Should you buy FX lenses for a DX camera?

I am an enthusiastic amateur, buying my first point and shoot in 1995, went digital in 2003 with a compact “Coolpix” and take a lot of photos since 2015. Currently I use the Fujifilm X100F and two Nikon DSLRs, both having a DX (APS-C, crop) sensor.

In this blog I want to dispel the notion that using a fullframe lens on a crop sensor is generally a smart choice.

There are cases where a lens you want is available only for fullframe (in Nikon terms, FX) so that is your only choice of course. Another reason might that be you just want to get the FX variant. For example an AF-P 70-300 lens is available both as DX as well as FX version. The latter is a tiny bit faster on the long end, and it has switches on the lens which the DX version omits. Does it worth it the additional cost and weight? Only you can decide.

Are there other upsides? Some reasons can be seen quite often.

  1. A lens performs best in the center. With DX, you use the center of a fullframe lens, which is the best part to use.

    True – but you still crop off parts of the projected image and thus, information. Even if the cropped border is not as good as the center, having access to it is better than a forced crop. If the lens is used wide open, even the center of the lens does not resolve the DX sensor pixel pitch. No lens does, but especially fast lenses will not fully resolve a modern DX sensor. If stopped down a bit, lens performance improves overall including in in the corners, making it even more desirable to get those image parts instead of cropping them off. In short, cropping a fullframe lens always means to lose image information.

  2. FX lenses offer higher quality

    That is usually true. If you talk about build quality. Regarding optical performance, a recent DX lens – even cheaper ones – usually performs very well when compared to older FX lenses used on DX. No wonder, those FX lenses were designed for then-common FX-sensor pixel pitch. Today’s DX sensor pixel pitch is much higher, with the DX lens design taking that into account.

  3. Buy FX lenses now because if you switch to an FX camera later, you already have lenses.

    True again. But if you want to go FX, why not buy the body now? If you buy FX lenses for a possible later switch, you forgo the DX advantage of a lightweight and more affordable system. When you finally switch, your fullframe lenses might be outdated already. And you might never switch to FX anyway …

Consider the lens performance wide open

The fullframe Nikkor AF-S 50 mm 1.4G lens is one of the betst portrait options for DX. But if used wide open on a modern DX camera, the center crop magnifies the sharpness issues and the problems with chromatic aberration.

On the other hand with a fullfame camera like D750, the lens performance is quite good at f/1.4. The bokeh is a bit harsh but otherwise, you can crank it up. Because the optical issues are rather small relative to the final frame.

To get good resolution on DX, one has to stop down a bit. You are still faster than with a zoom lens set to 50 mms, but a zoom usually has optical stabilization in its favor. Used correctly, the 50 mm 1.4 lens is an option for DX but usage wide open on DX is more difficult than on FX.

The Fujifilm world recently rejoiced because of the Fujinon 50 mm 1.0 lens. That lens has just marginally more background blur than 85 mm 1.8 … If you are crazy about maximum background blur, using a smaller sensor is an unnecessary hurdle. Using a fullframe camera instead, and a couple of f/1.8 lenses, you could even save money!

Iso performance drops with further cropping

If you have an already noisy image, magnifying it also amplifies the noise. There might be cases where you just have to crop to get the frame you want because an affordable lens is only so long – but generally you want a tele lens with enough optical reach to avoid crop/magnify in post.

Take the famous 70-200 2.8 lens, used on DX. How does it compare to the kit-type DX zoom, AF-P 70-300? On the short end of 70 mm, the DX lens only gives us f/4.5. With 2.8, the FX pro lens is some stop faster.

On the long end of the 70-200 we still get f/2.8, while the DX zoom only allows 5.3 for 200 mm. The FX lens is nearly two stops faster now.

On the long end of the 70-300, we only get f/6.3. In order to get the same field of view, we have to crop/magnify an image taken by the 70-200 mm lens at full length in post. While still taken at f/2.8, through that crop the noise performance suffers by some stop. That means, in terms of noise the FX lens is only about 1 effective stop faster.

If you compare the price and weight of these lenses, I would like to know how many DX users really need the pro-grade build quality of the 70-200 2.8. Paying the full price of that FX lens, carrying the weight, to use only half of the image it projects – why not get an FX camera instead and if money or weight is a concern, opt for the 70-200 f/4? With the larger sensor you can stop down this much and still have iso performance comparable to 2.8 on DX.

But …

There might be situations where the 70-200 2.8 is just the best choice for a DX camera. However those situations seem quite specific to me. 200 mm f/2.8 has a depth of field so thin, that the focus has to be extremely precise to get the sharpness where you need it. The depth of field might still be too shallow, so that you stop down. Then the DX 70-300 might be the better overall option. It is slower but has more optical reach.

What about the fullframe 200-500 5.6? There is no DX alternative, that FX lens is the only choice for this length. And 24-70 2.8? The DX 16-80 is not as fast on the long end, but allows much short settings giving you ultra wide angle on DX, whereas 24 mm on DX is only moderately wide.

What about the 14-24 2.8 for DX? It is a bit wider than the 16-80 but the 1.7x zoom of 14-24 costs a lot more than the 5x zoom of 16-80 which even comes with VR.

Is there any reason to get an FX lens for a DX camera?

The DX lens lineup leaves something to desire. FX lenses can fill those gaps, for a premium. And you might get less from those lenses than you hoped.

I have a couple of fullframe lenses including the 50 mm 1.4. Stopped down to f/2 it is my secret weapon for portraits. 50 mm f/2 has a depth of field still shallow enough for portraits. Why would I want to blow out the background completely? If I had a studio I could use custom backgrounds anyway. Since I shoot outside, I want to show something of the environment.

What about wide-angle prime lenses which are still wide angle if put on DX? Compared to a wide zoom lens, you get faster glass with those FX lenses, but forgo optical stabilization. Using a DX zoom lens with VR costs less and weights less, yet normally performs as well or better. Exceptions would include wide-angle action shots where you rely on fast shutter speeds. But if you buy those wide-angle FX lenses why would you use them with a camera cropping off a lot of that wide-angle image output you paid dearly?

There are of course cases like certain types of wildlife where you rely on super-telephoto. Even with the longest lenses available, you might only use cropped images. Getting a crop-sensor camera for that job seems more efficient. That is still not an argument per se to use FX lenses on DX glass, that is an argument to use the longest lens – though in this world, those are always made for fullframe.

Conclusion

An extreme FX lens becomes less extreme if mounted on DX: You crop image area, and if you use the lens wide open it might be not as sharp as you assumed from reading FX-based reviews.

If you determine that for some reason or no reason you want to get an FX lens, of course, get it! At least, I got a couple … but it is not a magic upgrade for your camera.

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