Archive for the ‘Digital Photography’ Category

Front Focussing of Sigma 24mm/1.8, Nikon 85mm/1.8 and Tamron 17-50mm/2.8

Surprise, Surprise! Got my Sigma 24mm back (remember the quick comparison with a Tamron 17-50 a few weeks ago). A professional photographer bought it and returned it a few weeks later, because of focussing issues. He suspected that it might suffer from Front Focussing. This is a problem with some third party lenses, where the autofocus focusses a point in front of the object and not exactly the object plane.  A short explanation and a quick test can be found at A more thorough test procedure is here. So I tested the Sigma and the two other lenses I own with the quick prodedure on my Nikon D80. This procedure also reveals tendencies of chromatic aberations in the defocus areas.

The Nikon 85mm/1.8 focusses perfectly fine, but shows strong vertical chromatic aberrations in the defocus areas: Green behind the focussing plane and magenta in front. The Tamron 17-50mm/2.8 focusses well, with – maybe – a slight tendency towards back focussing. There is nearly no chromatic aberration. The Sigma shows a clear front focussing affect and mild chromatic aberations.

Have a look at the test images:

Nikon Nikkor 85mm/1.8

Sigma 24mm/1.8 EX DG Macro

Tamron 17-50mm/2.8


Shock: My Nikon got its feet wet

On saturday we went on a first hiking excursion in the Bavarian forrest (the Lusen, ~1360m). I made some nice shots of the dying forrest at the south east mountain side. When we lunched at the Lusen hut with an incredible shock I noticed that my water bag had dripped and water had already entered my camera (Nikon D80). Fog was visible at the display and inside the viewer. Immediatly I removed the battery. Back at home I stored the camera and the lense at a sunny warm place. Both were open but covered with a black sleave. Today everything seems to be fine again. After carefully cleaning the body inside and the lense with a brush and a Rocket Air.
So branded I’m looking for a holster bag that is really waterproof. I think I’m going for a Ortlieb Aqua Zoom. Ortlieb gear usually is tough, absolutely waterproof and reasonable light. Also they provide a four point strap to fix the camera on your belly. Must be great for outdoor activities…

Comparison of Sigma EX DG 24mm/1.8 versus Tamron 17-50mm/2.8 XR DiII

Today I got my new lense. A Tamron 17-50mm/2.8. This will probably replace (and augment) my Sigma 24mm/1.8. Though I liked the handy focal width and fastness of the Sigma, I often suspected it to produce quite unsharp and dull pictures. A quick comparison with a few shots at aperture 2.8 exposes the truth. The Tamron is the better lense in any way (except that it’s slightly slower, of course). It is much sharper, brilliant and produces less chromatic aberations and flares even wide open. Have a look!

Sigma EX DG 24mm/1.8 at F2.8 Tamron SP AF 17-50mm/2.8 XR DiII at F2.8
DSC_3687.JPG DSC_3688.JPG
DSC_3688_crop.JPG DSC_3687_crop.JPG
DSC_3689.JPG DSC_3691.JPG

Measurement and Noise

Types of noise

Consider noise in digital photography (and find a well illustrated introduction here). I will summarize:
Noise is a deviation or error from the true picture, which is totally unrelated to the picture. Unrelated means, that one cannot predict the appearance of noise pixel if the true picture is known.

Basically, there are two types of noise on digital sensors:

  1. Noise that stems from unregularities of a particular piece of sensor. This noise is still unrelated to the picture, but it is predictable from one exposure to another.
  2. Totally random noise. Not predictable by anything.

Removing noise

It is easy to remove the totally random noise. Just take two or more exposures of the same true picture and average them. Since the noise is not correlated with the picture nor the camera it will diminish.

It is a little trickier to remove the sensor related noise. If you took several exposures of the same true picture with the same camera, the noise would also sum up and not be removed.
There are two things you can do: Substraction of the sensors noise profile or multiple exposures with different cameras.

If you choose the substractive approach, you first have to assess the sensors noise profile. How to do that? Simply take multiple exposures of different random pictures (e.g. sections of a white wall). Average the images. What remains wil be the noise profile of your sensor. You can now substract it from every future picture you make.

The alternative is to take the picture with different cameras. As the noise is only related to a specific piece of sensor, it will diminish with averaging.


“Why is this piece of blog in the category Usability Research?”

“Because there are phenomena regarding noise in usability evaluations. And we can learn from drawing the analogy.”

Up to this we can distinguish the following terms:

  • The measured object is what we want to represent as exact as possible
  • We use a certain instrument, …
  • … which produces instrument specific noise
  • Additionally we have to fight totally random noise.
  • We have the ability to make multiple independant measures with the same instrument …
  • … and multiple measures with different instruments

Multiple measures with the same instrument will remove totally random noise with the same object or will identify the instrument specific noise when different objects are measured.
Or we can measure the same object with different instruments to remove instrument related deviations.

How does this apply to usability evaluation methods? Next time!

Fat(t)ales Tonemapping: Effekte der Bildskalierung

Für das Tonemapping von HDR Landschaftspanoramen habe ich bislang die besten Effekte mit dem Fattal02 Tonemapping aus der PFStmo Toolsammlung erzielt. Allerdings tritt dabei ein unerwarteter Effekt auf, der die Vorhersage des Ergebnisses schwierig macht:

Das Ergebnis hängt stark von der Auflösung des verwendeten Bildes ab.

Um die Auswirkung der Tonemapping Parameter schneller einschätzen zu können, habe ich zunächst mit einer Skalierung von 0.2 gearbeitet. Auf der Kommandozeile sieht das folgendermaßen aus:

pfsinexr Pano1-Pano3.exr| pfssize --ratio 0.2 --filter MITCHELL | pfsgamma -g 0.7 |pfstmo_fattal02 -v -a 0.05 -b 0.8 -s 0.8 |pfsout Pano1-Pano3_ldr1.tif


Im Vergleich sieht das Ergebnis mit einer Skalierung von 0.8 deutlich anders aus:
pfsinexr Pano1-Pano3.exr| pfssize --ratio 0.2 --filter MITCHELL | pfsgamma -g 0.7 |pfstmo_fattal02 -v -a 0.05 -b 0.8 -s 0.8 |pfsout Pano1-Pano3_ldr1.tif


Ganz bizzar wird das Ergebnis, wenn man in der HDR Verarbeitungskette ganz auf eine Skalierung verzichtet (abgesehen davon, dass mein Rechner dann mehrere Minuten unter Volllast rechnet). Man beachte die Moiree-artigen Artefakte im Zentrum.
pfsinexr Pano1-Pano3.exr| pfsgamma -g 0.7 |pfstmo_fattal02 -v -a 0.05 -b 0.8 -s 0.8 |pfsout Pano1-Pano3_ldr2.tif


Fazit: Der Fattal Algorithmus liefert für mich immer noch die besten Ergebnisse beim Tonemapping kontrastreicher Landschaftsaufnahmen. Die Ergebnisse sind aber sehr schwer vorherzusagen. Insbesondere kann man sich nicht auf die Erscheinung einer verkleinerten Vorschau verlassen.

Ob die Effekte durch den Resizer in der PFStools Befehlskette entsteht oder erst beim Tonemapping entsteht ist momentan noch unklar und wird sich in weiteren Experimenten zeigen.

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HDR Panoramas with free tools (Part 1, Overview)

After my first naive trials of creating high dynamic panoramas I digged a little deeper. I started figuring out a workflow, which

  • is completely HDR, no intermediate Jpegs or Pngs
  • uses only free open source tools

At the moment my workflow is designed as follows:

  1. Exposures in RAW with my Nikon D80, triple bracketing for each panorama section
  2. Combination of each bracketing set with Qtpfsgui
  3. Panorama Stitching with Hugin (v0.7b4)
  4. Retouching and Cropping with Cinepaint
  5. Tonemapping and Jpeg Export with Qtpfsgui

Each of the above tools supports one or more HDR formats (hdr, pfs, openexr, tiff), but the intersection is incomplete. Thus, the following table shows, which formats to load and save in each step:

HDR Panorama Workflow with Input and Output Formats
Nr Step Tool Open as Save as Comment
1 Exposure Digital camera RAW
2 HDR Combination Qtpfsgui RAW HDR
3 Panorama Stitching Hugin HDR TIFF (IEEE 32bit) Automatic control points with autopano-sift and the PTStitcher don’t work with HDR. Define control points manually and stitch with nona
Intermediate Converting PFStools TIFF OpenEXR TIFFs saved from Hugin won’t show up in Cinepaint
4 Cropping and Retouching Cinepaint OpenEXR OpenEXR Adding an alpha channel required before saving
5 Tonemapping Qtpfsgui OpenEXR JPEG My favorite algo is fattal02

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Fat(t)al Tonemapping: Effects of image scaling

For tonemapping of countryside HDR panoramas I prefer the Fattal02 tonemapping algorithm of PFStmo Tools erzielt. However I recentyl noticed some strange effects, which makes it quite hard to predict the outcome of Fattal tonemapping:

The outcome strongly depends on image scaling before tonemapping

I noticed this effect, when I was using downscaled (factor 0.2) images to get some quick previews. On the command line this was:
pfsinexr Pano1-Pano3.exr| pfssize --ratio 0.2 --filter MITCHELL | pfsgamma -g 0.7 |pfstmo_fattal02 -v -a 0.05 -b 0.8 -s 0.8 |pfsout Pano1-Pano3_ldr1.tif


But, with a scaling of factor 0.8 this looks quite different:
pfsinexr Pano1-Pano3.exr| pfssize --ratio 0.2 --filter MITCHELL | pfsgamma -g 0.7 |pfstmo_fattal02 -v -a 0.05 -b 0.8 -s 0.8 |pfsout Pano1-Pano3_ldr1.tif


The results were virtually bizzare, when abandoning any scaling:
pfsinexr Pano1-Pano3.exr| pfsgamma -g 0.7 |pfstmo_fattal02 -v -a 0.05 -b 0.8 -s 0.8 |pfsout Pano1-Pano3_ldr2.tif
Notice the strange “macro moiree” artifacts in the center of the image


Conclusion: The Fattal algo is still preferable for high dynamic countryside images. But it’s hard to handle. Especially it’s useless to try to predict the outcome from smaller previews. As Giuseppe Rota (creator of the nice Qtpfsgui tool) remarked, this is due to the “locality” of Fattal. But I still haven’t any idea on the strange “macro moiree” effect and couldn’t reproduce it with other images either.

HDR Panoramas mit freien Tools: Nachbearbeitung des HDR Panorama

Bislang unterstützen nur zwei freie Bildbearbeitungsprogramme hochdynamische Formate: Krita und Cinepaint. Letzteres ist von GIMP 1.0 abgespalten und hat (noch) die leistungsfähigere Toolpalette. Damit gebe ich ihm zunächst den Vorzug.
Hugin liefert als einziges HDR Ausgabeformat TIFF (IEEE 32bit). Dieses lässt sich zwar mit Cinepaint erst lesen, nachdem es mit den PFStools auf der Kommandozeile in OpenEXR umgewandelt wurde:

psfintiff panorama.tiff|psfoutexr panorama.exr

Die neue Datei wird nun mit Cinepaint geöffnet. Oft sieht man zunächst ein völlig überstrahltes Bild. Dagegen hilft die Korrektur von Belichtung und Gamma mit View->Expose. Dabei wird nur die Anzeige, nicht jedoch das eigentliche Bild verändert. Nun kann man mit dem Crop Werkzeug den passende Ausschnitt wählen.

Danach machen wir uns an die Bearbeitung der Schnittkanten. Dazu eignen sich die Tools Clone Image Region (Stempel) und Smudge (Schmieren). Die Schnittkanten sind oft bei Wolkenformationen und Wasseroberflächen zu sehen.

Vor dem Abspeichern  muss auf jeden Fall  noch ein Alpha-Kanal angelegt werden mit Image->Alpha->Add Alpha Channel

Dann speichern wir als OpenEXR ab.

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HDR Panorama Walhalla, nahe Regensburg

Panorama_Walhalla_web, originally uploaded by mschmettow.

Auch dieses Panorama ist in einem vollständigen HDR Workflow entstanden. Zugrunde lagen 7 Abschnitte mit jeweils dreifacher Belichtungsreihe. Wie man sofort erkennt, hatte ich meine liebe Mühe bei der Nachbearrbeitung. Vor allem die beiden rechten Schnittkanten waren aufgrund der Reflexe im Objektiv der deutlich zu sehen. das Sigma 24/1.8 ist dafür aber auch sehr anfällig.

Diese Kanten habe ich so gut es ging mit den GIMP Werkzeugen “Schmieren” und “Stempel” retuschiert . Praktisch wäre natürlich ein automatisches Überblenden mit enblend gewesen. Dieses Werkzeug konnte ich aber bisher nicht zur Mitarbeit überreden.

Dieses und das letzte Panorama (Mariahilf) sind übrigens heute an Pixum rausgegangen – in einer Datei montiert auf 40*60cm. Bin gespannt.

HDR Panorama, Passau

Pano_Mariahilf_web.jpg, originally uploaded by mschmettow.

This pano is stitched together from HDR images with completely free tools (sse my previous post).
It is tonemapped with the Fattal algo, which I like most since it is nearest to the actual sensation.