What is causing this extreme glossy effect when using a polarizing filter?

Question

I have noted a few times at car shows and events when processing my images that some have what i can only call an extreme gloss effect - OK this car was shiny, but the image looks very striking and un-natural, not at all like this to the eye.

I believe it is a side-effect of my polarising filter - but how is it actually ENHANCING the reflections in this case?

I'm looking for a technical explanation here, not just a "its because of the polarising filter" answer.

for reference the equipment was: D800, Nikkor 18-35, Circular polarising filter, 0.9 grad filter.

Answer 1

When light bounces off a relatively nonconductive surface it becomes partially plane polarized, meaning the light tends to have the same polarization direction.

Polarizing filters can be used to counteract glare/reflections, by orienting the filter at 90 degrees to the polarized reflection so that it get filtered out.

If you orient the filter so that it is in line with the reflected light then this light can pass and you filter out about half of the remaining unpolarized light (which will have lots of different orientations but on average about half will be aligned with the filter). By reducing all light except the reflection you are in effect enhancing the reflections, hence the results you are seeing.

Answer 2

There are several problems with some of the answers you have already received. First, little of this effect is due to polarized reflections from the car itself. The reflection needs to be at a more glancing angle to get significant polarizing selection. Most of the reflections in this image are steep enough to not make much difference to the polarization of the reflected light.

Second, relections at the right angle from a dielectric are polarized. At just the right angle, you get 100% polarization. This effect does not happen when reflecting from a conductor, such as most metals. Part of the car body is made of metal, but, the reflections are off the paint so the underlying metal is irrelevant.

So what is going on? Sky light at the right angle from the sun is heavily polarized. Actually the blue part is heavily polarized, with the haze less so. Most of the effect you are seeing is your polarizing filter passing most of the polarized sky light, thereby effectively accentuating the rest relative to the polarized light. Rotate the polarizer 90° and the effect should reverse.

Answer 3

A polarising filter (ideally) lets through 100% of the polarised light of a certain angle, and 0% of the polarised light rotated 90 degrees from that angle.

Light that is not polarised is a mix of all angles, so the filter lets through 50% of that light.

Thus, it will let through twice as much of the polarised light at a certain angle compared to the unpolarised light, that's why some reflections are enhanced.

Answer 4

Your own eyes saw the car without polariser, unless you wore pol sunglasses, ie. the specular and diffuse part as well as the mirror image of the bright surroundings with some shiny highlights to grab attention and overload the dynamic range.

Then you took a shot with polariser, that theoretically removes reflections and specular highlights. Note that the standard theory on polarisation does not include metal, which you have here - partial metal/varnish, which may have a metal finish. This means that it tends to remove some reflections, and leave others there, in this case you got rid of the highlights that would shrink your dynamic range and contrast of the non specular parts, half of the diffuse part, and the mirror like reflection that seems intact.

Answer 5

in a sunny day, shooting a shiny car, how do I want my picture? it may be the first question we should ask.

Before that, we should understand there are 3 type of reflection : diffuse reflection, direct reflection and glare (polarized reflection).

Any light can produce any reflection, depending on the type of subject, and the angle of the light source related to the the camera, if the light source came within the " angle of family" (in short, the angle of incidence equals the angle of reflectance. search the web for more detail)you got maximum direct reflection and glare. polarized reflection cannot be as bright as an unpolarized direct reflection.

Back to the question, if you want the shiny car LOOK shiny, and rice of reflection of the environment:

1. Look for the "family of Angle" related to the light source (the sun), set up your camera there.

2. metering only on the "body" of the car, under 1/3 EV

3. Polarizing filter is an option, depend on if there is too much polarized reflection.

If you DON'T want the car look so shiny:

1. stay out of the "angle of family", in this case it is really hard for this subject (a car), but at least the part you are focusing on.

2. use Polarizing filter, cut the polarized reflection as much as you can.

3. wait for the cloud to block the sun. A "giant soft box" will help the most.