Moire Hunt
Moire occurs when particular circumstances occur. It is very easy to simulate via computer generated images, but it is rather difficult to consistently cause moire in images taken by a digital camera, even one without an anti-alias filter. Cameras with an anti-alias filter are not immune either but or less likely to see moire in their images. Ironically, when seeing images at less than 100%, moire is easy to see even if it is not present! This can occur when reviewing images on the back of the camera which is rarely done at 100%.
In order to compare the Pentax K-5 IIs and the K-5 II, we had to find subjects which would consistently produce moire. This proved to be challenging as we tried various fabrics, objects and targets. Targets were tricky since printing one often causes moire on the print! After all, printers use regular patterns to produce their output. So, printing one regular pattern with another causes moire too.
Success in causing moire finally done with targets downloaded from the Moire Demonstration Kit.
Distance versus Moire
Even with a target intended to produce moire, it does not always occur. One major factor in the setup is subject distance. Technically, it is the angle-of-view and resolution of the camera compared to that of the target. With a prime lens and fixed-sized target, the controlling factor is distance.
When the target is too close, the camera can resolve the details and therefore correctly render the pattern. For such cases, once may see moire when viewing the resulting image at less than 100%. When zooming to 100% though, the artifact disappears.
When the target is too far, the pattern is too fine to resolve and gets rendered as a uniform surface. In this case, no moire is visible regardless of magnification.
Aperture versus Moire
Lenses vary in sharpness with aperture. Stopping down from wide-open, lens usually get sharper. A good lens may even be sharpest wide-open or need less than one stop to reach its maximum sharpness. Typically, a lens is sharpest two stops down from its maximum aperture.
Stopped further down, a lens stays sharp until it hits the diffraction limit which depends on the physical aperture and the size of pixels. Passed the diffraction limit, images become gradually softer. Note that come cheap or kit lenses are sharpest beyond the diffraction limit of modern DSLRs, so they will always produce soft output.
Changes in sharpness according to aperture interacts with the occurrence of moire. Beyond the diffraction limit, a small aperture acts an anti-alias filter. It can have the same effect at wide apertures where the lens is not at its sharpest.
In Jip en Janneke taal:
- Ook met een K5II met AA filter kun je moiré krijgen.
- Moiré kan zichtbaar worden als je een foto sterk gaat croppen (zie de voorbeelden op het internationaal forum met de castussen en het golfplaten dak).
- Diafragemeren voorkomt moiré (blijkt ook uit een andere test).
Met andere woorden: als je weet wat je doet en goed glaswerk gebruikt, valt het dus reuze mee.
Topic: K5IIs beter qua scherpte en de moiré vrijwel afwezig? (afgesplitst topic) (gelezen 12677 keer)