Mystery loft find - The Other Lens

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Bosun Higgs Forum Member • Posts: 73
Mystery loft find - The Other Lens
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Some folk may remember me posting about discovering two lenses buried in the junk in my loft, at first I was taken by the unnamed, mysterious, and very old brass lens which I converted, modified, and described in a previous thread.

https://www.dpreview.com/forums/thread/4563208

Well, here is second lens of the pair, and it's a whopper!

Hmm, I don't think that's E-mount!

The lens is an 8 inch f2.9 Pentac of World War 2 vintage. It is huge, over 100mm in diameter and it weighs over a kilogram. The lens is uncoated and has an absolutely delicious 18-bladed iris that is perfectly circular at all stops, and it still works silky smooth after all these years. A shame really, as being a Bokeholic........... I will never use it.

The lens was supposedly designed by Dallmeyer, who did manufacture some of them, but a quick look at the optical layout shows that it is in fact simply a larger format version of the Voigtlander Heliar.

Some similarity there.........

The Heliar was designed in 1900 and is itself a development of the Cooke triplet via the Tessar.

Pentac/Heliar evolution.

During the war years lenses were produced by which ever optical firm had capacity, design rights and patents were ignored, so although the Pentac was allegedly a Dallmeyer design, my lens was produced by the National Optical company, a sister company of Taylor Hobson.

There are five elements (hence the Pentac appellation) in three groups, two cemented doublets on either side of a double concave lens.

The lens was used for aerial photo reconnaissance and was usually mounted in F24 cameras. At the start of the war Spitfires were used with a camera mounted in each wing to give a stereo view for photogrammetry. Later in the conflict Mosquito fighter bombers were used with three cameras, one pointing straight down in the fuselage, and one in each wing, the wing cameras were tilted at an angle in a configuration called Trimetrogon imaging.

My lens was a little dusty internally and I quickly learned that it was specifically designed for aerial use as it was nigh on impossible to disassemble! It seems that the old piston engine aircraft really shook a lot, so aerial lenses were made vibration proof to prevent them coming apart in flight.

After I discovered that every main threaded component had a cunningly concealed locking screw, I managed to get access to front air space for cleaning. The manufacturers had outdone themselves on the rear group however, as this was retained by a metal ring that had been swaged over the glass - no way that was coming out, so there were two glass surfaces I could not clean, luckily the dust on these was very light.

Another adaptation for aerial use was the replacement of brass with alumnium for the outer casing, presumably to save weight, brass was still used to house the elements however.

The rear focus was about 200mm, so I bodged up an amalgam of extension tubes, stepping rings, and heicoids to suit. The photo shows my Mark 1 Kludgemount, this was modified several times as the lens is extremely heavy, and handling is much better with the helicoid just behind the lens in the present Mark 4 iteration.

Also available in a caffeinated version.

The motorised F24 camera that the lens was used with had a 5"x5" frame, but the lens will cover 5"x7". As I have learned from previous experience of using old LF lenses on 35mm FF, a restrictive lens hood is a must to reduce contrast loss due to stray light.

100mm diameter, 150mm long lens hoods are rather hard to come by, but a quick rumage in the recycling bin resulted in the Mark 1 Kludgehood. The interior of Kludgehood is liberally coated with Black 3.0, it worked so well straight off the bat that it has stayed at Mark 1 so far. As an added bonus, Kludgehood also came complete with its own dedicated lens cap.

Some eagle-eyed folk will notice corner vignetting in some shots, this is caused by the hood, as you need it this long to maximise contrast.

The following images are all single exposures in ambient daylight, images are all uncropped.

Note edge outlining in bokeh.

Bokeh texture helps portray "3D" depth.

It's a bubbler!

Note "soft" outlining on bubbles.

Note how the bubbles "stack".

The lens is fairly sharp, especially when you consider that we are only using 3.8% of its coverage! There is some "glow" around white highlights, but this is usual for uncoated LF lenses used on smaller formats. There is no colour outlining on in-focus edges, but out of focus edges can show LOCA which is green behind the focus, but this is not too bad. Contrast is surprisingly good thanks to the restrictive hood.

Handling is not too bad once I got the helicoid positioned correctly, and it is certainly easier to use than some of my monster projection lenses. You do however need a specialised bag to carry it because of the length with the hood attached, luckily, because of aforementioned monsters, I have the necessary kit.

The main consideration for me is, of course, bokeh.

This lens delivers it in spades.

At medium distances it is a prodigious bubbler and shows slightly outlined bubbles that betray its triplet lineage. Although there is outlining, the softness of bubbles render this unobtrusive, no in-your-face Trioplan soapbubbles here! Bubbles layer well and the outlining helps them stay individually evident, producing complex patterns in dense areas.

At "macro" range, backgrounds are "smooshed" but edges and outlines produce a 3D texture in the bokeh that helps portray depth (see Daffodil shot). Outlining is present at this range too, but tends to emphasise edges rather than circles, this effect can be strong at times (see Tulip shot) whether you like it or not is down to personal taste.

So, all in all, I'm very happy about this lens, especially as it was an unexpected loft find. It will definitely be used a lot.

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