Friday, 23 November 2012

105mm f/2.5 AI-S: Another old skool Classic



The Nikkor 105mm f/2.5 coupled with a Nikon FM2, was reportedly the combination used to capture one of the most recognised portraits of recent times: Steve McCurry's "Afghan Girl". But how does this lens perform when not in McCurry's hands?


Continuing my digging of old 1980s film equipment, I came across the gem shown up top.

The lens has a long history, as noted in Nikon's inhouse "Thousand and One Nights" online magazine and also at the fantastic MIR site.

A general application for this lens and particularly the focal length is portraiture and across various internet forums, it is still held in high regard. From a gearhead perspective, what are it's basic characteristics?

The f/2.5 lens is nice, solid and compact with a built in retractable lens hood - the size however is deceptive given ultimate weight. The focus ring is wide with a long throw and being a Japanese made AI-S lens from the early 1980s, it's build is stunning. The lens is shortest at infinity focus and has a 52mm filter thread.

[updated at various times as I've had access to different/newer cameras - images below ranging from FX D750 and DX D7000/D300/D40]


Consider this mundane setting as the basis for examination - a Nikon RAW, converted to jpeg in CNX2 with standard sharpening. The focus target is about 1.5m/5ft away with the background extending another 4m to the window. Typical tight head/shoulders distance for FX. Images were shot with a full frame Nikon D750 on a tripod with the focusing via full magnification on liveview and captured with shutter release on selftimer.


D750 | f/2.5 | f/2.8 | f/4 | f/5.6 | f/8 | f/11

You may also get a feel for the colour rendition of the lens - the image above has been colour corrected with the WhiBal in the shot; in short the colours should be accurate. The scene was backlit with early winter sun and a slight mix of tungsten light from up above.

Sharpness

This is a series of 100% crops of the focus point, wide open f/2.5 and then stopped down to f/2.8, f/4 .. all the way to f/11.

D750

Wide open I've consistently found that the lens is a little soft although it could be my ability to focus. Even stopping down 1/3 of a stop to f/2.8 improves but only marginally. Going to f/4 and down yields significantly better results than the apertures around f/2.5. On the 24mpx D750 we can see the diffraction effects softening the images from f/8 onwards.


D40 | f/11 ISO400


Contrast is reasonable but the more modern lenses such the Nikkor 85mm f/1.4D are better in this regard. At f/4 and down this is less of an issue. This lens like many others of this era don't deal well with backlit subjects with flare and loss of contrast as a result.

A set of quick and dirty resolution/sharpness tripod'd, liveview focused images (DX D7100) all at 1.4m for reference:


D7000 | f/2.5 | f/2.8 | f/4 | f/5.6 | f/8 | f/11

Chromatic Aberrations

Purple fringing is common with most fast lenses, especially with older lenses.


D40 | f/2.5 ISO200 | 100% centre/focus crop

This image is backlit from overcast sky to exagerate high contrast areas.

D40 | top left (CA corrected) | bottom left (uncorrected) | top right (CA corrected) | bottom right (uncorrected)

As seen above, mild CAs can be reasonably well corrected in CNX2. However, extreme conditions are less successful as seen in the 100% center crop below.

Bokeh

I find the out of focus areas are pleasant overall, although I'm not so sure how I feel about the rendition of point light sources as they can seem a little jittery and especially when stopped down past f/2.8 but that may be due to the non-rounded aperture blades.


D300 | f/4 ISO200
Notice the slight swirly bokeh above. I tend to notice this when the background is close and less of a problem when further away (see below)


D300 | f/4 ISO200

D300 | f/2.8 ISO800

D750 | f/2.5 ISO6400 | no vignette correction

We can also see that on FX there's also no real problem with vignetting - whilst it's there and visible I haven't found it overly objectional.


Focusing is assisted with the size and throw of the focus ring and of course it has a physical aperture ring which is invaluable for those who want dual use on digital and film bodies. I still however find it difficult to manually focus this with good results wide open, particularly with the higher resolution bodies (D7100,D750,D800 etc) - this is probably a problem for me, although seasoned/long time film SLR users tell me that the image will 'snap' into focus so I need more practise.

Repairs/Dust Removal

The lens has a relatively simple optical formula and simple physical assembly which helps if you need to remove dust from within the elements.

105mm f/2.5 AI/AI-S lens cross section via Nikon
Detailed instructions for dis-assembly can be found online from fmlenses.com and from Richard Haw's repair site. The high level instructions involve removing the lens hood and then lifting out the entire optical assembly that also includes the aperture mechanism. After that access to the rear and front lens groups are physically possibly by unscrewing these groups from the optical assembly: note the need for acetone/nail varnish removal to undo any glue/thread-lock.

Access to lens groups:

  • extend the lens hood to uncover 3x small set screws - remove these screws
  • grab the exposed barrel hidden by the hood and the silver grab ring and unscrew - this gives access to the lens assembly which is a single block
  • hold the lens so the rear element points down and set focus to closest focus and aperture ring to wide open - you can lift the entire optical assembly out; this includes front group, aperture mechanism and rear lens group
  • front group (3 elements) - you can unscrew, there are no apparent thread locking/glue on threads
  • rear group (2 elements) - to unscrew, locate the hole on the rear group - there are 2; locate the one closest to the rear element (the other one is semi buried near the aperture mechanism; add solvent/nail polish remover to the hole via a qtip/cotton bud and shortly this group can be unscrewed
The instructions for rear lens group differs slightly to the MF lens forum post as he removes more than is needed, but which allows access to the aperture system - I need access to the rear elements so be aware.

Rear element group:

Be very careful with this group - the rear element is convex - CURVES OUT - so if you need to set this down, have the rear element face UPWARDS
  • hold the lens group, rear element facing down
  • use a lens spanner to remove the inner retention ring to gain access to the first of the 2 elements
  • pop element out with gravity or lens sucker
  • access to inside of rear element unknown - not known how to remove rear element from rear group casing

Re-assembly:

  • Rescrew the lens groups to give the single piece optical unit
  • notice the large silver screw on the side - this guides the optical unit into the lens barrel assembly, very similar to this image from Richard Haw's 105mm 105 2.5k repair guide. NB: disassembly is different to the 105mm f/2.5 AIS
  • ensure that the aperture blades are wide open, if not move the flat aperture level - if not, you will have problems with aperture or focus ring catching after re-inserting
  • ensure the lens barrel is set of closest focus and aperture is wide open
  • gently slide the optical group into the lens barrel
  • check the focus ring can turn from rear to infinity and aperture ring opens/closing - if either catches, see the note #3 above
  • screw on the lens hood assembly and attach 3x the grub screws and we're done

Overall the lens is very good, particular for the price that these can be found on auction sites when compared to the other portrait primes. However, it's maximum f/2.5 aperture does not make this a particularly fast prime so its not necessarily suitable for low light situations but nevertheless it's still a fun lens to use.

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