Thursday, 22 March 2012

Prime lens or zoom?

Inspired by having had a go with the Nikkor 45mm f2.8 pancake lens kindly lent to me by Tomek, I decided that a fixed focal-length (or prime) lens is worth getting, though one with autofocus. So last week I bought a Nikkor 35mm f1.8 lens.

Given the DX sensor on both my Nikons meant that whatever lens I was going to buy would have an effective focal length one-and-half times longer than its nominal length, I decided to go for the 35mm rather than 50mm prime lens. This would give me the equivalent of 52.5mm focal length on a full-frame camera, just a tad longer than a so-called 'standard' lens.

If you need shallow depth of field, a wide-aperture ('fast') lens of f1.8 or f1.4 is useful – for portraits. No good for street photography, where the you want to get a much into focus as possible. Being able to gather enough light at a fast shutter speed in dark conditions can be obtained by opening up the aperture (and losing depth of field) or by increasing the ISO (sensor sensitivity) at the expense of grain.

Digital camera are getting smarter. My 2006 vintage Nikon D40 produces decent images with a 6 megapixel sensor. Bear in mind that the new Nokia 808 PureView mobile phone has a built-in camera with a 41Mpx camera. Or, more conventionally (thanks Bob!) the Sigma SD1 Merrill DSLR with 46Mpx. Why so many megapixels , when a mere five or six gives you a decent image?

The answer is what you can do with such a huge number. Blow up the picture until it's big enough to cover your living-room wall from ceiling to floor. Now, assuming you don't want images quit that big, it means that you can zoom digitally rather than optically and still get outstandingly sharp images.

ISO ratings are rising; my D40 offers me 200, 400, 800, 1600 and 3200 ISO. The latter, incidentally, is this highest ISO rated film you could commercially buy in the old days. Current digital cameras offer 6400, 12,800, 25,600, 51,200 and now (with boost) even 204,800 ISO on the new Nikon D4. That's nine stops more than the 400 film ISO that I used to use for street photography on my Leica M3. That grain on 204,800 ISO film would make it unusable. But image noise reduction technology makes digital high-ISO images visually acceptable. And nine stops... there are only seven between f2 and f32.

And vibration reduction (Nikon's VR) or image stabilisation (Canon's IS) allows you to hand-hold at three to four stops slower shutter speeds without signs of camera shake. VR/IS might reduce the effect of motion in the camera, but not of the subject. Here, you will always get motion blur if you shoot a moving subject, no matter how good the VR or IS technology gets.

These factors taken together (more pixels, higher ISO and VR/IS) means that the quest for ever-faster lenses has ceased to be mainstream. Unless you are hunting for a specific quality of image ('bokeh' – the quality of light points that are out-of-focus), fast lenses are no longer necessary for good low-light photography. In other words, I can no longer see the need (as I once did when shooting film) for an f1.4, f1.8 or f2 lens. That extra stop or two of exposure can be bought by using higher ISO or slower shutter speed. And

So what does the 35mm f1.8 do that either my 18-55mm or 18-200mm lenses don't do? Low light? Let's take a real-life situation... Below: photo taken on the corner of ul.Krucza and Al. Jerozolimskie. Nikon D80, 35mm lens (52mm equivalent) at f2.0; 1/15th sec; 640 ISO. Now, the shutter speed here is dangerously slow; it should not be shorter than 1/52nd sec for this lens (without VR, the reciprocal of the equivalent full-frame focal length is the longest shutter speed you can hand-hold without risk of camera shake). Depth of field with the lens focused out to around 15m is shallow - a mere 30m, so the detail in the distance is out of focus.

Below: The unmodernised underground passage at the western end of Warszawa Centralna station. Nikon D80; 18mm (27mm equiv. full-frame) at f3.5; 1/20th sec; 320 ISO. The VR allows steady hand-holding with this lens at 1/7th sec, so it's within the comfort zone. Depth of field is from 4m all the way out to 72m. Nothing's out of focus.

So - having bought the 35mm f1.8 lens, I shall have to find uses for it outside of night-time photography - despite it being a whole two stops faster than my zoom, it loses out on lack of VR. Please let me know if you have any practical photographic applications for a fast lens!

I would say that for 90% of applications, an 18-200mm VR lens on a DX-format DSLR is the one lens worth buying. Something at the wider end, though not going fisheye, zooming out to 16mm or even 14mm is useful for big architecture or cramped interiors; a lens zooming to 400mm useful for wildlife, aviation or sports. Nikon will soon be bringing out a 18-300mm lens - this shall be very interesting, although I think the company should have stretched the zoom at the wide end (a 16-200mm would have been even more useful).

This time last year:
Warsaw's failed bid as City of Culture, 2016

This time two years ago:
Stalinist downtown at dusk

This time three years ago:
The End of an Age of Excess?

This time four years ago:
Snowy Easter in England


Unknown said...

You miss the point a bit - you buy primes to:
- have a shallow depth of field (for portraits or creative effecs)
- primes are far sharper when stopped down to the same aperture as an equivalent zoom
- primes are for a disciplined photographer, zooming with your legs (which isn't actual zooming, but learning composition).

And zooms for everything else ;-)

Mariusz eS. said...

Nikkor 35mm f1.8 is great lens, I used to have one. But I switched it to Sigma 30mm f1.4 which is also DX lens, but much better than Nikkor. Even wide open!