Nikon AI-S Nikkor 24 mm f/2.8 for UV and IR imaging

The AI-S Nikkor 24 mm f/2.8 is a good legacy lens with manual focus and manual aperture ring, mounted in a robust all-metal barrel. At 250 g, it is a little heavier than more modern primes of similar focal lengths and speeds. The 52 mm filter mount is almost standard for Nikkor legacy lenses. The optical formula with 9 elements in 7 groups has remained unchanged for over half a century.

The AF Nikkor 24 mm f/2.8 D uses the same optics (except for fancier coatings) in an updated barrel with less metal and more plastic, plus a mechanical autofocus coupling and a chip to interface with the camera electronics. As late as 2019, both the AF-D and AI-S models of this lens were still listed as in production.

While the front of the lens and its filter mount extend without rotating when focusing, a rear optomechanical subassembly does rotate as well as move forward. This is a consequence of this lens using a "floating" rear group for better optical performance when focusing close. Minimum focus is at about 29 cm, with a total rotation of the focus ring of less than half a turn.

Aperture clicks are only every full stop. The 7-blade iris has a little irregular shape at f/4 and f/5.6, but not overly so. At f/8 and beyond, the iris shape is still polygonal, rather than rounded.

The AI Nikkor 24 mm f/2.8 has a 52 mm filter mount. Given the short focal length, there is a chance that filters will cause some darkening of the image corners when the lens is used on full-frame sensors, especially fully open at f/2.8 and especially if a lens shade is also used.

The HN-1 is the lens shade recommended by Nikon for this lens. A 72 mm lens cap fits at the front of the HN-1, so you can use a lens cap of this size to close the lens without dismounting the lens shade.

In practical tests (see below), I found that the HN-1 lens shade is not enough to prevent the formation of central lens flare when shooting in the general direction of the sun. Flare still occurs even when the sun is not directly within the image circle of the lens.

Most generic petal lens shades for 24 mm lenses are better at preventing flare than the HN-1, since their top petal extends forward much more than the HN-1 and protects the front element of the lens much better from direct sunlight. Their drawback is that this type of lens shade with thread mount usually cannot be attached at the front of the lens in reverse orientation. Petal lens shades for wideangles are deeply indented, so they are less effective at protecting from lens flare when the sun strikes the front element through one of the indentations between petals.

Make sure that the petal lens shade you choose for this lens is of the "universal" type and has a threaded 52 mm attachment. Most petal lens shades, especially those on the second-hand market, attach instead at the front of a lens via a custom bayonet mount, which is absent in the AI-S Nikkor 24 mm.

It is highly desirable for a petal lens shade to accept a lens cap. The lens shade shown in Figure 3 has a female 52 mm thread deep within its petals, as well as enough room around this thread to allow mounting and unmounting a butterfly cap. This thread is exclusively meant for mounting a lens cap. A filter mounted on this thread strongly vignettes in the corners. Several universal petal lens shades on eBay seem to lack the female front thread, even though they look identical to mine in all other respects. Probably the manufacturer skips this step on most production batches to save money.

This lens shade causes a slight vignetting in the extreme corners at f/2.8. Closing the aperture a couple of stops greatly reduces this vignetting. A wider lens shade mounted on the lens via a step-up filter adapter might be better in this respect. However, only testing can show whether this is true, because the step-up ring may introduce vignetting on its own.

The lens shade of Figure 3 is entirely made of plastic and has a fixed (i.e., non-rotating) male 52 mm thread at its rear. A scalloped ring with a matching thread is mounted on the male thread. The interior of the lens shade is not particularly light absorbing, and could be better in this respect. However, this universal lens shade seems to be the only one broadly available from EU-based eBay sellers. I only rarely buy items from China-based sellers these days, because of greatly increased prices, customs fees and sales tax compared to a couple of years ago.

To attach this lens shade to the lens:

1. Screw the ring all the way against the lens shade.
2. Thread the lens shade onto the filter mount of the lens until the lens shade is properly aligned. In my case, this means approximately three turns.
3. Unscrew the ring until it reaches the front of the filter mount and locks against it.

The rest of this review centers on my use of the AI version of this lens for UV and IR imaging.

Using the Nikkor 24 mm f/2.8 in UV imaging

Virtually all modern filters for UV imaging use a combination of UV-pass stained glass and dielectric coatings to suppress the transmission of this stained glass in the red and/or NIR. Wideangle lenses like the Nikkor 24 mm can cope with moderately thick optical filters without major losses in image quality, although oblique light rays cross a higher thickness of filter glass than axial rays, which in turn causes some darkening of the image periphery. Dielectric coatings, however, have a transmission spectrum substantially dependent on the inclination of incident light rays, which, in the specific case of dielectric-coated UV-pass filters, often causes a significant false-color shift across the image.

A lighter, somewhat more grey, or green, or sometimes violet, central area of the image is the typical result. This can be confused with veiling glare caused by sunlight striking the front element of the lens. However, unlike veiling glare, this problem cannot be eliminated by judicious use of a lens shade, or by not shooting against the sun.

A focal length of 24 mm on full-frame sensor is well past the point where this problem is so evident that it spoils most images shot in natural sunlight. For this reason, the use of a UV-pass filter mounted at the front of this lens is bound to cause problems. This lens, however, uses a retrofocus optical design to provide a distance between the rear optical element and the film plane higher than the focal length of the lens. This implies that light rays that exit at the rear of the lens do so with a lower spread angle than light rays entering the front of the lens. In practical terms, this means that a dielectric filter mounted between lens and sensor works much better than the same filter mounted at the front of the lens.

On the large majority of DSLRs, rear-mounting a filter is not possible, because the registration distance of the Nikon F mount is relatively close to the sensor plane. This leaves no space available for the unimpeded movement of the camera mirror. On some DSLRs, special filters can be mounted in a thin metal frame immediately at the front of the sensor. While I am not aware of commercial UV-pass filters available for this purpose, it might be possible to cut a circular filter to make it fit in one of these special frames.

Rear-mounting a filter when this legacy lens is used on a mirrorless camera is relatively simple, and my solution is described in the following section. Another alternative is to add a filter mount within a lens adapter. This, however, makes it more difficult to swap filters. With M42 legacy lenses, I had better success by swapping among multiple lens adapters, each one with a different filter always left in the adapter.

Adapting the Nikkor 24 mm f/2.8 to Sony mirrorless and rear-mounted filter

The rear of the AI Nikkor 24 mm f/2.8 has a metal "stud" at the bottom (foremost in the above figure) that makes it a little difficult to attach a rear filter mount. There is also a less projecting "baffle" in the shape of a half circle in the upper part of the rear surface. The bottom stud has no obvious function (except possibly as a very small light baffle). For this purpose, I decided to use a 37 mm to 30.5 mm step-down ring, which required filing away some of its perimeter to accommodate the stud on the lens rear. In practice, a 37 mm to 30.5 mm step-down filter adapter is the right size for this modification. Since this adapter is not subjected to mechanical forces, I reversibly attached it to the rear of the lens with silicone adhesive. No permanent modification of the lens was necessary.

Rear-mounted filters always require the lens (or lens adapter) to be unmounted to swap filters. I find it much easier to change a filter attached at the rear of a lens, rather than one deeply set within an adapter.

Lens performance in UV imaging

The CoastalOpt 60 mm Apo is my reference lens for UV imaging. The test image with this lens (Figure 7) is uncompromisingly sharp corner-to-corner and displays the expected amount of false-color for a sunlit UV landscape. In comparison, the same scene imaged with the Nikkor 24 mm (Figure 8) is virtually monochrome. Applying a custom white balance (Figure 8) changes the color of the sky (which is overexposed and therefore in principle always tweakable with color balance, even in a monochrome image) but has little effect on the color balance of the buildings and trees.

The Nikkor 24 mm test image was shot at f/2.8. Even when substantially reduced in size, image resolution is clearly poorer in the corners. Stopping down to f/8 improves the overall resolution. Curvature of field in the UV is part of the problem with this lens.

Figure 10 was shot against the sun with the HN-1 lens shade recommended by Nikon. The sun was outside the picture, but illuminated the front element of the lens in spite of the lens shade. The unprocessed image shows a moderate center flare, which becomes much worse once a custom color balance is performed in post-processing (Figure 11). This problem has become less serious since I begun using a petal lens shade (Figure 3).

The above example, recorded with a 52 mm petal lens shade, shows no significant center flare, but a little darkening in the corners.

In NIR, the AI Nikkor 24 mm displays no trace of center flare. The above images of course contain also some VIS, given the short cutoff wavelengths of the filters, which is essential in achieving the desired false-color.

Alternatives to the AI-S Nikkor 24 mm