Today’s Question: If I am using my Nikon D850 with a 24-70mm lens in FX mode, I have the obvious de facto range of 24-70mm. If I then switch to DX mode at a 1.5X crop factor, in theory I would then have extended my range to 105mm. In this scenario am I correct in saying by carrying this one lens only a steep hike up the mountain I have the equivalent of a 24-105mm lens? From an image quality point of view what am I sacrificing when I am operating the 24-70 in the DX mode? What would be the difference between an image shot with my 24-70mm at 70mm in DX mode [105mm effective focal length] versus using a Nikon 105mm prime lens in FX mode?
Tim’s Quick Answer: Put simply, when you make use of this type of option that enables you to achieve the field of view of a longer lens, what you are actually doing is cropping the image in the camera. In other words, the image captured in this crop mode will have a lower resolution. You would get the exact same result in terms of image quality if you captured in the normal full-resolution mode, and cropped the image in post-processing.
More Detail: In this case I’m addressing a question about a specific camera model that offers what is effectively in-camera cropping to a lower resolution, effectively providing a field of view of a longer focal length lens in the process. This feature, however, is available in a variety of different camera models from different manufacturers.
The key thing to keep in mind is that when you switch your camera from full-resolution mode (FX mode in this example) to a cropped mode (DX mode in this case), you’re really just cropping the image circle projected by the lens in the camera. In other words, you’re capturing an image that includes only part of the pixels on the image sensor.
In this specific example, the Nikon D850 has a 45.4 megapixel resolution. When you use DX mode, you are cropping the image that the sensor is recording, and ending up with a 19.4 megapixel image.
You aren’t sacrificing image quality at all in this case. You’re only giving up resolution. You could achieve the exact same result (of using DX mode) by capturing the image in FX mode and then cropping after the fact to the same sizing represented by DX mode (a 1.5X cropping factor in this case).
Of course, you could keep the camera in FX mode, and use a lens with a longer focal length. So you need to consider the impact on resolution in terms of the in-camera cropping.
With the full resolution of 45.4 megapixels, you could print the image at its native resolution to about 18″x28″. That is assuming a 300 ppi print resolution, and no enlargement of the original image. You could obviously print even larger with great quality if you wanted to. For comparison, the DX image would have a native print size of about 12″x18″. Once again, you could print larger if needed.
But the point is that in FX mode the native print size is about 18″x28″, and in DX mode the native print size is about 12″x18″.
In FX mode using a 105mm lens, a capture will have a larger potential output size than if you were using the DX mode with a 70mm lens (105mm effective focal length). That could also translate into slightly better detail in the FX shot, since you have more pixels to work with. But the framing of the two images would be identical. Plus, the DX shot would have a smaller file size than the FX shot.
So, if you need to be able to produce very large prints, you may want to favor FX mode with a longer lens as needed. If you want to favor smaller file sizes (because you are cropping pixels on the image sensor) and don’t mind not being able to print quite so larger, using a shorter lens in DX mode provides a great solution. What that really translates to is that if you’re not intending to ever make especially large prints, you might favor using DX mode most of the time, with shorter (and lighter) lenses to match up with the in-camera cropping. But if you want to be able to make very large prints, you may want to favor FX mode, cropping in post-processing when you need a tighter shot than your maximum lens focal length allows.