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An Early Ultra Wide Angle Lens
The RE Auto Topcor 25 mm F/3.5
This lens was one of the first ultra wide angle lens intended for use on SLR cameras, and usable without mirror lockup and external viewfinder. The first lens of this type was probably the Angenieux Retrofocus, 24 mm F/3.5 made in 1957 to fit the Alpa camera. The Flectogon 25 mm F/4 for Exkata and the Distagon 25 mm F/2.8 for Contarex were also contemporaneous with this Topcor, but I do not have reliable information about the date of introduction of these two German lenses. It is probably safe to say that this Topcor was the world's first meter coupled ultra wide angle lens .
In many ways the RE Auto Topcor, 25 mm F/3.5, was a remarkable lens for it's time. A rather complex retrofocus (inverted telephoto) design, it not only offered an acceptance angle of 82 degrees but also good resolution, flat focus and very little barrel distortion. But because it was designed before the age of computer-aided ray-tracing, compromises had to be made resulting in a lens with a few particularities.
First, the front glass element is large, 51 mm--larger than for either the 58 mm F/1.4 or the 85 mm F/1.8 lenses. This combined with the large acceptance angle prevented the use of front mounted 62 mm filters without vignetting. For this reason, there are no threads for filter mounting at the front of the lens. To compensate for this short coming, Topcon built in a smallish bayonet mount at the rear of the lens and provided 10 tiny filters. (The 10 most commonly used filters at that point in time.) These rear-mounted filters are both a blessing and a curse-I make further comments about them below. When a filter other than one of the 10 was called for, an unusual combined lens hood and filter mount was available. While offering only limited service as a hood, it was capable of holding a series IX filter at the front.
This lens has the dubious distinction of being the only one of the RE Auto Topcors that never evolved into a last generation model. I suppose Topcon's rational was that with a 20 mm and 28 mm available, the 25 mm was redundant. And since the design was old, and the filter technique out of date, they could see no advantage to a costly redesign.
The image below shows the four variations of this 25 mm lens. The following matrix is laid out the same as the image and identifies each lens.
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Because of the included accessories, this 25 mm Topcor was always sold with it's own special lens case. This case is basically a leather cube with a zipper running around the top on three sides. The zipper fastening is a bit unhandy, but secure.
To the left, is an image of the case, opened to show the stowed lens, lens hood and filter tray. The filter tray is attached to the case top and folds down for access.
The image to the above above-right is a close-up of the filters installed in the tray. The years have taken their toll on the elasticity of the retaining bands and most no longer provide secure retention of the filters. Note that space is provided for only nine filters, the tenth is mounted to the lens.
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I do not find the rear bayonet mounted filters convenient to use and seldom carry them. The procedure for filter change is as follows:
Remove the lens and find some place to set it down.
Remove the filter from the lens and set it aside.
Open the lens case, pull down the filter tray and remove the desired filter.
Install the new filter on the lens.
Mount the lens back onto the camera.
Install the set aside filter into the filter tray and close the case.
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The filters are tiny and require care when handling. They are easy to drop and therefore, easy to loose. Its also easy to smudge the glass surface with oil from a finger tip.
The image above is a close-up of the rear of the lens with a mounted filter.
Note that the filter is a part of the optical design and one filter should always be mounted to the lens. Without a filter the lens will not properly focus at infinity and some second order optical aberrations may be present.
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The images above show details of the unusual black finished lens of the second generation (by my classification). Apparently a few second generation lenses were made with black finish and this lens shows all the attributes of this variation:
Mixed case lettering on the decorative ring and the focal length specified in cm.
Two infinity symbols on the distance scale with the 'feet' calibrations in white.
The milled recess on the rear plate.
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The image to the left shows the details of the unusual lens hood. On the left the hood is mounted to a lens, clamped to the outside of the front barrel. The little knob tightens the clamp. Of course the rectangular cutout in the front has to be aligned with the film image and the hood can either be rotated by loosening the clamp, or by slightly unscrewing the front element of the hood.
Below is the hood with the front element unscrewed, along with a series IX filter that fits into the part that attaches to the lens. Changing filters in the hood is somewhat awkward since the screw threads are of a fine pitch and five revolutions are necessary to free the front. Then, there is the tendency to cross-thread the front when reattaching it. And, of course, the rectangular cutout must again be aligned.
If you plan to use one of these series IX filters, be prepared for a high price. These are large pieces of optical glass with a diameter of 82 mm (3.23 inch)
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