Cheap tube lens

dmillard · Post by **dmillard** » Fri Jan 21, 2011 6:23 pm

morfa wrote:
A caliper read on the thread says it's 41mm. No idea about thread pitch etc. This thread doesn't fit anything I have.

I just received the achromat I ordered from Surplus Shed. Although they also list the OD of the threaded portion as 41mm, it screwed beautifully into a helicoidal Pentax extension tube (M42 x 1mm thread).

David

Craig Gerard · Post by **Craig Gerard** » Fri Jan 21, 2011 7:21 pm

David,

Excellent! Thanks for the update. Looking foward to a configeration image/snap

Craig

dmillard · Post by **dmillard** » Fri Jan 21, 2011 10:04 pm

Well . . . it screws nicely into the Asahi Pentax helicoid, and fits snugly when its in all the way, but it is a 41mm OD thread, and is too loose in other M42 mounts. I just threw together an ad hoc arrangement behind the front standard of the PB6 bellows: 52-52mm male-male adapter, Nikon BR6 ring, M42-Nikon adapter, Pentax helicoid, tube lens - not very elegant, but rigid, total thickness about 35mm. I'll post a picture tomorrow, along with some results.

David

Blame · Post by **Blame** » Sat Jan 22, 2011 2:03 am

dmillard

There is a tape used by plumbers for sealing loose threads. PFT... I think.

It is very thin. A few layers wrapped around the male thread might do wonders to tighten the connection.

morfa · Post by **morfa** » Sat Jan 22, 2011 6:38 am

Look forward to seeing your results David!

Blame wrote:Now I hope I am not insulting you by pointing out the obvious. How about your close up lens tests. Did you reverse mount them?

No, I actually didn't and I'm aware this isn't ideal. The following is taken with a reversed Raynox +5.9 diopter + 50/0.55 at 40x (measured 0.9mm horizontal FOV)

12MP: http://farm6.static.flickr.com/5288/537 ... d8cb_o.jpg

The edge degradation looks relatively symmetrical here. On an APS sensor (or smaller) the corner to corner sharpness of the combination above would be quite acceptable.

I also tried reversing the surplus tube and I saw no obvious difference in the resulting image quality. With my setup this meant that the objective->tube length distance increased with 25mm or so (the height of the tube lens)

Blame · Post by **Blame** » Sat Jan 22, 2011 9:08 am

morfa

Thanks.

I guess reversing lenses makes less difference than I thought.

morfa · Post by **morfa** » Sat Jan 22, 2011 10:27 am

Mitutoyo 10x 0.28 + surplus tube lens @ 8x (4.5mm horizontal FOV on FF)

Full size 21MP version: http://farm6.static.flickr.com/5001/537 ... dc72_o.jpg

100% crops:

Remarkably consistent performance considering the FOV.

The lighting is intentionally, partially non-diffused and not very pleasing. The reason for this is that I've found this particular butterfly wing to be notorious for generating stacking mush (stacked results often looks much worse than individual frames suggest).

Blame · Post by **Blame** » Sat Jan 22, 2011 12:33 pm

Morfa

That was the picture I was waiting for. 4.5mm - close on the limits of the Nikon 4x 160/0.2, and and 40% greater NA.

Wow.

Blame · Post by **Blame** » Sun Jan 23, 2011 12:20 pm

I would like to put this achievment in context.

You are now down to about f/15.

A check photozone.de (a camera lens review site) shows that the very best lenses can't give more than 25% improvement at sharpest aparture than they can at f/16 on a canon 5d Mk2. That is the best - the worst give little or no improvement at all.

You have reached the point of diminishing returns. The combination of tube lens + sensor + anti alias filter will be providing more blur than the objective! At least in the centre.