dimanche 27 février 2011

Nikon primes : 35mm f/2

35mm may be the most interesting focal length for a prime. On an APS-C sensor, they are equivalent to a "normal" 50mm. On a full frame 24×36, they give a really nice perspective from within the action, near the people and a view of the larger scene at the same time. My preferred. At the moment, Nikon makes a 35mm 1.8 but it doesn't cover a full-frame or a film. The 35 mm 1.4G does, but this pretty large ultra-fast lens punches a 2000 € hole in your bank account. And both are G's devoid of an aperture ring. No way here to set the aperture on a F4 or FM/2/3a camera.
Why not a wide-angle zoom, then ? There are for sure plenty of them giving outstanding image quality. Yeah, but the affordable ones have too small an aperture. And the faster ones would cost me an arm.
So, i already know that i'll keep my 35mm F/2, because i need it work on a D3100 and a FM2. But how does it behave ?

The lens reviewed here is an AIS, manual focus. It's said to be optically inferior to the 35mm f/2 AF-D (see review on www.kenrockwell.com/nikon/35af.htm). Let's see by ourselves (clic on the pictures to enlarge).

At f/2, the definition is already good in a large centered spot. If small details are clearly defined, the general contrast is spoiled by a slight haze. On the edges, at 100% crop, the image is seriously blurred, and the small letters are unreadable.

At night, very bright spots display a halo, the coma flare. I was able to shoot some very nice images with this 35mm in such low light conditions, although they may not have been technically correct.

At f2.8, central definition and general contrast are getting better. On the edges, the trend is the same, but it doesn't catch up. The quality is still poor.

At f/4, the centre image is precise and neat and the edges still lagging far behind.

At 5.6, is this is it? Not quite. The image is chiseled in its centre, the edges are not too bad, but not really good either.

At f/8, at last, the image is evenly sharp.

I bought this lens used twenty years ago and still use it today. It is beautifully crafted, all glass and metal, etc. I knew there was softness with the diaphragm wide open. It was obvious on prints 30×20 cm or larger. The pixel gazing trial shows the extent of the issue and is rather disheartening. The yield of this 35mm is not accurate in the whole field unless you stop down the aperture to f/8. Nevertheless, i have to say, i shot my best pictures with this very lens. With an average resolution film, like the HP5 Plus, it was just fine. But with a modern digital sensor, like this 14 million pixels in a 24×16 pack, the old design shows all its cracks. Not that all old lenses are passe. My 50mm 1.8 AIS passed the same test with flying colors.

If you plan to buy a 35mm, well that's a brilliant idea! But you should avoid this one. The AF-S 1.8G seems a good choice, but limited to DX cameras. The extra stop of the 1.4G should appeal to pros only, whose needs are worth fifteen times the price of the 1.8. Personally, I'd go for the lightweight and inexpensive Nikkor AF-D at f/2. But for now, I keep my old "special effects" lens and try to do with its shortcomings.

vendredi 25 février 2011

Nikon primes : the 50mm 1.8 AIS

This lens has long been the standard fitted on every moderately priced SLR until the cheap zooms came in the picture in the 1980's. I'm not sure this is a boon for the amateur who doesn't need many focal length in a very compact set. While zooms are the best answer to this specific need, they loose in many other respects. The design of a 10x zoom weighting around 400 grams and cheaper than 300 € assume some compromise. Modern ones with aspheric and high refraction index lenses can display very good sharpness throughout the focal range but they are usually not as good on distortion and vignetting. More troublesome is their aperture restrained at about 5.6 at 50 mm. This makes them a second choice for portrait, where shallow depth of field is necessary to isolate a subject from a distracting or ugly background or make nice bokeh (out of focus blur) effects. And at f/5.6, the lens let pass through less than a tenth the light it would do at f/1.8. A brute fact that shouldn't be overlooked. Where 640 ISO would do with a normal prime, you'd need to pump the settings up to 6400 with the zoom, a level where many sensors show a huge amount of digital noise.
The 50mm AIS is essentially the same as the current AF model. Same internal layout, weight about 130 grams and around 100 € used. As a AIS, it commands a manual focus. On an entry-level DSLR like the 3100, it hasn't got the light metering. Some tries and errors or an independant light meter may be useful. The AF model hasn't this limitation.

Now, has it got defects that spoil these nice features ?

At 1.8, at the center, the smallest details are still distinct but not well detached from the background, due to the lack of general contrast. On the edge, the names printed on the map, while readable, seem to be covered by a slight haze. The ability to render small details is here, but subtle shades may be lost, especially in the edges or corners. At 2.8, contrast makes a leap ahead, with a better rendering of shades, and the yellowish haze has vanished. One stop further makes it better and at 5.6 the lens is at its peak, showing sharpness and contrast across the frame. The colored artifacts in the small letters make me think it exceeds the resolution of the sensor.

How to use it at best ?

Obviously, the 50 1.8 is not a its best at full aperture or f/2 where it displays a pretty low contrast. It's not the ideal setting to shoot at maps or other very detailed flat objects. In such case, and if light is low, try to stick to f/2.8 and slow down the shutter or add some ISO. Full aperture won't be much of a concern though for portraits where most of the frame is out of focus and the main subject near the center. In this case, where shallow depth of field is key, you can try first to get near the subject, at f/2.8. A shorter distance will be more effective to reduce depth of field than a larger aperture. Scenes in low light conditions require extra caution with f/1.8. Bright lights against a dark background show a visible halo pointing outwards the image, the infamous coma. This optical defect, common to most fast lenses devoid of aspheric element, is strongly reduced from 2.8 and onwards. If you can't avoid f/1.8, well that's life. Don't expect to make huge prints out of it. I guess an A5 would be fair enough though.

I hope this test will help people use this lens at its best and avoid the rare pitfalls. Every lens has its own lot of defects and those without optical ones are usually heavy and horribly expensive. This one is lightweight, very cheap and gives from 2.8 outstanding images, on par with the micro-Nikkor 60 2.8 AF.

jeudi 24 février 2011

Nikon primes match: the 60mm 2.8 AF

Whether it's the need to be reassured on one's own choice or the mere curiosity about what makes a good lens, reviews about SLR lenses flourish on the web and seem to draw much interest and endless debates. The problem with these reviews is the lack of a universal yardstick that could be understood by anybody without a degree in applied physics. I'm not really fond of these links to a 15 pages PDF file where MTF, lpmm, LW/PH are supposed to be fully explained. In the analog era, when 24×36 TMax or TP 2415 were the standard for high definition films, the problem was somewhat simpler. But with the advent of digital sensors it's an other story. How can you compare a lens tested on a D3 with an other mounted on a D3x with 12 M more pixels ? Or a DX (APS-C) versus a FX (nikon parlance for Full-Frame format) ? Well you may, if you read that bloody PDF.
An other option is to sieve bits of truth in the fierce debates between those who advocate the Nikkor 50mm 1.2 AIS as a stunningly sharp lens and others who say they prefer use a blur filter than shelling out 300 € for a used special effect lens.
My choice is the third option : taking pictures in a very similar situation and look at minute details. Some call it the pixel peeper way. In French, it's fly fucking.
Anyway, my attempt here will appeal to your subjectivity, and to your good sense. The match is between Nikon primes, amongst which the 24mm f2.8 AF, 35mm f2 AIS, 50mm f1.8 AIS or micro-Nikkor 60 f2.8 AF. With each of them I shot a still life with fine details and the same (or about) magnification. This means that a given subject should have the same size on the sensor whatever the focal length is. The result is judged with the naked eye, on 100% crops. The objects are put in line on a shelf, their respective distance to the camera may vary by a centimeter and light may change. The camera, a Nikon D3100 shooting RAW at 100 ISO, didn't.
Despite my lack of scientific ambition, I have to admit the setup of such a confrontation is quite tricky. Even with a heavy tripod the perfect focus was hard to achieve. Many attempts were needed, it took days, hence the slight changes of subject and light.

I started with the 60 mm. At full aperture, the tiny characters on the center (DX sensor) are quite readable. On the edge (not the corners), the small letters are not so. At 5.6, the center is even better and the edge just slightly behind.

The film pack is situated somewhere in between the very center and the edge. Click to enlarge and see by yourself.

This kind of lens designed for close-up photos or reproduction of flat subjects are supposed to render details and contrasts with high fidelity. Well it seems to be true on the edges provided you stop down the aperture to 5.6 and below.

Now let's compare with an other lens, the 50 mm 1.8 AIS.
(to be continued)

lundi 14 février 2011

Matin sur Gassendi (La Lune, vue au télé)

Vue de Paris, la Lune. Prise avec un 300mm doublé (équivalent à 900mm en 24×36) au 1/80 sec, 100 ISO. C'est pas très net mais on voit bien les grands cratères (Tycho, Copernic) et, située entre les deux à l'ouest, la Mer des Humeurs où c'est le matin. C'est un cratère d'impact, bordé au nord par le cratère Gassendi, dont on perçoit le pic central.

dimanche 13 février 2011

Nouvel an chinois

Dans le 13 ème parisien, après les pluie, les lampions, les vitrines et les néons criards diffusent d'étranges couleurs.

Pour rendre les beaux dégradés sur l'asphalte, le capteur est limité à 400 ISO. À main levée, le temps de pose est de 1/25 ème de seconde environ, avec un 35 mm ouvert à f/2. À quoi servent les focales fixes ? À ça.

La mesure de lumière du Nikon 3100 ne fonctionne pas avec les objectifs AIS, elle est estimée à l'oeil. Le choix était de saturer les tons moyens, quitte à laisser une grande part de l'image dans la pénombre. Le capteur a une belle latitude d'exposition. Dans le fichier RAW, on trouvait assez d'information pour rendre une ambiance à la Nighthawks.