exposure time

All posts tagged exposure time

I remember when I started being serious about photography (quite some time ago already) there were a couple of things that I had to re-read in order to understand the technique behind it. DoF, Depth of Field that is, was one of them. I thought I could do a little write up about it including an example of DoF with different aperture settings.

So what is Depth of Field, really?
If you google the term a recurring definition you’ll find is “the amount of distance between the nearest and farthest objects that appear in acceptably sharp focus in a photograph”. That doesn’t tell you much, does it? It didn’t tell me much back in the day when I had to look it up in a book (you know, those rectangular shaped things with this funny stuff called paper inside on which text and pictures are printed 😉 ).
What this definition told me was that it had something to do with the distance between the objects in your picture. It does, kind of, but that’s not really the point.
For me DoF bluntly means: part of your image is in focus and the rest is not in focus. And it’s done on purpose 😉 The more blurred or out of focus the picture is, and the less of your designated object is in focus, the narrower (or shallower, or smaller, these are all terms used to indicate) the DoF.

Many things can affect the DoF, but the the DoF is mainly controlled by the aperture setting on your camera.
That was another thing that I just couldn’t remember: the larger the aperture, as in the smaller the number indicated for the F-stop, the larger the hole in your lens through which light is let through to the sensor. So larger aperture – smaller F number – larger opening in the lens to let light through. Without this getting completely technical, I’m trying to keep it simple, let’s suffice with saying that things with a small aperture are more in focus because the rays of light that are coming into your lens are less diffused, scattered if you will, by the small hole in the lens before they reach the sensor. The bigger hole with the later aperture allows for the rays to basically go all over the place and thus can’t create the sharp image on the sensor.

Do note that if you change the aperture, you will have to equally adjust the exposure time. A picture taken with f/8 and 1/500 sec exposure time will render the same result in terms of exposure as a picture taken with f/11 and 1/250 sec exposure time. If you stop down the aperture with one stop, you’ll have to open up the exposure time with one stop and vice versa.

Below is a series of images in which you can see what happens when you start with a large aperture and end with a small aperture.

Example of how DoF works

From top left to top right the camera settings were:

1/15 sec @ f/3.5; 1/15 sec @ f/4.8 (I didn’t adjust the exposure time, which shows in the image: it’s slightly darker than the first one); 1/8 sec @ f/6.7; 1/4 sec @ f/9.5;

From bottom left to bottom right the camera settings were:

1/2 sec @ f/13; 1 sec @ f/19; 2 sec @ f/27; 4 sec @ f/38.