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Back focus on Astro Camera wiith Canon EF Mount DSLR Lens

ZWO ASI533MC Pro Equipment CMOS Help Needed
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Eric Gagne avatar
Eric Gagne avatar
Hello,

I want to buy a ZWO ASI533MC Pro which I will use with my Samyang 135mm lens for a while.   I have read so many posts and articles about back focus in forums and blogs that I am not sure anymore that my brain can digest all the information.

My understanding is that the Canon EF Mount has a back focus of 44mm, so that would be a requirement for my Samyan Lens,  and that the ZWO Camera has a back focus of 17.5mm so since 44 - 17.5 = 26.5 I would need to somehow make up for that difference with extenders, filter drawers, whatever I want to put between the two as long as it comes up to no more and no less than 26.5mm in length.

Did I get this right or did I just write a lot of stupidies in that last sentence smile ?
Engaging
andrea tasselli avatar
andrea tasselli avatar
Eric Gagne:
Hello,

I want to buy a ZWO ASI533MC Pro which I will use with my Samyang 135mm lens for a while.   I have read so many posts and articles about back focus in forums and blogs that I am not sure anymore that my brain can digest all the information.

My understanding is that the Canon EF Mount has a back focus of 44mm, so that would be a requirement for my Samyan Lens,  and that the ZWO Camera has a back focus of 17.5mm so since 44 - 17.5 = 26.5 I would need to somehow make up for that difference with extenders, filter drawers, whatever I want to put between the two as long as it comes up to no more and no less than 26.5mm in length.

Did I get this right or did I just write a lot of stupidies in that last sentence ?

You got that right. Except that, technically, if you use filters you need to add 1/3 of the filter thickness to that number.
Dan H. M. avatar
Dan H. M. avatar
You are generally correct, but the 533 has a backfocus of 6.5mm, not 17.5mm. You’d need to attach the included 11mm M42F-F ring to make it 17.5. From that point, you can add the ZWO Canon filter drawer or any combination of adapter to reach 44mm.  I myself am partial to the ZWO filter drawer. It’s a really nice piece of equipment in my opinion.
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Daniele Borsari avatar
Daniele Borsari avatar
I have the exact same setup and if you're planning to use the 533MC and Samyang 135mm for a while, there is the ZWO EF-T2 adapter and also the ZWO EF-T2 with filter drawer adapter. These adapters add to the system 26.5mm of backfocus, so you'll have 44mm of total backfocus (required by Canon EF lenses).

Daniele
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andrea tasselli avatar
andrea tasselli avatar
You are generally correct, but the 533 has a backfocus of 6.5mm, not 17.5mm. You’d need to attach the included 11mm M42F-F ring to make it 17.5. From that point, you can add the ZWO Canon filter drawer or any combination of adapter to reach 44mm.  I myself am partial to the ZWO filter drawer. It’s a really nice piece of equipment in my opinion.

Only if you remove the included M48 to M42 ring. If you keep it than the math of the OP is right.
Daniele Borsari avatar
Daniele Borsari avatar
andrea tasselli:
if you use filters you need to add 1/3 of the filter thickness to that number


Actually, with my setup if I added 1/3 of the filter thickness to the backfocus I would have had the Red channel slightly out of focus. With the past full moon I experimented a bit and i finally got reasonable results.

Daniele
Eric Gagne avatar
Eric Gagne avatar
Daniele Borsari:
I have the exact same setup and if you're planning to use the 533MC and Samyang 135mm for a while, there is the ZWO EF-T2 adapter and also the ZWO EF-T2 with filter drawer adapter. These adapters add to the system 26.5mm of backfocus, so you'll have 44mm of total backfocus (required by Canon EF lenses).

Daniele

I have been wondering about this.  I think I can either use the filter drawer and get 2" filters or use the EF-T2 adapter and use 1.25" filters which (I think) can be screwed in the 11mm ring that comes with the camera.

I am just not sure about the length of the EF-T2.  ZWO's web site says it's two parts, one being 5mm but they say nothing about the 2nd part.  Is it 21.5mm to make 26.5mm total ?

Thinking about it though, I currently use an Optolong L-Enhance (clip-in) with my Canon DSLR, I want one with the new camera too but I also need another filter for shooting galaxies so I guess I would be better off wich the filter drawer if I want to be able to change filters easily.
Daniele Borsari avatar
Daniele Borsari avatar
@Eric Gagne Without the 5mm extender (which is needed for the ASI120 ASI224 ASI290 ASI178 ASI385 ASI462) you reach the right backfocus for the 533MC. Remember that the simple adpater without filter drawer let's you screw 2" filters, but it's not that easy if you keep changing them.
Eric Gagne avatar
Eric Gagne avatar
So it's going to be the filter drawer with 2" filters then.  

Thx everybody, I really appreciate all the help I am getting on this forum.
Eric Gagne avatar
Eric Gagne avatar


Somehow I completely messed up this post and can't figure out how to fix it so I apologize for the empty quote, I can't remove it.

I was asking about the filter and the 1/3 of its thickness to add to the back focus.  Would that be 1/3 of the outside dimensions of the filter or 1/3 of the glass thickness ?

Most of these seem to have a glass that is 1.85mm thick.  How would one add 0.6167mm ?
Daniele Borsari avatar
Daniele Borsari avatar
It is 1/3 of the glass thickness.
To add 0.6mm of backfocus there are little shims, both in the adapter and camera packages, that can add from 0.1mm to 0.6mm of backfocus.
As I said before, in my case, adding 0.6mm gave worse results.


​​​​​​Daniele
andrea tasselli avatar
andrea tasselli avatar
The 1/3 of thickness rule isn't a rule; is an approximation. The actual value depends on the glass refractive index (which in itself is a function of light wavelength).
Eric Gagne avatar
Eric Gagne avatar
So I changed my mind at the last minute and went with the T2 adapter and the 1.25” filter.  I got it today and I’m very happy with my choice….thx to everyone who helped me understand what I had to do.

How cool is astrophotography, not only do we take pictures of beautiful DSO objects but we get to do it with beautiful equipment.

Eric Gagne avatar
Eric Gagne avatar
After a month without clear sky I am finally getting to try my new ASI533 tonight.  I only took a few shots so far but I am quite happy with the results.  This is a 5 minutes exposure with a Samyang 135.

Well Written Engaging