I've received a lot of questions regarding long exposures. I see that there are lots of misconceptions and doubts in this issue, so I'll try to leave here some ideas and tips. What I'll explain is how you can do images like this one:



Even though I'm now dedicated to film photography, I'll concentrate this article around digital cameras.

It all resumes to "Exposure". Proper exposure is a result of 3 factors:

1. ISO
2. Shutter Speed
3. The light that goes through the lens, usually determined by aperture

I usually set the ISO at the lowest value possible, as it gives less noise. The shutter speed should be lower; after all, it's the main objective. So, that leaves the #3 to mess around.

Suppose you want to shoot a beach, like the one in the pic above, during the day. Let's imagine that you point your camera, in automatic mode, and it suggests something like 1/100s, f/16. It's obvious that if you put a speed like 2 seconds the image will be completely "burned", or white. So, what can be done?

Usually people start by closing the lens (f/22 it's the usual smallest aperture). However, this brings two problems: the aperture is not small enough to allow over one second exposure (at least in daylight) and diffraction appears. Diffraction is an optic effect, and i won't try to explain it. Let's just say that it worsens the image - you loose definition and detail. Usually all lenses have the best quality at half the aperture, around f/8 or f/11. So, if you want good quality, these are the apertures you should choose.

So, f/8 it is. So, the only solution is trying to lower the light that enters the lens by any other means: ND filters! These filters have only one purpose: to lower the light, without making any other transformation on the image. They exist in several "strengths", reducing more or less the light. I currently own two of them: a ND8 and a ND400, both from Hoya. The numbers/designation vary from manufacturer to manufacturer. To Hoya, ND8 means that the filter will only allow 1/8 of the light to pass trough. In the same way, ND400 will let 1/400 of light into the lens.

So, and returning to our daylight beach scene (now measured at f/8, 1/400s), how can we set the camera to shoot a long exposure?

First, you must use a tripod; otherwise, the image will be blurred, as the camera will surely tremble during the shot. It's good to invest in a good tripod, as it will be steadier and will last for a longer time. So, you set the camera on the tripod and compose the image. This must be done without the filters, as they will make the image very dark and impossible to compose and focus. You should set the camera in fully manual mode; compose and focus (manually). Then, carefully, attach the filter. Let's suppose we'll use the ND400.

Now, we must determine the new exposure setting. As I said, we're going to use f/8, and the camera metered 1/400s. As the ND400 lowers 8-9 f/stops (it's rather easy to calculate these values, but let's just say that those are given by the manufacturer), we'll have:

1/400->1/200->1/100->1/50->1/25->1/10->1/5->1/2->1s->2s

These are just near values, but they are good for the example. So, lowering 8 f-stops would bring the speed to 1s, and 9 f-stops would go to 2s. Try with both; for more precise results, it would be useful to have a spot meter. But this will work.

So, using either a remote cable or the timer on the camera, make the shot.

It's 2s not enough for you? Well, put the ND8 on top of the ND400. ND8 will lower an additional 3 stops. So, from 2s, we'll go to 2s->4s->8s->16s. Better, no?

So, that's it! I've tried to make it simple, as there are many other small details not mentioned. I hope it's useful for you...

Some more examples: