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Another Pentax 110 Test, this time with aperture control
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PostPosted: Wed Mar 28, 2012 4:37 pm    Post subject: Another Pentax 110 Test, this time with aperture control Reply with quote

This is my home-made Pentax 110 to m4/3 adapter. It is made from a c-mount to m4/3 adapter, the bayonet flange from a 110 camera and the body of a common 50mm f3.5 triplet enlarger lens. With suitable machining, the enlarger lens provided all the other mechanical bits required (bar a screw or two and a dab of glue). The screw sticking out is an ad hoc lever for turning the aperture ring. There are no click stops and the lens does not lock in place (but the lenses are so tiny and the bayonet grips quite firmly so i do not think this is necessary.



The aperture is positioned as close as possible to the back of the lens to mimic the original installation. The distance is about correct I think. certainly, it would hard to make the aperture closer.



This is how it looks with the incredibly tiny 24mm f2.8 lens



Some sample shots. I've stopped down by halving the shutter time with the camera in Av mode. No PP other than resize

F2.8


f4


f5.6


central sharpness is good wide open and superb wide open. But, as we shall see, that is not the whole story.

To be continued........


PostPosted: Wed Mar 28, 2012 4:46 pm    Post subject: Reply with quote

You're a genius Cool


PostPosted: Wed Mar 28, 2012 4:50 pm    Post subject: Reply with quote

Amazing work and looks worth it.
What with the softness in the corners?


PostPosted: Wed Mar 28, 2012 5:30 pm    Post subject: Reply with quote

ilguercio wrote:
Amazing work


Thanks

ilguercio wrote:

What with the softness in the corners?


Ah! Well there's the rub. Take a look at this sequence:

F2.8


F4


F5.6


It's not just corner softness but the whole short edge that is hopelessly soft. Just look at the top of the tree! Stopping down improves things, but not enough to make the edge anything like acceptable.

You will also notice vignetting in the stopped down pictures. Now, when i first made the adapter, the aperture was further away from the lens (about 5 to 6 mm away in fact) and I wondered if the vignetting was because the aperture was acting partly like a field stop. I experimented with a fixed aperture close to the back of the lens and the vignetting was not so bad. So I turned the aperture mechanism around to make it closer to the lens. As I mentioned before, it is now about as close as I can make it but still the vignetting is there.

So, is the vignetting because the aperture is not in the correct place or because of the lens design. I suspect the latter because the aperture in the 110 camera is actually square, and this will obviously reduce the corner shading.


PostPosted: Wed Mar 28, 2012 5:35 pm    Post subject: Reply with quote

A couple more shots to show the central sharpness

At f2.8, whole image then crop





Now at F4, whole image and then crop





These were taken at a distance of c. 50cm, and this may be why the wide open shot is just a touch soft. But one stop down it's like a razor!


PostPosted: Wed Mar 28, 2012 5:36 pm    Post subject: Reply with quote

GrahamNR17 wrote:
You're a genius Cool


Oh gosh! Embarassed


PostPosted: Wed Mar 28, 2012 5:46 pm    Post subject: Reply with quote

Center sharpness is pretty good for such a small piece of glass.
I hope you can fix the strangeness at the corners.
By the way, such a setup looks so compact and to think there's a big sensor behind the lens is even more astonishing.


PostPosted: Wed Mar 28, 2012 6:00 pm    Post subject: Reply with quote

I cured the corners with the 18mm version of the lens by placing the aperture as close behind the rear element as I could get it, almost touching. Excuse the grab shot, but just a demo.



The downside is it's a fixed aperture (about 2.5mm hole in a plastic disc)


Last edited by GrahamNR17 on Wed Mar 28, 2012 6:31 pm; edited 1 time in total


PostPosted: Wed Mar 28, 2012 6:19 pm    Post subject: Reply with quote

If you'd manage to get the aperture inside of the objective, between the lenses, you'd solve the problem. These little Pentaxes are magnifcent, but the aperture question is not easy. Bravo for trying! Smile


PostPosted: Sat Apr 07, 2012 10:43 am    Post subject: Reply with quote

GrahamNR17 wrote:
I cured the corners with the 18mm version of the lens by placing the aperture as close behind the rear element as I could get it, almost touching.


Thanks for the tip. I've re-jigged my adapter and the metal ring that holds the aperture blades now rubs the back of the lens bayonet - I really can't make it closer than that! The vignetting is now gone (or at least much reduced), but the corners and short edges are still very soft. This is the 24mm again.

Wide open at f2.8


One stop down from wide open by shutter speed (F4)


Two stops down from wide open by shutter speed (f5.6)


PostPosted: Sat Apr 07, 2012 10:59 am    Post subject: Reply with quote

I am finding the 24mm is difficult to get corner sharpness. Mine is at a fixed f7.45 at the moment, and I still get soft corners, but not quite as soft as yours. I'm going to make a new aperture of f8 and f16 and see what it's like...

The 18mm gets sharp from f5.6 Cool


PostPosted: Sat Apr 07, 2012 11:03 am    Post subject: Reply with quote

These are the results I get with the 18mm lens.

Once again, moving the aperture to the very back of the lens (and I only moved it about 0.5mm!) has fixed the vignetting. (I didn't post pictures with the 18mm before, but the vignetting was worse than with the 24mm).

Clearly, the aperture has to be placed against the back of the bayonet lugs with these lenses.

The 18mm shows the same dramatic softness at the corners and along the short edges as the 24mm. By f8-ish (judging by shutter speed relative to wide open). Things have got a lot better and I feel the lens could be used at this sort of aperture. With a fixed aperture of about 2mm, this lens could still make a cheap wide angle for m4/3 suitable for landscape work (where you would want to stop down to get the depth of field in any case).

18mm lens wide open (F2.Cool


F4 (One stop down from wide open by shutter speed)


F5.6 (2 stops down from wide open by shutter speed)


F8 (3 stops down from wide open by shutter speed)


PostPosted: Sat Apr 07, 2012 11:08 am    Post subject: Reply with quote

GrahamNR17 wrote:
I am finding the 24mm is difficult to get corner sharpness. Mine is at a fixed f7.45 at the moment, and I still get soft corners, but not quite as soft as yours. I'm going to make a new aperture of f8 and f16 and see what it's like...

The 18mm gets sharp from f5.6 Cool


I was composing my 18mm post whilst you were replying. As you will see from the above, I need f8 before I get decency with my 18mm. That's about as low as I can go with the 18mm with the aperture I have (maybe I can get f9).

I'll try the 24mm again and go smaller. I should be able to get to f11.


PostPosted: Sat Apr 07, 2012 11:21 am    Post subject: Reply with quote

This is what happens with the 24mm as I stop down further. Judged by shutter speed relative to wide open I can get to f16 with my aperture contraption as small as it will go. The three shots below are f8, f11 and f16 (approx, judged by shutter speed relative to wide open).

The corners do get sharper but slowly. Even at f16, there is still a bit of the softness left. The extreme corners are still very soft though.

14mm, approx f8


14mm approx f11


14mm, approx f16


PostPosted: Sat Apr 07, 2012 11:40 am    Post subject: Reply with quote

This is what i get with the 50mm f2.8. It's the same story really; very soft corners and short edges. Stopping down moves the zone of extreme softness further into the corners.


50mm wide open at f2.8


50mm at approx f4


50mm at approx f5.6


50mm at approx f8 (note, may be a bit of camera shake in this one as shutter speed was quite low. It is certainly a bit softer in the centre than the f5.6 shot)


PostPosted: Sat Apr 07, 2012 3:20 pm    Post subject: Reply with quote

Very similar results here. I think the 18mm will become my 'standard' lens on the camera with a f8 aperture disc most of the time.

Not very good tests at viewing size, but anyway:

18mm at f5.6


24mm at f7.45


50mm at f15.53


PostPosted: Sun Apr 08, 2012 9:27 am    Post subject: Reply with quote

Amazing jobs guys! Those little 110 lenses are special gems.