Category Archives: Software

Printer Diaries: Preparing Images for Print

The truth is in the print; what looks good on the screen doesn’t necessary hold in print (and vice versa). Having gone through the first set of cartridges and pile of paper, I am glad that I haven’t deleted my original files. In a number of cases I had to return to the RAW files and process them with adjusted settings. Let me explain:

Look at prints, not megapixels.

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Focus Stacking, Part 2: Artefacts

Now as part 1 of this post has had time to settle, let’s press on. Focus stacking is one of the techniques know as Computational Photography, which include panorama stitch, high-resolution sensor shift, multi-exposure HDR, and light field. All these techniques involve multiple images that are blended in post-processing to a new type of single image or scene representation; opposite to techniques in digital photography that work on a single capture, such as filtering and color adjustments.

These techniques may promt the question about ethics in photography. I have absolutely no problem with time-lapsing out people, for example. Over-saturation and over-sharpening, or turning the colors of a lake in Scotland to those of the waters at Anse La Digue is another matter. And nobody would consider the surrealistic photo-illustrations of David LaChapelle as dishonest work, while this is exactly what Steve McCurry, or his now fired assistant, are accused of producing. In my oppinion the problem lies not in the image itself, but its (incoherent) caption and the (false) message that the image is supposed to support. But I am digressing.

Focus stacking is particularly useful in situations where the scene has a large range of depths in the subject space compared to the shallow depth of field obtained for a given sensor size, focal length, and aperture combination. It will also be a way to make sense of the future FF sensors with 50+ megapixels, where diffraction will counteract the increase of accuaty by stopping down and compromise image quality for depth of field. Moreover, focus stacking can be useful to reduce noise in astrophotography. So there is nothing dishonest here; it is an attempt to optimize the conflicting elements of image quality:  noise, diffraction, lens-aberations, and depts-of-field.

There is, of course, the alternative of using tilt-shift lenses, but this is constrained to inclined focal planes (continuous depth maps), for example in landscape photography. The subject of the image below would also qualify.

gitarre-ref

Focus stack from 19 images. Automated capture using Helicon Remote and post-processed with Helicon Focus.

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Focus Stacking, Part 1: Introduction

Focus stacking, also known as z stacking or depth of field blending, is a technique that combines multiple images focused at different planes, to obtain a greater depth of field. This is particularly useful in situations where the scene has a large range of depths in the subject space compared to the shallow depth of field obtained for a given sensor size, focal length, and aperture combination. This is often so in macro, landscape, and architectural work.

Toscana

Toscana, Italy: Three stack composite along the horizontal lines of the fields. 85 mm, f/9.

It is important to recall that for every single, non-stacked, image there is one (and only one) plane in the subject space at which the image points on the sensor are exactly sharp, that is, point like. Any point of a subject in another plane will be imaged as a disk, know as the blur spot. If this disk is sufficiently small for a given magnification and viewing distance, it will be indistinguishable from a point. The diameter of a sufficiently small blur spot is known as the acceptable circle of confusion. The acceptable sharpness between two planes on either side of the focal plane is known as the depth of field (DoF). These planes are always at right angles to the sensor plane unless we explore view cameras or use tilt/shift lenses on DSLRs.

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