ZOOM OR PRIME - is there really any difference? And is it worth bothering with the 'kit lens?' Acutance.

NIKON Df SIGMA 50mm f/1.4 ART LENS 24-85mm ZOOM LENS OLYMPUS OM-D E-M5 14-42mm ZOOM LENS 25mm f/1.8

NIKON Df SIGMA 50mm f/1.4 ART LENS 24-85mm ZOOM LENS OLYMPUS OM-D E-M5 14-42mm ZOOM LENS 25mm f/1.8

It's the old question. is a prime lens better than a zoom at the same focal length and aperture? The accepted opinion is that yes it probably is. But haven't zooms got better these days? And since they are more popular, does the oft repeated doctrine that primes are better still hold true?

I decided to have a look with my Nikon Df DSLR and Olympus OM-D E-M10 m4/3 camera.

 OLYMPUS DIGITAL CAMERA

Olympus 14-42mm zoom at 25mm compared to 25mm prime. f/5.6 ISO 200.

So a centre and edge comparison. And yes the prime lens is sharper than the zoom and it's also quite noticeably sharper. So an advantage here. So what about the Nikon?

Again the prime is sharper than the zoom, but there is much less difference than with the Olympus. In fact it's quite marginal.

Now this is far from an exhaustive test, but I did it to illustrate something that I've noticed over the years. I also suspect that others have noticed it too. In the days when 35mm film cameras used to come as a kit with a 50mm f/1.7 or f/1.8, you knew that you were getting a good lens. Indeed some of those 50mm 'standard' primes were some of the best lenses made by the different manufacturers. But then gradually zooms started to take over and we eventually got the 'kit lens'. Usually made specifically for selling as a kit, these have often lacked the build quality of more specialist zoom lenses. Not surprising when you work out how much they cost when you subtract the cost of the camera from the kit.

And the difference above is an example of this. The Olympus is a 'kit lens' the Nikon is not. It's not particularly the difference between DSLR and m4/3 lenses but the nature of the lens itself. The Nikon 18-55mm DX kit lens and the terrible Canon 18-55mm EF-S would also compare badly with decent primes. Though having owned several copies of both I have to say the Nikon is some way better than the Canon, which is actually nothing short of a disgrace. The softest lens I've EVER used. 

So is it worth bothering with the kit lens? Have a look at the sample below.

With only a small amount of selective sharpening applied the kit lens suddenly looks a lot sharper. Now I didn't sharpen the whole picture, just the front of the golf ball. Since the background from the zoom shot is slightly softer than the prime, this has had the effect of making the name on the ball stand out more and the zoom shot now gives the impression that it's a sharper rendition when in fact it's not. 

So sharpness is to a large extent perception. And there is a good word that sums this up. Acutance.

'In photography, the term "acutance" describes a subjective perception of sharpness that is related to the edge contrast of an image. Acutance is related to the amplitude of the derivative of brightness with respect to space. Due to the nature of the human visual system, an image with higher acutance appears sharper even though an increase in acutance does not increase real resolution.' From Wikipedia.

So when I write about sharpness, I'm actually writing about acutance. A perceived sharpness rather than resolution. 

It's why when I get a new lens I'm always interested to see how that responds to sharpening. I've had lenses that initially seem slightly soft, but they sharpen up very well. 

Lets go back to the Nikon

When I add the same amount of sharpening to both images the 50mm prime 'responds' better and looks sharper. As it should since it is supposed a very good lens. 

The point of all this? Well it's that offhand judgements and endlessly repeated cliches about lenses is often complete BS. And rather than dismiss some lenses out of hand in Ken Rockwell fashion, with a little investigation and experimentation, lenses can seem to perform way above what would be expected of them. 

The application of sharpening is complicated and difficult to get right. I've had years of practice at it, including a time when I ran my own picture library and was sending out files to newspapers, magazines and book publishers for printing. And I have to say that when I saw copies of the finished pages there was no way I could tell whether an image had been shot on a zoom or a prime. 

Now I'm an unrepentant pixel peeper and also someone who upsizes images all the time, so because of that I still prefer primes with high acutance levels. And of course primes usually have faster wider apertures. But I'm still happy using zooms and kit zooms at that. In fact I've always liked what I can get from Olympus kit zooms. And that 'what I can get' is important. Because to just look at comparative jpgs. and decide that one lens is inferior to another can mean that you can lose out on a piece of of gear than can actually be very useful and produce good results.

Not all kit zooms are able to have their output 'enhanced' but many can. The Nikon 24-85mm is obviously a very decent lens, but then so is the Olympus and it will still find a place in my bag.

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There is an interesting article on the BBC website HERE, which deals with many of the issues above.

Many Thanks

David Taylor-Hughes