Dandelion test w/ Sigma 70mm DG macro Art

[GaryR]

I mainly use my Sigma 70mm macro Art lens + sd Quattro, for normal, color IR and monochrome IR, and was curious to see how this lens performs for UV. The sd Quattro H is resigned to monochrome UV, but surprised with the results.

 

sd Quattro H + B+W 403 + BG39 2.5mm

 

 

[colinbm]

Looking good Gary

[nfoto]

A dandelion "bull's eye" can be observed with most lenses able to go slightly below 390nm. In fact I can readily see it with the naked eye under overcast skies (not in direct sunlight).

 

However, seeing the familiar pattern is not the same as having it presented with strong, healthy contrast. A lens has to transmit lower than what I suggested above. Plus, more importantly, be able to make a good quality image not too impacted by residual aberrations. After all, using an ordinary lens for UV imaging is pushing it outside its designated operating domain, an act to which many lenses respond quite poorly to.

[Stefano]

A dandelion "bull's eye" can be observed with most lenses able to go slightly below 390nm. In fact I can readily see it with the naked eye under overcast skies (not in direct sunlight).

Are you referring to the slightly orange center? I usually can distinguish between “bull’s eyed” dandelions and “all black” ones just by looking for the orangish center.

 

Maybe you can see the bull’s eye pattern with violet light above 400 nm? I can try with my 405 nm LED (emitting closer to 400-401 nm), but my filter is basically a 400 nm shortpass filter. Polycarbonate goggles appear like 50% transparent with my camera/filter and this LED.

[nfoto]

I'm referring to what will appear UV-black in a capable UV-photo. Many microspecies of dandelion have a much smaller UV-black bull's eye than the centre seen more deeply yellow-orange.

[Stefano]

Many microspecies of dandelion have a much smaller UV-black bull's eye than the centre seen more deeply yellow-orange.

So not always what you see as orangish with the naked eye will appear black in UV? That’s interesting.

[nfoto]

Some microspecies of dandelion have almost no UV bull's eye whatsoever. However this applies mainly to the smaller species that occur in natural habitats such as dry slopes, upland and alpine regions, or sand dunes. The big "weedy" ones (Taraxacum sect. Ruderalia) regularly have large bull's eyes in UV.

 

Tussilago farfara Coltsfoot by contrast always has UV-black disc flowers, thus is in that sense a more reliable test target for assessment of UV/filter/lens response. Its ligules ("strap" or "ray" flowers) are quite UV-bright, but many dandelions are much brighter in those flower parts.

[Stefano]

I remember seeing dandelions (they looked like them, just with smaller heads) near the Parthenon, in Athens. They appeared uniformly yellow to my eyes, no orange center. They were completely black in UV.

[nfoto]

Asteraceae Daisy Family to which the dandelions and coltsfoot belong are the largest of all plant families with over 25.000 species. Many members of this family are yellow-flowered and a good deal of those appear UV-black all over.

[GaryR]

Thanks Colin, and thanks for your explanation Birna.

 

Our yard is covered with the standard Ruderalia variety this time of year, so I couldn't resist trying a non-UV lens. My Sony a7R/ Petri/Kuri or El Nikkor lens would likely fare much better, but surprised to get any level of UV response from the Sigma 70mm.

 

The Quattro sensor gets quite noisy at high iso's, but I pushed the sdQH to iso6400 for this set.

[Stefano]

I guess that identifying the exact species is quite difficult since most of them are very similar...

[nfoto]

Not more difficult than in any other grouping of plants (dandelions and hawkweeds both being apomictic genera are exceptions).