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01-14-2012, 07:05 PM   #31
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Is this too simplistic an idea? If a SLR lens (old manual primes mostly) has an IR-focus index mark, then it's probably OK for IR shooting.

Mea culpa: I've mostly shot digital IR with P&S's -- blame the 37mm thread filter set (780-900-939-1000nm) I bought some years back. I first used those on a non-modded camera: long exposures, nasty hotspots. Then I put them on my Sony DSC-V1 NightShot camera which has a switchable IR-blocking hot.filter: short exposures, no hotspots. Moral: use a hot.filter-less camera.

01-15-2012, 12:02 AM   #32
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QuoteOriginally posted by creampuff Quote
the possible internal reflections from a front mounted IR filter are responsible for exacerbating the hotspots and internal reflections.
I would say that is the primary cause for hotspotting, also the flatness of the rear element of the lens would also be a contributing factor to this. So any lenses that are prone to ghosting will be likewise prone to hotspotting.
02-24-2012, 12:59 PM   #33
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many factors are important, not only the lens

a technical approach to hot spots

Mainly, hot spots result from poor performance of the antireflective
coating of lenses in the infrared band. The coating is not supposed to do that job. Many will focus on lenses only, deviding in suitable/not suitable for IR photography, but the lens is only ONE player in that game.

So we should focus more on spectral knowledge than looking at arbitrary
properties of so many lenses. By the way, a simple kit-zoom Pentax 18-55mm II will outperform many more expensive lenses because of its simplicity.

Let us turn to antireflective coating. The change from an
effective coating at 700nm to a much weaker performance at 1100nm is gradual and continous. But we have to come along with that, so we simply try to use the shortest wavelenghts possible to produce the desired infrared look. There are some cameras with only modestly blocked
sensors, e.g Pentax K100/super, Nikon D40 or Samsung EX1.
With those cameras, there is enough energy to expose 1/10th of a second, even at low ISOs, using a Hoya IR72 or Schott RG 715 filter. The weak internal blocking of the sensor will limit the relative amount of longer wavelengths to an acceptable degree, advantage for the near
IR. So in contrast, today, many cameras come along with more effective
IR-blocking filters, models like K200 / K10, K-x / K20 /
K-r or K-5 will do worse IR jobs because of two facts: the sensitivity
to infrared radiation is dramatically reduced compared with K100
or istD.
If you manage to expose long enough to get an IR shot, the mean
wavelength of the effective exposure will turn out to be much longer, so the negative effects of hot spots will increase. In front of the sensor the multicoloured Bayer-pattern will transmit the longer IR wavelengths, preferably to the blue filter cells. Therefore hot spots are mainly milky blue.
What to do? Choose an older style sensor with only modest blocking (Schott BG 38 vs Schott BG39), avoid filters too extreme (IR 72 / RG715 is better than RG830, in summer, even a Schott RG 695 will work to produce the desired wood-effect, providing better sharpness and contrast, less risk of hot spots), and if possible, srew an additional Schott KG-filter of low intensity (Schott KG2) on the lens, thus suppressing longer wavelengths, and use a standard process of reducing / eliminating hot spots by post-processing software in an easy way.

There is a website (in German) not always easy to read, because the view is rather technical at times, but the multi-casual nature of hot spots may ask for that:Digitale Infrarotfotografie - Startseite
In contrast, doing false-colour infrared photography with appropriate filter glasses (violet or blue filters with very high IR transmission by HEBO) will reveal pictures of stronger blue, cyan, yellow, orange, red and magenta tones mainly free of any hot spot phenomena. But this is even more complicated to explain.
02-25-2012, 03:37 AM   #34
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QuoteOriginally posted by EX-Samsine Quote
hot spots result from poor performance of the antireflective coating of lenses in the infrared band. The coating is not supposed to do that job. Many will focus on lenses only, deviding in suitable/not suitable for IR photography, but the lens is only ONE player in that game.
quite correct - however you miss one very important factor: the Hot Mirror - every consumer oriented DSLR has one, and that is what I suspect is a major contributor to the issue of hot-spots. The efficiency of CCD/Cmos sensors recording Infra-Red wavelengths isn't in question here because we all know silicon is very good at it, sensors used for scientific purposes can be used to see far into the IR spectrum. The hot mirror reflects most of the IR that passes through the lens, but there will always be some NIR wavelengths that will slip through. But the problem is the IR that bounces back and reflects internally, since as you pointed out that the SMC coatings aren't designed for NIR these stray reflections can be rather problematic because we are using a lens that is for all intents and purposes optically un-coated. The geometry of the elements used in the optical design of a lens plays a part, if the lens elements are flatter then the probability for hot-spots to be reflected back is very high.

QuoteOriginally posted by EX-Samsine Quote
In front of the sensor the multicoloured Bayer-pattern will transmit the longer IR wavelengths, preferably to the blue filter cells. Therefore hot spots are mainly milky blue.
Actually, NIR wavelengths are so long that the colour filters used on bayer sensors do not have much impact on the total amount of light captured. This is because the wavelengths are so long they simply go around the molecules that make up the dyes that colour the bayer filter. The reason why some hotspots are different colours is largely due to the white balance used - since there is no such thing as colour (as we understand it) in the IR spectrum. The sensor architecture and software for image processing are just interpreting what the sensor is collecting.

these are a set of coloured flash gel filters photographed under sunlight:


this is what they look like when photographed through a Hoya R-720nm filter:




Last edited by Digitalis; 02-25-2012 at 03:42 AM.
02-25-2012, 05:06 AM   #35
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hot spots RENDERED as blue

It is right that our understanding of colour does not apply to the infrared range. When I say, hot spots are blue, of course they are not really blue. The fact is, infrared radiation resulting from internal reflection is capable to expose pixels that are covered by blue dyes, the blue pixels are preferred. They will be rendered as blue, that's the reason why some will say: the hot spot is bluish. This fact can be checked easily: if one looks at the three independant channels B, G, R in the file, then eliminates only the blue channel by software, e.g. by replacing it by the green channel, the hot spot will be weakened strongly. Then try this with the other two colour channels red and green. The result will be different. So finally we come to the point that the hot spot is mainly housed in the blue channel. Any white balance may give a slightly different look to the picture, but the fact is not changed: in most cases hot spots mainly make trouble in the blue channel.
02-25-2012, 06:54 AM   #36
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QuoteOriginally posted by EX-Samsine Quote
When I say, hot spots are blue, of course they are not really blue. The fact is, infrared radiation resulting from internal reflection is capable to expose pixels that are covered by blue dyes, the blue pixels are preferred
That doesn't make sense since NIR wavelengths can pass through all dyes, it doesn't have a preference for any of them. In my experience with IR, a hotspot affects luminosity - and therefore all the channels will be more or less equally affected by it. I suspect that hot spots in the NIR images are related ghosting, albeit the excess IR light is bouncing off the hot mirror as opposed to the sensor - but it is possibly a combination of both. I have shot IR film several times and I personally cannot recall seeing hotspots in any of my frames.
02-25-2012, 07:12 AM   #37
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much less reflection with film

It is true that hot spots are no issue with photochemical film. The surface of film is matte, there are much less regular reflections towards the lens. For digital photography, better lens coatings had to be developed since the pure sensor alone is already highly reflective, without any help of the hot mirror. It is not true that all dyes transmit IR the same way, some do more, some do less. It is You who gives the proof, because in your photos the green gel flash filter is significantly darker than the blue one. So I did a lot of metereing with spectral photometers and the resullts say the same: many dyes will transmit IR, but there are clear differences. You may simply do the suggested practical test to realize in which channel the hot spots are the most evident. Then You might believe.
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