Close-up lens - Bi-Convex Vs Plano-Convex@Sparky,
I am no expert in optics but can offer some comment.
I originally discovered the use of close-up lenses on desktop presenters (document cameras). These units use a standard CCD camera core and a No.2 bi-convex close-up lens with a focal distance of around 40cm. The lens does not introduce significant distortion.
I originally purchased 100mm Focal Length ZnSe Bi-Convex lenses for experimentation with my industrial FLIR PM695 that has a 0.5m minimum focus distance and a 24 Degree FOV lens. The 100mm ZnSe lens in combination with the 24 Degree FOV worked very well indeed.
Mike did some tests and found that the optimum ZnSe lens for the E4's wider 45 degree FOV lens was the 50mm Focal Length type. That makes sense as it is twice the FOV so needs half the FL for the same close-up FOV as my PM695 when fitted with a 100mm close-up lens.
The down side is that with the E4 fitted with a 50mm FL lens you need to be much closer to the target than when I use the PM695.
With regard to the differences between the Bi-Convex and the Plano-convex is minor in this case. My professional Inframetrics Close-up lens has the concave side facing the target and convex side facing the cameras sensor. I used the 50mm FL plano-convex lens the same way on my PM695 in order to test its performance and it performed well. HOWEVER it was discovered that the E4's lens behaved differently and the Convex side of the close-up lens needs to face the target to get the best image across the FOV. Both the Bi-Convex and Plano-Convex lenses will work as a close-up lens. I have attached the light path plots for plano-convex and bi-convex lenses.
My lens adapters are also ABS and there are two ways that the ZnSe lens may be held in place. Some users secure it with a thin bead on NON ACIDIC adhesive. Acid cure glues are a no-no with ZnSe ! Others have used a soldering iron to slightly melt the edges of the lens holder to create 'claws' like those used to retain gems in jewellery.
Depth of field will be a function of the interaction of the close-up lens and the E4's built in fixed focus lens. I regret that I know of no way to improve the DOF. I use the E4 on a tripod so the DOF is not such an issue for me.
I recommend having several lens adapters made and fixing the lens in place. I have considered making a lens adapter that takes my 19mm and 20mm photographic lens holders. These could then be changed out in the same way as on a normal camera lens. I have not bothered to date though.
Some usefull comment on Plano Convex Vs Bi-Convex may be found here:
http://www.physicsforums.com/showthread.php?t=318310A quote from the thread:
"If your 'telescope' is being used for image relay of small, nearby objects about a focal distance away from the lenses:
Plano-convex is fine, but the flat surfaces should now be facing outward. If the magnification of this image relay system is close to 1:1, then the use of biconvex lenses will eliminate most abberation. If the magnification is far from 1:1, then biconvex lenses will make things worse.
In any case, orientation and use of plano-convex vs. biconvex lenses won't matter terribly much. You will still form a fine image so long as the lenses are the correct distance apart. These details only affect small distortions in the image. "Note my comments on how the plano-convex does not work well with the convex side facing the camera lens. It should work OK but there is something about the E4 lens specification (FOV ?) that messes things up. (You get defocussing around the image periphery)