Noob question

[The_Camera_Poser]

Ok- this is embarassing.....

How does using a ND filter differ from turning down the exposure compensation?

[Doug Pardee]

Totally different.

Turning down the exposure compensation will make your picture darker in autoexposure modes (anything but M). If you're in Av mode and the shutter speed was 1/100, a -1 Ev exposure compensation will increase the shutter speed to 1/200.

Adding an ND filter will let less light into the lens. If you're using an autoexposure mode, the camera will automatically compensate for this to keep the picture from getting darker. If you're in Av mode and the shutter speed was 1/100, an ND2 filter (1-stop) will cause the camera to decrease the shutter speed to 1/50.

[SkipD]

Using a neutral density filter is much like turning down the lights that are illuminating the subject. Less light gets to (and, of course, through) the lens because the filter blocks some of it.

[The_Camera_Poser]

I'm (slightly) less ignorant. Thanks guys!

[rammy]

Two situations I can give you to use the ND filter is where the absolute lowest or highest settings (aperture/shutter/ISO) will not give you the exposure you need.

The lowest settings.
An example in this case is motion blur. Blur of cars, streams, waterfalls or anything that is moving. By slowing down the shutter speed, you are opening the sensor to more light (over time). But, by opening the sensor to the light for a longer time and having the smallest aperture (highest f-stop) some parts of the scene may still blow out. Especially if you are shooting during the day. So you can't reduce the aperture opening any further to reduce the light, to then slow down the shutter any further to give you the amount of blur you are looking for.

In this case you would use the ND filter to reduce the light further as you have already reached the smallest aperture (highest F-Stop number) on the lens.

The highest settings.
An example in this case is DOF (Depth Of Field). Shallow depth of field means a wide aperture (F2.8 or even lower, F1.4). But again in daylight, you may max out the shutter speed to get the lowest aperture F-Stop and so still blow out parts of the scene. So you can't increase the shutter speed any further for the wide aperture so that the scene does not blow out, for the shallow DOF you are looking for.

In this case you would use the ND filter to reduce the light further as you have already reached the highest shutter speed setting on the lens so that the scene does not blow out.

[The_Camera_Poser]

Excellent Rammy- thank you!