dewshields for Binoculars

Hi folks.

I want to make a set of dewshields for my 20X80's. I was thinking of having them extend about 3-4 inches from front of binoculars. My question is.

How wide do I need to make them to ensure I do not interfere with light path. Can I just fit them snugly against the outside of binoculars or do I need to make them extrawide?

Thanks

The only way you will really know Trevor is if you make a mock-up set by rolling a sheet of paper around the objective(s) and testing them that way.

Hi Trevor,
The field of view for a pair of 10x50 is about 5.5 degrees. That is 2.75 degrees a side. Therefore for a 100mm long dewsheild the end diameter would need to be only 9.6mm bigger than the lens to be totally clear of the field of view. (Tan(5.5/2)x100)x2

I just knew those maths would find a use some day!!

Michael

Thanks for the answers lads.

Michael

Can you show me the workings for that, Its been years since I did any trigonometry. I also need to work it out for 80mm objectives with a field of view of about 3.5 or slightly more degrees.

Also, as per your previous post, why would it be 2.75 degrees a side. Each side has a FOV of 5.5 on 10X50's and 3.5 in 20X80's

Hi,

5.5 degrees is the total field of view for most 10x50's. This is 2.75 degrees each side of the centerline (5.5/2)
The Tan of an angle is the opposite over the adjacent, (ie the gap from the edge of the lens), to the inside of the dew shield, divided by the adjacent (the length of the shield).

T=O/A, therefore O=TxA. This is one side, so multiply by 2 to get the amount of the diameters increase.

So Diameter increase is Tan(half the field of view) x length of dewshield x2

I will let you work out the 80mm binoculars...... Empower the people!

Michael

My figures still do not match yours..sent you a pm.