ART – Another RawTherapee

(Actually a fork of RawTherapee)

ART is written by the Italian programmer Alberto Griggio, who aimed to keep the best of RawTherapee (RT) whilst making it easier to use. One of the new features is local masking.

I recently discovered ART when I wanted to apply 2 grad filters to an image. (RT) only allows one application of the effect. There is the obvious workaround of applying one grad filter, save the image, reopen the saved image in RT and apply a second grad filter.

I was hoping that ART would allow more than one application of a grad filter but it does not. However for my immediate requirement the masking feature of ART (Not present in RT) solves the problem.

Above is the image with just some basic global adjustments. What I now want to do is to apply a grad to darken the sky. But the sky is reflected in the water so that will require some darkening too, probably a reflection of the first grad filter.

Enter the Mask

The required mask divides the image into 3 horizontal bands: sky, ground, and water by creating a blurred or feathered rectangular mask across the middle, i.e. the ground.

We would then expect any adjustment to affect the area that is not masked, but the author of ART seems to use the word in the opposite sense – no problem, just click the “invert” button. Whatever adjustment is then applied will affect the sky and the water in a similar way to 2 grad filters.

Whilst this is a very specific case of requiring 2 grads I am sure it is not uncommon; whenever a sky is reflected in a lake any adjustment to the sky must be reflected in an adjustment to the surface of the lake. Below is the result.

Diffraction Blur – in practice

How it affects your camera

Diffraction blur occurs naturally in every camera no matter how expensive the lens. How much blurring of the image depends on the aperture, specifically the f-number. It is not affected by the focal length of the lens; a 100mm lens at f/16 will produce the same “size” blur on the sensor as a 200mm lens at f/16 on the same camera. Perhaps surprisingly, the blurring increases as the aperture is reduced, i.e. as we “stop down” i.e. as the f-number increases.

So if we want to retain the maximum level of detail in our images – the best that our sensor is capable of – we should avoid using apertures smaller than a certain value. To take an example, my 16Mp, 1.5 crop Pentax K5 IIs will (theoretically) show diffraction blur at f-numbers higher than about f/10. This does not stop me using f/16 when the situation demands.

I referred above to the “size” of blur on the sensor. For a particular aperture this will be the same whether you have a 6Mp sensor or a 24Mp sensor, however the 24Mp sensor may be able to “see” the effect whilst the 6Mp sensor may not – simply because of its limited resolution.

My previous articles on this subject covered it in some detail. This article aims to present the essentials in a way that is useful to a wider audience. The following table allows you to find the “Diffraction limited aperture” for your camera – stopping down beyond that point will produce some blurring.

Stopping down to smaller apertures increases the amount of diffraction blur, the table shows at what point it becomes significant relative to the resolution of the sensor. This is a gradual effect.

Sensor size:
Megapixels (Mp)
1.5 crop
Full frame (35mm)


  1. Suppose you have a camera with a 16 Mp sensor and 1.5 crop factor. What is the Diffraction limited aperture (DLA)? The Mp column has a line for 12 and for 24 with respective DLAs of f/11 and f/8. Our 16Mp is nearer to 12 than to 24 so the best approximation for DLA is f/10. Note that this is all you need to remember – the rest of the table does not apply to your camera.
  2. Consider a full frame SLR with a 50Mp sensor. We could, of course, extend the table – the next line would be 96Mp – f/4 – f/5.6 (following the rule that we double the Mp and reduce the aperture number by a factor of √2, i.e. one stop) – however 50Mp is so close to 48Mp that we can accept the value of f/8 from the table.

Remember in all this we are referring to the aperture at which the diffraction blur is about the size of a pixel on the sensor. Thus a 6Mp camera will be more tolerant of diffraction blur at f/11 than a 24Mp camera at f/11 only because it can’t see it!

The focal length of the lens is irrelevant; it does not appear in the calculation of DLA. Also we assume a “perfect” lens. Real lenses have distortions which may be more serious than diffraction blur. Diffraction blur is a natural phenomenon and cannot be designed out by Canon, Nikon or even Pentax.

For further information on diffraction blur follow this link to my earlier article: How the Nature of Light affects the performance of digital cameras

Hugin 2017.0.0 — Stitching HDR and LDR Panoramas

Having been an occasional user of Hugin for many years, I have described my recent experience of stitching High-dydnamic-range (HDR) and normal, (Low-dynamic-range) panoramas from a set of 9 images shot one evening at Salford Quays. The article should prove interesting and useful to anyone new to Hugin, or to those, like me, who use Hugin infrequently and never quite become “experts”.

P1548 - P1559_hdr_pregamma_1_mantiuk08_auto_luminancecolorsaturation_1.4_contrastenhancement_1.71

As usual, Hugin did an excellent job of stitching, but I recommend outputting an HDR file in EXR format for tone-mapping in, for example, Luminance HDR.

Read the full article

Noise in Digital Camera Raw Files – How to Measure it

After writing two articles on the Nature of Light and its relevance to digital photography, I found that the subject of noise still fascinated me and decided that I had to make some measurements. Looking at the wiggly waveforms of my previous article might indicate that camera A is noisier than camera B but can we measure the noise in a rigorous way? This present article explains how to do that using free software. As well as presenting graphs of the measurements I have attempted to explain the results from physical principles – and evidently the noise is predominantly photon noise (aka shot noise).Selection_026

Click here to read the article

Test Cards for Monitor Adjustment – Revised

Test Cards - 1Test Cards - 2Calibration and Profiling

I have updated the page on this subject in order to clarify the distinction between calibration and profiling (or characterisation), and to remove reference to commercial operating systems now obsolete.

To avoid confusion I have now included only a version of each test card with sRGB profiles assigned and saved as jpegs. These should be viewed in an application that is colour aware (i.e. one that recognises and uses the embedded profile).

Click here for Test Cards page

How the Nature of Light affects the performance of digital cameras — (Part:2) Noise

G9, pixel plot across sample

In the second of two articles I look at another natural phenomenon, photon noise (also known as Shot noise). As with diffraction blur, the problem becomes more serious as the physical size of the sensor is reduced.

Whilst this is not the only source of noise, it is now the dominant one in the darker areas of an image where only a relatively small number of photons are incident on the sensor. It is the counting of photons, which is subject to Poisson statistics, which produces the noise.

Click here to read the article

Click here to read the first article: Diffraction blur

How the Nature of Light affects the performance of digital cameras — (Part:1) Diffraction

Image of a point source: Blue = Intensity, Red = Sensor output. Computer model of G9 at f/8

Reducing the physical size of a camera, even if the total number of pixels is maintained, inevitably reduces the quality of the images because of two fundamental properties of light itself. This technical article looks at diffraction, usually explained by considering light as waves. A future article will look at photon noise, explained by considering light as particles.

A simple rule-of-thumb is established for determining the “diffraction limited f-number” by relating this to pixel pitch on the sensor.

The earlier, pre-digital, criterion for diffraction limited aperture (based on required print sharpness) is revisited and considered to be still valid – perhaps with a little sharpening.

Click here to read the article

Test Cards for Monitor Adjustment – An Update

Test Cards - 1Test Cards - 2Basic adjustment of monitors and projection systems

When I last updated my page on this subject (2012) I decided that it was best to offer the test cards as .png files and leave it to the user to assign a profile (presumably sRGB) and to view the result in an application such as Photoshop. For convenience, I have now added a version of each test card with sRGB profiles assigned and saved as jpegs. These should be viewed in an application that is colour aware (i.e. one that recognises and uses the embedded profile).

Click here for Test Cards page

Raw Processors: Which Demosaicing Method?

Comparison of Demosaicing Methods available in Free, Open Source Raw Processors

My previous article included a table listing the various demosaicing algorithms offered by the four raw processors considered and I wondered why we (as users) needed such a wide choice. The table is reproduced below.DemosaicI decided to investigate those offered by RawTherapee by looking closely at the detail in an image of tree branches against the sky – the same part of the same raw file processed by each of the algorithms.

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Which Raw Processor?

Comparison of four free raw file processors: RawTherapee, Darktable, Lightzone and Photivo

With the exception of Darktable, which is not yet available for Windows, all of the applications are available for Windows, Mac and Linux. All are free and open source downloads.

I am looking for a raw file processor that will allow me to develop raw images to produce files ready for projection (at 1400 x 1050 pixels) and files at full resolution for further development, as necessary, to make high quality prints. I don’t expect to print directly from the raw processing application though this might be an advantage.

I have used both RawTherapee and Darktable for over a year and have recently tried Lightzone and Photivo so I will restrict my comments to these four.

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