Our discussion forums are a great resource for people looking to learn about photography, and it's therefore not surprising that certain topics are raised time after time. In many cases they are answered quickly and correctly, but certain topics are still subject to a great deal of confusion. One of these is the deceptively simple question of what the word 'standard' means in relation to lenses. It appears that there are certain misconceptions associated with this topic which reappear time after time, so in a bid to put the record straight, here's my attempt at an (almost) definitive answer.

At the simplest level, a standard lens is defined as one which produces images with a natural-looking perspective (the word 'normal' is often used synonymously in this context). This concept is perhaps best illustrated with regard to what it's not, that is to lenses which clearly don't meet this criterion. A telephoto lens, for example, renders distant objects larger in the frame, and has the effect of compressing the apparent spatial relationship between objects. A wideangle does precisely the opposite; in squeezing more content into the image, objects appear smaller and more distant. It's in the happy medium between these two extremes that the standard lens lies; the apparent sizes and spatial relationships between image elements appear natural, and much as they did in real life. So the question is, how de we define which lenses show these characteristics?

The answer lies in considering how we view images, and is perhaps easiest understood with respect to prints. Everything hinges on the fact that (with the exception of small prints) the most comfortable viewing distance is approximately equal to the print diagonal. For example, a 12" x 16" print has a diagonal of about 20", and it turns out that most viewers will choose to look at it from about 20". This allows them to take in the whole of the picture, while still being able to see fine detail.

So how does this relate to the focal length of a 'standard' lens? To understand this, let's conduct a thought experiment in which we take a picture, make a print on the spot, then see how it compares to the scene in front of us. Simple geometry (using the concept of similar triangles) states that a print viewed from a distance equal to it's diagonal will exactly match the subject in perspective when the focal length of the lens that was used to make the picture is equal to the diagonal of the sensor.

When we use this approach to calculate the focal lengths of 'theoretically correct' standard lenses for various sensor sizes, the results are slightly surprising. Full frame works out as 43mm, 1.5x APS-C as 29mm, and Four Thirds, 21mm. This flies in the face of received wisdom (and convention), which suggests 50mm, 35mm and 25mm respectively. How did that happen?

The answer goes all the way back to the earliest 35mm cameras, such as Oscar Barnack's Leica. For practical reasons of lens design as much as anything else, these were often fitted with 5cm lenses, and for various reasons this became a convention which firmly stuck. Oddly enough, the 35mm format counts as something of an outlier in this regard; the various 'medium' formats (such as 6 x 4.5cm) all stuck stubbornly to the 'frame diagonal' convention. Still even for 35mm there were honourable exceptions; many fixed-lens rangefinders (the progenitors of modern compacts) used lenses in the 40-45mm range, and Pentax famously launched its 'Limited' range of premium primes with a 43mm standard lens, which is still on sale today. And while it may sound like there's little difference between 43mm and 50mm, in reality it's the same as that between 28mm and 35mm, or 85mm and 100mm. There is, in fact, a fairly well-developed school of thought (which I must admit I personally subscribe to) that for a standard lens, that 40mm region is 'about right'.

The Olympus 35RC ca. 1971, with its 42mm F2.8 standard lens

Now that we've established what a standard lens really is, we can dispense with some of the misconceptions which continually reappear. A standard lens is not one which makes object appear the same size in the camera's viewfinder as they appear from the same position with the naked a eye; this is a function of both focal length and viewfinder magnification (indeed with a 50mm lens focused to infinity, it's used as the definition of viewfinder magnification). And a standard lens is not - quite - a lens with the same angle of view as the human eye; peripheral vision extends well beyond the angle of view occupied by the print at that 'comfortable' viewing distance.

So there we have it, the definition of a standard lens. Of course the punchline (and there always has to be one) follows on from my previous blog post, in which I decried the lack fast 'portrait' primes for APS-C, with the much-touted 50mm lens being a bit short for this role. Well to be honest, I wish manufacturers wouldn't build 50mm lenses for full frame either, but instead make their standard primes in that 43mm region instead (likewise rather than 35mm for Nikon's new DX prime, I'd have preferred 28mm). But at least for once I know this is an eccentricity, and I'm firmly in the minority in this regard.

There's something that's been troubling me a bit recently, and I want to get it off my chest. It's an irritation about the photographic equipment available today, and its fitness for the purpose for which it's most likely to be used. It has germinated over the course of testing an array of 50mm prime lenses, developed further with the recent arrival at the dpreview offices of an array of new fast primes, and finally crystallized with the relative lack of interesting new products and ideas at PMA. And it's this; most manufacturers seem be be thinking very conservatively, treating digital as though nothing's changed from the days of 35mm film, and simply aren't making the right lenses for photographers today.

My line of reasoning goes like this. Through the long history of photography, there's always been one constant; the most popular subject is people. The hardware, materials and techniques may have changed beyond all recognition, especially in the past decade, but when all is said and done our lenses are overwhelmingly trained on our family, friends and loved ones. And possibly the biggest advance over those ten years is the ability of the latest breed of DSLRS to operate effectively in low light, with such remarkable high ISO capabilities that it's now perfectly feasible to shoot indoors without the potentially ruinous effects of flash. This is in marked contrast to the days of film, as anyone who shot even ISO 800 colour negatives will attest; we can now use ISO 1600 or 3200 as a matter of course and achieve previously undreamed-of quality (remember Ilford Delta 3200 anyone?). Just throw on a fast prime, and Bob's your uncle.

The problem is though, which fast prime? The new age of the DSLR has certainly seen a resurgence in popularity of 50mm lenses, mainly because they are relatively cheap and gather lots of light. And the manufacturers appear to have cottoned on to this, with a new breed of 'fast fifties' appearing recently from Sigma, Nikon and Pentax (likewise Sony has been showing a 50mm F1.8 DT ‘concept lens’ at PMA).

But really, why 50mm? On an APS-C imager, that's equivalent to around 75mm-80mm in full frame terms. Problem is that this is a strange focal length, too long to be considered ‘normal’ yet just a bit too short for portraits (few manufacturers ever made a 75mm prime for film). The portrait range has classically been considered as being between 85mm and 135mm for comfortable 'head and shoulders' shots; personally in my book, 100mm counts as 'about right'. So it seems to me that manufacturers may be misinterpreting the appeal of those 50mm lenses; they’re popular just because they are fast and cheap, not particularly because of their focal length.

With a classic portrait lens you can shoot indoors in low light, and blur the background to oblivion

It’s not just the focal length that’s a problem, though, there's another issue with those 50mm primes, and that's the much-misunderstood concept of 'bokeh'. Now contrary to popular belief, a mere ability to blur the background is not enough; the concept of ‘bokeh’ refers to the aesthetic quality of that blur, which ideally needs to be smooth and attractive, not harsh and fussy as rendered by many 50mm lenses (a by-product of the traditional design goal of the 'standard' lens for maximum sharpness). True portrait lenses need a different optimisation, for attractive imaging at large apertures that balances sharpness against bokeh. And quite simply they need a large physical aperture, to achieve both that background blur and proper selective focus; 60mm F2.8 macro lenses don’t quite cut it I'm afraid.

Now the longer end of the portrait range can still be served quite reasonably on APS-C by 85mm lenses designed for full-frame, and at the lower end the Pentax DA* 55mm F1.4 looks like a step in the right direction. The problem is though that there's nothing much in the middle, in that 'just right' 65-70mm region. The only really convincing offering comes from Pentax (the one company, I think, that really understands primes right now, as evidenced by the new 15mm F4 ED AL), in the shape of the 70mm F2.4 AL Limited, although its compact design results in arguably too much compromise over speed. 

So what I'd like to see is the other manufacturers join in on the act, and produce a new breed of 70mm lenses optimized for shooting portraits on APS-C. These could perhaps come in two flavors, a ‘cheap and cheerful’ F2 and higher-end F1.4. No need for the manufacturers to obsess over pure sharpness or minimal vignetting wide open; let's just have something small, light and discreet to work in harmony with the new capabilities of our DSLRs.

So Sony, if you're listening; scrap that 50mm F1.8 DT, it's just 'old think' and a hangover from the days of the film. If you're really serious about making an APS-C prime, start afresh and offer something more suited to the digital era. The world will be a better place.