• Mon. Dec 11th, 2023

Why are modern 50mm lenses so damned complicated?: Digital Photography Review


May 10, 2021

Lenses like Nikon’s Z 50mm F1.2 S are bigger, heavier, more costly and more complex than older F1.2 designs. But why?
First, some optical history
Back in the days when men were iron and ships were wood, there were some things you could rely on. One of those things was your wide-aperture prime lens was a relatively simple double-guass design. Pre-1900, lens designers had already created five of the six lens types that are the basis of what we use today. (The retrofocus lens didn’t come around until the 1920s, and zoom lenses, for those who are curious, are basically modified Cooke triplets.)

Lensrentals.com, 2012
The double gauss has been THE predominant design for wide aperture primes since the late 1800s. Over the years, designers have added additional groups to bring in more light, correct aberrations, and generally make a better lens. But the double gauss part was there in the center of the lens diagram for wide-aperture prime lenses right up until modern times. Name a famous prime lens and it was probably a double gauss: Speed Panchro, Planar, Xenon, Summicron, Takumar, Ultron, and on and on.
There are some generalities about double gauss design that are of practical importance (note, I said generalities, not absolutes). An angle of view equivalent to 40mm to 60mm focal length on full-frame cameras is the easiest double gauss lens to design. Hence, 50mm tends to be ruled by double-gauss lenses, but longer and shorter focal lengths generally became telephoto or retrofocus designs. Second, aberrations in double gauss lenses are easier to control at F2.8 apertures and smaller. A decent 50mm F2.8 double gauss lens can be made with as few as five elements, while wider apertures require more elements to correct aberrations (notice the chart above).
But lenses change
In the last decade, 50mm lenses have gotten more complex. Compare the 30 year old design of the Canon 50mm F1.2 (right) with the more recent Sigma 50mm F1.4 Art and Zeiss Otus 55mm F1.4 (left) designs.

Lens diagram of (left to right) Zeiss 55mm Otus, Sigma 50mm F1.4 Art, and Canon 50mm EF F1.2L lenses.
Three major manufacturers have recently introduced even more complex-design 50mm F1.2 lenses, containing 13 to 18 elements with multiple aspheric and low dispersion elements. Any double gauss influence on the design is pretty hard to see.

Lens diagrams of (from left): Sony FE 50mm f1.2 GM, Nikkor Z 50mm f1.2 S, and Canon RF 50mm f1.2L. (from manufacturer).
Lens designers didn’t add all that extra glass just to charge more money and make the diagram look cool. These newer, more expensive, and more complex designs are supposed to overcome some of the limitations that we used to see in ultra-wide aperture, 50mm primes. At least that’s the theory and my expectation. We all know how my expectations usually turn out.
So, about those classic ultra-wide aperture prime lenses..
Speaking of expectations, for many years a lot of photographers paid a fortune for an F1.2 lens in the expectation that it would be a little soft wide open, but once stopped down, it’d be at least as good or better than a cheaper 50mm lens. Because stopping down fixes all evils, right?
Unfortunately, that’s not how it worked, at least not for double gauss lenses wider than F1.4. Stopping down made them sharper, of course, especially in the center. But away from center they never got wicked sharp; they got OK. Below, for example, is that classic 50mm EF F1.2L Canon lens compared to a newer Sigma 50mm F1.4 Art lens, both tested at F5.6. As you can see, the Sigma smokes the Canon.

MTF of a Canon 50mm EF 50mm F1.2L lens (left) and a Sigma 50mm F1.4 Art (right).
A field curvature plot shows us this isn’t a field curvature effect (although there is some field curvature). Rather, these lenses have some aberrations that aren’t really improved by stopping down all that much (3rd and 5th order astigmatism, elliptical coma, and other higher-order aberrations don’t improve much at smaller apertures).

These diagrams show that even stopped down, the outer areas of the lens just aren’t as sharp, and there is a lot of astigmatism.
That’s just how things were. A $1,500 ultra-wide aperture lens would give you lovely, dreamy images at F1.2, but stopped down to F5.6 or F8, a $300 F1.8 lens was usually sharper. I used the Canon EF 50mm F1.2L as an example, but the same used to be basically true of all ultra-wide (F1.2 or wider) lenses, including those $3,000 ones.
Testing the new Sony FE 50mm F1.2 GM
Sony sent me five copies of the new FE 50mm F1.2 GM for pre-testing. This is not complete testing, since it’s only five copies, but that should be enough to give us some information. And while I haven’t tested the Canon and Nikon 50mms, the designs suggest they should be rather similar.
Wide open tests
Compared to the old EF Canon 50mm F1.2…. well, 30 years of lens design sure does makes a difference. The Sony (right) is dramatically superior.

Lensrentals.com, 2021
Here’s a better idea of how good the Sony is; at F1.2 it has better resolution than the excellent Sigma 50mm F1.4 Art at F1.4.

Lensrentals.com, 2021

So here’s a win for the new design; wide open it’s as good as an excellent 50mm F1.4. That’s definitely a ‘worth the money’ point for many photographers. I was still curious though – how much it would sharpen-up when stopped down?
Stopped-down comparisons
Let’s do the obvious first, and compare one copy of the F1.2 GM to one of the Sony Planar 50mm F1.4 ZA, both at F5.6. The ZA is not an awesome lens, but a lot of FE shooters have used it. Stopped down, the MTF diagram below shows its bad astigmatism. I had expected the GM to be better than this, and it is.

Lensrentals.com, 2021
Let’s try a bit of a tougher comparison, between one of the GMs and a Sigma 50mm F1.4 Art (I flipped sides on this graph, the Sony is now on the right). There are some slight differences here, but if you’re tempted to say one is clearly better than the other, I suggest keeping quiet, because you’d be wrong. The Sigma is a tiny bit better at the edges, the Sony in the center, and the Sony has a bit less astigmatism. But the differences are very minor, to the extent that they’d be within the range of sample variation and probably not noticeable in photographs.

Lensrentals.com, 2021
This was what I was hoping to see. Historically, if you bought an F1.2 or wider aperture lens, you expected that it would be soft wide open and even stopped down it wouldn’t be as sharp as a less expensive, slower lens. The modern (and more complex) designs we’re seeing now allow F1.2 lenses be impressively sharp wide open, and just as sharp as a smaller aperture lens stopped down. And that is actually kind of a big deal.
As several people will kindly point out in the comments, resolution isn’t everything. It’s probably not even the biggest thing. But it is a thing that, at least in F1.2 lenses, has not really been a given in lenses like this until now.
And hopefully, you’re beginning to see why these new lenses have so many elements*.
*Yeah, there’s more to it, but this is a good start.

Roger Cicala is the founder of Lensrentals.com. He started by writing about the history of photography a decade ago, but now mostly writes about the testing, construction and repair of lenses and cameras. He follows Josh Billings’ philosophy: “It’s better to know nothing than to know what ain’t so.”

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