General Photography

Aerial Photography

Posted by: aliengrove on Fri Nov 25, 2011 15:25
 
I am lucky to have a job that allows me to take a lot of aerial photographs, and although it is a relatively obscure form of photography, I thought some members might be interested in how I go about capturing and processing images. These techniques will work from lower altitudes than I fly at, though the problem of haze increases dramatically with altitude. There are a number of challenges in aerial photography, both in capturing the image and processing it afterwards.

At the capture stage, the problems are mainly flare, reflections, distortion and haze.
 
 
Flare can be reduced with a lens hood, preferably a rubber one, as at times it helps to rest it against the window. On a light aircraft, vibration can be a problem, so a fast shutter speed will be necessary. Avoid shooting into the sun, which greatly reduces contrast as well as causing flare. Moving back from the window into a shaded area if possible is another way to reduce flare.

I deal with reflections by using the sun visor on the aircraft, which is heavily tinted, strategically positioned to block reflections. A piece of matte black card would be equally useful. I also make sure I take the same image through different parts of the window in case I have missed some reflections or dirt on the window.

Haze increases with altitude, and is mainly caused by the scattering of light at the UV and blue end of the spectrum, and gets worse with increasing amounts of dust or pollution. This was my main reason for buying an infra-red converted camera; haze is far less apparent in IR. A colour image ends up with a blue cast, which increases with distance. You would think a UV filter would help, but I have tried many, and they make virtually no difference. Tiffen do make one that claims to knock out most UV, but it is very expensive and not coated, and does not seem to work as well as advertised. Haze and UV dramatically reduce dynamic range, and this has to be dealt with in processing. For this reason it is essential to shoot in RAW. This also enables the white balance, which varies considerably at different times of day, to be set post capture.

It has taken me a while to develop a work-flow, and I hope some members will find it informative and perhaps helpful if they get a chance to shoot images from the air. Trial and error has led me to adopt this work-flow for the majority of my aerial photography. It also works on any image with a narrow histogram (though leaving out removing the blue cast if not necessary, obviously!)

As an example, I will start with a picture that typifies many of the problems associated with processing my images, and is more likely to be the type of image people would take from a light aircraft.

This is the RAW file straight from the camera (K5), opened in Lightroom.



I start with a lens correction, before making other adjustments to the image. As can be seen, the histogram is quite narrow. Some aerial images have an even narrower histogram than this, especially if shooting into the light. I select Auto-tone as a strating point. Auto-tone always seems to want to overexpose the image so I re-adjust the exposure to make sure the exposure puts the histogram in the centre. A touch of fill light lightens the darker section at the bottom, and recovery helps bring out more detail in the background. I increase blacks to add some contrast, then adjust the white balance (it's easier to adjust it after recovering some detail). The histogram is now much wider.




I apply noise reduction before opening the image in CS5, and duplicating the layer. I then add a levels layer, and select auto-levels. In images with a very narrow histogram, levels sometimes need manual adjustment; sometimes I do this separately in all three colour channels. For this image auto-levels works well. On some images I will use a large soft edged eraser at about 35% opacity to reduce the levels adjustment there, as a global levels adjustment is sometimes too strong for the foreground, where there is less haze. Large levels adjustments also sometimes cause vignetting, which can be adjusted back in Lightroom, by cloning, or by cropping the image a bit. On pictures with many areas of pronounced differences in contrast, I sometimes select different areas of the image, heavily feathered, and make several levels adjustments on different parts of the image. This can be a bit time consuming.

Next I want to remove some of the blue colour cast, so I add a colour balance layer. I select the top two thirds of the image and feather the selection 250 pixels.



I reduce the blue a touch and watch what effect this has on the other colours. In this image an increase in red and a tiny increase in green does the trick. For this image I am only going to adjust the mid tones. This has to be done by eye, so it's a good idea to have your monitor claibrated.

I then flatten the image and open it in Lightroom, and carry out any final minor tweaks that may be necessary. On this image I only reduce the white balance a tad. I then check if I have introduced any noise in CS5 and reduce it if necessary. The image is now ready to be converted to a Jpeg.



More of my aerial shots can be seen on Flickr
 
 
 

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