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115mm f/5.2 Linoscan lens as a tube lens for APS-C

 
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Beatsy



Joined: 05 Jul 2013
Posts: 1425
Location: Malvern, UK

PostPosted: Sat Aug 18, 2018 3:20 pm    Post subject: 115mm f/5.2 Linoscan lens as a tube lens for APS-C Reply with quote

Some time ago, I tried a Pentax 105mm as a tube lens for APS-C, crop mode and got a couple of pleasant surprises (thread here - http://www.photomacrography.net/forum/viewtopic.php?t=36977 )

A bit later I got a couple of scanner lenses from a Heidelberg Linoscan 1800. The 90mm worked really well and remains in regular use as macro lens at around 1.5x. The 115mm was relegated to use as a loupe in the end (one of the relevant threads here - http://www.photomacrography.net/forum/viewtopic.php?t=37121)

Around that time, mawyatt made an off-the-cuff comment about using the Linoscan 115/5.2 lens as a tube lens for Mitties. That stuck in my mind. So much so, I recently decided to give it a try. The image quality and distortion-free nature of the Linoscan lenses led me to believe (hope) the 115mm might make a stonking tube lens to push high power Mitties down onto APS-C (as with the 105mm Pentax, only better).

I ordered an extra 40mm tube to M52x1 male thread adapter (for focusing helicoids) from Raf and it arrived yesterday. I already use one of these to mount the 90mm Linoscan lens on the front of a Raynox DCR-150 (used as a tube lens, even though the Linoscan isn't an infinty corrected lens). The second adapter would allow me to easily try the 115mm as a tube lens as I already had other suitable adapters and parts. I'm sure there are better ways to do this, but this worked and was a relatively cheap solution. The 115mm lens, with a Raf adapter on each end, is marked with an arrow in this pic. It has various other adapters and tubes added to set the lens to infinity focus and attach any Mitty to the front (20x here).


At first, using APS-C mode, I wasn't impressed at all. Horrible pincushion distortion and a bit of field curvature in the corners too (though the latter is usually not an issue when stacking). Overall sharpness and resolution were very good though. Then I realised, despite only using crop mode (a 24mm x 16mm area of a FF sensor) I was still pushing the 20x Mitty way below it's rated mag, and way above it's rated image circle. Here's the whole (crop) frame. The circle represents the rated 1.2mm diameter FoV of a 20x Mitutoyo M Plan APO.


The red rectangle has the same 3:2 aspect ratio as the sensor, and represents the area that would (mostly) sit within the Mitty's "performance envelope". Here's a crop from within that area. A perfectly flat field with no distortion at all.


Here's a 100% crop from the top left corner of that smaller, "optimum performance" area. As good as I've ever seen from my 20x (in terms of sharpness, resolution and lack of distortions). Result!


And just for grins, here's a 100% crop from the top left corner of the full APS-C image. Very bendy, but image quality is surprisingly good considering how far it is outside the Mitty's specced limits!


Overall, the experiment was a success IMO, and I intend to use this setup for deep, high-magnification stacks, where smaller files are more feasible and empty magnification is a pain. But now, for obvious reasons, I'm looking for a 150mm-200mm "scanner lens" of the same quality. Any ideas?
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mawyatt



Joined: 22 Aug 2013
Posts: 1671
Location: Clearwater

PostPosted: Sat Aug 18, 2018 4:28 pm    Post subject: Reply with quote

Steve,

Wow, that's impressive performance indeed!! The Linoscan 115/5.2 works beautifully with your Mitty 20X!! You've put both of those lenses from the Linoscan to great use!! Sorry, can't help with the 150~200mm scanners lens (don't know much about them), I'll listen though.

Cheers,
_________________
Research is like a treasure hunt, you don't know where to look or what you'll find!
~Mike
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RobertOToole



Joined: 17 Jan 2013
Posts: 1051
Location: United States

PostPosted: Sat Aug 18, 2018 9:14 pm    Post subject: Reply with quote

Hi Beatsy,

I do have some recommendations for tube lenses in the 100-200 range but where do I start, I am just finishing up a tube lens test with almost 20 lenses, 11 of these are the <200mm range. I will post something maybe by tomorrow.
A lot of surprises coming!

Best,

Robert
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