Tag Archives: density

If You Want Shocks, You Don’t Need Fast Jets

Every once in a while, when photographing a fast jet at transonic speeds, you might get something in the background that allows the diffraction caused by the formation of shockwaves to be visualized.  I have posted about that here.  I was in Vancouver and shooting the floatplanes taking off from the harbor (since it is a Canadian harbor, perhaps I should write harbour).  As I was looking through the images zoomed in to check on sharpness, I realized that there was a visual effect of a similar nature.  (If you think this is a Schlieren effect, it is not.  That is a technique that involves a certain type of lighting to show the density differences but should not be applied to every time you see it in the wild.)

I don’t know whether what is showing up is the result of shocks forming on the props as they spin rapidly or just the tip vortices causing a similar effect.  You can often see diffraction in trailing vortices.  Whatever the reason, as you look above the aircraft at the patterns of structures on the shoreline beyond, you can clearly see some interesting effects.  Since the props are spinning fast and there is an overlap of the wakes from each pass of a blade, the shapes are rather complex.  Now I know that this is a thing, I might be tempted to take a longer lens and see what I can get in more detail of this interesting visual effect.

Variable Neutral Density Filter

Two of my shooting requirements have led me to the world of neutral density filters. Getting a low shutter speed on helicopters often results in very small apertures which can be a pain when it comes to sensor dust. Also, when I am shooting video I also want to be able to slow down the shutter speed. This can sometimes be hard to do and again shows up the sensor dust if it is there and dealing with it on video is a bit more effort.

Consequently, I decided to try out a variable neutral density filter. There are plenty on the market but I decided to go cheap! Why not? I never spend too much cash on photographic equipment!! This was from Amazon and was only about $40 so I wasn’t going to lose sleep if it turned out to be a bust. When the time came available, I decided to do some testing. I shot a clear sky with the filter set to varying levels of light reduction. I then imported these images into Lightroom to see how they looked. The answer was not good. In the mid range, things were actually reasonable. At the low and high ends, there was a significant variation in brightness across the image. This was not going to work.

I then decided to keep things simple. I already have a filter holding system for my old Cokin filters. I decided to buy a couple of neutral density filters for this. Then I can use either of them or both if the circumstances require. Not quite as flexible as a variable filter but pretty close and certainly more reliable without going to the expense of one of the top of the line variable models. We shall see how I get on.