William Optics 73 & flattener backfocus setup

[martinclayden]

I have these items (WO 73 & flattener) and I'm trying to find the correct backfocus procedure for the flattener, when used with a Canon DSLR with a clip filter fitted.

I've read that the thickness of the clip filter (Optolong L-Enhance) matters - true?

I've read that the flattener is to be set at 11.4mm - true?.

The clip filter thickness is to be added to this - true?

Do WO publish a detailed procedure ?

Thanks

Martin

[Crackabarrel]

Hi,

The thickness of the filter does indeed matter, typically you should add 1/3 the filter thickness to your flattener's stated backfocus.

So in this case it will (1.85/3)+11.4 = 12.05mm (L-Enhance thickness = 1.85mm)

You just screw out the flattener until it reaches 12mm and then you should be fairly close to having a corrected field.

 

[martinclayden]

Thanks for your reply.

I'll use those settings. I must admit, I'm struggling to get my head round why an optically flat filter can affect the flattener settings...

Regards 

Martin 

 

[Crackabarrel]

I think it has something to do with the refractive properties of the glass in the filter, although don't take my word for it.