[MJKoski]

I did not find such thread. Well let's make one. Maybe it will be useful for someone to save some money and hassle.

So far, my experiences with non-converted stock body:

Samyang 24mm f/1.4 K-mount
720nm Hoya R72 produces bright, small and clear hotspot when stopping down beyond f/5.6 and large mild one after f/2.0
Poor performer

Samyang 35mm f/1.4 K-mount
720nm Hoya R72 produces bright and quite large hotspot when stopping down beyond f/5.6 and large mild one after f/2.8
Poor performer

Pentax 31mm Ltd AL f/1.8
760nm and 850nm chinese filters produce faded-in and bright hotspot after f/4.0
Average performer

Zeiss 50mm Makro-Planar f/2.0
Works from 720nm to 950nm at all apertures using the same chinese filter brand as above
Excellent performer

I would say that, unless the hot-mirror filter on stock body is the culprit, Kolarivision hotspot database is wrong considering the mentioned lenses. At least the 35mm Samyang and 31mm Ltd. So far, not much luck with other lenses than the Zeiss 50MP. Should I try with old SMC manual primes? Or modern zoom lenses?

[baro-nite]

Interesting findings, thanks for posting. I routinely use SMC ("K series") manual primes on my 590nm-converted (by Isaac Szabo) K-5, usually at f/8 to f/11, and have not observed any hotspots. The SMC Pentax 1:3.5 35mm is probably my most used lens on that camera. Also the Samyang (Rokinon-branded) 24/3.5 tilt/shift.

---------- Post added 2019-06-03 at 09:30 AM ----------

I should add that after I had my K-5 converted, I did a quick comparison test between that, and the K-1 with an external IR filter attached. I found the results with the K-1 unsatisfactory; I guess the internal filter is rather effective at filtering out IR. Very different results my converted K-5.

[MJKoski]

How did the K-1 differ exactly? Exposure times go up of course, but what other side-effects did you notice?

[photoptimist]

These hot-spots are almost certainly an interaction between the hot-mirror (which is reflecting near-IR & IR) and the 720 nm Hoya R72 (which also strongly reflects near-IR around 720 nm). The light from the scene is passing through the filter, being focused by the lens, hitting the hot-mirror, reflecting back through the lens, hitting the backside of the Hoya, and being bounced back. (You can do a back-of-the-envelope ray tracing to see how this works.)

Hot spots like this will be a problem with "high quality" IR filters that using thin-film technology to create a sharp bandpass. Cheaper filters that use a bulk material that absorbs visible light (and passes IR) will have much less problems although these absorptive filters still need good AR coatings to avoid all problems with reflections between the IR-Filter and hot mirror.

One test of this hypothesis is that if you slightly tilt that Hoya IR filter while shooting with a narrow aperture, the hot-spot will move off-center move (for very small angles of tilt of less that 2-3°) and disappear (for angles of tilt exceeding a few degrees). If the lens is the cause, tilting the filter won't affect the location of hot-spot. But if the hot-spot moves and disappears when you tilt the filter, then the filter is the culprit.

Finally, even with a converted camera, there might still be some hotspot issues with light bouncing between the silicon sensor (which is fairly reflective) and whatever IR filter is in front of the lens (especially at ∞ focus). Again, a small tilt of the filter might help.

[MJKoski]

Yes, that is good theory what happens. But it does not explain how the Zeiss 50MP works with those chinese filters OR the Hoya R72. There are zero issues with the Zeiss. Only thing which comes to mind is that sensor microlens array works differently with non-wideangle lenses when it comes to IR imaging. Samyang lenses and the 31mm Ltd can be considered as wide angles. Zeiss coatings may also play a role here.

Zeiss examples:

1. Haida 720nm IR filter (no coatings), red+blue channel swap


2. Neewer 850nm IR filter (no coating, also passes "some" visible light)


3. Neewer 950nm IR filter (no coating, not a real 950nm either...)


I should have coated Heliopan RG780 filter soon to try it out. It was about 10x more expensive than those chinese pieces and double the price of R72.

EDIT: KolariVision article was very interesting and also confirms Zeiss 50MP results.
https://kolarivision.com/the-science-of-infrared-hotspots/

Indeed, my K-1 probably needs the conversion to fully work with the lenses which SHOULD work including 31mm Ltd.

[baro-nite]

How did the K-1 differ exactly? Exposure times go up of course, but what other side-effects did you notice?

I didn't save any of the K-1 images and this was a year ago; my sense was that the stock K-1 plus external filter (the Haida 720nm) gave a result that was much heavier on visible light than what I get from the K-5 with 590nm filter installed. Now I wonder, though, because your results look really good.

[enoeske]

Yes, that is good theory what happens. But it does not explain how the Zeiss 50MP works with those chinese filters OR the Hoya R72. There are zero issues with the Zeiss. Only thing which comes to mind is that sensor microlens array works differently with non-wideangle lenses when it comes to IR imaging. Samyang lenses and the 31mm Ltd can be considered as wide angles. Zeiss coatings may also play a role here.

Zeiss examples:

1. Haida 720nm IR filter (no coatings), red+blue channel swap


2. Neewer 850nm IR filter (no coating, also passes "some" visible light)


3. Neewer 950nm IR filter (no coating, not a real 950nm either...)


I should have coated Heliopan RG780 filter soon to try it out. It was about 10x more expensive than those chinese pieces and double the price of R72.

EDIT: KolariVision article was very interesting and also confirms Zeiss 50MP results.
The Science of Infrared Hotspots - Kolari Vision

Indeed, my K-1 probably needs the conversion to fully work with the lenses which SHOULD work including 31mm Ltd.

I love that second shot

[MJKoski]

Thanks! I think I will like this 31mm Ltd even if it is limited to f/4. Because...well it seems I hit a jackpot with these chinese things. This Neewer 760nm filter turns harsh noon daylight into magical winter-summer-sunset hybrid.

FA31mm Ltd @ f/4 & Neewer 760nm filter


I just ordered few backup sets of these filters.

[travelswsage]

Nice shots.


I did try the K-1 with DFA 28-105 with the Hoya R 72 on the wide side 28-35mm and had no hotspots. The Zeiss 25mm was also a good performer.

[Bob 256]

These hot-spots are almost certainly an interaction between the hot-mirror (which is reflecting near-IR & IR) and the 720 nm Hoya R72 (which also strongly reflects near-IR around 720 nm). The light from the scene is passing through the filter, being focused by the lens, hitting the hot-mirror, reflecting back through the lens, hitting the backside of the Hoya, and being bounced back. (You can do a back-of-the-envelope ray tracing to see how this works.)

Hot spots like this will be a problem with "high quality" IR filters that using thin-film technology to create a sharp bandpass. Cheaper filters that use a bulk material that absorbs visible light (and passes IR) will have much less problems although these absorptive filters still need good AR coatings to avoid all problems with reflections between the IR-Filter and hot mirror............

Are you sure about this? I have a Hoya R72 in front of me and it's a dye type filter - no thin films. Not doubting your theory, but the light would have to be reflected off the glass housing the filter material - it doesn't appear that the Hoya is coated to reduce reflections so that could be a source of unwanted reflections. If I look at this filter in IR (with my IR camera), it appears to be pretty transparent, not showing much reflection but the glass reflects just as it would for visible light.

I think some of these "hot spot" issues have to do with how lens coatings interact with IR since they are designed to reduce reflections of visible light, their IR performance can be quite different, and they could be reflective at IR wavelengths.

Most IR filters that I'm familiar with (for photographic use) are the dye type. Dichroic (thin film) filters are expensive to make and the narrow transmission bands they provide aren't needed for normal IR photography. It's pretty easy to make a dye that's opaque to visible light and transmits IR - most non-pigmented black printer ink has this property, so it's generally cost effective to make IR filters using dyes.

You could check this theory out by using a flat wall that shows a hot spot. If the IR filter is removed and the wall is illuminated with IR light (darkness needed otherwise), the hot spot should disappear if the filter is at fault. If it is still there, the lens/camera system is causing it. Putting the filter back on should produce the hot spot if it's at fault.

[Medex]

Thanks! I think I will like this 31mm Ltd even if it is limited to f/4. Because...well it seems I hit a jackpot with these chinese things. This Neewer 760nm filter turns harsh noon daylight into magical winter-summer-sunset hybrid.

FA31mm Ltd @ f/4 & Neewer 760nm filter


I just ordered few backup sets of these filters.

MJKoski, does your k-1 is full spectrum, or with stock hot mirror?

[photoptimist]

Are you sure about this? I have a Hoya R72 in front of me and it's a dye type filter - no thin films. Not doubting your theory, but the light would have to be reflected off the glass housing the filter material - it doesn't appear that the Hoya is coated to reduce reflections so that could be a source of unwanted reflections. If I look at this filter in IR (with my IR camera), it appears to be pretty transparent, not showing much reflection but the glass reflects just as it would for visible light.

I think some of these "hot spot" issues have to do with how lens coatings interact with IR since they are designed to reduce reflections of visible light, their IR performance can be quite different, and they could be reflective at IR wavelengths.

Most IR filters that I'm familiar with (for photographic use) are the dye type. Dichroic (thin film) filters are expensive to make and the narrow transmission bands they provide aren't needed for normal IR photography. It's pretty easy to make a dye that's opaque to visible light and transmits IR - most non-pigmented black printer ink has this property, so it's generally cost effective to make IR filters using dyes.

You could check this theory out by using a flat wall that shows a hot spot. If the IR filter is removed and the wall is illuminated with IR light (darkness needed otherwise), the hot spot should disappear if the filter is at fault. If it is still there, the lens/camera system is causing it. Putting the filter back on should produce the hot spot if it's at fault.

Interesting. I had assumed the Hoya R72 was thin film based on the sharpness and strength of the transmission curves but it would seem I'm wrong.

It's certainty true that the coatings on lens elements are optimized for visible wavelengths. But I would think that the impact of poor coating performance would be low contrast images, especially for more complex lenses. That is, the lens would act like an uncoated lens from the olden days before good coatings.

The fact that these hotspots are worse at narrower aperture settings and the spot size diminishes with narrower aperture settings suggests that it's caused by something in front of the aperture iris.

Your idea for testing for hotspots with an IR-lit wall is a good one! Putting the IR filter on the light source, not the camera, would show if the hotspot is intrinsic to the lens-sensor combination or if the filter really is causing the problem.

The other test is to tilt the filter. Quickie pencil sketches of the ray tracings bouncing around in the optic system suggest that the hot spot only happens if the filter is parallel to the sensor. (Note: that quickie tracing also suggests that the further the filter is from the lens, the dimmer the hotspot -- the deeply recessed optics of the in the Zeiss 50/2 Makro may help it avoid this.)

[MJKoski]

MJKoski, does your k-1 is full spectrum, or with stock hot mirror?

It is stock body. No conversion done, yet. But I think I have reached some kind of limit what can be done without conversion. After sunset the exposure times go through the roof all the way to 5-10 minutes.

[Medex]

It is stock body. No conversion done, yet. But I think I have reached some kind of limit what can be done without conversion. After sunset the exposure times go through the roof all the way to 5-10 minutes.

I use cnverted pentax k-5. With 640 nm filter on the sensor. It is very convenient. I shoot landscapes and portraits. And now I want to buy second k1 and do conversion. But cant find not expensive IR filter to use because I need bigger on compared to APSC sensor

[Bob 256]

I considered using my K-1 for IR before having a K-01 converted, and there's no comparison. The K-1, with its IR rejection filter in place is terribly sluggish for capturing an IR image and in some cases, as MJKoski pointed out, is useless. Also, with an IR filter over the lens, the viewfinder can't be used (Liveview only) and autofocusing is real iffy. That's what pushed me to have a K-01 converted and I've been happy ever since. It is mirrorless so shows the IR image on its viewscreen (could be a bit brighter for daylight viewing) and the autofocusing works great. I haven't tried it with too many lenses yet so haven't come across any that hotspot - have to test some in my collection. Hotspotting is probably a lens camera combination so might differ from individual to individual, hence the different reports for given lenses, but there are some lenses out there that are notorious from reports seen.

---------- Post added 06-04-2019 at 09:01 AM ----------

.............................................
Your idea for testing for hotspots with an IR-lit wall is a good one! Putting the IR filter on the light source, not the camera, would show if the hotspot is intrinsic to the lens-sensor combination or if the filter really is causing the problem.

The other test is to tilt the filter. Quickie pencil sketches of the ray tracings bouncing around in the optic system suggest that the hot spot only happens if the filter is parallel to the sensor. (Note: that quickie tracing also suggests that the further the filter is from the lens, the dimmer the hotspot -- the deeply recessed optics of the in the Zeiss 50/2 Makro may help it avoid this.)

I did note a strong glass to air reflection from the Hoya (you can see the reflection of a light source when viewing a tilted filter through the IR camera) so it does appear they didn't use anything to suppress these with coatings. That could well be one possible cause. Will test further with some of my other lenses to see if I can get a baddie for reference, and then to narrow down the culprit. Right now, I'm using a single zoom for the IR camera and it is spot free, but I'm sure it's the exception.