Tuesday, July 9, 2013

D800 does 850nm infrared panoramas

Infrared photography with modern DSRLs needs a infrared passing filter and extremely long exposure times - several minutes is not unheard of. Newer the camera, the less infrared the IR blocking filter lets thru. Nikon's D800 is no exception to this rule; whereas trusty old D40 could shoot 850nm black and white infrared photos hand-hold, the D800 needs routinely 30 seconds or more exposure. The choice of filter affects the outcome the most.

Popular 720nm filters such as the Hoya R72 let thru some color, resulting in "typical" infrared look where grass is yellow and sky is blue (after channel swap - or the colors will be reversed turning sky yellow). 850mn filter however yields thoroughly black and white image, since the color filter array atop of the sensor is wholly transparent at these wavelengths - turning foliage white and skies and water black or dark shade of grey. Incidentally this means that a 850nm filter can be used to set a UniWB white balance for the camera; it's a good idea to save that WB setting in of the camera's banks for later retrieval.

This post mostly discusses using 850nm filter, since using and post-processing B&W infrared shots is easier. Filters can be acquired for around 20 euros like this 850nm one from Fotga. Results with 900nm - 1000nm filters should look similar, but the exposure times might get longer since the filter on sensor will block more and more of the light.

(Nikon AF-S 50mm f1.8G wide open.)

Lenses for infrared

Not all lenses are created equal for infrared photography. Older and simpler designs tend to perform better at infrared wavelengths, less glass elements there are, the less there reflections off them. Personally I've tested some lenses:

* Nikon 18-55mm f3.5-5.6 II (the all plastic kit lens): good lens for infrared if mechanically the build quality leaves lot to be desired. No hot spot but flares quite a bit when pointed in the direction of the Sun.
* Tokina 11-16mm f2.8: unusable for infrared if fine lens otherwise, strong hotspot in the middle. Flat field correction might fend off some of the hot spot, but that's untested as of now.
* Samyang 35mm f1.4 and its variants: mixed experience, sometimes there's hotspot, and sometimes not. Flares easily. Worth a try though, it's optically marvellous lens.
* Nikon AF-S 50mm f1.8G: no hot spots, quite a bit of flare. Panoramas on this post are all shot with this lens.
* Nikon 105mm f2.8 VR Micro-Nikkor: no hot spots, "slow" aperture can make focusing difficult on newer bodies.

Kolarivision.com maintains an extensive list of lenses and their hot spot status. Surprisingly, they claim that the 50mm Nikkor and the Tokina ultrawide zoom in the above list is a bad lens for IR photography. Maybe the results depend on camera too, since the coatings and filters above the sensor can change between makes and models.

Auringonpalvojia Kuopionlahden rannalla
(Exposure times were 30 seconds for each image, this caused the passers-by to disappear from the image.)

Shooting and post-processing

One thing that takes patience when doing infrared photography is focusing. Since the filter only passes thru light that's invisible to the naked eye, live view must be used to focus. And the sensor's being not very sensitive to infrared, so using bigger apertures help by passing more light. For example, by f/4 even the D800's live view mode becomes useless for focusing because the signal gets drown under the noise. And that's at noon, when the Sun is brightest - even bigger apertures are needed when overcast or it's otherwise dimmer.

Some - mostly older, manual focus lenses - have an infrared focusing mark in the distance scale of the lens. It's usually small red dot or line next to the visible light focusing mark. In a pinch it can be used to perform scale focusing. None of the lenses mentioned in the list above such markings, sadly.

Next thing causing grey hairs to the photographer is the white balance, which if left on auto, will be way off. Easiest way to fix this is to take a white balance reading off from grass or tree, since they should appear as white. For some reason, Adobe's Camera raw can't adjust the white balance so much that it could be corrected in post. However, for example Raw Therapee has no such limits in its white balance tool. Since the images straight off the camera are quite dull, adding more contrast is usually needed.

Kuopionlahden venevalkama aurinkoisena kesäpäivänä
(Setting the white and black point correctly adds some nice contrast to the otherwise bland image.)

Theoretically a camera equipped with Bayer filter souldn't need demosaicing if the filter is equally transparent at each pixel and color. However, no filter is perfect meaning that some shorter wavelengths are passed thru the filter creating faint false colors in the image. These faint colors make passing the pixels without demosaicing impossible, creating bad artefacts in the final image. This I tested with Raw Therapee by setting the demosaic method to none (Method=none under heading RAW) in the .pp3 file.

Sunday, March 24, 2013

D800: 12 bit vs 14 bit raw images

I compared how well the Nikon D800's can take extreme adjustments and if there's difference between 12 bit and 14 bit raw files and when is it okay to use the lesser bit depth. Below you can observer how the shadows get abruptly clipped in 12 bit files when adjusting the by ten(!) stops in Raw Therapee since it does its magic in 32 bit float ensuring maximum image quality. In 14 bit raw files there's still some detail left although the image gets a tad noisy. However, if shooting above approx. ISO800, there's no discernible difference between the formats, the camera's noise floor rises so high that the clipped parts of the image are drowned in the noise.

D800: 14bit vs 12bit
(When measuring the dynamic range of digital sensors, this is it. The difference is almost always in the shadows since linear sensors such as CCD and CMOS always clip when the photon well of a sensel is full or ADC gets clipped.)

Northern lights with D800 and Samyang 35mm f1.4

Awesome show of northern lights, probably the best since the early 2000's when was the last solar maximum. According to Spaceweather.com the planetary Kp index peaked at six out of ten. All these images were shot with Samyang's 35mm f1.4, it's an awesomely sharp and contrasty lens, especially factoring in the price of just under 300 eur.

Samyang 35mm f1.4 northern lights

Sadly, the infinity stop wasn't properly calibrated as they usually aren't in these Korean miracles. Live view was used to focus instead of relying to the distance scale. At f1.4, less than half a millimetre of focusing ring movement is enough to ruin the sharpness of the image.

Samyang 35mm f1.4 and D800 shoots violet northern lights

It's a big lens, but so is the camera (Nikon D800) I'm using too. I also tried the lens on a older film camera (Nikon FM or somesuch) and the lens honestly felt just too big for that body. I couldn't get proper grip of the camera and the focusing was a pain in the shooting hand because of that.

Sunday, January 20, 2013

Couple of big panoramas presented with OpenZoom

Yön sineä ja kultaa Siilinlahdella This post shall show some panoramas I've shot recently using OpenZoom. It's a nice way of showing large images on the net as it is both fast, responsive and good quality.

You can press the icon in the lower right corner of the image to go to full screen mode.

First, a panorama I shot couple if days ago from Siilinjärvi's Siilinlahti. It was supposed to be an astrophotography-oriented trip but alas, the clouds rolled in just as I got everything set up. Such is life sometimes. The artificial yellow light creates a stunning contrast between the dark, blue skies. If you look closely, you can spot the waxing crescent Moon there too.

The last two images are from Kuopio's Valkeisenlampi, which is a beautiful place especially in autumn.

Saturday, January 19, 2013

Messier 42 with D800 and Astrotrac

Messier 42 aka M42 or NGC 1976 or Great Orion nebula is the first photographed deep sky (object outside the solar system) subject. First time it was photographed in 1880 by Henry Draper and the picture can be seen at Wikipedia. So, 133 years later I was in the path of Mr Draper, only with much more modern equipment that is operable by a single man.

M42 & M43
(The faint nebula above M42/M43 pair is the triplet NGC 1973, 1975 and 1977. It's an emission nebula divided in three by a dark nebula.)

Due to the immense brightness (relatively speaking) of the center of the nebula this was photographed at ISO 400. Bigger sensitivities would only lead to the center being overexposed and offering no advantage noise-wise. The dynamic range of a camera is always biggest at the lower ISOs as you can see in these graphs. Canon sensors seems to make an exception there though.

If there was less light pollution (white snow reflects terrestrial light sources back to the skies too well) and the D800 would bit more sensitive to deep red h-alpha light, the nebula would indeed appear a bit bigger and some more details would be recorded. See example image here.

Twenty images were stacked together from 16bit TIFF files made by Raw Therapee. Images were aligned in Hugin and stacked with Enfuse. Final histogram stretching was made also in Raw Therapee. Histogram stretching is the step that brings out the details in astrophotography imagery; black level is limited by light pollution and the white point is limited by camera's sensor and sensitivity. It is not uncommon to clip out almost 80 percent of the histogram, mostly from the dark section of the image.

By the way, if you ever wonder if a bright spot on the night sky of light is star or a planet, you can tell them apart very easily. Just look for twinkling - planets will not twinkle as they are disks and not point sources of light. Below you can see comparison picture with two stars: Betelgeuse and Sirius. They are the bayerian alpha stars of Orion and Canis Major - respectively. At the right is the yellowy light of Jupiter.

How to differentiate between planets and stars

Note how the stars have changed color during the exposure. It is the scintillation effect of the turbulent air between observer and the star that causes these rainbow-colored streaks. Since the planet is so big (again, relatively speaking), it has drowned the twinkling effects in its big size.

Saturday, January 5, 2013

Last year's best images

Retrospective into year 2012. Images are in chronological order, oldest first.

Little planet panorama of some artificial light pillars accompanied by rare upper tangent arcs.

Vuodenvaihteen keinovalopilaripikkuplaneettapanoraama

Then for some northern lights from last January. I like how the fine structure of these fairly faint aurorae is visible.

Revontulia illalla 22-01-2012