Neutral Density filter on the XL1
an article by Ross Lowell
The XL1 has a built-in ND that is activated by a button on the lens. Fast and clean. It subtracts, I calculate, six stops. A "bit" much, even for the desert.
There are both creative and technical reasons for prefering an ND, or in some situations, a polarizing filter, to shutter manipulation. Higher shutter speeds will affect the look of moving subjects or cameras; you'll want to run tests to make sure you don't dislike the results for video, not still capture uses. Filters can introduce some slight quality loss, or in the case of the XL1's extreme internal ND, some slight color shift, it appears to my eye.
Three reasons to consider introducing filters or other methods of reducing exposure:
- The lens can not be stopped down enough (obviously),
- The lens CAN be stopped down enough BUT you don't want to do so, because lenses are generally designed for optimum performance in their mid-range, typically between T 4 and T 5.6 to 8. This is independent of depth of field issues, only resolution at the plane focused on.
- The creative part: Often you will WANT shallow depth of field and one of several ways to acheive it is to keep the aperature large (the T # small) in order to throw backgrounds out of focus so your subject will separate from an otherwise distracting and confusing background.
Jason Powell adds:
Your ND filter should be used whenever you would like to get a smaller f-stop (more wide open lens) without raising the luminence values of your scene too high.
ND=neutral density, therefore it is a filter which decreases the amount of light getting to (through) the lens without affecting color temperature.
I've always been taught that video cameras like to be exposed between f-stop f/4 and f/5.6 and if you are running f/11 your iris is closed down too much. If you are running f/2 then you probably need to add light. I use my DSR-200's internal ND, as well as a Tiffen .6 ND when I am in a high lighting (outdoors) situation to get the proper luminance at the f-stop I want.
If you are running the camera in auto iris and engage the ND filter, you will get the same luminance values at a different f-stop. That's why you don't see any dramatic difference in the picture. What you may be able to see, however, is the decreased depth of field with the ND filter
engaged. Use ND filters to decrease your depth of field and throw a background out of focus.
Don Palomaki sums up:
Neutral density filters are most useful in very bright light: They allow the iris to open up a bit and reduces the depth of field.
Also, they allow you to get the lens operating in the range of its best optical perfomance, say around f/5.6 to f/8, rather than a very small opening where diffraction or other optical artifacts might become an issue.
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