What Lens to Put In Front of an Complex Lens to Push Focal Plane Back?

[cajer]

I have an extremely fast (f/1.05) night vision optic that I scavenged from a old night vision unit which is faster than current production lenses (f/1.23). However due to the design of newer night vision tubes, the lens will not focus to infinity on the night vision tube. This is because the old tubes had an imaging plane flush with the surface of the tube, but the new tubes have an imaging plane that is slightly recessed (~3 mm) from the surface. Even at it's closet travel (touching the front of the night vision tube) it will only focus to a point 20~30 cm away. Additionally the night vision optic is unit focusing.

As such, I am wondering what type of optical element I can put in front of the night vision optic to push the focal plane ~5 mm further away from the night vision optic? Also how will this effect image quality and fstop?

 

[tech99]

Are the two lenses the same diameter? It sounds as if the new lens has an increased f number as a result of having a longer focal length. So if you add another lens to shorten the focal length you may decrease the f number back to where you started.

 

[cajer]

Are the two lenses the same diameter? It sounds as if the new lens has an increased f number as a result of having a longer focal length. So if you add another lens to shorten the focal length you may decrease the f number back to where you started.

What do you mean by diameter? Do you mean entrance pupil? According to the spec, the lens has an effective focal length of 27mm (not sure what they mean by effective focal length) with a 40 degree field of view (the sensor size is a 18mm diameter circle.). It appears that the older lens have a faster f number because of a larger entrance pupil as the front of the lens is much larger than the new lens despite both of them sitting in the same lens housing. Additionally both of them should have the same focal length as they have the same field of view and magnification.

I was thinking the reason I can't focus to infinity using the old night vision lens and the newer tube, is because they recessed the night vision tube sensor into on the newer night vision tube vs the sensor being flush with the surface of the tube on the old night vision tube. So would this mean I need to increase the focal length of the lens by adding a -negative achromatic doublet lens. If their effective focal length is the real focal length, and I want to move the imaging plane back ~3mm I should be looking for a plano-concave lens with a -3.7 diopter or a -270mm focal length? However with a focal length of 30mm, that would give me f/1.16 which is not that large of an improvement.