Tuesday, January 8, 2013

Adapter Modifications for Nikon 1 18.5mm f/1.8 - 08 Jan 2013

Having played w/ the rubber sleeve adapter for the Nikon 1 18.5mm f/1.8 lens on the 20-60X Zoom eyepiece of the Zeiss 85T*Fl I've made some startling (and disappointing) realizations.  First, the system is not quite as stable as I originally thought; the sleeve has a tendency to misalign, resulting in shadows across part of the image.  Second, and more disappointing, is that that the combination is soft across the outside half of the view at magnifications of 20 - 40X on the zoom eyepiece.  This was also evident when I used the 10-30mm lens on the Nikon 1 V1 attached to the 20-60X Zoom eyepiece of the Diascope.

Using a resolution chart I took a series of images from 20' of the 18.5 mm f/1.8 lens on the 20-60X Zoom eyepiece. At 20X zoom the center is sharply resolved, however, resolution drops off quickly to either side.  This is also evident at 30X zoom.  At 40X zoom the effect is not horrible, but still somewhat noticeable.  Images acquired at 50X and 60X zoom appear to be sharp across the entire image, but magnifications are a bit high.  Also, shutter speeds drop to a level where hand-held digiscoping is extremely difficult.

Similar results were observed when I tried the 10-30mm f/3.5-5. lens on the 20-60X zoom. Zooming the lens to ~18.9mm f/4.5 produced similar 'soft' edges when I zoomed the eyepiece from 20 - 40X.  Unfortunately I don't have a suitable adaptor for this combination of lens / eyepiece, so resolution testing couldn't be performed.

Conversely, both the 10-30mm f/3.5-.6 and 18.5 f/1.8 are sharp across the entire field of view when coupled with the 40X W eyepiece of the Diascope. So I decided to modify the adaptor for the 18.5 mm lens so that it would fit the smaller-diameter 40X W eyepiece.

I was able to accomplish this by purchasing a 1 3/4" PVC (4" long) coupling tube from Lowes ($0.79), which fits inside one half of the rubber sleeve adaptor that is attached to the 18.5 mm lens.  I had to cut 1" off the end so that the 18.5 mm lens would almost touch the eyepiece. Place the cut end of the tube inside the rubber sleeve. 

In order to fit snug over the eyepiece I wrapped a single layer of electrical tape around the base of the 40X W eyepiece.  The PVC tube now slides over the eyepiece and forms a tight, snug fit.  It should also just barely extend beyond the eyepiece itself.  The beauty of this is that the tube can stay on the eyepiece and not impede normal scope viewing.  Now, its just a matter of sliding the camera w/ the rubber sleeve attached over the PVC tube for a stable connection.

There should be no shadowing in the corner of the viewfinder - if so, sand down the cut edge of the PVC tube a bit more until the shadowing disappears. The inner ridge of the sleeve adaptor will prevent the camera lens from contacting the eyepiece lens if the PVC tube is sanded too much.

I then performed resolution comparisons between the 18.5 mm f/1.8 and 10-30 mm f/3.5-5.6 lenses on the 40X W eyepiece using the ISO 12233 resolution chart set up 20 feet away.  I used the 10 sec. self timer on the camera, since the shutter speeds were only on the order of 1/5 to 1/15 sec. at ISO 100.  I set the 10-30 mm lens at 18.9 mm and f/4.5 to compare w/ the 18.5 mm at f/1.8.

Surprisingly the exposure differences were not as great as I expected from the two lenses.  The 18.5 mm f/1.8 showed a 1/6 sec. shutter speed, while the 10-30 mm at 18.9 mm f/4.8 showed a 1/5 sec. shutter speed. If I bump the aperture on the 18.5 mm lens to f/4.8 the shutter speed also shows 1/5 sec., but I find that the White Balance starts to get affected; the image is a bit brighter, but also shows a bit more blue.  Bumping the aperture to f/16 causes the image to be really overexposed.  So the lesson w/ the 18.5 mm f/1.8 is to leave it at f/1.8.  You gain no depth of field by stopping down.

I took the 18.5 mm f/1.8 out to test it on a Ring-billed Gull in a nearby park, and took these images from ~100', 50' and 30' away. 

This modified adapter appears to be stable and provides sharp, full frame views w/o corner shadowing. Image quality is quite good, and by setting Autofocus to AF-C I tend to get more accurate focal lengths.  But also note that fine-tuning focus w/ the EVF will improve keeper rate - just make sure the diopter is set to give the sharpest view through the EVF!

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