The real reason you shouldn’t mix crop lenses with full frame camera bodies

I wish I’d known way back not to invest in decent but cheap crop sensor lenses. I was shooting with a Canon EOS 7D, which has an APS-C sensor, so it made perfect sense at the time. Besides, back then, I was still a hobbyist photographer, mostly shooting wildlife and landscapes on the occasions when I was travelling. It all made sense.

Fast-forward a couple of years, and I’d upgraded to a full-frame camera and was studying to be a commercial photographer. Many of those lenses I’d bought for the 7D were no longer useful on the full-frame sensor. Weirdly enough, I kept both the camera and the lenses as a backup, even when I later upgraded to the Canon EOS R, Canon’s mirrorless system.

I was pleasantly surprised to see that these new mirrorless cameras have an in-camera crop ratio option. This means that you can finally use those crop lenses again (albeit with a cropped sensor). But the questions remain: why is there this difference in lenses, and should you use them between the two systems? In this video for Adorama, David Bergman explains the ins and outs of the two sorts of cameras and lenses and explores whether it’s a good idea to mix the two or not.

Full-frame vs. crop-sensor cameras

Full-frame cameras (like the Canon EOS R3) have sensors the size of 35mm film, while crop-sensor (or APS-C) cameras, like the Canon R50, have smaller sensors. Crop sensors capture a narrower portion of the image projected by the lens, effectively “zooming in” on the subject. This effect is quantified by a crop factor (e.g., 1.6x for Canon APS-C, 1.5x for Nikon DX, and 2x for Micro Four Thirds).

Impact on field of view, not focal length

When a full-frame lens is used on a crop-sensor body, the lens’s focal length doesn’t change, but the field of view appears narrower. For example, a 16mm lens on an APS-C camera with a 1.6 crop factor produces a field of view similar to a 25.6mm lens on a full-frame camera. It sounds confusing but the lens’ focal length doesn’t change, it just feels like it does because the image appears magnified.

Using full-frame lenses on crop bodies

Full-frame lenses can be used on crop-sensor bodies, providing a narrower field of view, which can be advantageous in sports and wildlife photography for added reach. However, this setup loses some wide-angle capacity.

Crop-sensor lenses on full-frame bodies

Crop-sensor lenses project a smaller image circle. On full-frame cameras, this results in dark vignetting around the edges. Some full-frame cameras have an automatic crop mode that uses only the central portion of the sensor when a crop-sensor lens is attached. For example, Nikon’s DX crop mode activates automatically with DX lenses, reducing resolution but avoiding dark edges.

Full-frame cameras often allow in-camera cropping with image area settings, such as 1.2x or DX (1.5x) crop modes, which narrow the field of view and lower resolution. For example, on a Nikon D850 (45 MP), using DX mode reduces resolution to 19.4 MP.

Conclusion

To summarise, yes, you can use a full-frame lens on a crop sensor body with no issues at all. You will lose some of the wide-angle capabilities. However, for some subjects where you want a little extra zoom (figuratively speaking), the crop ratio can be helpful, making your subjects appear closer. This is great for sports or wildlife photography.

However, the same is not the case for using crop sensor lenses on a full-frame body. There will be an obvious circular vignette around the edges of the frame. It is entirely possible to crop in (either in camera or in post-production); however, you are cutting off a large amount of sensor area by doing so and losing pixels by cropping.

If you really want to get the most from your equipment, then my advice would be to buy the best full-frame lenses possible for your budget. This way, you can future-proof yourself in case you want to upgrade to a full-frame body or swap between bodies using the same set of lenses.

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