PDA

View Full Version : Ross Homocentric 15” Convertible Lens - wet plate and ULF!



Tim Meisburger
21-Nov-2012, 21:36
I was thinking about selling this lens I picked up in an antique store in Burma, and started reading about it and learned it is convertible. I couldn't find any information on how to convert it so, it being Thanksgiving and nothing else to do as my wife cooks the turkey, I tried to test it for focal length and image circle using a piece of white foam core. Although I make no claims to super accuracy, my results should be close, and frankly I was astonished. Converted, this lens can be either super fast (f/1.5), or cover a huge area (35"). Focal lengths ranged from about 100mm to about 850mm, with every configuration covering at least 5x7 wide open. Below are my results, with a, b, c, and d representing the four interchangeable lens elements from film side (a) to subject.

Ross Homocentric 15” Convertible Lens (normal configuration covers 12x10 according to old catalog)


84102

Tim Meisburger
22-Nov-2012, 02:13
I've been playing with this a bit, particularly the 6" and the 3.5", as they work on my 4x5. Super soft and no depth of field wide open, but they sharpen up nice when you stop down. There is a pronounced focus shift when stopping down, but these two are so bright I was easily able to focus the 3.5 at something like f/32 with just a loupe (no dark cloth). Fun stuff!

We have some people coming over tonight for turkey and I will try the 6" with my last sheet of color fuji instant film (its the smaller format, not 4x5). I have some black and white but it is asa 2000, so it may be too fast for that lens, unless I shoot in the dark!

Dan Fromm
22-Nov-2012, 06:40
From the VM:


Homocentric from 1902.
Ross seem to have begun to introduce proprietary anastigmats from 1900. The Aplanatic f5.6 seems to have
been the first, made to a 4-glass Gauss design , and 'improved' in 1901 and 1902, and then replaced or
renamed the Homocentric in 1902. There is a Brit. Pat No20,349/1900 which may cover this. The name
Homocentric however covers a number of different lenses. In 1905, the B.J.A. p917 reviews the lenses as 3
series at f8.0, f6.3 and f5.6, and comments that it will not stop below f16. Test results showed longitudinal
errors at 0.5% of focal length for the f8.0; 2% for the f6.3 and 4% for the f5.6. (This is a very unusual type of
comment at that time. It may explain why the f6.3 was the longest lived, being still made into the late 1940's
or 1950's.) The designer was Richmond and the f4.5 was issued later in 1912 (B.J.A. 1912,p712, p1914) again
with a 4-glass layout, as was the f5.6 and f6.3 (Photography 13/07/1909). The inner glasses were rather thick
and this may have helped the design. It is a lens best liked in the slower versions, and covers a good angle,
but is rather prone to flare in uncoated examples. All these seem to be 4-glass Gauss types, the f8.0 being
the choice for hand cameras with blade shutters. For a note see Photography, 13/07/1909. Homocentric has
the advantage of covering a good angle and of being free from focus shift on stopping down, and was regarded
as well corrected. All the Gauss type lenses were separable.

Further on the f/6.3:


Homocentric f6.3 It was made in 2.75, 3.5, 4.0, 5.0, 6.0, 7.0, 8.5, 10, 12in. and was available in shutters
such as Koilos and Compur. A 6 or 5in lens was suggested for 5x4. The drawing shows a lens with thicker
inner glasses and this may have helped the corrections, and been easier to use on the slower design.
(Ros032). In use it was a sharp lens, but seemed to have a high flare level. It was found as a f6.3/6.5in
No69280 in a Compound shutter No371,939. It was seen at No128,36x in Rimset Compur No712,95x of
c.1925-6 which had been fitted to a c.1936 Linhof Technika II; and No108,86x in dialset Compur, and as a
coated barrel mount lens at No223,64x from the later 1940's. This was also referred to as Series 111.
Incidentally this shows it was fairly common. It was a popular lens as it covered a wider angle (see 1939 list
for coverage) and was continued for a long time, being coated postwar. It was also sold in blade shutters such
as the Compound and later the Compur during the 1930's so a Linhof user could buy a shuttered Homocentric
but not we think a similar Xpres. Adding in that a 5in would cover 5x4in or more when stopped down, it was
probably a better choice though the flare level was higher due to the extra air-glass surface. Certainly the lens
used on the 9x12cm Linhof ST2 gave a sharp image but was high in flare even though it seemed to be in very
nice order. A coated one would be much preferable for use. The B.J.A. 1928, p8advert. gives:
5in for 4.25x3.25 at f6.3 5x4 at medium stops.
5.5in for 4.75x3.5in 6x5in
6in for 5x4in 6.5x4.75in
6.5in for 5.5x3.5in 7x5in
7in for 6.5x4.75in 8.5x6.5in
8.5in for 7.5x5in 8.5x6.5in
10in for 8.5x6.5in 10x8in
12in for 10x8in 12x10in
15in for 12x10in 15x12in
While the full aperture coverage is interesting, the original customer probably was interested in using camera
movements and as a professional, would probably stop down so the last column for use at say f11 was
probably the important one and shows it to be a good lens for most technical and architectural uses.

Do what you will, but the single elements probably won't give good image quality. Single cells, possibly. I'm a little surprised that the cells aren't the same focal length.

Struan Gray
23-Nov-2012, 02:28
I have an uncoated brass 12" Homocentric which I've used whole for portraits on 6x6. It's sharp, sharp enough wide open for portrait use, but very flarey. So much flare that the look is something special - in flat lighting with a pale background you end up with a desaturated, watercolour feel. Fewer air-glass surfaces will help tame some of the flare, but at the expense of point-blur from increased aberrations.

If you get something you like, use it. There are no rules.