After sitting on a standard filter wheel cover and canon lens mount adapter for my STT-8300 for a couple of years, I decided to give it a go for the Perseids this year. Cover replacement from the guided to the standard cover was straightforward. The lens adapter was already mounted. I popped an 8mm fish-eye (Boken) on the adapter without any problem. However, at the len's infinity setting, there is a large airy disc. The disc gets smaller as I set the focal distance down. At the shortest focal distance, it still isn't coming to focus. I switched out a different lens (Canon EF-S 10-22mm) and got the same result. The lens was pointing straight up so I removed the lens and placed it on the adapter without letting it pop in -- increasing the focal distance by a few millimeters and found it was even further out of focus. So, it appears that the lens needs to mounted closer than what my configuration has in place. I have filters installed in the filter wheel and was tested using the stock baader filters that came with the SBIG bundle -- no filters would increase the back-focus and make things worse I expect. Looking at the stack-up on the camera adapter, it looks like the lens adapter still has plenty of clearance before the back of the lens extends beyond the standard cover plate interior -- this is with an EF lens and might have less (but still some) with an EF-S lens. Any advice would be appreciated. Thanks!
Jason, Look at the chart for STT http://www.sbig.com/support/product-documentation/backfocus-table/ I could not find anything on Boken... TheEF-S naming refers to Short-back focus design which is only compatible to APS-C EOS SLRs starting with the EOS 300D (Digital Rebel). EF-S lenses feature a protruding rear element which requires a special mirror design to avoid a collision of mirror and the rear part of the lens. The principal idea is the reduce the distance of lens to the sensor The BFD or "Flange focal distance" in this case is 44mm. It may be you have the wrong lens adapter. Canon EF-mount 44.00 Canon EF-S-mount 44.00 You can email or call me at the info below.
