It seems like the debate of DX vs FX for wildlife and sports photography is a never ending one. DX shooters argue that they get more reach, stating that DX is like a “built-in 1.5x teleconverter”, or that DX setups are lighter due to smaller lenses and less expensive, or that DX chops off the corners of lenses, thus reducing vignetting and other optical issues. On the opposite side of the fence, FX shooters argue that they get better image quality at pixel level, better viewfinder, less diffraction issues, better AF performance in low-light, etc. Seems like we have two camps, each defending their own side for various reasons. Having spent a number of years shooting both DX and FX starting from the first generation Nikon FX cameras and every single DX camera manufactured by Nikon to date, and having talked to a number of other photographers that shoot for a living, I came to a conclusion that there are some myths surrounding the DX format that need to be debunked. In this article, I will provide my personal insight to this topic and explain why I believe that FX is always better for photographing sports and wildlife. This article evolved as a result of recent discussions of the subject with some of our readers.
1) The Myth of the DX Built-in 1.5x Teleconverter
A lot of people seem to be very confused about the effect of a crop sensor on the focal length of a lens. Stating that a crop sensor increases the focal length of the lens or acts as a teleconverter is completely wrong, since focal length is an optical attribute of a lens and has nothing to do with the camera. I talked about this in detail in my “Equivalent Focal Length” article that I published a while ago. Simply put, a DX sensor can never change the optical parameters of a lens, so if you are shooting with a 300mm lens, it stays as a 300mm lens no matter what camera you mount it on. The confusion of “equivalent focal length” comes from manufacturers that initially wanted to make people understand that the field of view on a cropped sensor camera is tighter than 35mm, because the image corners get chopped off. The word “equivalent” is only relative to 35mm film. So you cannot say that your 300mm lens becomes a 450mm lens on a DX body. It does not and never will. All you are doing, is you are taking an image from a 300mm lens, cropping it in the center area and magnifying that center with increased resolution.
2) DX Pixel Size and Resolution
The only reason why some people thought that DX provided longer reach, was because DX sensors historically had similar resolution as FX. For example, both Nikon D300 (DX) and D700 (FX) have about the same resolution – 12 MP. So despite having sensors of completely different sizes, the two cameras produce images of similar size / resolution. Ultimately, this means that the D300 can resolve more detail from the center of the lens (which is typically the sharpest on any lens) and thus magnifies the subject more, which led people to believe that DX was better than FX to get closer to subjects. One aspect that was rarely talked about, however, was the fact that the D300 has a lot more noise at low ISO levels than the D700 due to smaller pixel size. So despite having this magnification advantage, photographers had to constantly deal with cleaning up apparent noise even at low ISO levels. I personally had to constantly down-sample images and clean them up via noise-reduction software to get rid of the artifacts visible at anything above ISO 800 (and noise was visible even at base ISO!). So at the end of the day, taking a DX image and down-sampling it aggressively, versus simply cropping an FX image produced somewhat similar results, with a slight advantage on DX that resulted in more detailed shots, thanks to the down-sampling process.