SMC Pentax M 40mm F/2.8 Pancake Lens Review

Lilliputian. Svelte. Teensy and weensy, and of course, teensy-weensy. All words I’d never expect to write in a lens review, yet here we all are and I might as well get these silly synonyms for tiny out of the way. Because today we’re talking about one of the smallest legacy lenses I’ve ever used, and the smallest SLR lens that Pentax ever made – the Pentax-M SMC 40mm F/2.8.

Produced from 1976 to 1984, this full-frame lens offers an incredibly compact form factor that was rare in its own day and downright impossible to find in today’s era of auto-focus lenses. A pancake in the truest sense, you’ll be hard-pressed to find a more compact lens from any manufacturer to mount on your Pentax or mirrorless digital camera. But size isn’t everything, and this lens does have some issues. Whether or not these imperfections will be enough to detract from the value proposition inherent in its portability will depend on your needs and preferences.

Let’s get down to details, and find out if this Pentax short stack is right for you.

Okay, it’s small. But how small is it? At 68mm it’s similar in diameter to any other legacy lens, but its length of just 18mm is astoundingly short and quite rare. For reference, that’s less than three-quarters of an inch of lens sticking off the front of your camera. I’ll give you a moment to estimate that distance with your thumb and forefinger. Pretty small, eh?

And not just tiny, it’s also dense and well-made. Like most Pentax lenses from the SLR era, build quality is right there with the likes of Nikon and Minolta. A beautifully damped focus throw, an aperture ring that clicks into its detents with mechanical precision, and robust, metal construction throughout the lens barrel, lens mount, and filter threads, all combine to create a lens that, in 2017, is uncommonly precise and solid. At 110 grams it’s lighter than most standard primes, and the five elements in four groups optical configuration does extremely well to correct for any and all distortion. This compact size and weight coupled with quality construction means that when mounted to a small Pentax SLR or modern mirrorless camera, the lens feels fantastic.

The 40mm focal length is just about perfect for general-purpose photography, and even excels in some specialty shooting styles. Street photographers, for example, should enjoy the way it splits the difference between wide-standard 35s and the tried-and-true 50mm. Arguably the street focal length, subtracting 10mm from the 50 gives us just a bit more to work with, as far as capturing more stuff in the frame. The additional background can provide greater context in certain street environments, yet it’s not so wide that we need to get right up to the nose of our subject. Snapshots, family photos, travel shots, landscapes and architectural shots, as well as everyday adventures should all look fantastic through the 40mm eye.

Where this minuscule lens starts to stumble is when we bend a critical eye toward image quality. Sure, certain optical ailments are well-handled by Pentax’s fantastic SMC (super multi-coating) tech, such as flaring and ghosting, and the SMC glass continues the trend of coaxing punchy and colorful images from the world around us. But other optical issues are prominent. When shooting at maximum aperture there’s substantial chromatic aberration (color-fringing). While this won’t be a problem for those who shoot black-and-white or those who know how to mitigate the effect in post-processing, it might stand to distract from other folks’ images. When we stop the lens down to F/4 things get just a bit better, but to be honest, the chromatic aberration doesn’t totally resolve until F/5.6, and even then we can still make it happen in extremely contrasty areas of the frame.

Light fall-off (vignetting) is pretty extensive, with corners of lighter exposures being noticeably darker than the center of the frame. This is worse on full-frame digital sensors, such as the one found in my Sony a7. The phenomenon is less prevalent with film, and won’t really show up at all on crop-sensor machines such as Fuji’s X-series where the lens will give a field of view similar to that of a 60mm lens. All this taken, vignetting is one of the easiest problems to solve in photo editing software – slide a slider and you’re good to go.

What’s worse than these troubles is that the lens just isn’t super sharp, an issue that’s especially egregious, again, when shooting wide open. Corners are spongier than a rum cake, and even the center of the frame lacks the clinical precision of something like a Nikkor 50mm, or Minolta’s Rokkor 45mm F/2. And with the Pentax’s maximum aperture being a relatively sluggish F/2.8, this lens will need to be shot wide open in more situations than you might expect. Shooting Portra in the city at night? Better bring some diazepam. Or a tripod. Pick your poison.

Stopping the lens down will naturally improve sharpness, to a point. When we’re shooting at F/4 or F/5.6 we’re making images that are acceptably sharp, and at F/8 I’m happy to compare shots made with the Pentax with many other legacy lenses. It’s true that even at F/8 or F/11 shots aren’t as sharp as those made at equivalent apertures by some non-pancake lenses, such as the Zeiss 45mm F/2 Planar I’ve been shooting on my new G2, but we can’t really expect them to be, can we? This is a pancake lens of tiny proportions – we shouldn’t be surprised if image quality suffers a bit. And sharpness isn’t everything. Some of my favorite shots I’ve ever made are ones that you wouldn’t necessarily describe as “sharp”. In the real world, the Pentax lens will be sharp enough, just don’t expect anything special.

The five-bladed iris creates some fairly distracting bokeh, and this combines with the far-flung minimum focus distance of 60 centimeters (nearly 2 feet) to make for a lens that won’t be mistaken for an exceptional subject isolator. Highlight bokeh at any other aperture is geometric and rather unpleasant. Just as you wouldn’t buy this lens if sharpness was your primary metric for quality, so too should it be avoided by those for whom bokeh is the only thing that matters.

All these imaging caveats might make you think twice about hunting out one of these lenses, and before I’d spent a good deal of time with the 40mm I’d have understood if this lens’ reputation had you finding more fault than virtue. But after a few months shooting this pancake from Pentax I feel it’s a lens that defies the kind of normal lens assessment to which we all mostly subscribe. For sure, this lens is not a perfect lens, and it doesn’t make perfect images in challenging situations (low light, close subject, traditional portraiture). But there’s something special about it that stems directly from its size, form factor, and usability. There’s a certain character and quality about this lens that may get lost in translation when reviewers put their pen to paper.

It’s just a great lens to carry around, and paired with the right camera it’s an absolute dream. Mount this lens on something like the Pentax K1000 and you’ll likely miss the boat. That camera is large. But put it on something like the professional-spec LX, or the mid-level ME Super, cameras that are among the smallest SLRs in the world, and you’ll have discovered a pairing of lens and camera that is simply divine. These pairings create a machine and lens combination that just feels so, so good.

Concise, precise, and sophisticated, I found myself simply setting the lens to F/4 and firing away, caring very little if things weren’t quite sharp enough and caring even less about potential vignetting and chromatic aberration. Out in bright light and shooting the street, I often realized I’d been simply pointing and shooting with the lens set to F/8 and the focus ring set someplace in the middle of its action. It worked. Shots were sharp enough for my taste, contrasty, vibrant, and most important, I was happy to take those shots. In fact, this lens fitted to the Pentax LX became my surprisingly luxurious go-to set-up for the final few months of 2016 (LX review coming soon, Pentaxians).

Mounted to the Sony a7 the lens is truly capable of making excellent images with little fuss. Unfortunately, mounting this lens to a Fuji or Sony digital camera will require an adapter (as do most legacy lenses), so the size benefit is somewhat negated. That said, this combination is still a smaller overall package than mounting competitive and comparable legacy lenses. In truth, this 40mm mounted to my Sony presents the smallest legacy lens profile I’ve ever used. And I should mention that this lens uses Pentax’s K-mount, which in similar fashion to Nikon and their F mount is still being used by Pentax on their modern DSLRs. That means this lens can mount to your new Pentax DSLR with only minor concerns (it’ll be manual-focus only, and the aperture will not be controllable via the camera’s in-body controls). If you’ve got a modern Pentax DSLR, try looking for the newer version of this lens, called the SMC Pentax-DA 40mm F/2.8.

All this said, it seems that the SMC Pentax-M 40mm F/2.8 is one of those rare creations that is, clichés be damned, greater than the sum of its parts. It’s inexpensive, a capable image maker with uncommon fit and finish, and a lens with fantastic feel on the street. It offers a standard focal length that’s just abnormal enough to keep things interesting, great optics capable of making punchy shots, and the smallest package available to vintage Pentax fans and modern legacy shooters alike. If you’re a photo geek using a Pentax film camera, a traveler with a digital machine, or anyone interested in compact precision, this lens is worth a try.

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