Nikkor 10.5 mm f/2.8 fisheye:

Get back the hidden complete Angle of View

(Sawing off the sun-shield)

This wonderfull fisheye lens is blessed with very good optical properties and has a generous Angle of View as well.

These features were described in detail here.

The sun shield that is an integral part of the outer barrel of the Nikon lens, is made of hard black plastic. It was designed to protect from possible light flare and fits exactly the coverage of the (APS-C sized) sensors of the Nikon DSLRs.

There is a 0.15 mm gap between the outer part (i.e. sun shield) and the internal front moving cylinder of the lens body. This cylinder moves axially forward or backward for focusing: it is retracted inside the sun shield when the lens is focused to infinity and it is in the most forward position when the lens is focused to the closest distance (i.e. 0.14 meters). The overall displacement length is only about 4 mm.

To remove the front part of this sun shield that is an obstacle to the full opening of the Angle of View, I have used the tools that are photographed:

It is adapted with a thin steel profile that makes impossible to saw more than a preset depth (d). The sketch on the upper right corner described a cut view of this principle.

This saw will be used as a first tool to make a grove that nearly cuts off the plastic but not quite all of the thickness of it.

These clamps shall have two purposes: as a lateral guide for the manual sawing action and to keep the paper protection in place (have a look at the photo below) .

You may have an idea of the result by looking at the following photos (also shown on the page here for another reason i.e. installation of an external aperture setting manual control).

Nikkor lens mounted on a Canon EOS 20D before transformation Nikkor lens mounted on a Canon EOS 5D after transformation.

Personnal point of view and conclusion:

Nikon has claimed on all written commercial documents that I could read about the 10.5mm fisheye lens, that it is compatible only with the DX type of Nikon DSLR line. This statement is also implicitely consistent with the patent written by the inventor of the 10.5 mm fisheye lens. An extract of it follows:

(Quote)

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fisheye lens sufficiently securing an equivalent air distance between the last lens surface and the image plane, in spite of its compactness, suitable for a SLR camera and in particular for a SLR digital camera.

2. Related Background Art

Almost all fisheye lenses for a SLR camera have an image size corresponding to 35-mm films (For example, Japanese Patent Application Laid-Open No. 54-32319.) Even if these fisheye lenses are directly used in a SLR digital camera, the angle of view of 180 degrees cannot be secured because due to the image size the relation between the focal length and the angle of view of a SLR digital camera is different from that of a SLR film camera.

Generally, the size of the imaging plane of an imaging device for a SLR digital camera is a little smaller than the film frame size of 35-mm films. Accordingly, "the diagonal of the image size of 35-mm film frame" divided by "the diagonal of the image size of the imaging device" multiplied by the focal length of the lens becomes the focal length (converted focal length to 35-mm film format) of the lens used in a SLR digital camera.

Therefore, when a fisheye lens for a SLR film camera having the angle of view of 180 degrees is directly used in a SLR digital camera, no more than n effect of a super wide-angle lens can be obtained.

As described above, the angle of view of a SLR digital camera whose image size is a little smaller than that of a SLR film camera inevitably becomes narrow. Moreover, when you want to take a photograph of a wide area with use of a fisheye lens, the focal length has to be set even shorter. As a result, since the back focal length of a fisheye lens for a SLR digital camera is required to become more than 3 times as large as the focal length, an extremely strong divergent lens group has to be located to the object side of the lens system.

In a fisheye lens, a retrofocus type in which the principal point is located backward is used. However, since an extremely strong divergent lens group is located to the object side of the lens system, optical performance tends to be deteriorated due to curvature of field and astigmatism.

Moreover, in the retrofocus type, the diameter of the negative lens in the front lens group tends to become large, so that it is a problem that the fisheye lens becomes larger and heavier.

(Unquote)

Furthermore, the excellent resistance to flare of the lens does not justify installation of a sunshield at all as it is almost immune to this plague.

The shield is a way to protect the exposed front lens from scratches and Nikon has also explained this. However, this cannot be an absolute requirement as Nikon has produced many other (much larger and heavier) fisheye lenses that were not fitted with such a protection. Other manufacturers have also done that.

I and others have found the Nikon posture to be at the least misleading: The Nikkor 10.5 mm is an excellent performer especially in Full Frame 35 mm format and probably the best in the 7 mm to 12 mm focal length range. While it is generally used by "action photographer reporters" making panoramas with 3 or 4 shots, it also allows to make a high quality 360 degrees cylindrical panorama with 140 VFoV with two shots only! The latter case would not be possible if the lens had be optimized for higher resolution and then would have a focal length of about 12 mm, making it the equivalent of 8 mm in the APS-c DSLR world.

I believe this ackward and naive technical protection solution was in fact mostly intended to formally (but not functionnally) restrict the use to the Nikon (or Sony) APS-C (DX) size sensors that are only presently existing.

Wondering what Nikon shall decide to do when they possibly sell Full Frame 35 mm DSLR in a (distant?) future. Using the same (Patented) design to make and sell a 12 mm Fisheye lens would then be the best news for Nikon FF and... EOSDSLR users as well;-)

Michel Thoby

2 April 2006