# Understanding neutral density filter types, ND2? ND4? ND8?

Discussion in 'Casual Photo Conversations' started by brodie_thomson, Jul 1, 2009.

1. ### brodie_thomson

I am currently looking around on eBay to purchase some neutral density filters, but am having trouble understanding what the ND2, ND4, and ND8 mean in terms of how many stops down it is. And to anyone who discourages getting cheap filters off ebay; I don't wish to spend a lot of money on good quality Hoya filters or anything like that, I simply want something affordable to experiment with long exposures in daylight. On one of the eBay items for a 3 piece set it has in the information:
• ND2 - reduce one gear of the aperture,average transmittance of 50%;
• ND4 - reduce two gear of the aperture,average transmittance of 25%;
• ND8 - reduce three gear of the aperture,average transmittance of 12.5%;
Does the 'gear' refer to a stop down in terms of aperture?
And average transmittance and that i don't understand either.
Basically, I'm asking how many stops down ND2, ND4, and ND8 are?
Thankyou

2. ### rainer_t

http://en.wikipedia.org/wiki/Neutral_density_filter

3. ### steve_levine

As you surmise, ND2 ,4, 8 equal 1, 2, 3, stop reductions respectively. I personally have never heard the term "gear" used in that manner though.

4. ### matt_needham

A stop is a x2 or /2 of the light. ND2 = 1 stop. ND4 = 2x2 = 2 stops. ND8 = 2x2x2 = 3 stops, etc....

5. ### brodie_thomson

Thankyou Rainer, that Wikipedia article answered everything.
Steve, yeah the use of the word "gear" threw me off. Thanks for your response.
And Matt, thanks! that explains exactly how to work it out

6. ### mark_q

You wrote you want to experiment with long exposures in daylight. In case you won't get the desired/required exposure times with these filters then you need serious ND filters, like ND64, ND400 or ND1000, which I have and use. These are made by serious companies like Hoya, Heliopan and B+W, and hardly "affordable" (whatever that means).
For my personal use I made following charts to have inside the filter cases for quick reference which filter to select when exposure time w/o filter is "X" and should be "Z" (on left column w/o filter, on right with ND). Feel free to print them for later reference:
ND64: http://img523.imageshack.us/img523/5619/nd64lutaw1.gif
ND400: http://img156.imageshack.us/img156/8916/nd400lutfz2.gif
ND1000: http://img523.imageshack.us/img523/5107/nd1000lutsj4.gif
Allthough high quality filters as such these all do cause magenta cast, seen on this comparison I made: http://img254.imageshack.us/img254/2048/ndfiltersab1.jpg
Some color correction is thus required.

7. ### alan_marcus|2

When it comes to labeling neutral density filters there are several customs.
The most popular is called a “Filter Factor”
By tradition exposure changes are given in an increment known as the f/stop. This method is based on making adjustable to the circular aperture that controls how much light can enter the camera. It copies our eye, the colored portion, that contracts in bright light and expands (dilates) in dim light. In our eye this wonderful control is called the Iris after the Greek goddess of the rainbow.
Many years ago it was deemed best if the camera’s aperture adjustment allows chances in increments of doubling or halving. Thus each f/stop change equate to a 2x change in light energy.
If 1 f/stop = 2x change then
2 f/stops = 4x change (two f/stops compensation)
3 f/stops = 8x change (three f/stops compensation)
4 f/stops = 16x change (four f/stops compensation)
This notation is called a “filter factor”.
When we mount a filter we check the data sheet. If it says “filter factor” = 8 we count on our fingers in powers of 2, thus 2 – 4 – 8 that’s three fingers or 3 f/stops.
If the “filter factor” was 4 we count 2 – 4 that’s two fingers or 2 f/stops.
Sidebar: Most modern cameras feature a built-in exposure meter system that reads scene brightness right through the camera’s lens. Thus if a filter has bee mounted camera senses the change and automatically adjusts exposure to compensate.
Photo scientist are somewhat snotty, they use a different method based on advanced math. This method has advantages however it is somewhat difficult to follow. I will tell you about it because many neutral filters are sold using this notation. It is based on the 0.30 equals the number 2.
Table of filter factors based on this system:
0.10 = 1/3 f/stop
0.20 = 2/3 f/stop
0.30 = 1 f/stop
0.60 = 2 f/stop
0.90 = 3 f/stop
1.20 = 4 f/stop
Neutral density filters are often sold using this notation and likely the decimal point is omitted. Thus a 60 ND = 2 f/stop reduction.
These values have a legitimate usage in photo science. The values are a logarithm base 10 notation the value of the filters are an exponent. Thus 100.30 = 2 100.60 = 4 etc. This notation is traditionally used whenever advanced measurements of films is called for.

We word gear refers to the iris adjustment on a camera. One click or one notch is 1 /stop. Gear means the adjustment is a gear chain or linkage. Substitute “stop” for “gear”