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.

    And if you compare performance stopped-down, DX lenses are optimized for performance over the DX frame. So, no general advantage with FX glass on DX bodies here.

  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.

The true fullframe advantage over crop

In low light, the larger sensor shows less noise, so it is iso performance, right? Wrong.

The iso performance of a sensor is measured for the full sensor area. Other things being equal, a sensor with higher resolution has more noise per pixel but since each pixel – hence, its contribution to noise – is smaller, not more noise overall. For an iso rating of a sensor, the single pixel does not count, the whole image counts.

Other things being equal again, iso 100 on a fullframe sensor looks better than iso 100 on a crop sensor. So far it looks reasonable: Larger sensors = less noise. But if you use a lens on each camera yielding the same field of view and depth of field, you have to use longer focal lengths on the larger sensor and thus stop down further on the larger sensor to keep the same depth of field. In low light, when iso is above the camera’s base iso, you have to use a higher iso value to compensate. It still looks as good as the lower iso value on the crop sensor. But you don’t have an iso advantage in terms of lower noise on the larger sensor in this situation.

Let’s go deeper. If the fullframe user uses the lower f-stop, too, but also the smaller focal length like on the crop sensor, and crops the image in post to get the same field of view, the remaining image pixels get enlarged and so the noise gets amplified as well. Still no noise advantage!

The only option to get less noise with the larger sensor is if you can live with a shallower depth of field.

Again, a fullframe sensor only gets you less noise if you can live with a shallow depth of field, or if you are at very low iso values already. In the latter case, today’s crop sensors are quite good though, more quality is usually not needed for an amateur.

Is sensor size useless then? Of course not

Other things being equal, optical issues of a lens have a certain extent. The larger the sensor, the smaller the the lens issue compared to the final image. Meaning you get more undistorted image information just through the larger sensor area.

On top of that, with the larger area you use longer focal lengths and stop down further to get some depth of field. Stopped down a bit (but not too much) the lens performance will improve, getting you even higher resolution. That is the real advantage of fullframe versus crop! You get your lenses more often to the peak performance, and the larger area captures more information even if you shoot wide open.

Another reason for fullframe: At least in the Nikon world you get the best lenses here. The DX range consists mostly of consumer-grade products, and while you can mount FX lenses on a DX camera, you cannot fully use their performance with a crop sensor. I am told it is different in the Fujifilm world where a lot of APS-C lenses are made to professional standards.

Do you need fullframe? I personally look for a reason to finally give in. But honestly? 1.5x crop is just some stop in terms of light and thus iso performance. Meaning yes it can be noticeable if you look at individual pixels – but since when has a good photograph to be be pixel perfect? If you think it does, why being satisfied with “full”frame, which in film terms is amateurish small frame? Why not a larger sensor? Such cameras are available, look at Pentax and Fujifilm …

Conclusion

I am a crop-sensor user but of course admit that systems with larger sensors can offer better technical quality. But the common idea that this advantage would be less noise in low light only applies to situations where not a lot of depth of field is required.

Limiting native iso is wrong

I am one of the guys insisting that with year 2020, while the new twenties begun, a new decade begins with 2021. The first year AC was year 1. Math cannot be cheated.

Take this blog entry in this light.

You cannot cheat the exposure

Using the Raw format one can push the exposure and recover shadows quite a lot. Some think it would be good to limit iso in order to prevent getting too much noise and then to use the Raw file to recover dark areas. But of course this increases the noise. The result is slightly more noise than if optimal native iso would have be used to begin with.

If lowest noise is the prime concern, one should rather pull the exposure (recover highlights) using Raw. Of course this has to be balanced with clipping, one should not expose so bright that one gets clipping where not wanted.

If there would be somehow a trick to get less noise with pushing an underexposed image, by now it would be done in the camera already in order to get good reviews.

Detailed settings discussion

If the minimum shutter setting is a bit too fast for what it is possible and the max iso value set just a bit below the optimal iso while you use the aperture priority mode, the iso limitation results in longer shutter speed and gets you an image with less noise. But in practice this situation rarely occurs, where an iso limit just compensates for an overly cautious minimum shutter speed.

If in low light the camera is used handheld, one can just adjust the minimum shutter speed in order to first take some shots with fast shutter, and then try to push the luck with somewhat longer exposure.

If timing is critical, one cannot take multiple shots until it works out. You probably don’t want to deal with shake blur in the photo, so you probably want to use even quicker shutter speed than normally in order to really not risk any shake blur. Using an iso limit within the native iso range does not help, as with fixed shutter speed the image is underexposed and while some underexposure can be fixed in post, using native iso would yield a slightly better result.

For best sharpness one usually stops the aperture down. Full aperture often includes less than ideal sharpness but if weighted against iso noise, having some sharpness issues caused by the lens is normally better than having the sharp image drowned in noise. But if a certain depth of field is needed to get every important sharp, you probably prefer noise to having an important image part out of focus, or to risk motion blur for the whole frame.

More about iso choices

The higher the iso, the less dynamic range is provided. Using an underexposed image also reduces dynamic range though. If highlight recovery is the main concern, meaning if you look only at the top-end of the range, you might want to live with the disadvantages of an underexposed Raw, but that is a different discussion.

I suspect, this is how some decide their maximum tolerable iso: They look on 1:1 crops and might say iso 3200 is still good, 6400 not so much, 12800 clearly compromised so I use 6400 as max iso.

But at the end of the day the whole photograph counts, not a 1:1 crop. A sensor with high pixel density gives you more noise (larger random color offset) per pixel but not necessarily for the whole image. A 1:1 crop might let a sensor with higher pixel count look worse but that does not tell you something about the photograph as a whole.

In almost all cases, the published photograph will be downscaled anyway. This also reduces noise.

Some sensors are said to be iso invariant in that pushing in post or using higher iso in the camera does not make a noticeable difference. For that sensors, limiting iso and getting an underexposed results does not hurt, but does not help either. In my experience, even “invariant” sensors are in fact slightly variant, so using native iso is still, if just a tiny bit, better than pushing an underexposed Raw.

This blog entry is about the native iso range, not about an extended range offered by some cameras. Photographing with in-camera extended high iso is comparable to pushing a Raw in post, meaning no advantage but also not disadvantage.

It appears some think that extended low modes, like “Low-1” for example converting a base-iso 100 to iso 50, would be useful because the lower the iso the better. While that would actually slightly decrease noise, it also risks blown-out highlights because all is done here is using an overexposed image with digital exposure correction.

Dealing with noise

On film, higher iso values affect all levels of brightness. With 135-format film you can change iso only with using a new roll, and iso 800 film is substantially more expensive than iso 400. This is why I use iso 400 film even in low light and have the laboratory push it. That gets more noise than paying up for iso 800 film but it saves me a lot of bucks.

In digital, such cost concerns don’t exist. Digital photographs show noise mostly in dark areas, making it even easier to deal with. A contrast curve can pull shadows, making those levels darker and conceal or reduce the noise there. A digital noise filter applied for the whole frame then can be used with less aggressive settings.

Addressing noise in color and brightness separately allows to remove the most distracting noise artifacts in color, while keeping most details and with it some noise in luminance. Even more sophisticated tools exist, but I am not sure why some noise is seen as bad in the first place. Perhaps because newer technology provides less noisy images and is advertised to be desirable.

Of course, the lower the light, the more noise, up to a level where it removes too much detail. But limiting auto-iso within the native iso range or using a manual iso setting leading to underexposure does not help you to get a result which shows more detail.