The Odd Lens' Bigger Brother

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Bosun Higgs Regular Member • Posts: 103
The Odd Lens' Bigger Brother
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A while ago I posted a piece about the Kershaw Type 250 4 inch lens which I had found had several odd features, not the least, its optical formula. To my surprise I discovered that Kershaw had made a longer version of this lens, and I managed to obtain one.

This is a Kershaw (I had my reading glasses on this time!) Type 250 6 inch f2.8 projection lens intended for use with the Kershaw Type 250 35mm slide projector. The 4 inch lens I described in my earlier post was supplied as the standard lens with this projector, this 6 inch version seems to have been an accessory option, which may explain its comparative rarity.

The Nuts and Bolts

I was surprised by the large size of the 4 inch lens, but the 6 inch is even bigger. The lens is 160mm long and 65mm in diameter, the tube is the same odd 57mm diameter (2 1/4" in old money), as the first lens.

All of the glass is in the front "bulge", making the handling very front heavy.

The optics are all contained in the front "bulged" section, which screws off from the main tube. As with the 4 inch, the 6 inch has the same odd 4 element 4 group "back to front Ernostar" formula, but the elements' shapes are actually much closer to those of the original Ernostar layout and this causes some problems for conversion to a taking lens.

The arrowed shoulders reflect light strongly, causing a bright internal "halo" and lowering contrast.

The biconcave element has more angled rear shoulders (arrowed in diagram) than the one in the 4 inch. These ground surfaces are not in the light path when used as a projection lens, but when light is coming the other way this ground surface reflects light strongly, forming a bright halo deep in the lens

Although the lens is coated, this amount of light scatter inside the lens is Not a Good Thing. So, I blackened these surfaces and the halo disappeared, resulting in an increase in the (already good), contrast.

I retained the empty rear tube as the back focus is quite long, conversion was to simply clamp a M65 helicoid to the rear of this tube. Handling on my A7rII was not as good as the 4 inch, the optics are all in the front "bulge" and with the long back focus, this makes the lens very front heavy.

The Image

Focusing was a bit of a surprise, with the longer focal length and same aperture, I expected the much shallower DOF to give a "pops into focus" effect, instead, focus was rather uncertain. The lens has a broad range of "sharp" focus in which there seems to be little change. You know exactly when you enter the range and leave it, but inside that zone it is very difficult to find sharpest focus, even using full magnification.

This is not the "focus spread" problem that is common with Petzvals, in that case the optimum sharpness and contrast points do not coincide, and as you focus you can see the effects of both processes working. With the Type 250 there is just a wide zone that seems equally sharp and contrasty until you drop outside of it.

Sharpness is good, though not exceptional, and the 6 inch does not show the rapid fall off outside of the central area that I observed with the 4 inch. As with the smaller lens, the 6 inch has almost no field curvature, and with this lack of fall off, the whole frame of the 6 inch can be used to allow off-centre subject placing.

Although native peak sharpness is only "good", the lens shows no fringeing and the images respond extremely well to sharpening in post, so very good results can be obtained.

As expected for a 35mm slide projection lens, there is no hard vignetting and corner vignetting is limited to that caused by the usual restrictions of the E Mount used with large coverage, or "telecentric" rear focus lenses.

Indeed, with the rear tube removed, the image circle of the lens is over 150mm, so Fuji GFX, and even large format bods, might find this lens interesting.

As with the 4 inch lens, catseyeing is almost non-existent, even in the corners. This is probably due to the large front element and massive coverage.

The Bokeh

Firstly, I apologise for the rather uninspiring images, but it's been breezy every day for weeks here. Wind is a pain for manual focus, and deadly for my stereokeh shots, so these are best I could manage under the circumstances.

Nice 3d bokeh depth in this "stereokeh" image.

With an extra 50mm FL and the same aperture, I was expecting a much smaller DOF and bigger bokeh bubbles with this lens compared to the 4 inch, and in tests using similar subject/background distances, this was the case.

Note large bokeh bubbles with almost no "catseyeing".

In real world use however, the increased FL means that you chose a more distant position to get the coverage you want and this seems to result in bokeh bubbles smaller than you would get with the 4 inch for the same subject.

That said, bokeh is quite respectable for the modest f2.8 aperture. Bokeh bubbles on close focus subjects are large and remain nearly circular into the corners. With medium distace subjects the bubbles are smaller and have very tight edge highlighting.

It looks still, but it was really blowing a gale when I took this shot!

Because of the lack of field curvature mentioned above, the bokeh effects are even across the frame and do not increase toward the edges. Similarly, the DOF is constant across the frame and there is none of the enhanced subject isolation that can be obtained with most Petzvals.

The lens blurs well, but the bubble overlay creates a "lumpy" bokeh effect.

Like the 4 inch, OOF backgrounds are blurred effectively and smoothly, but with this lens the bubble pattern overlays this smoothness and creates a "lumpy" bokeh texture. If you look at the pathway image above on your monitor at normal distance, it looks lumpy. Now back off from you monitor about three metres and look at it again, the bubble pattern becomes less noticable and the underlying blur takes over.

If you are looking for a native background smoother/remover lens, the 4 inch version will be a better choice.

Reasonable subject sharpness in this unsharpened shot.

Throughout the bokeh testing I was constantly reminded of the 200mm Hektor that I wrote about recently . Both lenses are four element designs which resemble, or are an adaptation of, a triplet. Both have flat field, and their bokeh shares many similarities. The Hektor gave even "lumpier" bokeh than the Kershaw, although in that case it seemed to be caused by a "edge emphasis" rather than simple bubbling.

The Bottom Line.

I'm usually a big fan of textured bokeh, but I don't find the "lumpy" results from the 6 inch lens attractive. The 4 inch lens has better bokeh with none of the lumpiness, it also has better handling on my camera body. I'm hard pressed to think of a situation where I might be more inclined to pick up the 6 inch rather than the 4 inch. But I'm glad that I managed to obtain the 6 inch and give it a try, otherwise I'd always be wondering.....

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