@tinwhistle wrote:
I hate to confess this, especially since I've been "taking pictures" for about 60 years and even been paid to take some, but I have no idea of what the following statements mean: dual ISO, dynamic range, moire, aliasing.
tinwhistle aka Chris
Dual ISO isn't really a universally understood term even among hard-core photographers. It was a trick created by the authors of Magic Lantern who used it in an attempt to reduce the amount of digital "noise" in an image shot at high ISO.
Dynamic range: If you meter a subject and it turns out you realize you can get a good exposure of this subject at (I'll make up something) ISO 100, f/8, and 1/400th second exposure, it turns out that if your subject will still probably be fine even if over-exposed or under-exposed by just 1 stop. Most cameras would have no problems with up to 3 stops in either driection (even a camera that isn't thought of as being very good). When you shoot some types of scenes (landscapes are noted for this) you can meter the brightest object in your scene (say... some puffy white clouds in the sky) and also the darkest element in your scene (the tree-trunks in the forest shadows at the end of a meadow). If you expose for the puffy cloud, the tree-trunks are just black. If you expose for the tree-trunks in shadow the sky is blown out. So you try to expose for the middle and HOPE that your camera sensor has enough "dynamic range" that neither the puffy clouds nor the shadowy areas are clipped or blown out. Some camera sensors do better at this than others. That's the idea behind "dynamic range" -- how far can you go (under or over) your intended exposure without losing details that get clipped or blown out.
Moiré: If you lay a one piece of screen on top of another piece of screen and then slightly rotate the top piece (so that the rows and columns of wires are not parallel) you'll get an interesting pattern. That pattern is called "moiré". See: https://en.wikipedia.org/wiki/Moiré_pattern
This wasn't a problem with film because the film isn't made up of light-sensitive material all in neat little rows and columns. It was a chemical coating in which each molecure was more or less randomly positioned. But since digital cameras DO use light-sensitive photo-sites which are neatly aligned in rows and columns, you can get a moiré pattern if your subject has a pattern which is almost-but-not-quite, parallel to the sensor rows and columns.
To combat this problem, the camera employs a "low pass filter". The filter very slightly softens the sharpness of the image and this combats the moiré pattern. This slight softening is sometimes referred to as anti-aliasing.
Consider that the reason you do NOT see moiré pattern with film is because the surface coating on film is composed of molecures of light-sensitive material which are randomly distributed and not in neat rows and columns... and you only see the pattern when one pattern of rows and columns is placed on another pattern of rows and columns -- but without perfect alignment. Therefore, if we slightly degrade the perfection of our rows and columns in just one of the layers, we wouldn't see the moiré pattern. This is basically what the low-pass filter is doing.
Aliasing is the notion of getting a jagged edge when, in fact, you know the subject you photographed has a straight edge. This effect is particularly noticeable at low resolution. In images it is also known as "spatial aliasing". See: https://en.wikipedia.org/wiki/Aliasing
The 5Ds r (specifically with the "r" suffix) removes the low-pass (anti-aliasing) filter which allows for a sharper final image... but at the risk that should you photograph something with a neat row/column pattern (anything with parallel or vertical lines close together) you can get the moiré pattern. Nature and lanscape photographers are less likely to encounter this. Fasion photographers, on the other hand, probably encounter this problem in fabrics fairly often. Canon makes the camera both with and without the filter and the photographer can make their own (hopefully informed) decision as to which they prefer.
Tim Campbell
5D III, 5D IV, 60Da
Absolutely! As usual post is where it is at.
