I remember how in the times of classic photography I used to receive in my mailbox catalogues advertising different photographic filters. You could find virtually any type of filter – blur filters, speed filters, soft filters, gradient filters, effect filters, star filters… Dozens of filters which were replaced and substituted by Photoshop and the other editing software programmes. Only the polarizing filters and neutral density filters cannot be emulated with the help of a software programme. They are very often used in landscape photography.
The neutral-density filter is in practice a dark glass which limits the light passing through the lens.
This is necessary in case that we want to shoot at slow shutter speed during the day when it is light but we also want to blur moving water or clouds. In this way the combination of sharp and blurred objects within the frame will convey the feeling of dynamics and movement more convincingly. In some cases when exposure time is too long – a sense of peace.
Neutral-density filter can have varying density. The difference between the various densities is in the different amount of light passing through the lens.
ND 2 regulates 1 f-stop of light (reduces one aperture, speed or ISO stop).
ND 4 – 2 f-stop
ND 8 – 3 f-stop
ND 16 – 4 f-stop
ND 32 – 5 f-stop
ND 64 – 6 f-stop
ND 100 6.2/3 f-stop
ND 128 – 7 f-stop
ND 256 – 8 f-stop
ND 400 – 8.2/3 f-stop
ND 512 – 9 f-stop
ND 1000 (1024) – 10 f-stop
In this way we can calculate what speed settings we need to use for the camera after placing the respective ND filter. Cameras can calculate and adjust automatically speeds of only up to 30 seconds. For longer exposure times we need to make the calculations ourselves.
For example, with the aperture and ISO settings being the same, if the camera shows shutter speed of 1/125 sec and we place an ND 8 filter, the speed will now be 1/15 sec (because it brings it down by three stops, the number 125 must be divided in half). For ND 1000, the speed will already be 8 seconds.
We can do these calculations ourselves, or much more easily, we could download an app for the telephone which even comes with a timer.
This really simplifies things. We set the camera, note the shutter speed value, switch to BULB mode (if we are going to shoot at speeds lower than 30 sec), we place the ND filter, set the initial speed indicated by the camera in the application and the number of the ND filter and we turn the timer on. At the same time using a remote control we activate the camera shutter. When the timer beeps, we use the remote control to stop the exposure and the photograph is ready.
The good thing about long exposure times is that it is very difficult to overexpose or underexpose the photograph if you get distracted.
The longer the exposure time, the greater the tolerance. When the exposure time is for example 4 minutes, if you go wrong by a whole minute, it would still be 1/4 f-stop, which will hardly be noticeable in the end result.
In this way we can use ND filters and very long exposure times to shoot overcrowded places without the people in them. Because people are moving and they leave no traces in the final photograph. Here is a photograph of Venice in the evening when the streets are full of people and the canals are full of boats. Only a couple of boats left bright traces.
When you buy ND filters it is good to select from ones by well-known established brands. They may be indeed more expensive, but it is not worth spending your money on some glass produced somewhere which can irreversibly change the colours in the image to the point where even if we shoot in RAW format we could not reverse the damage. I have come across poor quality ND filters which literally block a part of the light spectre with the end result being that certain colours were missing in the image.
I would not recommend the use of variable ND filters, i.e. filters with variable density, especially of cheaper ones. They are two polarizing filters one on top of the other, two grids overlaying one another and very often they also alter the colours of the image. Some even reduce the resolution of the lens blurring the image to some extent.
Using neutral-density filters requires that the camera should be placed on a tripod.
The polarizing filter is a grid which lets through light coming at a certain angle and blocks light coming at another angle from passing through the lens. In this way you can clear some of the unwanted light reflections on the surfaces. Therefore, when shooting with a polarizing filter our photographs have more saturated truer colours. This effect is particularly evident when shooting water surfaces.
This filter cannot be emulated using a software programme because if the camera did not see what was behind the reflection, there is no software to reveal what was hidden in the photograph. In the next two shots, the sea on the left is blue because the sky is reflected in it. In the right photograph using a polarizing filter, the sea is green because the reflection from the top layer of the water was removed and the camera could see the actual colour of the water and the sand underneath.
By blocking some of the rays, a polarising filter reduces the total amount of light reaching the sensor. The effect is that the lens becomes darker by about two aperture stops. That is why it is advisable to take the filter off when the light is poor – in the morning and in the evening. In such cases its effect will be poorer.
It is possible to use a combination of polarising and an ND filter one on top of the other.