[biz-engineer]

Using an IR filter in front on the lens, on a conventional digital camera can produce stylish images. Since the IR filter will cut most of the visible light, such images come at the cost of long exposure times.
Beside the blur effects created by longer shutter speeds (foliage, water..), I'd like to know if converted cameras* deliver sharper images (vs using same lens IR filter with a conventional digital camera)?
Or, working the question differently: for long exposure IR photography, is there an advantage using a converted camera over a conventional camera?

*removal of IR filter in front of digital sensor

[robgski]

Not Pentax, but this video could answer some of your questions.

Youtu.be

Youtu.be

[pepperberry farm]

there is always the option of a Sigma Foveon-sensor camera, where the swap to a full-IR camera is just the removal of the filter in front of the sensor (it snaps in and out in seconds)....

[photoptimist]

IR photography on an unconverted camera depends on the camera having a low-quality IR-removal filter (so some IR light does get through) and the user adding a very high-quality IR-pass filter so that no visible light gets through.

In general, an unconverted camera's internal filter blocks the vast majority of the IR and the added IR-pass filter leaks a small percentage of visible light. The result is that the image mixes the low-levels of IR (leaked by the camera's IR-block filter) with low-levels of visible light (leaked by the added IR-pass filter). If the lens is not apochromatic all the way into the IR spectrum, the images will be fuzzy because the visible image has a different focus point than the IR image.

Thus, a converted camera will perform much better although there is still the issue of finding lenses that are sharp in IR.

Finally, it's worth noting that IR images will generally be less sharp than visible light images both because the longer wavelength of IR creates more diffraction effects and because IR photons tend to penetrate deep into the silicon chip which can mean their photoelectrons end up in adjacent pixels.

[biz-engineer]

Not Pentax, but this video could answer some of your questions.

The video doesn't answer my question. My question is if converted cameras (without IR blocking filter) deliver IR images sharper than IR images delivered by a conventional camera with an IR filter in front of the lens. Thomas Heaton had his camera converted to IR 720nm, so he's having shutter speeds like 1/1000th.

---------- Post added 05-07-22 at 14:28 ----------

In general, an unconverted camera's internal filter blocks the vast majority of the IR and the added IR-pass filter leaks a small percentage of visible light.

I see the tradeoff , getting an image out of residual wavelengths that are all intentionally blocked at some stage in the optical path

Thus, a converted camera will perform much better although there is still the issue of finding lenses that are sharp in IR.Finally, it's worth noting that IR images will generally be less sharp than visible light images both because the longer wavelength of IR

You answers my question. Thanks a lot. The lens should be focused for the dominant IR image, and the visible light should contribute much less to the image on a camera converted to IR.

[EssJayEff]

On a related note, I recall from film camera days that infrared wavelengths come into focus at a different point than visible wavelengths, so you had to adjust your focus manually or use the infrared guides marked on some lenses. I'm presuming that is still the case with digital, since the film plane and the sensor surface are essentially the same thing. Is that (i.e., the focus shift) correct?

---------- Post added 07-05-22 at 08:44 AM ----------

Just found this: Film Infrared Photography – Focus Techniques.

[ProfessorBuzz]

Sharpness is a function of the optics, and the ability to bring all wavelengths of interest to focus at the same point on the image focal plane.
Removing the internal UV/IR blocking filter will change the focus distance, and this will have an effect.
If you want sharper, you need a great lens and go with a narrowband filter that passes a limited range of wavelengths. Astrophotographers often use a Hydrogen alpha bandpass filter on the moon and shoot monochrome.

[Medex]

I think converted camera is better if you want sharper images, and one aspect that was not mentioned is removal of anti-aliasing filter (most photographers prefer to convert their older cameras that have AA filter).
If you like black and white infrared, then your best choice for details is monochrome + infrared converted camera. I have one at this time (K-30). Still did not have possibility to shoot more with it. And I have same K-30 converted just to IR (both cameras 640 nm filter).
What I can say from my observations using Ricoh GR 16 mpix monochrome and GR III 24 mpix normal camera, monochrome gives at least same amount of details even the picture is enlarged to match 24 mpix picture size.

Full size pictures available...

Monochrome infrared (MIR), K-30, 640 nm


Monochrome infrared (MIR), K-30, 640 nm


Monochrome infrared (MIR), K-30, 640 nm


GR1 monochrome 16 mpix on the left enlarged to 24 mpix dimensions vs GR3 normal sensor 24 mpix

[kypfer]

This page might prove helpful https://www.pentaxuser.com/article/infrared-photography-with-a-pentax-digital-slr---part-1-466

[Bob 256]

Typically, you're going to get sharper images with a converted camera for one reason, the lens can be stopped down considerably further compared to a non-converted camera (due to the increased camera sensitivity to IR), and you can make use of the increased depth of field thus obtained. So the simple answer is yes, you might expect sharper images with a converted camera.

Since you're shooting a relatively narrow range of wavelengths, there are no issues with color correction in the lens. In fact, you can get fair results with a non-achromatic lens since you aren't dealing with a range of colors as in normal photography. All this presumes an IR filter on the lens.

In my K-01, the UV/IR blocking filter was replaced with a like thickness of glass so the autofocus wasn't affected. The AF works just as it would for visible light and seems to get the focus pretty close. It can then be modified if needed using Liveview with magnification (and switching to manual focus).

[biz-engineer]

In general, an unconverted camera's internal filter blocks the vast majority of the IR and the added IR-pass filter leaks a small percentage of visible light.

There is also the RGB CFA in the way of IR, IR being out of RGB bands. Is Bayer interpolation still needed to form the image?

---------- Post added 06-07-22 at 08:35 ----------

This page might prove helpful https://www.pentaxuser.com/article/infrared-photography-with-a-pentax-digital-slr---part-1-466

Interesting read thanks.

[photoptimist]

There is also the RGB CFA in the way of IR, IR being out of RGB bands. Is Bayer interpolation still needed to form the image?

"It's complicated"

The red, green, and blue filters in the typical Bayer filter camera each have different IR sensitivities that vary across the IR spectrum (see graph below as an example from a Sony sensor but not one used by Pentax). Those different sensitivities imply: 1) that if you don't interpolate, a RAW image translated directly into a TIFF will have checkerboarding; 2) if you only develop a single channel, you'll still need to interpolate to fill in the missing value; 3) who knows how the white-balance algorithms are going to handle it.

(If I had to guess, Bayer-interpolated "color" IR images using the sensor in that graph would have a color space that spans brownish yellow for near-IR to white for far-IR subjects.)


TLDR; Yes, you'll want to interpolate but the colors will be muted and a bit strange.

[newmikey]

Check one of my recent posts as I just had a K-5 converted with a 630Nm filter. My experience is that shooting handheld increases your chances of a sharp image but at the downside of having some lenses not performing very well in infrared light. First few days of shooting show that my DA16-85mm is plenty sharp, DA*50-135mm ditto but my Sigma 8-16mm will only deliver fuzzy images and has hot spot issues. I've tried AF finetuning but besides the fact this is really hard on an UWA, the potential range seems out of bounds for the -10 to +10 scale in the K-5. That would require setting a different finetuning basepoint in debug mode - doesn't seem worth it if all other lenses perform fine.

As to subject matter, broad landscapes and architecture with some sky in the image seem to fare best as far as sharpness and detail microcontrast are concerned whereas dense foliage in f.i. forests turn out fuzzy at best. Mind you, I'm still taking baby steps here so there may be a tweak or trick I am oblivious to but those are my initial observations after shooting for less than a week.

[EDIT] This was a great reason for me to test my Tammy 90mm macro (the oldest AF version, I believe it's the 172E) on a flower vase sitting on the dining room table. See how much detail I could extract. Color range is really flat, almost (but not entirely) monochrome. Detail sharpness seems great and the lens AF is spot-on despite the IR filter.

[tuco]

Examples on a full spectrum camera with just a R72 filter. I don't get hot spots with these older lenses I use on this camera with shutter speeds at base ISO typically ranging from 1/2 to 2 seconds on sunny days at f11. Focusing is not a problem with the filter on becasue the old M9 is a rangefinder. On my full spectrum DSLR I would get hot spots with the modern lenses on this system and shutter speeds around 30 seconds at base ISO, f11 and sunny days. So much depends on the camera and what filters the manufacture puts in front of the sensor for an unmodified camera. Click through for larger images.


Voigtländer Nokton 50/1.5


Maryhill, WA by tuco, on Flickr


Ferns by tuco, on Flickr


Zeiss 25/2.8


The Sphere by tuco, on Flickr


Arthur Temple by tuco, on Flickr


Grand Avenue Park by tuco, on Flickr


Zeiss 35/2


The Wild Cat Covered Bridge by tuco, on Flickr

[Wheatfield]

Interestingly enough, there isn't a whole lot of visual difference between an image shot with an IR converted camera and a software conversion from a non converted camera. They aren't the same, but the effect, visually, is.