning to digital to 'relearn' how focal length related to FOV. "Conversion factor' seemed like the perfect choice. And it was. At least back then.

    But today, we have an entire generation that has never even SEEN a 35mm film camera, let alone understand focal length, sensor/film plane size and how the two relate to FOV. So today, the 'conversion factor' has royally muddied the waters. Not to mention the internet, and it's ability to spread incorrect information that will be accepted as gospel.

    And next, the assumption (and even firm belief with some) that the 'conversion factor' applies to other properties of the lens, such as aperture and minimum focus.

    I say it's time to deposit 'the conversion factor' to the dustbin of history. Let it reside the Hall of Useless Technology, next to ice picks, buggy whips and 8-track tapes.
     
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  12. john.margetts

    john.margetts No longer a newbie, moving up!

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    No, not right. It will still be a 105 mm lens. What you need to think is not what happens if you buy a 105 mm lens. What you need to think is "I want a lens to do so-and-so. Which lens will do that?". When you buy a new lens, buy one that produces the pictures you want without worrying about a FX camera you do not own.

    That 105 mm lens will not behave the same on your camera as it will on an FX camera but that does not matter as you do not have a FX camera. It will behave the way that a 105 mm lens will behave on your camera. Comparing it to a camera you do not have does not help you - you might as well compare it to how it would behave on my medium format Bronica where it would be hellishly wide angle with a crop factor giving it a 'focal length' of 50 mm.


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