How to attach flash diffuser when using lens adapter?

[xariatay]

Need advise on how to attach flash diffuser when using lens adapter? The 2nd-hand lens adaptor I bought is something like this FUN-IN Underwater Photographic Equipment & it blocked the flash diffuser's catch-holes on the Canon housing WP-DC21... As I have no strobe the flash diffuser is quite important...
Thanks!

 

[M_Bipartitus]

I don't have any experience with that type of lens, but I'd suspect an on-camera flash wouldn't be very useful as the lens is so large it would black most of the flash's light and that which the diffuser does diffuse is less powerful since it is farther from the camera's flash. If your lens is a wide angle adapter, I find it unlikely that the on-camera flash will prove useful. For macro you may be able to get some use out of it, but also limited.

 

[xariatay]

Thanks! My intention is to use a macro lens...

 

[M_Bipartitus]

The main problem you are likely to run into is losing a lot of flash light that hits the lens and never makes it to the subject. If you can find a way to mount the diffuser you should try and get the diffuser as close to the end of the wet lens as you can. This way the light that is diffused won't just hit the lens reducing the usable light and likely leaving a shadow in your photo.