Richard Kilpatrick tested the Sigma 50mm f/1.4 on both the cameras it could almost have been built for, the Nikon D3X and the Sony Alpha 900. To the Sony, it added – for the first time in his hands – HSM to an Alpha mount body. Here’s what he found out.


Here, I will be linking at the end of this article to an entire series of studio tests at full 24 megapixel size hosted on my own pBase space, which first allow comparison of the Nikon D3X and the Alpha 900 for this type of ISO 100 work, and secondly compare results at full aperture and f/8 from three very different lenses: the 1985 designed classic Minolta 50mm f/1.4 AF, the 2008 designed Nikkor AF-S 50mm f/1.4 G, and the 2008 radical rethink of what a 50mm f/1.4 should be, Sigma’s flasgship prime.
But first, a review of the Sigma – a lens intended to be a true ‘prime’, a first among equals. It’s certainly a first. No maker has ever built a 50mm standard with a 65mm diameter front element and a 77mm filter thread, weighing over 500g and costing almost £500.
The Sigma concept
Here is that 1985-ish Minolta next to the new Sigma. Despite appearances, they were indeed separated at birth. Both are basically the same type of Gauss design, the Sigma using just 8 elements in 6 groups. The difference is that in front of the iris, the design is progressively scaled up to improve illumination at the edges of the field, a critical issue with digital sensors.

This is what a light-table view of the old Minolta design looks like viewed at an angle equal to the corner of the frame, with the aperture held wide open.

This is what a similar view through the Sigma looks like. You can realise why from this outfield angle the Sigma is putting far more light on to the silicon, and indeed, when comparison photos are taken of a plain sky using both lenses wide open, the difference in field illumination is clear:


The Sigma is top, the Minolta bottom. During the period of testing this kit, I also had the Nikkor AF-S G, and it’s very similar to the Minolta, not the Sigma, in size and in full aperture vignetting.
This is not the end of the story. The Sigma is also made (like the Nikon) to have almost zero aperture related focus shift. This is a complex issue, which affects AF and manual focusing accuracy. Most AF sensor arrays will ‘see’ the lens at between f/2.8 (centre sensors on Alpha 900, for example) and f/7.1 (outer sensors on the 900) with f/5.6 being very typical (Nikon MultiCAM 3500, most Canon sensors except for central). Most focusing screens ‘see’ the lens around f/4.5, as do metering systems, which is why the vignetting issues don’t affect metering (the field is evenly illuminated by the time you stop down to f/4.5 with all such lenses).
With most lenses, including the old Minolta, the plane of exact focus shifts between f/1.4 and f/8. Cameras equipped with f/2.8 AF sensors will provide a better compromise, but full aperture may not be focused precisely as intended (stopping down masks the effect of shift, even when it happens). With the new lenses like the Sigma and Nikon, there is hardly any shift so they focus very accurately for shots all all apertures, both via AF and manually on-screen.

Here’s a shot at f/2 on the Sigma, 1/30th hand held at ISO 400 on the Alpha 900. One stop down is always a good idea, even with the excellent performance of this lens. The pub sign shows how well it focuses and how sharp it is:

Testing the Sigma at full aperture on a distant subject with the Alpha 900, here are some results:

This is the centre of the frame – trust me, at 24 megapixels 100% and without any sharpening, this is state of the art for a 50mm f/1.4 used wide open. The corners of the frame, all photographed in turn right into the extreme corner as the 100% finder of the Alpha 900 allows this, show both the slight darkening and the slight softening present:

Click the image for a 100% view of this composite.
The Sigma 50mm f/1.4 is a great lens to use for its bokeh – which is best between f/1.4 and f/2.8, here shown at f/2.2:

The iris is nine-blade, which keeps things pretty tidy even when stopped down, but the design itself does produce an amount of colour bokeh and can produce some colour fringes, so processing carefully from raw files is the best way to handle its output.

Finally, this expensive lens comes with a good petal hood and a case. I will not show examples here, but be warned of one phenomenon – the view through the lens at f/1.4 when the sun is just out of the frame can be perfect, no flare. Then your final picture at f/8 is seriously affected by flare! You must check each shot in these conditions, as the stopped-down aperture blades create the flare which is absent in a wide open lens for viewing.

If I wanted to use f/1.4 for a high proportion of shots, I would get the Sigma in place of my old faithful Minolta. It has HSM focusing and it’s every bit as good as Sony SSM (not available on the new version of the Minolta, Sony branded), working perfectly with smooth manual focus over-ride and very fine adjustments in virtual silence. It is also built like a tank and the two (new) samples I tested on Nikon and Sony were equally good.
The studio tests
On to the final part, the studio tests of the various 50mm lenses and of the two 24 megapixel bodies. What I show here is a fraction of the many tests done, and raw processing exports produced using a whole range of raw converters and different parameters. The ‘native’ results of the Nikon and Sony look so different you would believe they were different sensors, but by careful alignment of the default settings in Adobe Camera Raw 5.3 it was possible to see they are clearly identical or very similar. Nikon treats exposure calibration differently, and I believe they pay Sony for the best test-selected sensors from the production. I don’t think they add a ‘magic pizza topping’ as some have suggested (if anything, they reduce the flavour – the density of the RGB filtration). Everything in the difference between the A900 and D3X, including the price, points to selection of the absolute pick of the sensor crop after fabrication.
That is perfectly possible, it happens already in most fields of similar production, every sensor has to be tested anyway (each one has a different calibration table) and with the D3X, you get a premium quality imager for £6,000 where Sony give you a standard quality for £2,000.
Here is the basic set-up used for the test shots, and the first shot, on the Sigma at f/8 using the Sony:

To view the full size image (’original’ size) go to:
http://www.pbase.com/davidkilpatrick/image/109053091
It is very large – over 15MB of Level 12 JPEG data – so be prepared for a long wait unless you have fast broadband. The image can be viewed or downloaded. Full caption details are provided on pBase.
This is the first image in a series of ten. They include two examples where the Sony and Nikon are set to totally flat, linear output to show with accuracy the dynamic range (these will look dull and desaturated to the eye, but are a bit closer to the Bayer value contrasts than a normal raw conversion – sure, nothing like as dark and dull as a literal Bayer readout looks!).
They also include full aperture results from all four lenses tested (two Sigma 50s, Minolta and Nikkor), and f/8 results as above. The small differences in framing you will see are partly due to the true focal length of the lenses varying – the Sigma is around 48.5mm, the Nikon is around 52mm – and their different methods of focusing, internal or whole lens. Also, the D3X is a different shape of body and the tripod column had to be raised to place the A900 in the same position. Differences in focus are down to the way the AF worked and how the locked manual focus held. In all cases the intended focus point was the label/wool right hand edge of the pastel multicoloured ball of wool.
To view the sequence, just click NEXT after each one. Tip: to compare images, after they have been cached/loaded, you can click PREVIOUS/NEXT and your browser will flip between them rapidly.
The Sigma lens, though it focuses down to 45cm, is not at its best with close-up subjects and the other two conventional 50mms render fine detail rather better across the frame for this setup. Also, when focused close enough for test charts, the Sigma distortion appears to be around -2% but when used at infinity it’s very low – more like -1%, not worth bothering with correction. Here is an uncorrected typical medium close shot:

As someone who shoots 90% of everything at ISO 100-200 (200 is a bad light setting, 400 is dusk!) I’m in no way unhappy with the Alpha 900 despite relatively high noise levels. It has a sweet spot at ISO 320, which is noise-free in good light and almost looks like ISO 100. Adding a very high quality 50mm f/1.4 to such a camera seems the best way to squeeze the most from 24 megapixels and full frame.
Since writing most of this report, I have started using video-capable DSLRs and invested in a Nikon D5000 to learn more about V-DSLR technique. The Sigma 50mm f/1.34 is firmly back on the shopping list as this must be the ultimate fast lens for video capture with powerful differential focus effects. Also, I’ve invested in a 70-200mm HSM f/2.8 for my Sony A900 – the HSM focus on the 50mm won me over, I already knew the quality of the 70-200mm in previous incarnations. I’m not disappointed (I ordered the 2X HSM compatible converter within 24 hours of testing the 70-200mm) and I will report on the 70-200mm in a further article.
– David Kilpatrick FBIPP Hon FMPA