itsallart wrote in post #17426474
I'm still relatively new to some issues and have a question: what happens if you shoot in a manual mode and stop down the aperture manually by let's say 2-3 stops which is what the ND filter would do? Would
'n the effect be the same as with an ND filter?
yes and no.
If you are setting all the exposure parameters manually (so the camera makes no adjustments based on metering) then stopping down the aperture by two stops, without compensating with a slower shutter speed or higher ISO, will result in two stops less light getting in and so reduce the exposure by two stops.
Keeping the same starting manual exposure, not adjusting any of the 3 values, but using the ND filter would also reduce light by the two stops and result in the same reduction of exposure by two stops. So, they have the same effect in that sense.
However, because they are doing it in very different ways, there are other effects to consider. Stopping the aperture down will also affect your depth of field and typically we select the aperture we want to use with that in mind. More commonly you would be likely to leave the aperture where it is, but use a two stop faster shutter speed to reduce the exposure. In general shutter speed is desired to be fast enough to avoid any motion blurring (from either the subject or the camera) and going faster will then make no difference other than to further reduce the risk of motion blur. Or, you may prefer to reduce the ISO for better quality. It is normal to select the best compromise between these three exposure parameters that you can get, based on the amount of light you have available, rather than to use ND filters for general exposure control.
An ND filter is normally only used when there is too much light in the scene to let you use the settings you want. Although it is more common to want to avoid motion blur in an image, there are many occasions when it is desirable (to show aircraft propellers spinning, background blur in panned shots of moving vehicles, smoothing out fast moving water etc). At such times you may want a very slow shutter speed and you are already stopped down to the smallest aperture you can, and have the slowest ISO set, but the shutter speed is still too high. That is when the ND filter gets used.
The typical way of controlling exposure is to decide on the aperture you need to give you the amount of depth of field you want, then set a suitable shutter speed to freeze motion, or allow blur, whatever you need, then having decided on those you set the ISO required to give correct exposure. You would normally aim to have the lowest ISO that gives you the desired aperture and a suitable shutter speed.
You need to consider that altering aperture or shutter speed affects the look of the image, they are controls that allow you to be artistic when you create your work. An ND filter has no effect on the look of the image, it just cuts down the light, and the less light you have available the higher the ISO you will typically need to use to get a fast enough shutter speed, for example (unless shooting in blazing light).