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.