You've discussed the roll of
perspective projection distortion, but no one seems to have considered
optical (lens) distortion.
Quote:
The radial distortion can usually be classified as one of two main types:
* barrel distortion, in which image magnification decreases with distance from the optical axis. The apparent effect is that of an image which has been mapped around a sphere. Fisheye lenses, which take hemispherical views, utilize this type of distortion as a way to map an infinitely wide object plane into a finite image area.
* pincushion distortion, in which image magnification increases with the distance from the optical axis. The visible effect is that lines that do not go through the centre of the image are bowed inwards, towards the centre of the image. In photography, this aberration is often seen in older or low-end telephoto lenses.
A mixture of both types, sometimes referred to as moustache distortion, is less common but not rare. It starts out as barrel distortion close to the image center and gradually turns into pincushion distortion towards the image periphery. It is observed with certain retrofocus lenses, also more recently on large-range zooms such as the Nikon 18-200mm.
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In the context of the above quoted material, it can be taken it as a given that your digital camera's
integrated (not accessory) zoom lens would be considered
low-end telephoto lenses. With virtual certainty -
- Non-zoomed, you have some amount of barrel distortion.
- Somewhere in the zoom range you have some mustache distortion.
- At the far end of zoom you have some amount of pincushion distortion.
Recent versions of PaintShop Pro ( and I believe Photoshop, Photoshop Elements as well) have manual tools to correct for pincushion and barrel distortion. There are commercial plug-ins available for the same programs that provide more automatic correction based on camera/lens model (but probably don't include you consumer grade digital in their lists.)
It's still winter here and I'm waiting for warmer weather. When that happens, I'll find a wall to back the car up to such that there is a long, level, unobstructed expanse in front of the car to position my camera shot. Prop a 4x8 sheet of plywood against the wall - vertical but in landscape position orientation. Position my
tripod mounted camera* such that the plywood sheet fills about 90% of a picture taken with camera set to maximum Safety Digital Zoom (no in camera pixel interpolation - only cropping) at 1600x1200 resolution. This picture will be used as a reference to determine the best numeric setting to use in the pincushion correction filter.
* Tripod mounted camera: To minimize vertical perspective distortion, height of camera lens center to be 1/4 (height of car + 48")
Moving on to the picture of the car. As controls
- A small object (soup can?) will be positioned on the roof of the car at the point of it's maximum height.
- Measured lengths of fluorescent flagging tap, will be taped to key points on the car's body.
- The measured distance between outer tips of the side mirrors will also be used as a control.
These controls will be used to decide any application of perspective correction after application of lens correction.
When I actually do this, I'll post a thread with pictures.