For long daylight exposures, should you use an ND filter or just stop down the aperture?
Asked 4/23/2013
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When shooting long exposures in daylight, you can reduce light either by stopping down the aperture or by adding a neutral density (ND) filter. Stopping down increases depth of field, but past a certain point image sharpness can decline due to diffraction. If depth of field is not important, is an ND filter generally preferable once the same exposure reduction could be achieved either way? Are there situations where using an ND filter is actually worse than simply stopping down more?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
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There is a point where lenses transition from being aberration limited to being diffraction limited. This means that peak sharpness will increase up to a point as you stop down but will then start to decrease again. If you don't care about depth of field then use of an ND filter is preferable to stopping down past this point.
Of course ND filters aren't perfect and constitute another pair of surfaces to create reflections. So there is a small penalty for using an ND filter. Really powerful filters may also induce a slight colour cast as they are never perfectly neutral. So going slightly past the diffraction limit might be wise before getting out the ND filter. Vari-ND filters suffer from all the problems of a polarizing filter.
In reality you're probably going to stop down and use an ND to get the right shutter speed unless you have a huge number of different ND filters or a variable filter.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
13y ago
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In general, stop down only until your lens reaches its best balance of sharpness, then use ND if you need more light reduction. Past a certain aperture, lenses become diffraction-limited, so further stopping down reduces overall sharpness even if depth of field isn’t a concern.
ND filters avoid that diffraction penalty, but they have tradeoffs: they add extra glass, which can increase flare/reflections; strong NDs may introduce a slight color cast; wide-angle lenses can show uneven darkening or vignetting with some filters; and very strong ND filters can sometimes show IR leakage on long exposures. Variable NDs also inherit the usual issues of polarizers.
So neither is always “better.” Mild stopping down is often useful, but if you need substantial additional light reduction, ND is usually the better choice than closing the aperture far past its sharpness sweet spot. In practice, many photographers use both: stop down to a sensible aperture, then add ND as needed.
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