Back focus help please.

[Nugs]

I have recently taken the plunge and picked up a AA269 protec camera.

My current scope is a WO73ii, and i have both the 73A and 73R. On the William Optics website, it states a back focus of 56.8 for the 73R..... happy days. Yet for the 73A, it gives 2 distances, 66.2 and 67.7. Am i being particularly dense here (probably), or is WO just not telling me what i need to know in a way i can understand?

Any help on the differences in these two dimensions would be greatly appreciated. 

Dave

 

Edited April 28, 2022 by Nugs

[newbie alert]

To me it's showing the measurement from the first of the threads B and the last of the threads A

Since you can't measure the distance A when you have your equipment installed

I could be wrong, but that's how I'm seeing it

[Knighty2112]

Looks to me like there are 2 measuring points. A on the external lip seems easier to be able to measure than point B which unless marked up on your equipment looks like an internal measurement.

[Nugs]

2 hours ago, Knighty2112 said:

Looks to me like there are 2 measuring points. A on the external lip seems easier to be able to measure than point B which unless marked up on your equipment looks like an internal measurement.

And we have a winner!!!!

Having just measured the threads where i would attach my extension tube (M48 internal, M54 external), the thread length on each one is 4.2mm external and 5.7mm internal from the outermost face..... the difference of 1.5mm aligns with the difference between the two WO back focus dimensions stated.

Every day is a school day.

Thanks both. ✌️

[Knighty2112]

6 hours ago, Nugs said:

And we have a winner!!!!

Having just measured the threads where i would attach my extension tube (M48 internal, M54 external), the thread length on each one is 4.2mm external and 5.7mm internal from the outermost face..... the difference of 1.5mm aligns with the difference between the two WO back focus dimensions stated.

Every day is a school day.

Thanks both. ✌️

Thanks. I’ll PM you my address so you can send my prize! 😉