 Eddie Lagos |
ND filters I'm about to purchase a ND filter but am a bit confused about my choices. I went to Adorama's site and looked up filters of the 62mm variety. I'm choosing between Hoya or Tiffen, but I don't understand what the numbers mean exactly. For instance, the Tiffen says 2x (0.3). What does this mean? Perhaps 2x means two filters? Then, there is also multicoated glass filters by Hoya. There is also ultra thin. Can any one clarify this?
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Jon Close |
2x (.3) is the filter factor. It describes how much light is blocked by the ND filter. Every .1 = 1/3 stop, so .3 means a 1 stop reduction in light - hence "2x" since each full stop is a doubling or halving of the light. Plain glass is very subject to light loss and flare/glare. Lens coatings increase light transmission and counter flare/glare. Single coating is better than none, multicoating (several layers of coating) is better than single coating. Standard filters have a rim with screw threading (male and female) on each side. This gives the rim enough depth that it can cause vignetting (cutting light to the corners of the image frame) when used on wide angle lenses. "Thin", "Slim", or "Wide-angle" filters will have narrower rims, and perhaps eliminate the outer female threads so that they will not vignette on wide angle lenses.
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Alan N. Marcus |
Hi Eddie, ND filters, if fact, all filters must have a “filter factor”. When we mount a filter, no matter how transparent or dark (opacity), or what color (hue), it stops some measure of light. The filter factor tells us how many f/stops of compensation will be required. As John has so skillfully has told you, each f/stop increment of adjustment is a doubling of halving (cut-in-two) of the amount of light passing through the optical system. Thus FF (filter factor) 2 = 1 f/stop. FF 4 = 2 f./stops – FF 8 = 3 f/stops etc. I count on my fingers in powers of 2 starring with the pinky finger – like thus: 2 – 4 – 8 – 16 – 32 . FF 8 is the third finger thus the lens needs to opened 3 f /stops – FF 16 is the fourth finger thus 4 f/stops FF 6 = 2 ½ f/stops and so it goes. In modern cameras you need now worry about how to make the lens adjustment. Modern automated cameras use chip logic and sensors and they read the light after it has passed through the optical system. Thus the modern camera makes this adjustment and compensates automatically. One more thing: There is a second method used to measure and state the value of ND filters. This method uses a more advanced density unit. Below is a table that will help should you encounter these values. 0.10 density = ⅓ f/stop Hope this did not confuse Regards, Alan Marcus
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