# How fast is the rotor of a Helicopter

With an exposure time of 1/100s and a roto diameter of 11m one estimates about 220m/s for the speed of the blade tips.

One often reads again that the blade tip of a helicopter reaches supersonic speed (over 340m /s). In hover, this is not the case for this helicopter. The helicopter, Eurocopter BK117-C1 has a rotor diameter of 11m. During the exposure time of 1/100s the tips moved about 2.2 meters. The rotor moves at approximately 220m /s and turns nearly 7 times per second.

# Defraction limit using a digital camera

Die Auflösung eines Objektivs ist beubungsbegrenzt von der Blende abhängig.

In the picture you can see small sections (about 100 pixels wide) of a photograph that was taken with a 50mm lens from a distance of 5m from a laptop screen. We see significant uncertainty in the larger aperture numbers (22). The lower quality in Aperture 1.8 is due to other aberrations.

Due to the diffraction limit the diameter of a pixel is d = 2.44 · λ · B, where F-number B and the wavelength λ determined. With a typical value of λ = 555nm we obtain d = B · 1.3μm. The typical pixel size of today’s digital SLR cameras is about 5μm, so that we reached at f / 4, the theoretical limit. Digital Cameras have in some cases significantly smaller pixel sizes because they work with smaller image sensors.

# Polarized light in nature

There is more polarized light than you think

The 20 regular readers are probably a little tired of the polarization, but the big difference between horizontally and vertically polarized light at a normal view I was not aware of. The sun was low, and about 90 degrees to the left of the view direction. The sunlight is therefore scattered in the mist by approximately 90 degrees. In this case, the polarization can only be perpendicular to the direction of incidence and outgoing beam. Therefore in the picture it is almost vertically. Both images were taken with the same aperture and shutter speed. To get an idea of where polarized light appears in the picture, I have determined the difference with the GIMP. In the fine structures in the tree, the leaves probably did not stay exactly in the same place. The brightness at this positions are probably artifacts.

Here the difference between vertically and horizontally polarized light is shown

In landscape photography you should always have a polarizing filter it.