Polarizing Lens Filter, Part I. (LO 2,3)
This blog is dedicated to “a learning element” about polarizing (CPL) filters, when and how to use them.
The main reasons why to use a CPL filter are:
bring out the saturation/colours & contrast
reduce the glare from reflective surfaces such as glass, metal and water
Today I write about how CPL filters bring out colours and contrast.
1. Bring out colours & contrast
A polarizing filter brings out colour and contrast but it also reduces the amount of light coming into the camera. CPL filters reduces the amount of light by 1-2 f-stop. My CPL filter reduces the amount light by 1 and 1/3 f-stop. It means that if I put a CPL filter on then I need to adjust my exposure (exposure triangle) by either adjusting shutter speed or aperture (sometimes ISO).
I took couple of images with and without polarizing filter to show the difference.
Image 1: 1/320s, f/2.8, ISO 100, no CPL filter
Image 2: 1/125s, f/2.8, ISO 100, CPL filter
Image 1 is shot without a CPL filter and Image 2 with a CPL filter. In order to get the right exposure on Image 2 I decided to change the shutter speed by slowing it down by 1 and 1/3 f-stop. By slowing down the shutter speed I allowed more time for light coming to the camera. Image 2 has bigger contrast and deeper saturated colours.
f-stop
a fraction that indicates the diameter of the aperture. The f stands for the focal length of the lens, ,the slash (/) means “divided by”, and the number represents the stop in use.
The smaller the f-stop number, the larger the lens opening;
the larger the f-stop number, the smaller the lens opening.
Exposure triangle
A correct exposure is a combination of three factors: aperture, shutter speed and ISO.
The aperture controls the volume of light that reaches the camera sensor during the exposure.
The shutter speed controls the amount of time that the volume of light coming through the lens. Each halving or doubling of light is referred to as a f-stop. The same halving and doubling principles that applies to aperture applies to shutter speed.
I took the following pictures in my garden. The aim is to show how to balance the exposure.
Image 1: 1/250s, f/3.2, ISO100, CPL filter
Image 2: 1/250s, f/3.2, ISO 100, no CPL filter; by removing CPL filter and keeping the same camera set up tas for Image 1, he image 2 is overexposed
In this case the Image 1 is shot with a CPL filter. Then I took it out and took the same shot again. It is visibly overexposed as my CPL filter reduces an amount of light by 1 and 1/3 f-stops (on Image 1). Based on the reciprocity law I changed the shutter speed on image 3 and then I changed the aperture on image 4 to balance the exposure triangl. Both options are possible and it really depends what you want from your final effect.
Increasing the exposure by one-stop will double the exposure and decreasing the exposure by a one-stop will halve it.
With aperture, shutter speed, and ISO, there are more intervals than just doubling and halving exposures. For example, between f/1.4 and f/2, there is another f-stop, f/1.8. These are third stops, which give me more control over my exposure.
To get the balanced exposure without a CPL filter I could increase shutter speed (Image 3) or narrow aperture (Image 4).
Image 3: 1/640s, f/3.2, ISO 100, no CPL filter - I increased shutter speed to balance the exposure by 1 and 1/3 f-stop compared to Image 1
Image 4: 1/250s, f/5.0, ISO 100, no CPL filter - Compared to Image 1 I narrowed the aperture by 1 and 1/3 f-stop to balance the exposure based on a reciprocity law
The last images are another example of using a CPL filter to boost the colours and contrast.
The Image 1 is shot without a polarizing filter. The second image has bigger contrast and deeper colours which is typical when using a CPL filter.
Image 1: 1/250s, f/2.8, ISO 100, no CPL filter
Image 2: 1/125s, f/2.8, ISO 100, CPL filter
Source: book Understanding Exposure by B. Peterson; book Light&How to photograph it by M.Freeman; course materials; internet
