# Neutral Density Filter question with a twist.



## MintChocs (Jun 6, 2014)

This a theoretical question. Using a fast lens at say f1.4 in full sunlight requires a high shutter speed, so if you run out of a high enough shutter speed you then use a ND filter to reduce the light. Now I'm not talking about the quality of such filters but does having a filter reduce the sharpness of the image as a ND filter obviously reduces the light by blocking some of it hence you are not getting all the light reflected from the scene. Is the reduction so small so as to be negligible or with stronger filters affecting it more?


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## gfoulk (Jun 6, 2014)

Well, if you make it so dark as to require high ISOs, that would lead to noise, which will reduce sharpness (both directly by obscuring details, and of course if you apply noise reduction.) But assuming your ISO is pegged at base (because if not why are you using an ND filter?) and your resultant image is well exposed, there's going to be more than sufficient light to not impact sharpness. If you get dark enough, sure, the limited number of photons is going to at some point reduce sharpness, but we're a looong ways away from that. 

(Also, any lens wide open isn't going to be as sharp as it could be stepped down, but I assume for the purpose of the question we're assuming an ideal lens in addition to the ideal filter.)


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## neuroanatomist (Jun 6, 2014)

The loss of light will not directly cause a reduction in image quality (unless, as stated above, you need to raise the ISO which will add noise). However, an ND filter is a piece of glass in front of your lens. A good quality filter (e.g., B+W) will have no detectable impact on IQ. A poor quality filter (which includes most variable ND filters) will result in a loss of sharpness.


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## MintChocs (Jun 7, 2014)

Thanks for your replies, I was looking at this, more from a scientific point of view.


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