Where do I measure the distance from a lens's focal plane from?

[Stub Mandrel]

OK 400mm f/l lens cell, two-element ED lens.

The lens is about 2cm thick.

Where on the lens do I measure from to find the theoretical focal plane? Front, back, middle, somewhere else?

For a simple lens, its the middle, but this isn't a simple lens... with 2cm between possible correct positions, it would be easy to make the OT too long or short or evenb end up with insufficient focus tube travel one way or the other.

The chance of having a convenient local star/natural satellite appearing from behind thick cloud and giving me the obvious way to find this accurately is very small in teh next week...

 

[Thalestris24]

I think, probably, from the mid-point. As long as you have the focusser travel to cover the potential range + eyepiece/camera distances, you should be fine  It does tend to annoy me that quite often you buy a scope and then have to also buy an extension tube...

Louise

[Stub Mandrel]

Thanks Louise.

More careful measurement shows my 2cm thick was an exaggeration.

[Stub Mandrel]

I have had some useful feedback from an ED66 owner which suggest the Skywatcher non-reducing field flattener doesn't seem to change the focus point by much.

Alternatively I have been musing about a field flattener off eBay but it might be bit big...

[Gina]

Ooh arr - I got one o' they!

[Astro Imp]

17 hours ago, Stub Mandrel said:

I have had some useful feedback from an ED66 owner which suggest the Skywatcher non-reducing field flattener doesn't seem to change the focus point by much.

Alternatively I have been musing about a field flattener off eBay but it might be bit big...

Looks like overkill to me.

[AKB]

18 hours ago, Stub Mandrel said:

Alternatively I have been musing about a field flattener off eBay but it might be bit big...

Is that a light bucket at the end?

[Macavity]

To be strictly correct, I recall that you would have to measure from
the rear nodal point ?!? Recalling Dad's 50's Photography manual...  

https://en.wikipedia.org/wiki/Cardinal_point_(optics)#Nodal_points

The latter would probably have to be determined "by experiment"?  

 

[Stub Mandrel]

2 hours ago, Macavity said:

To be strictly correct, I recall that you would have to measure from
the rear nodal point ?!? Recalling Dad's 50's Photography manual...  

https://en.wikipedia.org/wiki/Cardinal_point_(optics)#Nodal_points

The latter would probably have to be determined "by experiment"?  

There's always one clever b...

According to that link the 'effective focal length' is measured from the 'rear principal point'.

So the lens can be modelled as if all diffraction takes place at the 'Principal plane' and the 'principal point' is where teh 'principal plane' crosses the 'optical axis' but:

" For a thin lens in air, the principal planes both lie at the location of the lens. The point where they cross the optical axis is sometimes misleadingly called the optical centre of the lens. Note, however, that for a real lens the principal planes do not necessarily pass through the centre of the lens, and in general may not lie inside the lens at all. "

To find the 'rear principal point' it seems I have to focus the lens at infinity and measure back.

To do this I need to know the 'effective focal length' which other sources point out can be different to the nominal focal length.

To find the actual focal length I could build the lens into a telescope, take a picture, plate solve it and calculate the actual image scale and work back to the field of view and focal length. Armed with this information I can then work out where the rear nodal point is, allowing me to build a telescope to put the lens in.

Galileo Galilei (Bismillah!) I takes me 'at off to you...

 

 

 

[Astro Imp]

1 hour ago, Stub Mandrel said:

There's always one clever b...

According to that link the 'effective focal length' is measured from the 'rear principal point'.

So the lens can be modelled as if all diffraction takes place at the 'Principal plane' and the 'principal point' is where teh 'principal plane' crosses the 'optical axis' but:

" For a thin lens in air, the principal planes both lie at the location of the lens. The point where they cross the optical axis is sometimes misleadingly called the optical centre of the lens. Note, however, that for a real lens the principal planes do not necessarily pass through the centre of the lens, and in general may not lie inside the lens at all. "

To find the 'rear principal point' it seems I have to focus the lens at infinity and measure back.

To do this I need to know the 'effective focal length' which other sources point out can be different to the nominal focal length.

To find the actual focal length I could build the lens into a telescope, take a picture, plate solve it and calculate the actual image scale and work back to the field of view and focal length. Armed with this information I can then work out where the rear nodal point is, allowing me to build a telescope to put the lens in.

Galileo Galilei (Bismillah!) I takes me 'at off to you...

 

 

 

Stop it, you're making my brain hurt 

[The Admiral]

7 hours ago, AKB said:

Is that a light bucket at the end?

Looks as though it's made of plastic, so should be pretty light. Ba-boom!

Ian

[michael8554]

Focus the sun (that bright object sometimes seen in the sky) onto a non - combustible surface and get  the missus to measure where 400mm is on the lens.

Or a combustible surface to relive those long- forgotten childhood days.

Heavy rollers are only needed for Australian Test wickets.......

Michael

[Stub Mandrel]

That was a the fall-back plan, and lo! Ten minutes ago a star appeared despite weather warnings for snow!

As missus Stub is in Tenerife (where there is plenty of starlight night and day but no lens cell) I had to work single-handed

You won't believe this but the focus plane is (within 1mm) 400mm from the front of the lens cell.