Joined: 10 Aug 2014
|Posted: Mon May 18, 2015 2:20 pm Post subject: Tube lens comparison
|There has been a certain amount written about the possibility of adapting Minolta Vectis lenses for use on small, light cameras with aps sensors.
The Vectis lenses , although presumably of high quality, generally are not mountable on any but the Vectis aps film cameras and one older Minolta digital, although a rare adapter converting the Minolta V mount to A mount apparently does exist, so a physical mount is possible on other cameras, with some searching and adapting.
I acquired a Vectis 80-240 mm apo, very cheap for obvious reasons, and wondered how it would fare as a tube lens mounted on a Sony alpha 5000.
Getting a physical mount turned out to be not that hard. I removed the NEX E mount, male bayonet from the smaller of a $7.00 2 piece macro tube set I bought on ebay. Although the 4 mounting holes on that bayonet are close enough to the 4 holes on the Vectis male bayonet on the rear of the lens, that they could be swapped, I opted for just slipping the Sony NEX E mount bayonet over the Vectis mount. It just barely fits and sits on there with a nice and snug pressure fit. The Vectis 80-240 zoom apo now mounts on my Sony alpha 5000.
All of the Vectis lenses had electronic control so the aperture is not adjustable, nor is the focus. This one is fixed at f: 4.5. due to that. There is a manual focal length control ring, and only on this one model of Vectis lens can the barrel be rotated against the focus motor to focus it. It is cumbersome but functional.
The Vectis zoom lens works quite well as a zoom lens on the Sony , although all pictures are at wide open, so it's not likely it is giving it's best showing.
However as a tube lens for microphotography, here are some comparisons. All pictures were taken with the Sony alpha 5000.
The subject are stained potato starch grains on a potato section in dark field. The objective is a Reichert planapo infinity corrected 25X N.A. .65
The specular points from the scattered rays of the dark field, are particularly difficult to resolve and contain a lot of chromatic artifacts, even with the planapo objective.
When both telephoto camera lenses are used uncropped, the degree of severe chromatic aberration and edge distortion is quite extreme but the Nikkor Q especially, when cropped in such a way as to represent the original microscopic visual field, has only a minor amount of chroma. The Vectis zoom lens, even though it is termed an apo, although not terrible, is definitely inferior to the Nikkor Q, as a tube lens, showing poorer resolution and more chromatic aberration.
1) relative size of the Vectis 80-240mm zoom, compared to a 200mm
2) Friction fit Sony E bayonet on the back of the Vectis lens.
3) Vectis lens mounted on the Sony Alpha 5000
4) Afocal image of the starch, through the stock Sony 16-50 lens zoomed to 39mm. The coverage is right out to the edge of the visible microscope field( 20mm f.n.)
5) Reichert 25x planapo infinity corrected beam, direct to sensor. 1.6 crop factor.
6) Nikkor Q 200mm as a tube lens . f.4, focused at infinity. crop factor 1.3x.
7) Minolta Vectis 80-240mm as a tube lens. f.4.5, focused at infinity. set at 240mm. crop factor 1.4x
Minolta Vectis lens uncropped, giving it's full image capture. The field is approximately the equivalent of a 28mm f.n. microscope field.
9) Nikkor Q lens uncropped.