By understanding a filter’s factor, we can determine if the exposure needs to be adjusted. Some manufacturers indicate these factors on their filters — they might be located on the top right corner of rectangular or square resin filters or the outer rim of circular ones (see Image 1). However, not all do.
Regardless of whether the factor is provided, it’s essential to determine it for each filter before use. While not all filters come with specified factors, many do.
In Image 2, you can see some of my LEE polyester filters housed in frames. Since these filters don’t have factors imprinted on them, I’ve noted them down on a card and tucked it in the case’s front pocket. This makes it easy to reference the factors when shooting.
Image 3 displays factor numbers hand-inscribed on the cardboard protectors provided by the manufacturer. I acquired this set second-hand and am grateful that the previous owner marked the factors. Their annotations align with my needs, saving me time.
This article delves into filter factors, their purpose, and how to utilize them. Let’s dive in.
Filter Factor Calculation
The mathematics of photographic exposure is governed by the Inverse Square Law of Light Intensity. This principle indicates that when you double the distance from a light source, the light intensity you receive drops to a quarter (1/4) of its initial value.
The same principle applies to filters. If a filter reduces the light intensity by a certain factor, it means it allows only a fraction of the original light to pass through.
Filter manufacturers determine filter factors by calculating how much the light transmission is reduced when using the filter compared to when it’s not in use.
The formula for the filter factor (FF) is as follows:
FF = 1 / T
T represents the fraction of light transmitted through the filter. It’s usually expressed as a decimal. For example, if a filter allows 50% of the light to pass through, T would be 0.5.
You get the filter factor when you take the reciprocal of T (1 divided by T).
Examples are shown in the graphic below.
Three Methods of Applying Filter Factors to Exposure
(1) ISO Adjustment with Filter Factor (FF)
Procedure: Divide your ISO speed by the filter factor to determine the effective film speed.
Example: With a filter factor of 2 and a film ISO speed of 100, the effective film speed becomes 50 (since 100 ÷ 2 = 50). By setting your light meter to a film speed of 50, you effectively adjust for an additional 1 f/stop in exposure.
(2) Shutter Speed Adjustment
Procedure: First, identify the necessary exposure without a filter. Then, multiply the shutter speed without the filter by the filter factor.
Example: If the exposure without the filter is 1/60 second and the filter factor is 3, the required exposure becomes 1/20 second (calculated as 1/60 x 3). However, since most cameras lack a 1/20 second setting, you’d typically use 1/15 second or the closest slower shutter speed available.
(3) TTL (Through-The-Lens) Metering
Procedure: For SLR cameras equipped with TTL metering, simply attach the filter to the lens and adjust your exposure as you normally would.
Note: Some filters, particularly those in dark blue, red, or orange, might produce inaccurate readings with TTL systems. This is because the meter typically reads 18 percent gray and might not detect certain light colors transmitted by the filter.
Should you ever feel uncertain, referring to a filter factor equivalent exposure table, like the one provided below, can offer clarity.
Filter Factor Equivalent Exposure Table
[ FILTER FACTOR EXPOSURE TABLE – CLICK FOR LARGER IMAGE]
In essence, the filter factor quantifies the reduction in light that passes through a filter. It’s derived by taking the reciprocal of the proportion of light the filter allows through. This factor assists photographers in adjusting their exposure settings, ensuring their images turn out as intended when filters are used.
NOTE: Filters with identical names and types from different manufacturers might have varying filter factors, even if they’re crafted from the same materials.
Once you’ve acquired your filters, it’s prudent to test their accuracy in terms of exposure factors. If the factor isn’t printed on the filter, jot it down on the filter case or a handy card. You’ll appreciate the foresight later!