Tuesday, February 10, 2009

A Prime Lens

That, is Nikons new "fast 50" for DX sensor cameras. Technically speaking it's the "AF-S DX Nikkor 35MM f/1.8G", so it's not really a 50mm lens at all, but in effect it is.

The fast 50 prime lens is the most basic SLR lens for all photography. It's the default general photography lens, and has been since the invention of the SLR.

... and up 'til now, Nikon DSLRs have been missing one.

Oh yeah, there's been a 50mm available, in fact a really quite decent 50mm kit lens that came with the D40 kit, and an 18-55 kit zoom that was decent (though it's an f/3.5); but on a less than full frame digital camera (any camera less than 2 grand) a 50mm isn't actually a 50mm.

With any digital camera where the sensor is smaller than a full 35mm film frame, lenses have what's called a cropping factor, or a multiplier. The DX is 66.6% full frame area, therefore it has a cropping factor of 1.5 (Canons sensor is 62.5% for a 1.6x cropping factor). so a 50mm lens used on a DX will have the same effective focal length of a 75mm lens used on a full frame camera. In order to get an effective 50mm focal length on a DX, you need to use a 34mm lens.

That's a pretty significant difference; and it's why it's such a pain to get good fast primes (a prime is a non-zoom lens), and especially a pain to get good wide angle lenses.

For example, the 24mm wide angle is the standard for full frame photographers. In order to get the same effect on a DX body, you need to go down to 15mm; which is INSANELY wide, and thus insanely expensive, especially for a fast lens (a lens with a large usable aperture).

Zeiss makes some really spectacular fast 50mm lenses, with f/1.4 models runing around $500 and even an f/.7 at around $4k; but nothing wider than 50mm in those speeds, leaving DX users out in the cold.

Generally speaking, the shorter the focal length below 50mm, or longer above 135mm, the more expensive glass is. Also generally speaking the faster the lens, the more expensive it is.

Ok so why is this important?

Wide lenses, let you capture more of a scene, with greater sharpness, and less distortion. Fast lenses gather more light, which also means more sharpness and less distortion; and they let you shorten your depth of field more (keep the subject in sharp focus while blurring the background). Prime lenses gather more light, and distort less than zoom lenses; and they're also smaller, lighter, and more reliable (and generally cheaper than a zoom of equivalent speed and quality, but not always).

What exactly do I mean by "fast"?

Basically the smaller a minimum f stop number a lens has, the larger its aperture can open. The larger the aperture opens, the more light it lets in; therefore the faster it will expose film. Lenses with f/2.8 or below are generally considered "fast", and anything below f/2 is very fast.

F stops are based on powers of the square root of 2 (1, 1.414, 1.999, 2.827 etc...) rounded, and each progressive full stop allows twice as much light to come in; thus doubling the speed of the lens. So 1, is twice as fast as 1.4, which is twice as fast as 2, which is twice as fast as 2.8, which is twice as fast as 4 and so on.

A 1.4 is the fastest common lens of any focal length, and lets in 4 times as much light as a 2.8; which is 2 stops (or 6/3 stops, as most lenses are calibrated in 1/3 stops) slower. A 1.8 is 4/3 faster than 2.8 so it lets in about 3 times the light.

So a very fast, relatively wide, prime, at a low cost (about $200), is a big deal. This new lens, at 35mm and f/1.8, will give DX shooters the fastest first party 50mm class lens (effectively 52.5mm) available for less than $1500.

I say "first party", because Sigma does have a 30mm 1.4, which is both wider and faster; but it's a $500 lens, and its optical quality and build quality aren't spectacular (significant aspheric and chromatic abberation, and edge distortion); and it's lens coatings are inferior to Nikon, for signifcantly worse light transmission at a given aperture.

I don't mean to say the Sigma is a bad lens; any lens that wide with that large an aperture is going to have some distortion; it's just that it's not up to the same quality as Nikon.

Now admittedly, I'd love it if it were a 30mm f/1.4; but Nikon doesn't make anything wider than a 50mm any faster than 1.8 (at least not anymore. They used to, but it was $2,500)... Oh, except for a 35mm 1.4 manual focus; but it's designed for stationary studio photography.

Up 'til now, I thought in order to get into the 50 range with acceptable speed, I'd need to buy one of the $1,000 plus high dollar f/2.8 ultrawide zooms; and they have unacceptable edge distortion and chromatic abberation (an inherent issue with ultrawide zooms). Now for $200 I can get three times the light gathering capacity (or be considerably sharper at the same aperture), without the distortion.

I'll be picking one up as soon as the pipeline allows it. Knowing Nikon they won't make enough to cover demand for the first year.

Time for some twilight shots; and some nice fixed interiors under natural lighting.