FX vs DX (Full Frame vs Crop Sensor)

Fstoppers posted a great question we’ve all heard many times.. If you were forced to only use one lens for the rest of your life, what would it be? Check out their full post here http://fstoppers.com/poll-the-one-lens-to-rule-them-all and let them know what you think!

Personally I chose the 50 prime. Arguably the best lens for your money on the market, no matter what brand you shoot with! Relatively cheap, plus super light weight, great F/Stop range, and amazing macro lens. But thats not the only reason I chose the 50 prime. I chose the 50 prime because of Crop Factor (Crop Factor is not specific to the 50mm, it applies to all lenses and all cameras). Personally I shoot with the D3 & hopefully some time soon the D800, both FX (full frame) cameras with an option to shoot in DX mode. DX is Nikons code for a cropped sensor. In a nutshell a cropped sensor will automatically zoom any lens you put on the camera. Bam! This lens is 50mm on my FX camera, and 75mm on my DX camera! Two lenses in one!! It does the because the sensor isn’t large enough to cover the entire area of the glass. (on a lens designed for FX bodies) DX lenses are designed for cropped sensors, but the focal length measurements remain the same. Focal length is the result of a formula comparing the distances of the front and rear glass elements of the lens to the camera’s sensor (of film). Read more on focal length on Wikipedia. So mechanically the focal length of a lens never changes when you put it on a FX or DX camera. Its specifically the size of the sensor that magnifies the image on a  DX body. This is where it gets slightly confusing.. Different lenses are designed specifically for FX or DX cameras. If you are Nikon, you can use any lens with any body. Other brands do not offer this flexibility. There are however some limitations you should be aware of.

For this example I’m sticking with 50mm, to my knowledge there are no 50mm lenses designed specifically for DX (cropped sensors), but for simplicity I made one up.

  • A FX 50mm lens on a full frame camera will give you an image at a true 50mm
  • A FX 50mm lens on a cropped sensor camera will give you an image at approximately 75mm (depending on the ratio of the crop, some are smaller than others..)
  • A DX 50mm lens on a full frame camera is still 5omm, but you will not be using the full sensor because the lens is designed for a cropped sensor camera. (you will not zoom in or out, but you will not use all of the camera’s megapixels. For example the D800 is a 36mp camera in FX mode. In DX mode since you are only using a portion of the sensor, you will only shoot images at 16mp.)
  • A DX 50mm lens on a cropped sensor camera is still approximately 75mm. (nothing changes between a FX 50mm and a DX 50mm)

Starting to make sense? Essentially the only thing a full frame sensor does is give you the true focal length for each lens you put on your camera. It makes every lens a bit wider when compared to the same lens on a DX camera. Now this does affect several other aspects of your photography! For example, depth of field on each lens will change. So be careful not to get to used to focal lengths on your DX camera if you ever plan to upgrade to a FX body, everything will change! Personally I love having one FX and one DX camera in my bag at all times. Why, because it doubles the lens options I have without paying for additional lenses! Previously I shot with the Nikon D3 and Nikon D300s, but recently sold the D300s to replace it with the D800, which is a FX body, reasons behind this can be found in the blog post Gear.. I wrote a while ago.  (little insight, the D800 is a beast and produces great DX shots!) If you have no experience with either a FX or DX camera, check out BorrowLenses.com and rent one!

Rent something, play with your focal lengths & Keep Shooting!

Chris W ‘WhonPhoto’

Remember to ask any questions in the comments or on FormSpring! I don’t want you leaving confused!

5 Comments on “FX vs DX (Full Frame vs Crop Sensor)

  1. Hey Chris, great post. For some reason, I have been running into an endless string of photographers gushing about 85mm primes, especially the f/1.2 variety. Needless to say, pricey lenses. Well, I also have a post regarding “the only lens you really need” (which I agree is a 50mm), and this was my first purchase to match my new Nikon D610. Then I read about this nifty little button on the front that will instantly switch between FX and DX, and as you say, bam, now I have a 50mm f/1.8 that at the flick of a switch becomes a 75mm f/1.8! Not too shabby — I think I can live without the extra 10mm for the price of “free”, haha. Keep up the fine posting,

    • Thanks Dave, that was an old post that I should do some updating on…
      If we are thinking pure composition this is correct. However a true 85mm will give you a significantly smaller depth of field simply due to the fact it is a longer lens. Cropping into your sensor like a DX body does to full frame lenses will give you the 50mm DoF vs the 75mm-ish.

      • So to clarify, shooting portraits with my 50mm 1.8 on my Nikon D800 in DX Mode, will not gain me the much sought after bokeh of the 85mm in FX Mode.

        Is this correct?

        • Correct, it will only crop in on the 50mm image. Its simply a faster way to crop in if you need the extra reach in the field and don’t want to bother with it in the computer. Also drops file size significantly!
          (I should really re-write this post..)

Leave a Reply

Your email address will not be published. Required fields are marked *