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Bushnell 21mm f/3.8 Automatic

Lens
5 replies to this topic

#1 UlfW

    Ulf W

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Posted 22 March 2021 - 09:04

Last Update: 03 Aoril 2021 agb/label
Finalized: Work in progress.


Bushnell 21mm f/3.8 Automatic

Manufacturer: Bushnell
Lens Label: Bushnell Automatic f:3.8 f=21mm No.<serial number>
Currently manufactured: No

Lens type: Manual focus, Prime, Ultra Wide-angle
Focal length: 21mm
Aperture range: f/3.8 – f/16, Manual / Automatic with pin-actuation , 6 blades,
Design: TBD
Flange Focus distance: Mount dependable
Mount: M42 x 1.0**, others?
Sensor format/coverage: up to 24mm x 36mm
Front filter: 72 mm x 0.75 mm
Introduction year: TBD
S/N of test object: 7000206
**Mount of test object


Image of test object:
Attached Image: IMG_2734.jpg Attached Image: IMG_2735.jpg Attached Image: IMG_2738.jpg

Transmittance Summary
Definitions of the parameters below
  • Range: The Bushnell 21mm f/3.8 Automatic lens transmits 1-55% in an increasing slope from 354nm to 400nm.
  • TVISmax (%) =74%
  • T400nm (%) = 55%
  • T365nm (%) = 15%
    This rather low percentage is an indicator for a longer exposure time under typical UV-pass filtration peaking around 365 nm.
  • λUV HMvis(nm) = 378nm
  • λUV HM400 (nm) = 371nm
  • λUV Zero (nm) = 354nm
    These three values indicate that the lens is working best for UV-A photography with filters with a good transmission closer to 400nm

Spectral transmission graphs:
UV-NIR, Bushnell 21mm f/3.8 Automatic
Attached Image: Screen Shot 2021-03-19 at 12.00.14.png
The transmission measurement accuracy into the end of NIR range is less good due to limitations in the light source.

UV, Bushnell 21mm f/3.8 Automatic
Attached Image: Screen Shot 2021-03-19 at 11.59.23.png

UV-Log, Bushnell 21mm f/3.8 Automatic
Attached Image: Screen Shot 2021-03-19 at 11.59.02.png
Numerical Spectra Data available: Pending


Filters and how to use them on this lens:
The front filter thread is 72mm standard filter thread. A step-up filter ring and bigger filters will be needed to avoid vignetting with a filter stack.
This wide angle makes it unsuitable using front mounted dichroic filters, causing a big colour shifts towards the corners.
It might be possible to use such filters rear mounted instead ti decrease the problem. (tbd)

It is possible to use rear mounted filters, either in lens mount adapters for mirrorless cameras like Sony A-series etc, or placed directly in the camera.

An alternative, if space in the camera allows, is to putty-mount a filter directly against the lenses rear flat parts.
The rear lens element is recessed and thus safe and the ring provides a good reference plane for orienting the filter normal to the optic axis.

My Omega 330W80 Improved Ø25mm, is mounted in a 27mm-filter ring and would work well to mount as it is.

Handling and focussing:
This lens has a focusing helicoid that smoothly turns almost a full turn for focus from infinity to around 0.3m.

Flare and sun-stars:
TBD

Sharpness:
TBD

Lens distortion:
TBD

Chromatic Aberration / fringing in UV:
TBD
Ulf Wilhelmson
Curious and trying to see the invisible.

#2 dabateman

    Da Bateman

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Posted 22 March 2021 - 11:08

Ulf did you have the rear filter glass issue that Cadmium posted:
https://www.ultravio...est-comparison/

#3 StephanN

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Posted 22 March 2021 - 12:48

Would it be worth noting that the same lens probably also goes by other names, e.g. the Soligor 21mm f/3.8 (https://www.ultravio...ligor-21mm-f38/ ) looks pretty similar?
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#4 UlfW

    Ulf W

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Posted 22 March 2021 - 13:28

View Postdabateman, on 22 March 2021 - 11:08, said:

Ulf did you have the rear filter glass issue that Cadmium posted:
https://www.ultravio...est-comparison/
No I did not.
This is a different type of lens with f/3.8 while Cadmium's lens is a f/4.5.
They do not share that problem.
Ulf Wilhelmson
Curious and trying to see the invisible.

#5 UlfW

    Ulf W

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Posted 22 March 2021 - 13:37

View PostStephanN, on 22 March 2021 - 12:48, said:

Would it be worth noting that the same lens probably also goes by other names, e.g. the Soligor 21mm f/3.8 (https://www.ultravio...ligor-21mm-f38/ ) looks pretty similar?
Yes I saw that when writing about the Bushnell-lens.
Except for some external differences like the focus ring design they appear very similar.
That made me glad when I got the Bushnell-lens as your images look sharp.
Frankly I have not used it at all. I have been more into close-ups of flowers.
However I like the wide angle perspective and after getting a FF Sony A7 III that lens makes more sense to use like you do on your Canon 6D.
Ulf Wilhelmson
Curious and trying to see the invisible.

#6 OlDoinyo

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Posted 25 March 2021 - 03:18

This could be another decent lens in the 20-21mm range; there are not a lot of those (The Tamron and Asahi offerings are similar, but with slower maximum aperture.) Now if there were only a 14mm that is usable in UV....

I suspect rear-mounting dichroic filters will also lead to problems. These old wide-angle lenses have small rear elements and high corner divergence angles in the rear. Stick to alternatives if possible.

Edited by OlDoinyo, 25 March 2021 - 03:21.