Many of you are aware that you can use the beloved 35mm f/1.8 DX lens with an FX sensor. I have an on-going love affair with this lens, but if anybody mentions it to my wife, I’ll flat-out deny it. There are many discussions about the ‘vignetting’ levels when you try this lens on FX, but there is precious little data on the “corner sharpness” when you decide to misuse/abuse this lens by mounting it on an FX body. Not to mention how the focusing distance affects vignetting. This article explores the practicality of using the 35mm f/1.8 DX lens with an FX sensor.
The news is good. You don’t need to abandon what (to many) is their absolute favorite DX lens that Nikon produces. The coverage (the image circle) of the 35mm f/1.8 DX shows how this lens is an over-achiever. It goes above and beyond what is required for a DX sensor; it’s almost as if Nikon designed this lens for FX, but accidentally labeled it as “DX” instead. Almost.
To be honest, the corners do get a bit dark, especially when the lens is focused near infinity. The secret sauce is to use the “vignette control” in image-editing software to lighten corners, and to not stop down the lens too much. The post-processing solution isn’t perfect, but for many photographs, it will make it turn vignetting into a “don’t care” situation.
Believe it or not, the corner resolution isn’t that bad, either. It’s not as good as an “FX” lens, but most people won’t even notice the difference between this “DX” lens and an “FX” equivalent. At a fraction of the cost and a fraction of the size and a fraction of the weight.
The bottom line, assuming that you want to be able to focus at infinity, is to keep your lens between f/1.8 and f/4.0. This isn’t much of a hardship. If you want to stop down further, then yes, you’ll need to crop a bit, but only just a bit.
35mm f/1.8 AF-S DX mounted on an FX Nikon D610
Since talk is cheap, let’s look at the data.
Nikkor 35mm AF-S DX at f/1.8, with no distortion correction, using Nikon D610.
You can barely make out any vignetting in the resolution chart (4.6 feet away). The vignetting was corrected using Capture NX2, but the correction is almost identical using other tools, like Photoshop. You can tell there’s some barrel distortion, though, so let’s try to fix that next.
Nikkor 35mm AF-S DX at f/1.8, barrel distortion removed, Nikon D610.
Now, there’s no visible distortion and any vignetting is almost gone. For normal photography, you’d not notice any vignetting.
Nikkor 35mm AF-S DX at f/4.0, vignetting/barrel distortion removed, Nikon D610.
Note that vignetting is still almost imperceptible at f/4.0, and I’ll bet nobody would be complaining about edge sharpness, either. You can even get away with f/5.6 and not have vignetting trouble, unless you try to focus at a long distance. I'd say that the vignetting here is about the same as the Nikkor 18-140mm AF-S DX lens on a DX camera.
Nikkor 35mm AF-S DX at f/1.8, focus near infinity, Nikon D610. No free lunch.
Now, it’s time to look at some warts. The picture above was focused near infinity, and the lens had both a hood and a UV filter on it. Cropping is becoming a necessity under these conditions, but the angle of view after cropping still far exceeds the Nikon in-camera “automatic DX cropping” that the camera offers.
Using a UV filter and a hood don’t make a perceptible difference with vignetting; the lens image circle is the limiting factor.
So, how does the resolution at the FX frame edges stack up? The following charts tell all.
MTF50 lp/mm at f/1.8. The center, of course is already terrific. Corners aren’t.
MTF50 lp/mm at f/2.8. The center is very, very good. Corners still aren’t.
MTF50 lp/mm at f/4.0. The center is stellar. Corners are now acceptable.
With only a few concessions, this lens is quite useable on FX. Speaking for myself, it’s a keeper for either DX or FX.