Focus Shift with Nikon AF-S Nikkor 28mm f/1.8G

I'm currently reviewing the new Nikon AF-S Nikkor 28mm f/1.8G and one of the first tasks with any new lens is to check focus accuracy and to test for focus-shift. This particular lens and camera, a Nikon D800, showed consistent front-focus using AF and the combination also revealed focus shift. Both are problematic, though focus shift is the more difficult to deal with (requires conscious effort). To my knowledge there are no Canon or Nikon cameras that make any attempt to compensate for the effects of focus shift (unlike some MF cameras I could mention).

I've used the Michael Tapes Design Lens Align Mk II to demonstrate the focus shift - it's a first class focus calibration tool which was originally designed to highlight issues with DSLRs and AF lenses but it's perfectly suited for this particular purpose. The following images are unretouched JPEGs displayed at 1:1 in Adobe Lightroom 4.

F/1.8

This photo, taken at maximum aperture and focused manually using Live View on the targets (left) at a distance of 1m, shows the zone of focus to be over +6 to -2. This is, arguably, front-focused but it's nothing like the obvious front focus when using AF (the same targets are clearly out-of-focus).

F/2.0

There's no real difference at f/2.0, as you might expect.

F/2.8

Neither is there much difference at f/2.8 save for an increase in DOF; the zone of focus extends from +12 to -4.

F/4.0

By f/4.0 things are starting to get interesting, as there's a shift rearwards (back-focus). The zone of focus covers +14 to -11

F/5.6

It is at f/5.6 that focus shift is most noticeable, with the zone of focus covering +16 to -20. In fairness, the DOF is masking the issue.

F/8.0

At f/8.0 the effects of the rearward focus shift are quite apparent, with the focus zone covering +24 to -36

F/11.0

At f/11 the depth of field masks the focus shift on this scale, although the Lens Align moniker on the scale is clearly out.

F/16

At the minimum the DOF extends rearwards so that the chart behind is practically in focus while the front of the ruler/scale is barely in.

 

Summary

These results aren't quite as severe as the focus shift on the Voigtlander 35mm f/1.4 CRF lens tested last month but there is clear evidence of focus shift - we can see the plane of focus move rearwards indicating back focus (under-correction of spherical aberration). This could negatively impact on sharpness with high-resolution cameras, such as the Nikon D800 but I wouldn't call it 'severe', as other internet sites have, as it's mainly masked by DOF. I suspect one of those sites were seeing the effects of focus-inaccuracy rather than focus shift.

In all other respects, the lens performs well. I'll have more to say in a rolling review.

The Nikon AF-S Nikkor 28mm f/1.8G can be bought from Amazon in exchange for $699.

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Focus Shift with Voigtlander 35mm f/1.4 Classic MC

Focus Shift with Voigtlander 35mm f/1.4 Nokton Classic

I am currently reviewing the Ricoh GXR A12 M-mount body and have the super-compact not to mention super fast Voigtlander 35mm f/1.4 Nokton Classic MC rangefinder lens for review also. Before the reviews are published I thought I would show the focus shift that this particular lens exhibits. On a camera like the Ricoh GXR M-mount (or, for that matter, any mirrorless camera using an adaptor) this particular characteristic isn't really an issue, providing you focus at the taking aperture. It is, however, a distinct issue if you focus at the maximum aperture and then stop down to the taking aperture or if you're using a coupled rangefinder such as the maker's own Bessa models or a Leica M series, which do not compensate for focus shift.

I've used the Michael Tapes Design Lens Align Mk II to demonstrate the focus shift - it's a first class focus calibration tool which was originally designed to highlight issues with DSLRs and AF lenses but it's perfectly suited for this particular purpose. The following images are unretouched JPEGs displayed at 1:1 in Adobe Lightroom 4.

F/1.4

At maximum aperture, this is about as accurate as we can get (at a distance of around 3 1/2 feet), using focus peaking and the somewhat poor resolution of the Ricoh's VF-2 viewfinder in magnified mode. We've focused on the target to the left. If you look at the ruler to the right, focus is centered evenly on the +4 0 -4 digits.

If you're concerned by the amount of LoCA I can say that photographing targets with high contrast edges like this at close distances easily reveals this trait and others (such as SA) but real world images aren't likely to look quite so aberrated.

Besides, as you can see from the photo above, I can remove nearly all traces of LoCA in the latest version of Lightroom 4 - it's one of the most powerful features of the recent upgrade and one of the reasons why I recommend the utility for professional users.

F/2.0

Focus shift is immediately apparent at f/2.0, as the focus plane has shifted rearwards (exhibiting back-focus) with the depth of field over +12, 8, 4, 0 -4 (the +12 and -4 being equally unsharp)

F/2.8

By f/2.8 the depth of field continues to expand but the red ±0 is now out of focus, with the focus plane being over +24 to +4 range and the sharpest point of focus being around +20 to +16

F/4

At f/4 the focus shift continues rearward with the sharpest focus point being around +26 to +20, note the red colored rear target intersecting the large 4 is clearly in focus.

F/5.6

At f/5.6 the depth of field is beginning to encompass the original focus target (it's still a bit soft however) but the point of sharpest focus remains to the rear at around +36 to +20.

F/8

Much the same can be said at f/8, only now the depth of field now encompasses the original focus target (though the sharpest point of focus remains rearward at +36). And things don't change at f/11 or f/16 (there's little point showing those). Apart from the focus shift there's a lot to like about the Nokton, which we'll reveal in our forthcoming review so please check back for that.

The Voigtlander 35mm f/1.4 Nokton Classic MC can be purchased online in the UK at pro-dealer Robert White.

Nikon AF-S 17-35mm f/2.8 best wide-angle zoom for Nikon D800?

Introduced at the same time as Nikon's first digital SLR the APS-C format D1, the Nikkor 17-35mm f/2.8 was made not only to replace the earlier AF 20-35mm f/2.8D IF but to compete with and out-perform the rivals. Nikon may be slow in producing top-quality lenses, at times, but they look closely at competitive offerings before announcing new models that can out-do their rivals optically and mechanically.

Such is the case with this particular optic, and is still available today in spite of the more recent and impressive AF-S 14-24mm f/2.8G and 16-35mm f/4G VR models. Both are said to be mighty fine performers (I have no personal experience of the 16-35mm f/4 VR) but I've not been inclined to replace the 17-35mm f/2.8. First, the AF-S 14-24mm f/2.8 is large and heavy, especially on a pro-body but it''s still a handful on a D700/D800, while the 'compact and affordable' alternative, as Nikon claim the 16-35mm f/4 VR to be, just doesn't seem to be as compelling as the 17-35mm f/2.8. Not only can it serve as a formidable landscape lens, which the 14-24mm can not (due to the wildly convex front element and lack of a front filter thread) but because of the manual aperture ring, will also be of immense interest to those using video.I don't need to tell Canon users that Nikkor lenses can be fitted (courtesy of an adaptor), and, indeed, are favored for their manual aperture rings, along with Nikon's reputation for low chromatic aberration (especially on wide-angle primes and zooms).

Like Canon EF lenses, Nikon's newer G-series lenses (such as the 14-24mm / 2.8 and 16-35mm f/4 VR) lock out rival maker's bodies and the one or two Canon to Nikon G-series adaptors that are available commercially (we use a high quality Novoflex version) don't allow you to select a particular aperture. There is no aperture readout in the cameras' viewfinder as the lens adaptors simply operate a lever opening and closing the diaphragm (albeit steplessly, which is a plus).

Pricing

Perhaps that's the reason why the 17-35mm f/2.8 continues to be made and is available from US retailers for about $1,770 (with USA warranty) or around £200 less as an (gray) import. Part of the reason that this lens is expensive can be attributed to lower production volumes now the newer models exist but there are other reasons as well. This lens has two large precision ground aspherical elements (the front element, for instance), one composite asphere and two ED glass elements, instead of the usual one (if you're lucky). Due to thermal expansion characteristics, this lens focuses beyond the infinity mark.

Performance

Abbey Altar. St Albans. England. May 2012. Nikon D800 with 17-35mm f/2.8 @ 17mm f/4, ISO 6400

Nearly of the reviews I've read about this lens make some reference to the AF-S 17-55mm f/2.8 as result of there being only DX camera bodies available at that time but things have moved on and there is renewed interest in the 17-35mm f/2.8 with FX bodies, especially now that the D800 has been introduced. However, if you're using a DX camera (and as of now that means either the D7000 or D300s) and have no intention of buying a full-format (FX) model, then between the two you should choose the AF-S 17-55mm f/2.8. Besides the extra range and disproportionately lower price, it's close in both AF performance and optical quality, color, and sharpness. That said, I still prefer the contrast and drawing style of the 17-35mm f/2.8. Images from the AF-S DX 17-55mm f/2.8G, in my experience, look excellent on a technical level but appear somewhat sterile (and I have both).

The 17-35mm is less prone to flare, but it's not impervious to it as I was reminded only recently when a shot taken using a D800 directly facing the sun coming through a window just above the subject caused some veiling glare. Like most modern Nikkors, the 17-35mm has a nine blade diaphragm to produce attractive circular highlights and improve bokeh. Few zooms have attractive out of focus planes and wide-angles less so (in any-case, strongly out of focus backgrounds and foregrounds are far easier to achieve with longer focal lengths, such as a 50mm or short tele) but the 17-35mm f/2.8 has none of the harsh double-edged effects seen in less capable optics and is quite acceptable. Longitudinal CA (sometimes refereed to as purple fringing, most noticeable at wider apertures) is low on the Nikon D800 but there is some lateral CA, though this is easy to remove in post (using Lightroom 4) There's also some vignetting at the maximum aperture, but this too can either be removed in-camera (on JPEGs) or in software later. It's not unexpected and no worse than rival offerings. As you can see from the picture above, there's some barrel distortion but it's reportedly lower than the Nikon 16-35mm f/4 VR (I've not tested that model but I've seen sample photos of distortion).

Mechanical Quality

Ford. Redbournbury. England. May 2012. Nikon D800 with 17-35mm f/2.8 @ 17mm f/4, ISO 6400

If you're familiar with Nikon's older manual focus lenses then you'll be pleasantly surprised by the quality of the 17-35mm f/2.8. The only really noticeable difference between them is that this lens, like others of the same period, has a plastic aperture ring though it is engraved still thankfully. This is an autofocus lens, however, so there are no brass helicoids, but the focus ring is smooth and not too highly geared, allowing large but precise adjustments. Auto-focus is very fast, near-instant and, unlike others that adopt a silent wave motor (SWM), practically noise -free.

The outer barrel is made from heavy-duty aluminum alloy and has the same durable construction that can only bee seen today in Nikon's top pro-quality super-telephotos, such as the AF-S 400mm f/2.8. If there's a shortcoming it's that there's no weatherproof sealing, and no rubber seal at the lens mount. The lens comes supplied with an old-fashioned rigid lens-case, pinch front and rear caps.

While well-made, the case isn't much use, to be honest. I prefer the soft, draw-string pouches but even those can be a pain to work from, so they're best used for storage. The supplied (HB-23) hood is really the only disappointment out of the whole package, and I rarely use it. It is the same one that's supplied with the 12-24mm f/4G and 18-35mm f/3.5-4.5D lenses. Nearly all Nikon hoods appear to be made by a third-party, and can all be characterized by fitting too tightly (the exception to this is the huge hood for the AF-S 17-55mm f/2.8 (28-70mm equivalent) for DX cameras). Fortunately, the hood bayonets onto the outer barrel so it can't do any damage as it might to the extending barrels of lesser lenses. Still, it can be recommended to protect the front element, to a degree.

Shrine of St Alban. St Albans. England. June 2012. Nikon D800 with 17-35mm f/2.8 @ 17mm f/5.6, ISO 6400

Of course, now Nikon has the 36-megapixel D800 and D800E variant, interest in a suitable high-quality wide-angle zoom is paramount. For photojournalism and human interest / documentary work, the fast-focusing AF-S 17-35mm f/2.8 is the obvious choice. This lens performs remarkably well even at maximum aperture the centre has high sharpness and good contrast, meanwhile the low chromatic aberration and easily correctible barrel distortion make it equally suitable for landscapes, interiors and architectural photography alike.

Links


The Nikkor 17-35mm f/2.8D ED-IF AF-S is available from:

B&H Photo (USA warranty) $1,769.00

Adorama (USA Warranty) $1,769.00

Amazon (in-cart price) $1,719.95 (USA warranty)

Amazon UK £1,495.25

Tokina AT-X 16-28mm f/2.8 Pro FX review

Quality. Build quality of the Tokina ATX-Pro 16-28mm f/2.8 FX is good, with a mix of robust metal and plastic outer components and excellent ridged rubber grips.

You might think Nikon and Canon users have a good choice of ultra-wide zooms available to them but Tokina believes there's room for another.


Tokina has been making high-quality photographic lenses for various camera makers for over sixty years, with only the last three decades using the Tokina brand name. It's claimed the company was founded by a group of ex-Nikon engineers, and perhaps that might explain why many of their models take their design cues from Nikkor lenses to this day.

Certainly this lens, with its bulbous front element, built-in hood and trombone-shaped body could easily be mistaken for the AF-S 14-28mm f/2.8 Nikkor. Aside from the distinctive silhouette, the finish, focus grip and markings not to mention the lack of an aperture ring make it look uncannily similar up close too; only the widely ribbed zoom control covering gives it away. As a full frame model, the lens is also offered with a Canon mount, but the design looks somewhat incongruous on EOS bodies.

With a slightly longer focal length and 107-degree diagonal angle-of-view at its widest the Tokina 16-28mm f/2.8 can't match the extreme 114-degree view offered by the Nikkor but then the price isn't close either.

A quick look at the list (inclusive of VAT) shows there's quite a difference, £1500 for the former as opposed to £1075 for the latter. But, the disparity in street prices is even more favourable. The Tokina can be bought for a shade under under £665 (ex VAT), while the highly regarded Nikkor is commanding around £1100 (exc VAT). Nikon has since introduced a 'more affordable' AF-S 16-35mm f/4 but even though the VR-enabled lens is a capable performer it's aimed at different demographic.

Canon users face a similar dilemma. Their EF 16-35mm f/2.8L is smaller, lighter and has a wider range. But the main drawback is price. It sells for only a £100 (ex VAT) less than the big Nikkor. And while significantly cheaper than the f/2.8 model, Canon's widely respected EF 17-40mm f/4 L targets the same users as the new Nikon VR option yet costs around the same price as the Tokina.

As an ATX-Pro model, the Tokina is well built mechanically, only the built-in petal shaped hood, focus ring and mid-section are made from engineering plastic while the rest of the external components (essentially, the rear half) are metal. It's all rather impressive but a cheap plastic push-on cap is supplied, and a concession to the price no doubt. It marks the built-in hood, but at least it's not easily dislodged when placed in a bag. A clip-on cap was offered as a replacement for Japan market and, hopefully, that might now be made available for the UK.

To maintain the image quality at the wider end of the range the focal length at the longer end is reduced from 35mm to 28mm. The coverage is still useful but Tokina argue this lens is likely to be supplemented by a 24-70mm f/2.8. Even so anyone expecting the 16-28mm to be small and lightweight will be disappointed. With a length of 133mm and weighing 950g, this is a large and heavy lens.


The long body allows plenty of room for well-spaced and sizeable zoom and focus controls. Despite the otherwise excellent ergonomics though the action of the zoom ring was heavy compared to rivals. No claims are made of its resistance to the environment in any UK PR material but with translation the maker's Japan site makes reference to 'dustproof and waterproof rubber sealing the outer periphery'. It has a seal around the mount, but it's discrete enough to overlook without closer inspection.

Viewed against today's electronic real-time manual override options of rivals Tokina's One Touch Focus Clutch push-pull design of the focus-ring appears somewhat outdated. However, it's ergonomically superior to the switches of rivals and manual focus is well-weighted allowing ultra-fine adjustment.

Tokina says it has a newly developed silent DC motor, but that's a stretch as there was a noticeable rasping-sound in use from the sample tested. The lens is also the first model to feature a new GMR magnetic sensor in place of the more usual rotational type. These sensors relay focus movement of the lens to the body, but the accuracy of rotational sensors is subject to mechanical wear, typically, backlash from the reduction of gears.

This new design should help maintain accuracy with use, while a contact for the company told me it's more precise any way, and faster in operation due to the GMR sensor being located immediately above the focusing cam. Real-world use showed the Tokina to be in the same class in accuracy and AF speed to my AF-S Nikkor 17-35mm f/2.8, a lens still in production but one that bizarrely sells with a 25-30-percent premium over the 14-24mm f/2.8.

In terms of optical quality, the Tokina is unlikely to disappoint. As you might expect, wide-open at 16mm the edges fall behind but throughout the zoom and aperture ranges the resolution in the centre of the frame is very high.

Three SD (low dispersion) glass elements are used, with properties and performance close to fluorite say Tokina. But, this doesn't prevent chromatism completely, after all this is a complex zoom with 15-elements arranged in 13 groups. All the same, levels are low and easily correctable.

So to is light fall off wide-open at 16mm; a substantial 2.4 stops to the corners according to my testing with Imatest software, but it's not as noticeable as the figures suggest. Modern Nikon bodies remove this with out-of-camera JPEGs, though it's an important point to bear in mind when using a RAW workflow.

Care must be taken when shooting towards the sun as the large convex front is highly prone to flare. Like other modern designs the front group extends from the petal shaped hood during zooming and is more exposed at the wider-end.

With the Sun in the frame there will be the inevitable patches and even with oblique rays, you can expect dramatically lowered contrast. The propensity to flare is perhaps its weakest point as geometric distortion is low especially at mid distances and beyond. After a while it becomes obvious that this lens hasn't just been developed for a particular niche. The drawing style has an appealing aesthetic quality and out of focus elements are rendered smoothly with little of the harshness associated with zooms. Considering the broad range of potential uses the efficacy is impressive.

Lens design is all about compromise, though. The 1.75x zoom range is more modest than rivals, however, the addition of three aspherical elements; one a large (56mm) PMO and the high number of SD elements combine to make a very highly corrected lens. The large size and weight count against it to a point and some users might find it difficult to work with instinctively when switching between makes, but few can argue the Tokina 16-28mm f/2.8 isn't an attractive option.

Distortion. This indoor handheld high ISO shot won't easily convey the excellent micro-contrast this lens is capable of but it does show the excellent geometric correction as well as the propensity for flare (at the window, upper left) and ghosting (seen at the base of the column, lower right). Taken with a full-frame Canon EOS 1Ds Mk III.

Contact


More information, including a list of UK stockists, can be found at www.kenro.co.uk.

Tokina is distributed in the US by THK Photo, www.thkphoto.com.

Sales


Shop for the Tokina ATX-Pro 16-28mm f/2.8 FX at the following retailers:

Adorama US - $899 Nikon, Canon.

Amazon (all countries) Nikon, Canon

Warehouseexpress.com UK - £794.99 (inc VAT) Nikon, Canon

For lens hire in the US try here, and the in UK, try here (please mention us).

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