Reflected UV Light/Color Comparison of Flower lit with Sunlight vs. Sunlight+365nm Flash vs. 365nm Flash

[Pylon]

 

The white balance was set in Adobe Camera RAW using a custom white balance profile created with the Adobe DNG Profile Editor application.

 

With the sunlight-only image, it seems there is mainly just blue.... but when the 365nm light is introduced, another shade of color comes in. What is the explanation for this? I would think the sun would be lighting the flower with a broader span of wavelengths (ie. a more-or-less even graph from 300-400nm), whereas the flash is peaked at 365nm, and overpowers the sunlight at the 365nm peak but the wavelengths outside of the 365nm peak are limited/suppressed, rendering the outer wavelengths as blue on the sensor (after the white-balance set in post-production). Whether the blue is representing wavelengths before or after the 365nm peak, I don't know.

 

To see an image of the same flower photographed with just the flash alone with no sunlight, view this post: Nikon 50mm Series E vs Coastal Optics 60mm APO (Reflected UV Photography testing for transmission and color)

[Pylon]

Here is an even more dramatic comparison, where the saturation has been increased on the same images above:

 

Sun only (entire frame was selected for white-balance):

 

Sun + 365nm fill-light (uses same WB settings as previous image):

 

Sun + 365nm fill-light (entire frame was selected for white-balance):

 

and just for reference, here is the transmission graph for the filter on the flash (the red line) and the filter on the lens. I don't have a transmission graph for the lens itself or the sensor; or an emission graph of the sun, though!

[ulf]

The effect is caused by the different spectral characteristics of sun and flash, dominating different parts of the image.

It can give artistically interesting images, but have nothing to do with floral pattern, only how shadows are falling.

The flash's light is able to fill in in the areas where the sunlight is shadowed and have more UV there.

The directions of the light from sun and flash are different, making the effect more pronounced.

 

This is a typical problem / feature when mixing different light sources.

It can sometimes even be seen with sunlight only.

The effect then appear in the shadows where the fill-light comes from the sky.

[Cadmium]

I would sure like to see those white balanced in NX-D or Photo Ninja, just to compare...

[dabateman]

Your sunlight is mostly 380nm to 400nm.

Your flash is giveing you more 365 to 370nm of light. Thus the shift to yellows.

400nm to 405nm is purple, violet.

380 to 400nm is blues

355 to 380nm is your golden yellows

300 to 340nm goes green.

 

So your sunlight is more 380 to 395nm, as selected your Baader venus filter and camera sensitivity.

As you add the flash you are giving more deeper 370nm light allowing you to see the typical yellows.

[Andrea B.]

No David, not every flower has false yellow after white balance. Ulf has given the correct explanation. Pylon has supplied 365nm light at a greater intensity than the sunlight because the flash is up close to the flower.

 

To prevent the mottled false yellow, the flash should have broader filtration that doesn't peak at 365nm. I use ny modded SB-14 flash to fill outdoors and get the same look from a flower as I would from sunlight. But get shorter exposures, of course, with the flash.

 

Yes, there is a drop off in intensity from 400 nm down to 300 nm, but when photographing under a broadband filter like the BaaderU you are not recording 380-400 nm. You are recording mostly 325-370 nm. The BaaderU suppresses quite a lot of the input between 380-400 nm.

 

The results here illustrate why we encourage white balancing against a white standard stable under UV/Vis/IR. "Average" white balance is less accurate. You shouldn't use average white balance for such comparisons.

[Pylon]

New white balance test coming tomorrow, hopefully.

[Cadmium]

Pylon, If you ever need a Kuri or Kyoei 35mm to test, I am close by.

[Cadmium]

Yes, there is a drop off in intensity from 400 nm down to 300 nm, but when photographing under a broadband filter like the BaaderU you are not recording 380-400 nm. You are recording mostly 325-370 nm. The BaaderU suppresses quite a lot of the input between 380-400 nm.

 

This would depend also on the lens being used. The above statement may well apply to a UV-Nikkor or Coastal Optics, but some of even the best 'UV friendly lenses do have tapering transmission plots the deeper they go,

and that will cut down on lower UV just the same way that the Baader U may cut down on higher UV. Also higher UV is stronger, and lower UV is weaker, both in amount (depending on light source, and in sensitivity by the sensor,

but just the lens alone will work like a filter, limiting the amount of lower UV that is really being actualized because of the tapered transmission curve of the lens.

Most of the UV only filters we use (Baader U and stacks) are fairly equal to many of the transmission plots of the 'friendly UV lenses' we use, reaching down to about 320nm with the best case scenario (other than the special lenses I mentioned).

Therefore, based on the lens, filter, source, and sensor, I don't think we can expect much from the lower part of that range, at least when the peak part of that range in also engaged.

 

Here is a kind of example.

The Baader U (red plot), the Kuribayashi 35mm (blue plot), disregard the rest.

If we take 50% of the combined peak at ~360nm (~80%) = ~40%, this gives us about 335nm to 380nm FWHM range.

The lens transmission has a lot to do with that range, even when using a good UV friendly lens like a Kuri 35mm.

Of course, if you have a special and expensive deeper transmission lens, with a flat transmission profile (such as a UV-Nikkor)... then you can recalculate the range.

The rest of us will be happy to have a lens as good as the Kuri, and given the limits of the filter, and sensor...

I often wonder what the point is of having the UV-Nikkor or the like (yes, except of course for exploring deeper filters and the limits of the sensor and such), not that I wouldn't like to find out, but for the usual UV filter range, lens, etc....

 

 

[ulf]

The sensor response combined with the light source spectrum are often the most limiting factors, even if the lens can contribute and must be reasonably good for UV.

The 18mm in the graph above is not OK for UV

Appart from that I fully agree with all Cadmium is saying above.

 

I think I'd liked to use an UV-Nikkor too, but only because of it's sharpness and low focus shift.

The transmission range it has is totally unimportant with our type of cameras.

[Cadmium]

Yes, "disregard the rest", the Nikkor 18/4 being part of the rest. However, the Nikkor 18/4 was at one time anyway, one of the best 18mm lenses for UV, it was Birna's 18mm UV pic, however I don't see it listed on her lens page now.

I am sure some of you have other 18mm lenses at this point in time that work better for UV? Maybe.

However, in no way does it reach the standard threshold of UV transmission.

I didn't put it on that graph for this discussion. It is just there. :-)

Let me see if I have some UV shots using that lens.

Here is a comparison using the Nikkor 18/4, Baader U vs U-360 stack.

Cold, cloudy, with a dusting of snow it looks like, and no nice yellow flowers, boo hoo. :-(

But the Sparticle test tells more.

 

Baader U

 

U-360 stack

 

The 18mm has as good of Sparticle UV transmission as does the Bushnell 21mm f/4.5 (AKA Tamron 21mm f/4.5)

https://www.ultravio...ndpost__p__9137