Pentax Asahi Takumar SMC lens infinity focus with Canon 600D dslr

[Elp]

Having searched for a while I managed to acquire the legendary Pentax Asahi SMC Takumar 6 element 135mm f2.5 m42 lens to use for wide-ish images, I've also got a 200mm f4 to try. I'm using it with a modded Canon 600D.

I'm wondering if there is a way to improve the infinity focus, the below was shot at roughly f5.5 with the 135mm to reduce star bloat (around a 10 image stack of 30s each, no calibration frames), but the focus is still off despite the lens being at it's infinity focus stop with the focusing ring. It could be the m42 to ef adaptor I have maybe too thick/thin (though I don't really see how much thinner/thicker it can be to make a difference), the lens is fully home against the front face of the ring and the ring is fully connected to the DSLR body.

Any suggestions or is this the best I can expect from such an old and wide-ish lens?

The image has been level stretched around 3 times, and the red channel brought down in line with the green and blue.

Edited May 17, 2022 by Elp

[DaveL59]

don't know for the Canon but for the Sony a-mount the thickness does make a difference, so I've read. Might be worth trying one from a different maker and see if it slightly thinner and helps give that extra shade of focus you're looking for?

[alacant]

The Takumar eos infinity is slightly displaced from the physical stop.

Assuming you have a well fitting M42 to eos adapter with the correct 1mm optical path but without a pin retaining flange: using a bright-ish white star at 10x live view,  move away from the hard stop until the red halo just disappears. 

If you're not going to use the lens for terrestrial photography, you can loosen the focus barrel, twist, and re-tighten to go way past where the physical stop allows. That way gives you a larger oscillation either side of infinity.

HTH

Edited May 19, 2022 by alacant

[Alien 13]

As Dave mentioned, I have also found some adapters can be a little off. I have noticed that on my 50 mm Takumar that the M42 thread stops just before the back plate face resulting in a tiny gap when screwed in, this was fixed by adding a small chamfer to the adapter with a dremil.

If all this fails then the adjustment screws (often hiding beneath the rubber grip if fitted) can be tweaked but you will have to search online to find which ones.

Alan

Edited May 17, 2022 by Alien 13

[Elp]

This is the adaptor I've got, the brass coloured insert is removable but checking the fitment the front face of the brass insert is level with the flange face when assembled. Looking at the rear, there's clearance for the lens to screw further in, but I assume it's not supposed to as the upstanding rectangular peg on the rear of the lens sits level with the back of the ring, and due to the front face of the ring sitting flush with the opposing face on the rear of the lens it's physically impossible for it to screw in any further without destructive mods.

[Elp]

4 hours ago, alacant said:

The Takumar eos infinity is slightly displaced from the physical stop.

Assuming you have a well fitting M42 to eos adapter, preferably without the flange: using a brightish white star at 10x live view,  move away from the hard stop until the red halo just disappears. 

If you're not going to use the lens for terrestrial photography, you can loosen the focus barrel, twist, and re-tighten to go way past where the physical stop allows. That way gives you a larger oscillation either side of infinity.

HTH

Can this be set back to how it was before I get my surgical implements out?

[alacant]

10 hours ago, Elp said:

the adaptor I've got

That's a flanged version which causes issues with the auto pin on the lens body and loses you that precious mm or so of infinity back focus. Best to use a properly machined non-flanged fully threaded version, forget about the pin and so gain valuable infinity focus manoeuvrability.

10 hours ago, Elp said:

Can this be set back to how it was before

Mark the position of the focus barrel -e.g. masking tape- relative to the lens body before you begin;)

Cheers

Edited May 18, 2022 by alacant
adaptador adecuado

[Elp]

Some useful suggestions overall, I'll try an unflanged adaptor first before making any mod to the focus ring.

[tompato]

Flange distance for Canon EF is 44mm and the Pentax M42 is 45.46mm so your adapter needs to be pretty thin, if yours measures over that then that might be why.

Not related to the issue directly, but I have a couple adapters from Urth and they've been good.

[DaveL59]

the downside of using the un-flanged adaptor is that there's nothing to push the aperture pin so you're stuck with fully open, unless your lens has a lever/switch to swap between auto-manual aperture control. If not then you can always remove the lens end plate and fit a short length of wire insulation over the pin to keep it depressed. I've done this on several of my M42 lenses (zeiss etc) to force manual aperture control, regular UK mains ring wire provided the insulation for those 😉 I prefer that to other mods I've seen where the pin is glued as this way is easier to reverse the mod.

[alacant]

1 hour ago, DaveL59 said:

there's nothing to push the aperture pin so you're stuck with fully open, unless your lens has a lever/switch

Hi

AFAIK, both the OP's lenses have the A -> M lever, hence the no-flange recommendation.

Cheers

[Elp]

31 minutes ago, alacant said:

Hi

AFAIK, both the OP's lenses have the A -> M lever, hence the no-flange recommendation.

Cheers

Yes, both have auto manual switch (though they're a bit tight, best leave it in manual). I've ordered a generic no flange adaptor and will see how it goes and will update whenever the clouds decide to shift.

My next thought is how much better will a Samyang 135mm f2 be as it's newer? The wider ones I've got are decent but I can't use them often due to how wide they are and the light pollution they also capture.

Edited May 18, 2022 by Elp

[tompato]

I'd have thought that the Samyang 135mm f2 will be superior.

Some old glass is very good, especially for photography where you generally don't need a massively flat field, but not so great at the corners.

Also the coatings have come a long way from back in the day, as I understand it.

Also also, the Samyang 135mm is a wonderfully sharp lens even at f2 so it must be packing some fine glass.

[Ratlet]

Had a look at my bahtinov focus attempt from my takumar 200 and I think it was just slightly off.  I opted to do the adjustment to infinity myself.  It's quite quick and painless.  Just don't over tighten the screws.

I marked the case lightly where the focus was originally so I can return to normal if required.  I suspect/hope that I'll keep using this lens for a while.

[Elp]

Update: got to try out my new rig 135mm Tak is on the 183mm Pro on the LHS, the 200mm Tak is on the Canon 600D RHS, with this combo their image scales match close enough where I can combine the data:

 

 

The 135mm f5.6/183mm luminence image (1 hour stack - level stretched around 3 times) with the old flanged m42 to eos adaptor ring I have, focus position is a little odd as its set at 5.75M rather than infinity:

 

 

 

The 200mm f5.6/Canon 600D set at infinity image (1 hour stack - level stretched around 3 times) using a flangeless m42 to eos adaptor ring:

 

 

The RGB image does suffer from red star bloat, when I tried combining the luminence on top of the RGB the outer stars have an offset ring around each of the luminence stars (like a hoola hoop):

 

 

Overall I think the lenses are quite good considering their age, need more time on a DSO to see how well they perform in greater detail. RGB imaging in general from my location is difficult.

Happy with my dual rig too which I'm going to use more often.

I know with this test I didn't try the 135mm with the Canon going back to the topic at hand, a trial for another night.

Edited May 27, 2022 by Elp

[alacant]

25 minutes ago, Elp said:

The RGB image does suffer from red star bloat

This is because the Takumar prescription focuses the blue and green to the same point, leaving the red out of focus. For this reason you cannot use a focusing mask.

The method for focusing is described here.

Nice shots.

HTH

Edited May 27, 2022 by alacant

[Elp]

Thanks, I don't use a bahtinov, I use the asiair focusing routine as it's easier to judge the minute focusing adjustments, but what you say makes sense.

[alacant]

13 minutes ago, Elp said:

the asiair focusing routine

Not sure if you can see colour using that, otherwise it's also going to leave you with red halos:(

Edited May 27, 2022 by alacant

[Elp]

It's in mono unfortunately. I can do what I otherwise do, take image previews and review each image after each focus adjustment.

[alacant]

On 27/05/2022 at 23:20, Elp said:

It's in mono

Pity. I think you'd probably get better compromised focus using the 600's focusing screen in real time.

Edited June 9, 2022 by alacant

[Elp]

Well, did the infinity focus ring adjustment on the 200mm, reds still there around stars, when I focussed up to remove the red rings a lot of the stars seemed to blur a bit, I felt a little red halo allowed for sharper stars, especially the smaller tiny ones. Overall happy with this considering its only 1 hour of exposure with all calibration files (flats, dark flats, bias, dark), the L-Extreme helped a lot. Makes me think if these lenses produce decent results do I really need the heavier Samyangs (I have a few)?

Now to figure out how to remove the banding artifacts...

[VectorQuantity]

Is this dithered? Are you able to?

18 minutes ago, Elp said:

Well, did the infinity focus ring adjustment on the 200mm, reds still there around stars, when I focussed up to remove the red rings a lot of the stars seemed to blur a bit, I felt a little red halo allowed for sharper stars, especially the smaller tiny ones. Overall happy with this considering its only 1 hour of exposure with all calibration files (flats, dark flats, bias, dark), the L-Extreme helped a lot. Makes me think if these lenses produce decent results do I really need the heavier Samyangs (I have a few)?

Now to figure out how to remove the banding artifacts...

Is this dithered?  Are you able to?

[Elp]

8 minutes ago, VectorQuantity said:

Is this dithered? Are you able to?

Is this dithered?  Are you able to?

Dithered every 5 by 5 pixels

[Ratlet]

What F stop was that at?  Stars look really good

 

[Elp]

One stop down from the lowest f4, so I think f5.6. The lextreme helps a lot to keep them small, taking 10 second previews when adjusting focus it looks like you're shooting through a narrowband filter (which it kind of is). The L-Enhance was also similar though not as light restrictive.