Camera Lenses and Testing

I don't have any fancy optical bench equipment, so "testing" is a practical issue: how well do the images look when I use this lens? It never really occured to me until reading Covington and Reeves.

There are two main things to look for when testing a lens. First, is it any good at all; i.e., do the star images focus fairly well across most of the field of view. Second, how much does it have to be stopped down before the images as the edge of the field of view are sufficiently sharp.

My 50-mm f/2 Pentax SMC (used with my Pentax A3000) shows aberrated images when used wide open. I haven't adequately tested it to find the minimum f-stop where the images look good, but at f/4 the star images are quite sharp all the way to the edge of the 35-mm frame. My 50-mm f/1.4 Takamura (used on my Pentax Spotmatic) is another matter....

I took a set of test shots of Lyra ranging from 5 seconds to 120 seconds. All where done from third-floor front porch here in Forest Hills, NY (Queens, NYC). Although the conditions were good (very clear skies, low humidity) for New York City, on the Bortle scale this would rate an 8 or 9. Visually, I could see down to about magnitude 4 at the zenith with averted vision, magnitude 3 with direct vision.

Rather than keep you in suspense, lets just say I was diappointed with the lens' performance. Still, this is preliminary; I need to take a second set of pictures to try to find which part of the optical system was at fault because I was using the lens with a 2X tele-extender and had a Skylight 1A filter in place. I doubt the filter did anything, but I don't think it is coated, so some of the reflections may be due to the filter.

Lens Test, 50mm f/1.4 2X @ f/2.8Lens Test 50mm f/1.4 2X @ f/4
Object Lyra
Location Forest Hills, NY
Time 2001 Aug 25 0200 UT
Camera Pentax Spotmatic, 50 mm f/1.4 lens + 2X tele-extender
Film Fuji Superia X-tra, ISO 800
Exposure 60 seconds at f/2.8 (first) and f/4
Comments Splotch on right hand side of first photo is a development flaw on the print. An airplane flew through the field of view on the second photo.

Severe aberrations affect both images. With the lens at f/1.4 (photo at f/2.8 due to the tele-extender), Vega looks like a brush-stroke. The streak across the middle of both appears to be a reflection but may be a roller mark from the developer. Note that it appears to arc in opposite directions on the two images. Although the second image appears much better, a close inspection of Vega (see below) reveals in interesting, not-quite-symmetric, 6-pointed star. I suspect I have some interesting reflections off the lens' iris which has 6 blades.

Lens Test, Crop of Vega
Object Vega
Comments Full-scale crop from the better (f/4) image

There are other aberrations visible at the edge of the frame. At first, they might look like field rotation but I was polar aligned more than good enough for this short exposure. Plus, they become very apparent in the close-up shown below: even the airplane's lights are distorted!

Lens Test, Edge Aberrations
Object Frame Edge (Vega)
Comments Full-scale crop of the airplane track from the second image showing the lens aberrations near the edge

This particular lens combination is definitely a loser for me. I'll probably try again without the tele-extender and with a multicoated Skylight filter (as well as without any filter) to see how the lens performs with settings from f/1.4 to f/4. This camera, the Pentax Spotmatic, is a fully-manual camera which makes it more attractive than my newer Pentax A3000 which requires batteries to hold the shutter open; i.e., the "B" setting is not a mechanical shutter. Unfortunately, they are not lens compatible. The Spotmatic takes the older Pentax screw-mount lenses while the A3000 takes the more common bayonet mount. If it weren't for that, I would just take the newer lens and put it on the older camera.

One thing which is nice to see is that I can easily make out Lyra. The limiting visual magnitude near the zenith was about 3.5; with averted vision it was about 4.5. Near the center of image, there are stars as faint as magnitude 8.

References