Test-driving the Tamron 21/4.5: Historic Firefighting Equipment

[OlDoinyo]

Thanks to Cadmium, I now was able to install a composite UG11/S8612 filter disk in the rear of the Tamron lens, finally bringing it into service. I mounted the lens on the A900 and photographed some old wildfire-fighting vehicles at Holmes Educational State Forest as a test. A few frames with comments follow, shot at ISO 100 and my basic UV color balance setting. Display intent is BGR. I used an auxiliary sight mounted in the flash shoe to help aim.

 

The first frame depicts old Helicopter #1, which was cast-off from the Vietnam War and placed in service with the North Carolina Forest Service for a number of years. It is painted bulldozer-yellow, with some red decals and clear plastic windows and nose panels.

 

 

This frame, shot at infinity at f/4.5, is quite blurry at all detectable distances, not really in focus anywhere. It is clear that shooting wide-open is even less of an option here than with the Enna 28, except possibly at very short distances. The fact that the rear filter disk is much thicker than the skylight disk it replaces undoubtedly plays some role; it is possible that intrinsic UV focus shift is also part of the issue. The yellow paint shows less dark than I expected; coming out a medium chocolate brown. The chopper's windows are bright orange, indicating considerable short-wave absorption. Interestingly, the red in the Forest Service decal also is rendered reddish here.

 

 

The next frame shows the helicopter from a more distant perspective, at f/16. Sharpness is respectable at this aperture.

 

 

The last frame shows a fire-line tractor rig, painted with the same paint, also at f/16. This gear towed a plow-like rig through the woods to help establish a firebreak to stop a wildfire. I guess the cylinder-like thing in back is to help maintain downforce on the plow.

 

 

Again, the paint shows brown, indicating some pigmentation.

 

It is fun to have this kind of field of view in UV; chromaticity is somewhat limited compared to the best-bandpass lenses, but short wide-angle lenses that are usable for UV at all are not the most common; even the specialist quartz optics tend to be of longer focal length. Compared to the Enna 28, I would say that focusing is less forgiving (having an opaque lens is a bit of a challenge, even with a sight) and the wider apertures are less usable; however the field of view is 92 degrees rather than 68. The filter stack seems to do its job quite well; there is little sign of filter cosine rolloff, something that would probably be more of a problem with a front-mounted filter. The two lenses seem to have comparable bandpass properties.

[nfoto]

I'm using my Tamron 21/4.5 on a D3200 with built-in Baader U (gen.2) filter and found it surprisingly sharp in UV applications. A future project would be using it on a "full-frame" mirrorless camera and put rear filtration into the camera adapter. Problem is finding an acceptable camera ...

[Cadmium]

To show sharpness I added a couple 'actual pixels' crops to post #30 here:

http://www.ultravioletphotography.com/content/index.php/topic/1626-bushnell-21mm-vs-nikkor-18mm-uv-transmission-test-comparison/page__view__findpost__p__12455

This was using the same filter as Clark. 13.8mm diameter, 2.75mm thick.