Using DX lenses on an FX camera

What happens if you use a lens designed for the common 15×22 mm sensor on a full-frame (24×36 mm) camera?

To start with, I will regularly use the Nikon naming conventions below: DX for “digital” or 1.5× or 1.6× sensors. And FX for “full frame” or 24×36 mm sensors. Alternative names for the smaller sensors are “EF-S” (for Canon’s DX lens series), APS-C (after a particular format film), “reduced image circle” (the main issue), or “digital” (which was correct at the time that there were no FX sensors yet).

Can you safely mount a DX lens on an FX camera?

• On Nikon you can mount a DX lens on an FX camera. And the camera (Nikon D3, D700, D3x) responds by automatically reducing the coverage of the viewfinder images and cropping the resulting JPG or raw image. This is a sensible thing to do as the lens was designed for smaller sensors: the extra field of view covered by the larger sensor will have (often much) lower quality than the DX part.
• Canon’s EF-S lenses, in contrast, are mechanically blocked from mounting on an FX camera. Canon’s story is that, because the FX mirror is larger than the DX mirror, there is a risk of the larger DX mirror hitting the back of the EF-S lens and destroying the mirror and maybe the lens. In other words, this mechanical safety measure allows EF-S lenses to be designed that are physically unsafe to use on an FX camera like the Canon 1D or 5D series. This mechanical safety measure is actually a special plastic ring at the back of every EF-S lens. If you are brave enough, you can remove that ring with a screw driver (at your own risk) according to some sources (link1, link2, link3).

Third-party lenses and early cameras

There are a few special cases:

• Canon’s first DX camera modelswere released before Canon decided to create a dedicated EF-S line of lenses. These early models (the Canon D30, D60 and 10D) were a bit of a transgender thing: a DX camera trapped in an FX casing. Adapting Canon’s EF-S 10-20 mm lens to fit on these pre-EF-S DX models should be pretty safe (small mirror).
• Third-party lens suppliers also sold DX-type lenses, but probably all without the mechanical interlock. This was because the same optics was typically sold in Canon, Nikon, etc versions and particularly Nikon didn’t have the equivalent safety feature to block using a DX lens on an FX (film) body. One example is the Tamron 11-18 mm lens which was designed for DX-type cameras. It is possible to safely mount this lens on a full frame camera – the lens does not have parts that protrude further into the camera body than FX lenses, and it worked for me.

Shots with a DX wide-angle on an FX body

The following images show the results when you use this “Tamron AF 11-18 mm f/4.5-5.6 Di II LD Aspherical (IF) SP” on a Canon 5D Mk 2 body. The same should apply for other Canon FX bodies. And likely a Nikon FX body with the Nikon version of the lens.

Tamron AF 11-18 mm at 18mm f/5.6 1/15s 400 ISO

Note that in the 18mm image, the left half shows the original vignetting from the camera, and in the right half vignetting has been corrected in software (Lightroom 2). Although the camera also has an in-camera PIC feature to compensate for vignetting, it by default only kicks-in for 25 specific (Canon) lenses.

One can argue that the version with the vignetting looks better: apparently the eye doesn’t mind vignetting in many cases. Note that in these images, chromatic aberration has been corrected using Lightroom. Chromatic aberration is a major problem with wide-angle lenses, especially far from the image center.

Tamron AF 11-18 mm at 14mm f/4.5 1/20s 400 ISO

At 14mm, the vignetting becomes a bit disturbing at the top and probably needs to be cropped there. But again, the vignetting at the bottom is fine.

Tamron AF 11-18mm at 11mm f/4.5 1/30s 400 ISO

At 11mm, the vignetting is extreme. Unfortunately you cannot capture the entire image circle on this particular lens. If you crop enough to show a rectangular area without any vignetting, you might have been better off with taking the same shot at 14 mm or  more. But it might be beneficial if you intend to create a wide view, but need a square image rather than the sensor’s 2:3 aspect ratio.

The cropping itself is not much of an issue on the various 20+ MPixel high-resolution FX cameras (Canon’s 1Ds, 5D Mk2, Nikon D3x) as the resolution is high enough – but it is still unlikely that zooming and cropping will give you better quality than using a longer focal distance and not cropping.

Image quality

You shouldn’t expect miracles from a DX lens when used on an FX body. Here is a 100% crop of the 14 mm image shown above. The image might look better with additional sharpening.

100% crop near the center of the 14 mm image

Conclusions

A DX lens wide-angle lens can be usable on an FX body if you can get it to mount without risk of damaging your mirror (an issue for Canon EF-S lenses) and are more interested in getting the shot than worrying about edge sharpness. The edges of the image are probably unsharp – especially with this particular Tamron lens which is already weak for a DX image. Unsharp edges may be undesirable for landscapes or nearby buildings, but may look natural for a picture of the subject (castle, person) in the context of its surroundings (fields, room, street). In those cases, edge unsharpness just strengthens limited depth-of-field and helps focus attention on the subject itself.

Thus arguably this particular 11-18 mm Tamron lens can be used as a full-frame 18mm lens for some kinds of shots, and the even more extreme 11-14 mm settings might more be interesting as “special effects”. In general, vignetting often doesn’t harm an image and may even strengthen it.