Lens coatings and IR focusing`

[Herbc]

I heard from my IR converter that they have had some problems with more modern lenses focusing at infinity due to the coatings they are now using.
I had an issue with a Loxia 35mm focusing on IR on my A7R (converted), but it resolved itself at f5.6 and smaller.
Is there a way to predict this?

[Telecaster]

Coatings throwing off focusing accuracy should only be an issue—if there's an issue—with AF, no? As the Loxia is a manual focus lens, don't you just magnify with the EVF and focus visually using the IR light detected by the sensor? I've never used Sonys for IR pic taking, but this method works great with various m43 cameras (using 720nm IR filters) and MF lenses.

(Attached IR selfie via Oly E-M1 & 12mm lens.)

-Dave-

[Herbc]

I did have a problem with the loxia wide open- I could not get infinity focus with it at f2- that is what prompted the post.

[BrianVS]

Modern multi-coated optics will cause more light loss in the Infrared region compared with an uncoated lens or single-coated lens. The inability to focus to infinity when used wide-open is more a function of the longer wavelengths coming to focus in a different plane. If the focus of the lens stops at infinity and the IR index is beyond infinity- the image will be out of focus. stopping down works as increased DOF covers the error. For lenses that are over-corrected for spherical aberration stopping down causes the focus to shift towards infinity, which also helps.

[Telecaster]

I did have a problem with the loxia wide open- I could not get infinity focus with it at f2- that is what prompted the post.

Oh, of course… All my native manual focus m43 lenses can focus past infinity (and the FE adapters I own all allow this with non-native lenses on the Sonys should I ever try IR with them) so I've never run into this issue.

-Dave-