FOCAL REDUCERS ( SPEEDBOOSTERS )

[Robert Hart]

Just a little hint for folk to do their own fixit if you happen to have a camera with a focal reducer fitted in the throat. My arrangement has a Caldwell (speedbooster) in the throat of a "big" URSA.

If you dismantle for a clean, be aware that the Caldwell optical cell can be refitted back to front and it will still work.

Rather it will seem to work but you will soon find the collimation (what some call backfocus) of the lenses is way out.

The long lenses will work. The zooms will go nuts, The ultrawides in the ballpark 12mm to 25mm will be way off infinity focus and be very furry in the corners as well.

I have just sent myself a little insane thinking I had messed my shimming up. I pulled things to bits and totally missed the Calwell optical cell having been reversed.

It is probably good practice to take notes, even phone photos as well as systematically laying out parts.

 

[Brian Caldwell]

Just a little hint for folk to do their own fixit if you happen to have a camera with a focal reducer fitted in the throat. My arrangement has a Caldwell (speedbooster) in the throat of a "big" URSA.

If you dismantle for a clean, be aware that the Caldwell optical cell can be refitted back to front and it will still work.

Rather it will seem to work but you will soon find the collimation (what some call backfocus) of the lenses is way out.

The long lenses will work. The zooms will go nuts, The ultrawides in the ballpark 12mm to 25mm will be way off infinity focus and be very furry in the corners as well.

I have just sent myself a little insane thinking I had messed my shimming up. I pulled things to bits and totally missed the Calwell optical cell having been reversed.

It is probably good practice to take notes, even phone photos as well as systematically laying out parts.

When I saw your post I tried reversing the optics in the design file. Pretty bad performance that way - not unlike reversing just about any photographic lens, although not quite as horrendous as I expected. I suppose an adventurous person might try it to intentionally mess up the image.