Filter effect on back focus distance

[Albut]

So if you can’t achieve exact focus with thin spacers, is it better to be just over or just under under?

[ollypenrice]

9 hours ago, Albut said:

So if you can’t achieve exact focus with thin spacers, is it better to be just over or just under under?

This will be the time to experiment...

Olly

[Adam J]

Oh this one again. The mistake everyone makes is thinking that you need to move the sensor by adjusting back focus so that the sensor lays on the new focal plane as dictated by the added glass. This thinking leads to confusion. What you actually need to do is ensure that the focal reducer is on the correct part of the light cone when the sensor is in focus i.e you don't want the change in focal length to result in the position of the corrector changing. 

Hence the thing to remember is that the focal length of the scope is slightly extended by the introduction of the filter (and sensor cover glass that everyone forgets to add) so to keep the corrector in the same place as it is without that glass you need to add that difference in focal length onto the back focus quoted for the corrector. That distance normally being 1/3rd of the thickness of all flat glass elements in the light path between corrector and sensor. 

If none of that made sense stop thinking about it and just add 1/3rd the glass width to the back focus. You don't need to understand it for it to work. 

Adam

Edited June 25, 2023 by Adam J

[andrew s]

To be pedantic a plane parallel glass element doesn't change the focal length it shifts the focal plane.

Regards Andrew 

[Adam J]

12 hours ago, andrew s said:

To be pedantic a plane parallel glass element doesn't change the focal length it shifts the focal plane.

Regards Andrew 

Depends on if your frame of reference is the mechanical path length or the optical path length. As we are talking about spacers and the physical position of focus changing we are talking about the mechanical path length between the objective and focus and that has changed. 

In fact both have changed, the thing that has stayed the same is the F-ratio of the system as the angle of the light cone is the same. Its a bit like how some corrector designs with also change the position of focus and hence the mechanical path length. 

However, looking at the definition on Wikipedia the focal length is defined as the physical distance between the focal plane and the principal plane of the optical system and so I am going to go ahead and say that the focal length has by this definition been changed by the addition of a parallel optical surface.  

Now if you want to talk about the focal length of the objective, thats the same, but we are talking about the focal length of the entire optical system including filters and corrector and cover-glass etc..

 

Adam  

Edited June 26, 2023 by Adam J

[andrew s]

4 hours ago, Adam J said:

However, looking at the definition on Wikipedia the focal length is defined as the physical distance between the focal plane and the principal plane of the optical system and so I am going to go ahead and say that the focal length has by this definition been changed by the addition of a parallel optical surface.  

Just consider where the principle plane is in a compound telescope. It is not always one of the lenes or mirrors! 

This illustrates how effective focal length of the whole system is defined using marginal rays. 

Regards Andrew 

 

 

[Adam J]

2 hours ago, andrew s said:

Just consider where the principle plane is in a compound telescope. It is not always one of the lenes or mirrors! 

This illustrates how effective focal length of the whole system is defined using marginal rays. 

Regards Andrew 

 

 

Yes but the point is that it is a physical measurement from one point to another that changes when you add a filter. 

[andrew s]

4 hours ago, Adam J said:

Yes but the point is that it is a physical measurement from one point to another that changes when you add a filter. 

Absolutely, it, as the early posts pointed out,  displaces the focal plane.

Regards Andrew 

Edited June 26, 2023 by andrew s