How to use Samsung 135mm F2 lens

[Jkulin]

Hi Carole, not sure if this helps but if you were thinking of going to the PAS I could bring my kit as I have a 135mm and spend an hour or so showing you the basics?

[carastro]

Quote

Stopping down is reducing the physical size of the aperture (which means going to a larger f-stop number). The f-stop is the ratio of the focal length to the physical aperture size. E.g. 135mm focal length at f/2 is 135/2=67.5mm aperture. Stopping down to f/4 would result in the lens having a 135/4=33.75mm aperture.

Thanks for that I have copied it into my useful information file.  Some of the rest of the info was a bit over my head, though I do understand the Inverse square law thingy as it's the same for telescope imaging.

Quote

Hi Carole, not sure if this helps but if you were thinking of going to the PAS I could bring my kit as I have a 135mm and spend an hour or so showing you the basics?

Thanks for the offer and would be nice to meet you, but what is the PAS?  Sounds like an Astronomy show. 

Carole 

[ApophisAstros]

1 hour ago, carastro said:

Thanks for that I have copied it into my useful information file.  Some of the rest of the info was a bit over my head, though I do understand the Inverse square law thingy as it's the same for telescope imaging.

Thanks for the offer and would be nice to meet you, but what is the PAS?  Sounds like an Astronomy show. 

Carole 

https://practicalastroshow.com/

DSC weekend and its near Leicester?

Roger

 

[carastro]

Quote

https://practicalastroshow.com/

DSC weekend and its near Leicester?

Yes as Roger says we are going to a star party that weekend.

Carole

[carastro]

Can I get an understanding of the back focus of this lens. Is this the distance from the base of the lens to the sensor?  (is it the very rear end of the camera lens) 

My Camera Atik460 = 13mm back focus and the EFW = 22mm = 35mm 

I think I read somewhere that the back focus for this camera is 44mm so I need a further 9mm is that correct?  

With the Geoptik adapter I can't get closer than about 46mm because the foot of the adapter fouls the EFW when trying to screw it on and can only fit it with a small adapter.  Have tried to remove the foot but it won't budge. 

Carole 

Edited February 29, 2020 by carastro

[geordie85]

56 minutes ago, carastro said:

Can I get an understanding of the back focus of this lens. Is this the distance from the base of the lens to the sensor?  (is it the very rear end of the camera lens) 

My Camera Atik460 = 13mm back focus and the EFW = 22mm = 35mm 

I think I read somewhere that the back focus for this camera is 44mm so I need a further 9mm is that correct?  

With the Geoptik adapter I can't get closer than about 46mm because the foot of the adapter fouls the EFW when trying to screw it on and can only fit it with a small adapter.  Have tried to remove the foot but it won't budge. 

Carole 

Have you thought about using one of these? 

https://www.firstlightoptics.com/adapters/astro-essentials-samyang-lens-to-m48-adapter.html

[carastro]

Quote

Have you thought about using one of these? 

https://www.firstlightoptics.com/adapters/astro-essentials-samyang-lens-to-m48-adapter.html

I have seen it, but first of all my camera is an Atik460Ex and this seems to be made for a ZWO camera, I am rubbish with threads and no idea if it will fit and knowing what the threads are.   2ndly I believe you have to remove the bayonet from the lens and I have DIY skills that are likely to mess it up!!!! 

Finally I might want to use my DSLR with the lens sometimes i.e. going on holiday.  

Plus would this distance be right for my camera anyway. 

Many years as a telescope imager, but technical stuff is not my forte.

Carole   

Edited February 29, 2020 by carastro

[Adreneline]

1 hour ago, carastro said:

I think I read somewhere that the back focus for this camera is 44mm

The distance from my Samyang end plate to the camera sensor is 46.25mm and that takes account of the 1.25" Baader filters I use.

I currently use the lens with a ASI1600 (6.5mm backfocus) but I have also used it with difficulty with an Atik428ex (13mm backfocus).

This site gives some useful info:

"So the first thing you need to do is check to see if your lens has an ∞ symbol printed on the barrel. Some lenses actually have an additional marking that tells you how far you need to go backwards after that hard stop in order to focus to infinity. This is typically shaped like a sideways “L.” If you line up the distance indicator (a vertical line) with the infinity sign, you won’t get the correct focus. If, however, you line it up with short vertical part of the “L,” you’ll be focused to infinity. Theoretically. Remember that if you’re off even a little it could mean the difference between tack-sharp stars and pancake shaped stars, so you’ll need to double check using the method below."

My lens focusses here:

Edited February 29, 2020 by Adreneline
Clarification

[sploo]

2 hours ago, carastro said:

Can I get an understanding of the back focus of this lens. Is this the distance from the base of the lens to the sensor?  (is it the very rear end of the camera lens) 

My Camera Atik460 = 13mm back focus and the EFW = 22mm = 35mm 

I think I read somewhere that the back focus for this camera is 44mm so I need a further 9mm is that correct?  

With the Geoptik adapter I can't get closer than about 46mm because the foot of the adapter fouls the EFW when trying to screw it on and can only fit it with a small adapter.  Have tried to remove the foot but it won't budge. 

Carole 

Canon EF has a 44mm registration distance (https://en.m.wikipedia.org/wiki/Flange_focal_distance) so if this is a lens with an EF mount then the distance from the rear face of the lens flange surface to the sensor plane does indeed need to be 44mm.

[carastro]

Quote

 I have also used it with difficulty with an Atik428ex (13mm backfocus).

Thanks Adrian,  I have both an Atik460EX and an Atik428EX (dual dig at times) both with a backfocus of 13mm.

When you say with difficulty, what difficulty do you have?  

Carole 

[Adreneline]

15 minutes ago, carastro said:

what difficulty do you have?

Hi Carole,

I found it very difficult to get a combination of spacers that allowed me to get the 'infinity' focus position in the right place, i.e. within the bounds of the 'L' mark on the lens focus ring. In the end I had to take a 5mm spacer and grind it down using a belt sander (! - nearly ground my fingers away at the same time!) until it was just a shade over 4mm. I had a 3mm ring spacer but that wasn't enough - a 5mm spacer was too much. My experience is that the spacing is super critical. In my current setup I have 0.75mm of spacer between the EFW and the ZWO 16.5mm spacer; if I remove the 0.75mm the focus point is before the 'L' on the focus ring on the lens.

I know the quoted spacing is 44mm but my experience with this lens, filter wheels with filters and astro cameras is that 44mm is a good starting point but it will not give an optimum focus position within the 'L' mark. I had exactly the same experience with the 85mm lens that I used to demostrate the effect of gravity!! I also understand there is a tolerance on the 13mm quoted on the camera but I am not confident to quote the figures I have seen.

Theory is one thing but as with most things in this game it is careful experimentation that generally gives the best solution - and that coming from a theoretical physicist! I should go wash my mouth out.

Adrian

 

Edited February 29, 2020 by Adreneline

[carastro]

Whilst I know that the L mark is for infinity, isn't this just a guide as focus surely could be slightly one way or another.  

Quote

it will not give an optimum focus position within the 'L' mark.

I am just wondering why it is so critical to get focus on part of the L, surely focus is focus, and the spacing is in order to get a flat field, or am I not understanding something vital here?

Carole 

[happy-kat]

is it not possible to use home cut rings of say acetate between the spacers to pack thread when looking for minute adjustments?

[Adreneline]

3 minutes ago, carastro said:

I am just wondering why it is so critical to get focus on part of the L, surely focus is focus, and the spacing is in order to get a flat field, or am I not understanding something vital here?

Without my 0.75mm of spacers the 'infinity focus' position was before the 'L' mark and I had significant star shape problems in the extreme corners of the image with very noticeable 'coma' type star shapes in the top right and bottom left of the image. If I add another 0.25mm spacer the focus point moves beyond the 'L' mark.

I cannot be certain my current spacing is 'perfect' but the star shapes are acceptable (to me). On modern lenses the lens can be set to focus beyond infinity - although I am not sure exactly what that means.

As a final complication the focus point is different for all my filters. Using the ZWO EAF as a reference the focus position varies from 28302 to 28382. Now I've not done the maths to work out exactly how much that has moved the focus ring on the lens but I do know that even at the extremes it still stays within the 'L' so I am happy I've got it about as good as I am likely to get it without becoming obsessively compulsive about it.

I hope this helps.

Adrian

P.S. As a footnote I spent two entire evenings stripping the camera and lens apart and messing around with an array of spacers before I got to where I am now. I'm in no rush to change anything! Good luck!

 

[Adreneline]

14 minutes ago, happy-kat said:

is it not possible to use home cut rings of say acetate between the spacers to pack thread when looking for minute adjustments?

I am sure it is but when you buy a ZWO camera it comes with a whole bag of shim type spacers as well as various metallic spacers with various threads - unlike an Atik camera which in my experience comes with nothing!

Adrian

[Gina]

I've been very pleased with the ZWO ASI1600MM-Cool cameras since upgrading from Atik 460EX.

[wxsatuser]

12 hours ago, carastro said:

Whilst I know that the L mark is for infinity, isn't this just a guide as focus surely could be slightly one way or another.  

I am just wondering why it is so critical to get focus on part of the L, surely focus is focus, and the spacing is in order to get a flat field, or am I not understanding something vital here?

Carole 

The vertical part of the L is infinity the horizontal part is compensation for temperature.

Wether the mark is actually at infinity is debatable and depends how well it was calibrated at the factory.

My 135mm is more towards the 10metre mark which threw me when first setup.

[carastro]

Thanks, wow that seems a long way off.

Carole 

[Gina]

This Samsung lens would appear to move lens groups relative to each other and only produce a flat field, properly corrected across the entire field, when both internal distances and the back focus is right. 

The lenses I use which were produced by the Asahi Company of Japan move the entire lens set to focus - there is no internal relative movement between lens groups.  This make setting up simple - slightly wrong back-focus is corrected by focussing.  The 135mm lens I use is smaller aperture of f2.5 rather than f2 so the Samsung passes nearly twice the amount of light.

For those interested, these Asahi lenses were produced for Pentax cameras and with versions of Takumar, Super Takumar and Super Multi-Coated Takumar (SMC Takumar) as lens coatings progressed.  They come in two mount versions M42 1mm pitch thread and (later) bayonet mount.  The M42 thread is slightly different pitch from T2 which is 0.75.  A so-called "Russian Adapter" converter can be bought for a few pounds, though in most cases the Pentax thread will engage several turns in T2.  Back-focus of the thread mount lenses is 45mm.  These lenses are available from that well known auction site at prices ranging from around £50 to £100.  Focal lengths in general go from 28mm to 200mm and occasionally 300mm telephoto lenses are available.  I'm talking prime lenses here - I don't use zoom lenses for astro imaging

Edited March 1, 2020 by Gina

[Albut]

On 26/02/2020 at 13:10, drjolo said:

I think there are two things worth to mention about this lens.

One is aperture: it may happen that you will get little bit better image quality when you stop it down a little. But then bright stars will get spikes that comes from aperture blades. Some users (including me) attach "aperture stop ring" in the front of this lens to stop it a little and keep circular aperture shape.

Second one is mounting to camera. This connection needs to be stiff, because that is fast optics and do not forgive tilt or bending. And also backfocus distance needs to be kept, so you focus on the stars as close to infinity mark at the focusing ring as possible. This lens has internal focusing, so it is designed to have specific lens configuration at specific object distance. In my setup the ring is set about 5mm left to that mark when I focus at stars. And then I have pinpoint stars in the whole frame, but I have small chip - QHY163M. And SY135 is full frame lens. 

Then I think you should be very happy about this lens Here are my thoughts after (now) three years of having it https://astrojolo.com/gears/thirty-months-with-samyang-135-f-2/ 

Is my Samyang 135mm faulty? Just not enough travel to reach infinity focus. I have asi183mc pro + the supplied 11mm spacer, and the Zwo canon filter drawer 26.5mm. With the 6.5mm built into the camera this makes it 6.5mm+11mm+26.5mm=44mm. I have also measured it and it’s bang on. The only way I can achieve infinity focus is by putting a 2mm thick filter in the drawer, which obviously adds by its refraction.

[Albut]

On 29/02/2020 at 21:19, Adreneline said:

Without my 0.75mm of spacers the 'infinity focus' position was before the 'L' mark and I had significant star shape problems in the extreme corners of the image with very noticeable 'coma' type star shapes in the top right and bottom left of the image. If I add another 0.25mm spacer the focus point moves beyond the 'L' mark.

I cannot be certain my current spacing is 'perfect' but the star shapes are acceptable (to me). On modern lenses the lens can be set to focus beyond infinity - although I am not sure exactly what that means.

As a final complication the focus point is different for all my filters. Using the ZWO EAF as a reference the focus position varies from 28302 to 28382. Now I've not done the maths to work out exactly how much that has moved the focus ring on the lens but I do know that even at the extremes it still stays within the 'L' so I am happy I've got it about as good as I am likely to get it without becoming obsessively compulsive about it.

I hope this helps.

Adrian

P.S. As a footnote I spent two entire evenings stripping the camera and lens apart and messing around with an array of spacers before I got to where I am now. I'm in no rush to change anything! Good luck!

 

I find adding shims or increasing the distance makes it worse with my samyang 135mm. Just not enough travel to reach infinity focus. I have asi183mc pro + the supplied 11mm spacer, and the Zwo canon filter drawer 26.5mm. With the 6.5mm built into the camera this makes it 6.5mm+11mm+26.5mm=44mm. I have also measured it and it’s bang on. The only way I can achieve infinity focus is by putting a 2mm thick filter in the drawer, which obviously adds by its refraction.

[Adreneline]

1 hour ago, Albut said:

I find adding shims or increasing the distance makes it worse with my samyang 135mm.

This is very frustrating for you - it can be a very frustrating lens to set up and I can only hope your lens is not defective.

To illustrate the point I decided to swap my ASI183MM for my ASI1600MM on the Samyang last night. I made the not unreasonable assumption that the sensor position would be the same on both lenses so I just took the 183 off and screwed the 1600 on to the EFW. My preview image was very out of focus - blobby stars! I found I could not achieve focus at all - I was getting close but the lens was up against the end-stop. I decided to remove the 0.5mm spacer between the EFW and the camera (used purely to get the orientation of the sensor so it was orthogonal to the Vixen bar - in 'landscape'). Removing the 0.5mm spacer allowed me to achieve focus but with the focus mark approx 3mm before the 'L'. After much messing around in the dark at midnight I finally achieved focus with 0.3mm of spacing - the lens focussed on L, Ha, OIII and SII all within the confines of the 'L'. I can but assume the sensor position on the 183 is different to the 1600, i.e. the back-focus is not 6.5mm - I assume it 6.7mm. I read somewhere that ZWO state the sensor is positioned to with ±0.5mm - that is a lot when used with the Samyang 135mm (and have seen the same quoted on CN). I suppose the back-focus could be anywhere between 6.00 and 7.00 mm - again a huge range when trying to set up the Samyang.

I can only suggest to persevere but bear in mind the 44mm figure is purely a starting point that will be affected by filter thickness and sensor position. The gap between my ASI1600MM camera front face and lens end plate is 38.73mm, so the spacing is (38.73+6.5) = 45.23 (±0.5mm cos' I don't really know where the sensor is!). Finding the right spacing for your setup is based purely on trial and error and a range of 1mm for the sensor position is a real challenge for the Samyang. (I also read somewhere that 'top-end' dslr manufacturers place the sensor to within ±0.01mm of the flange plate interface with the lens).

Sorry I cannot be of more help.

Good luck!

Adrian

 

Edited July 8, 2023 by Adreneline

[Adreneline]

3 minutes ago, Adreneline said:

To illustrate the point I decided to swap my ASI183MM for my ASI1600MM on the Samyang last night

This was a single 300s Ha frame from last night to check image train alignment - I am pretty pleased with this:

... and the result of stacking 10 frames in APP:

.. all cut short by the early arrival of clouds!

Adrian

 

[Peds]

On 08/07/2023 at 10:53, Albut said:

Is my Samyang 135mm faulty? Just not enough travel to reach infinity focus. I have asi183mc pro + the supplied 11mm spacer, and the Zwo canon filter drawer 26.5mm. With the 6.5mm built into the camera this makes it 6.5mm+11mm+26.5mm=44mm. I have also measured it and it’s bang on. The only way I can achieve infinity focus is by putting a 2mm thick filter in the drawer, which obviously adds by its refraction.

I don't have one myself but have heard about this issue and how to fix it here:

 

 

 

[900SL]

On 08/07/2023 at 12:53, Albut said:

Is my Samyang 135mm faulty? Just not enough travel to reach infinity focus. I have asi183mc pro + the supplied 11mm spacer, and the Zwo canon filter drawer 26.5mm. With the 6.5mm built into the camera this makes it 6.5mm+11mm+26.5mm=44mm. I have also measured it and it’s bang on. The only way I can achieve infinity focus is by putting a 2mm thick filter in the drawer, which obviously adds by its refraction.

I thought adding a filter increased the backfocus? Are you sure you cannot reach focus without the filter?

It worked the other way round for me... I could reach backfocus with the 533MC and appropriate spacers, but when I added an NBZ filter I couldn't reach focus, so had to add a 1mm spacer. The focal position is very sensitive to backfocus.