As part of attempting to be more precise in matching my Canon EF200 lens to an ASI6200 camera, I've been puzzling about the effect of the UV/IR cut off filter in the DSLR body. I've developed a working theory on this and want to test it here if I may.
The Canon EF200 EOS lens has a stated set back distance of 44.0mm. This is with the lens mounted on the camera body. So when we go to mount this on to an astronomy CCD or CMOS camera, we target the set back distance as 44.0mm. However, I am now wondering if this is correct because it ignores the possible effect of the UV/IR cutoff filter in the DSLR.
I am going to make a couple of assumptions here. The first one is that the UV/IR cutoff filter will affect the length of the optical path in the same way as the filters that we use for monochrome imaging. So if this assumption is correct then, based on traditional rules of thumb, the length of the optical path will be lengthened by an amount equal to 1/3 of the thickness of the glass in the UV/IR cutoff filter.
I have been unable to find any specifications for the thickness of the glass in the UV/IR cutoff filters so the second assumption that I am going to make is that they are 1mm thick. The increase in the length of the optical path due to the filter then is 1/3 of 1mm or 0.33mm. If anyone has any detailed knowledge of this please share.
So then, if the EOS lens is mounted on a DSLR body, the UV/IR filter is adding in an extra .33mm. If we then remove the EOS lens from the DSLR body to use it on another camera, the actual set back distance expected by the lens is 44.00mm - 0.33 mm = 43.67mm.
So, therefore, if I am using my EOS lens on a CCD or CMOS camera, my target set back distance should be 43.67mm NOT 44.0mm.
I would be happy if others could pull apart my assumptions and analysis above and comment accordingly.
I am going to make a couple of assumptions here. The first one is that the UV/IR cutoff filter will affect the length of the optical path in the same way as the filters that we use for monochrome imaging. So if this assumption is correct then, based on traditional rules of thumb, the length of the optical path will be lengthened by an amount equal to 1/3 of the thickness of the glass in the UV/IR cutoff filter.
I have been unable to find any specifications for the thickness of the glass in the UV/IR cutoff filters so the second assumption that I am going to make is that they are 1mm thick. The increase in the length of the optical path due to the filter then is 1/3 of 1mm or 0.33mm. If anyone has any detailed knowledge of this please share.
Hi R,
As an exercise of the grey matter, alegbra, trig, optics/physics etc... I decided to flesh out what I thought was the basis for the 1/3rd thickness of the filter rule of thumb....
It looks like it's quite a valid supposition if you take the refractive indices involved as ~1 for air (1.0003) and ~1.5 for the glass (this could be anywhere from 1.4 to 1.7 depending on glass used). Anyway if you're interested in the derivation of the difference in focal length of an optic WITH and WITHOUT an intermediary optical layer (glass) just completed, then see the attached.
Long story short .... SEE THE RED BOX. If you substitute typical values for refractive indices of air and glass of 1 and 1.5 respectively you will note that the focal point shifts backwards from POINT A in the diagram to POINT B in the diagram by a distance of 1/3rd the filter thickness, when the filter is included in the optical path. Given by the derived expression ...
b = x [1 -(n1/n2)]
where
b = distance focal point moves with and without filter (see diagram)
x = glass / filter thickness
n1 = refractive index of medium before filter (i.e: that of the air)
n2 = refractive index of the filter medium (i.e: that of the glass)
As an exercise of the grey matter I decided to flesh out what I thought was the basis for the 1/3rd thickness of the filter rule of thumb....
It looks like it's quite a valid supposition if you take the refractive indices involved as ~1 for air (1.0003) and ~1.5 for the glass (this could be anywhere from 1.4 to 1.7 depending on glass used). Anyway if you're interested in the derivation of the difference in focal length of an optic WITH and WITHOUT an intermediary optical layer (glass) just completed, then see the attached.
Long story short .... SEE THE RED BOX. If you substitute typical values for refractive indices of air and glass of 1 and 1.5 respectively you will note that the focal point shifts backwards from POINT A in the diagram to POINT B in the diagram by a distance of 1/3rd the filter thickness, when the filter is included in the optical path. Given by the derived expression ...
b = x [1 -(n1/n2)]
where
b = distance focal point moves with and without filter (see diagram)
x = glass / filter thickness
n1 = refractive index of medium before filter (i.e: that of the air)
n2 = refractive index of the filter medium (i.e: that of the glass)
Yes R.
I had to reformat it due to size. it's now attached. Over and above any such assessment in choosing the right flange focal distance I would try using something like an adjustable spacer or photographic Canon bellows and fine tune to the exact distance and then get the exact spacer, using 44mm as your starting point. Or simply try the ZWO to EOS adapters if (??) they accommodate FF sensors.
Yes R.
I had to reformat it due to size. it's now attached. Over and above any such assessment in choosing the right flange focal distance I would try using something like an adjustable spacer or photographic Canon bellows and fine tune to the exact distance and then get the exact spacer, using 44mm as your starting point. Or simply try the ZWO to EOS adapters if (??) they accommodate FF sensors.
Best
JA
Thanks, given that there is a piece of glass within the DSLR body between the lens and the sensor, then it is a reasonable conclusion that this will push back the optical path further away from the lens. My other critical assumption is the thickness of this piece of glass which I am currently basing on 1mm. This is a pure guess on my part so I will see if anyone has a better figure.
I am using the Astro Mechanics adapter to match the EF200 lens to my ASI6200 camera. The Astro Mechanics adapter is adjustable for spacing and tilt which is perfect for the job. I found however that I am having to wind back the spacing quite a lot more than I calculated and was chasing down reasons for this.
One of the reasons was that Astronomik originally had information on their web site which gave an incorrect value for the optical thickness of their filters of 1mm where in fact it is .33mm. See this post here for a discussion on this. Allowing for the additional effect of the actual set back distance required for the EOS lens helps further.
I'm still fine tuning the lens set back distance and I probably now down to the level of variations due to the manufacturing tolerances of the various components in the image train. At some stage soon I will (must) stop fiddling and leave it alone
The position of the sensor inside the camera can be moved.
The sensor position is therefore set to match the optical length including the built-in filters.
When the Canon is modified ie removal of filters (either one or both) requires the position of the sensor to be changed or additional filters re-installed.
This is well known and documented on the web.
The position of the sensor inside the camera can be moved.
The sensor position is therefore set to match the optical length including the built-in filters.
When the Canon is modified ie removal of filters (either one or both) requires the position of the sensor to be changed or additional filters re-installed.
This is well known and documented on the web.
Thanks Ken. Just for the avoidance of doubt, I'm not looking at modifying a DSLR. My intent is to look at the correct set back distance for an EOS lens when it is not being used on its 'native' DSLR body. Having said that, I believe your observation confirms my hypothesis that the glass in the filters in front of the sensors affect the optical length between the lens and the sensor. The big question is, how much does it push back the set back distance.
Yes Ken, that's certainly what one would expect it to be, but expectation and reality may not meet perfectly. Rodney mentioned he had issues with getting things perfect with standard values earlier in the thread. Measuring the distance will remove any doubt.
Actually Rodney, that begs another question.... What was the spacer adapter thickness when you realised those concerns. Does that thickness together with the ZWO 6200 flange focal distance give you the nominal 44mm?
I’ve used Canon lenses on various CCD camera, with suitable spacers.
From memory no issues.
What’s the real problem you face?
Hi Ken,
I'm not sure if you're asking me or Rodney, but I'm not facing any problems with such, I'm responding to Rodney's concerns and his quest for more information.
I’ve used Canon lenses on various CCD camera, with suitable spacers.
From memory no issues.
What’s the real problem you face?
This enquiry Ken is as much academic driven by curiosity but with a practical aspect as well. It can be informative at times to follow theoretical queries purely for just wanting to know the answer. In my experience the knowledge gained will help us down the track somewhere
@JA - My calculated adapter spacing for my set up is 11.5mm but I find that I need to reduce the adapter spacing to about 10.5mm to get a reasonably flat field. This is taking into account the sensor set back in the ASI6200 (12.5mm) and the ZWO 2” filter wheel (20.0mm).
There is no problem as such in that I can adjust the adapter spacing to get the result that I want. I do how ever get curious when the theoretical values differ from the results in practice and I start looking for answers.
I know I'm getting down into the minutia here Ken but I'm going to argue that the answer is not as definitive as that. Yes, the set back distance is 44.0mm for a Canon EOS lens when installed on a Canon DSLR body. However, if my calculations are correct, the set back distance I should be using when I connect the lens to a CCD or CMOS camera should be 43.67mm. I realise that I'm only talking about .33 of a millimeter but it can, and will, make a difference.
Specifically as it relates to the components in my image train, we have the following:
ZWO ASI6200. The sensor is set back 12.5mm
ZWO 2" EFW = 20.0mm
Astronomik 50mm filters - optical thickness = 0.33mm
(The Astronomik filters have a glass thickness of 1mm for an optical thickness of 0.33).
If I take the target set back distance to be 43.67mm for the Canon EOS lens, then to calculate the adapter length I have:
43.67-((12.5+20.00)-.33) = 11.5mm.
If I were to use the 44.0mm figure as my target set back distance then I would get an adapter size of 11.83mm. Not much but can make a difference.
Last edited by Ryderscope; 02-02-2022 at 11:14 PM.
Rodney,
You’re over thinking things.
The back focus of the Canon lenses is 44mm.
Nothing to do with the secondary filters etc inside the Canon body.
With the removal of filters the focus is still 44mm.
(I have to add internal filters - in my case clear, to allow the camera lenses to give infinity focus on a fully moded camera.)
Rodney,
You’re over thinking things. Possibly, but I would still like to explore the issue.
The back focus of the Canon lenses is 44mm. According to my analysis above, only when mounted on the DSLR body
Nothing to do with the secondary filters etc inside the Canon body. See previous answer
With the removal of filters the focus is still 44mm. see previous answer
(I have to add internal filters - in my case clear, to allow the camera lenses to give infinity focus on a fully moded camera.) This sounds like you are confirming that the filter is changing the length of the light path
Ken,
My starting positing in this thread was to present my analysis which concluded that moving a lens from a Canon DSLR body to a CCD or CMOS camera will mean that the stated 44mm set back distance does not apply. I have provided details of my analysis above. The reason for starting this thread was to test my analysis and subject it to peer review. I'm more than happy to be proved wrong but so far though, nothing has been presented that conflicts with my conclusions.
I have attached a diagram which provides a pictorial representation of my theory. Placing a filter in the light path must alter the point at which the light comes to focus because glass has a different refractive index to air and, therefore, the optical length of the light path will change. The response from JA above is consistent with this position. If there is something wrong with this analysis Ken please point out where it is.
Your post #6 above states: The sensor position is therefore set to match the optical length including the built-in filters. Exactly, on that we agree. So if you take the filter out (or remove the lens from the DSLR body, which does the same thing) the optical length will change.
When the Canon is modified ie removal of filters (either one or both) requires the position of the sensor to be changed or additional filters re-installed. This is confirming that the filters in the DSLR body are changing the length of the optical path which is exactly my conclusion.
This seems to be inconsistent with stating that the "backfocus of the Canon lens is 44mm" and has nothing to do with the "filters etc inside the Canon body.".
If the filters inside the Canon body are not affecting the 44mm set back distance then why is it necessary to change the position of the sensor when they are removed
We maybe (hopefully) are getting crossed wires on semantics here Ken. I'm keen to continue to explore this to make sure I haven't got my own wires crossed
In your diagrams the top one is correct, the others are in error.
The second diagram - the lens is still 44mm and no longer focuses on the sensor, hence the need to move the sensor or add back a filter element
The third is incorrect as the Lens has a fixed focus of 44mm.
{edit} This is based on the lens being focused on infinity. Obviously you can refocus the lens (within limits) to compensate, but the infinity focus doesn't change.
In your diagrams the top one is correct, the others are in error.
The second diagram - the lens is still 44mm and no longer focuses on the sensor, hence the need to move the sensor or add back a filter element
The third is incorrect as the Lens has a fixed focus of 44mm.
{edit} This is based on the lens being focused on infinity. Obviously you can refocus the lens (within limits) to compensate, but the infinity focus doesn't change.
Thanks. Clarifying the assumed setting at infinity focus is a good idea.
I see Ken that you are sticking doggedly to the fact that the lens has a 44mm set back distance when removed from the DSLR body. You note also that the second and third images in my diagram are wrong because of this. From my viewpoint the diagram is correct and there is nothing in your response that refutes this. Simply sticking to the fact that the 44mm set back is not changed without a supporting analysis does not take us anywhere.
Can I trouble you maybe to produce a similar diagram to mine demonstrating how the 44mm set back distance can be maintained when the lens is removed from the DSLR body?
Rodney,
We’re going round in circles.
The lens independent of whether or not it’s mounted on a camera body has a 44mm infinity focus.
Just focus a solar image on a card to verify.
Sorry I can’t add much more to the discussion.
