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Infrared
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#21 nfoto

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Posted 25 May 2021 - 06:58

I gather the different wavelengths were present within the same frame?

#22 Andrea B.

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Posted 25 May 2021 - 09:08

oh, now I get what Andy meant !!

I seem to have been thinking of focus shift only in conjunction with changing filters or changing apertures at which time one might have to refocus with certain lenses.

Having two kinds of IR reflectivity within one photo could also cause a kind of "focus shift". But wouldn't it be more of an axial aberration?

I don't think I've ever seen that happen in any IR I've photographed.
Either I've been just lucky or I haven't photographed enough IR. La!! :lol:
Probably both.
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#23 nfoto

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Posted 25 May 2021 - 09:57

Yes, a kind of axial colour aberration or longitudinal chromatic aberration. Which, in fact, is the most unpleasant kind as the colour fringing will differ front and rear of the focused plane thus making later correction difficult., if not downright impossible. We see a lot of such wave-length dependent focus shifts just outside the visible range for many lenses, in particular when entering the UV range.

However we ought to keep in mind that blaming a lens not designed for use outside the visible spectrum for its behaviour when (ab)used in that manner, is not fair.

#24 Andy Perrin

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Posted 25 May 2021 - 15:25

I defer to you all on the proper terminology, but yes, two different wavelengths of light in the same image (from different light sources) will end up focusing at two different planes even if they are the same physical distance from the camera. Meaning that there is no way to adjust the lens to deal with it and no way to fix it in post processing either. The only solution is to find a different lens which is corrected for the range of wavelengths we are interested in.

Andrea, it’s entirely possible that the lenses you use are well-corrected enough in IR that you didn’t see it, or you just never encountered the kind of mixed lighting situation that makes the effect most obvious (hint: nighttime IR photos have that a lot...).

Anyway, back to the original question, leaves reflect IR so well that if you had a filter with a shorter cutoff like a 550nm on there, it’s possible you would see some leaf blur relative to other objects in the scene if they were reflecting mostly shorter wavelengths.

Edited by Andy Perrin, 25 May 2021 - 15:34.


#25 Andrea B.

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Posted 25 May 2021 - 18:40

Andy, you are likely spot on that the lenses I use most often do not have any serious IR-aberrations.
And also, it is true that I have not made many night IR photos.

On a side note: Here I don't have much to shoot at night unless I go into town. Must do this. And I would love to have an IR critter camera to see all the rodents around here who have wonderful parties at night, lots of dancing and hijinks.

So your example was *very* instructive to me. Good !!! :grin: (...and thanks, as always....)

The leaf problem, to me, is probably just the typical lack of fine detail in IR because most leafy foliage does not differ greatly in its IR reflectivity. We should look up that value. I used to know it, but it escapes me at the moment. :rolleyes:
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#26 Andy Perrin

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Posted 25 May 2021 - 21:14

Quote

The leaf problem, to me, is probably just the typical lack of fine detail in IR because most leafy foliage does not differ greatly in its IR reflectivity. We should look up that value. I used to know it, but it escapes me at the moment. :rolleyes:
The issue isn't that the foliage differences in IR reflectivity, it's when you have, say, a car and a tree in the same scene next to each other with a 550nm long pass filter. You focus on the car (reflecting visible red at 600nm, cause it's a Miata), but the trees are reflecting 700-1000nm. So you get a nice sharp Miata and a blurry tree.

Of course in addition to this you have diffraction in both.

Edited by Andy Perrin, 25 May 2021 - 21:15.


#27 nfoto

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Posted 25 May 2021 - 21:30

This scenario will play out only if the lens fails to keep its focus shift under sufficient tight control across the spectral range used for the capture.

#28 dabateman

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Posted 26 May 2021 - 04:48

View PostAndy Perrin, on 25 May 2021 - 21:14, said:


The issue isn't that the foliage differences in IR reflectivity, it's when you have, say, a car and a tree in the same scene next to each other with a 550nm long pass filter. You focus on the car (reflecting visible red at 600nm, cause it's a Miata), but the trees are reflecting 700-1000nm. So you get a nice sharp Miata and a blurry tree.

Of course in addition to this you have diffraction in both.

But isn't that how you make the car look to be moving faster than it really is?
Parked cars aren't always so interesting.