step down mask for canon 200mm F2.8

[Astroblagger]

Hello

im wanting to step down my canon 200mm f2.8 lens to F4 for LRGB imaging can anyone point me in the direction of a screw on mask for this, iv googled but cant really see anything coming up?

[Alien 13]

All you need is a "step down filter ring" the size would have to be calculated from the lens filter size to a size to give you the required f/ratio

Alan

[Astroblagger]

Should it be 72mm to 58? Wonder would this give about F4? 

[Alien 13]

With a lens of 200 mm focal length a 72- 58 mm step down ring would give about f/3.4 so you would need around 50 mm for f/4

Alan

P.S. I think standard sizes are 52 or 49 mm, a quick e-bay trawl found this http://www.ebay.co.uk/itm/72mm-to-52mm-72mm-52mm-Stepping-Step-Down-Filter-Ring-Adapter/251746313586?hash=item3a9d3fe572:g:D0UAAOSwaNBUgv9a but have not found a 72-49mm yet.

[zicklurky]

When I used that lens I used a 77mm-> 52mm step down ring which brought it to f/3.85. I'm been thinking of going back to that to that lens, it's cracking!