spacing between ASI1600 camera and Samyang 14 mm lens

[alcol620]

Hi folks. All sorts of confusion here. I have just obtained the Samyang 14 mm f2.8 lens and at first light obtained images with really bad star distortion, all 4 corners and halfway to the centre of the images. The lens is connected with the EOS-T2 Adapter and the 11mm T2 extender to the camera body.

Can someone tell me what the distance has to be between the lens and the ASI1600 body?? The more I read the more confused I become.

Thanks

[Adreneline]

I can't speak specifically about the 14mm lens but I have exactly the same arrangement with a 135mm Samyang with EoS fitting. I worked on the basis that the distance from the end of the lens (the silver ring on your photo) to the sensor (for all Canon cameras) is 55mm; that distance needs to change if you introduce a filter into the imaging path (1/3 of the thickness of the filter?).  I think the 1600 back focus is 6mm so you need 49mm of spacer. I am more than happy with star shapes on my images and no one has ever made comment about star shapes on images I have posted on this forum.

HTH

Adrian

[alcol620]

17 minutes ago, Adreneline said:

I can't speak specifically about the 14mm lens but I have exactly the same arrangement with a 135mm Samyang with EoS fitting. I worked on the basis that the distance from the end of the lens (the silver ring on your photo) to the sensor (for all Canon cameras) is 55mm; that distance needs to change if you introduce a filter into the imaging path (1/3 of the thickness of the filter?).  I think the 1600 back focus is 6mm so you need 49mm of spacer. I am more than happy with star shapes on my images and no one has ever made comment about star shapes on images I have posted on this forum.

HTH

Adrian

Thanks Adrian, could be the reason for such bad images. I only have the 11 mm T-extender and the EOS-T2 adapter totalling 34 mm, then 6.5 mm ASI back focus, thus requiring a further 55 - 40.4 = 14.6 mm.  Does this seem correct, you must have the same or similar?

I took my info off the ZWO site that suggested that the 11mm T-extender and the EOS-T2 adapter was all that was required!

Thanks

Alec

 

[Adreneline]

It is all very confusing! I agree with your calculation that you require another 14mm. I have a filter holder in the image train so I have a lens end plate to sensor space of ~56mm. I cannot measure the spacing that accurately - at best I'm within 0.5mm - and even that might be wishful thinking.

Good luck!

Adrian

[alcol620]

1 minute ago, Adreneline said:

It is all very confusing! I agree with your calculation that you require another 14mm. I have a filter holder in the image train so I have a lens end plate to sensor space of ~56mm. I cannot measure the spacing that accurately - at best I'm within 0.5mm - and even that might be wishful thinking.

Good luck!

Adrian

Thanks Adrian very much appreciated. Now need to get hold of the required T2 spacer. Will put in an oversize spacer tonight and see what happens, then get the correct one during the week

Thanks again, will update in due course

Best wishes Alec

[Adreneline]

24 minutes ago, alcol620 said:

Now need to get hold of the required T2 spacer.

Hi Alec,

i've just been checking my spacing rather than going by memory and my memory is not what it should be!

I'v just meansured the end of lens to sensor on my Canon 70D and it is 45mm not 55mm - sorry for the mistake. It is 55mm from the end of my reducer/flattener on the ZS71 and ED80 to the sensor. So 45mm is the spacing you are looking for.

Age is a terrible thing when it comes to memory.

Apologies again.

Adrian

[alcol620]

No problem Adrian. I have a feeling that the ASI1600 I have requires a distance of 55mm. Presumably your's is a Canon?

[Adreneline]

30 minutes ago, alcol620 said:

I have a feeling that the ASI1600 I have requires a distance of 55mm

Hi Alec,

I've removed my setup from the mount to show the spacing I have between my Samyang 135mm and my Atik428ex.

The back-focus on the Atik is 13mm (the red tape shows where the sensor is in the body of the camera). There is a 2" Ha filter in the holder so the spacing from the end of the lens to the sensor is ~46mm.

Sorry again for the confusion I caused but this spacing works well for me.

HTH

Adrian

[alcol620]

Thanks Adrian, appreciate you taking time out to provide the info. I will up date when I get it sorted.

Take care Alec

[melsky]

With camera lenses it’s fairly simple, set the lens to its infinity mark and take a shot if it’s in focus the spacing is correct, If the lens is at anything other than infinity you could have problems because moden lenses use internal floating elements that change their spacing for different distances. Wide angle lenses are particularly critical for this, I have the Sigma Art 14mm F1.8 which I use via a Sigma adapter on a Sony A7S and it works brilliantly, I initially was using a Commlite adapter and getting terrible coma so much so I thought I had a faulty lens (still showed it at F8) after contacting Sigma they of course suggested using their adapter which cost £130, I was sceptical because both are just tubes with no optics but it cured the problem. I measured the tubes to see what the difference was between them, 0.2mm

[michael8554]

Canon EF-S and EF lens backfocus is 44mm, not 45mm

So a 11mm T2 Adapter would give you that 55mm figure you thought of.

Michael

[alcol620]

Sorry guys, many apologies, but I'm still a bit confused. Can someone simply tell me the distance requirement from the ASI sensor and the end of the lens?  ASI camera has a back focus of 6.5mm.

Sorry for being thick, I put it down to my age.

[Laurin Dave]

If the lens has a Canon fit you need 44mm between the back of the lens (ie the mounting plate) and the sensor..  ZWO do a number of different EOS camera adapters and spacers and there are diagrams on their website showing how they fit together for correct back focus..  If it's Nikon fit  then its 46.5mm 

[alcol620]

thanks Laurin much appreciated, this distance seems to be far more importent than i thought

[Laurin Dave]

Yep... it’s critical for being able to get both focus and star shape.

Dave