One of, if not the most important decision that anyone will make regarding their images is exposure. Does your image appear to light or too dark? does it have the full tonal range you want? However, it is not just a case of getting the exposure right. It has to be right for you and what you want to say about your subject and how you, and others relate to it.
It is generally accepted that there are 3 main facets to exposure these are
1) The Shutter Speed
2) The Aperture
3) The ISO
The correct exposure of the image rests on a balance of these three “legs”.
This is how long the film or sensor is exposed to light and is normally measured in fractions of seconds or seconds. A shutter speed of 1/30 sec is longer than 1/60 sec and, half as much light reaches the sensor with 1/60 sec than 1/30 sec. Moving from 1/30 to 1/60 is one stop of light. In real terms, this controls the amount of movement that takes place in the shot. More movement will occur in slower speeds while faster speeds will freeze the action.
The big question is how much movement do you want in your shot. Do you want the soft creaminess of very slow movement or do you want to flatten everything out with a very long shutter speed conversely do you want the action frozen with a very high shutter speed say 1/2000 sec?
To give you an example. If you are taking an image of a waterfall or shoreline. You have to decide how much movement you want in the water. Freeze it with a fast shutter and it may look unnatural. A slow shutter may look equally unnatural with all the movement removed. All this depends on what you, as the photographer, want to show the viewer or your images. What story do you want to tell?
Somewhere, on your camera or lens, there will be some numbers that run. 2,4,5.6,8,11,16,22. These are aperture sizes and the larger the number the smaller the diameter of the hole the light has to go through. F22 is a tiny hole, f 4 is a big hole. Less light with f22 and more light with f4. So this will have an effect on shutter speed. Faster shutter speeds need more light so need a larger amount of light going through the aperture to make up for the smaller amount of time the shutter is open. With me?
Let’s give you an example. sunny day. f5.6 with a shutter speed of 1/125. You can go down to f4 and the shutter speed goes to 1/250. Alternatively, you can go to f8 and the shutter speed falls to 1/60sec.
In the same way that the shutter speed works in stops, so does the aperture so f4 to f5.6 is one stop with half the amount of light reaching the sensor at f5.6 than at f4.
So why would you chose an aperture size that does not correspond to the shutter speed you want. This comes down to optics. the larger apertures of f4,f2 etc have a smaller depth of field than f11 or f22.
Depth Of Field?
When you focus a lens at a given point everything that is on the same plane as the focal point is in focus. So focus at 6 ft and that 6ft arc away from the sensor or film will be sharp. However, one foot behind or in front will no longer be in critical focus. So focus on a berry and forget which one you focused on the rest could be slightly blurred. F22 has a greater depth of field than f 4 so a section in front and behind your focus point will be acceptably sharp although not in focus.
The best way to think about this is an expanding section of the image which is sharp and you select the depth of range that suits your image.
There are other things that affect depth of field but as of now let’s just think about aperture.
Examples. A portrait photographer may choose a shallow depth of field to blur the background behind his model to all the focus is on the sitter. A landscape photographer may choose f22 because he wants a lot of his image sharp.
So Far So Good
So we have 2 of our three components for exposure. Both work in conjunction with each other if one changes you have to change the other. Aperture increases, (gets bigger/ smaller number) shutter speed goes up, and Vice Versa. Aperture decreases (gets smaller, bigger number) shutter speed goes down. We can use these to our artistic advantage. Changing movement within the image, and blurring aspects within the image to concentrate attention where we want it.
The third section of this exposure tripod is ISO. Typically you will find this in numbers of 100, 200, 400, 800 etc although with modern cameras there will be in between numbers as well. The normal 100 – 200 is again one stop of light. 100 reacts to half the amount of light as 200 does. So changing from 200 down to 100 with either requires the shutter speed decreasing say from 1/60 to 1/30 OR the aperture opened up say from f5.6 to f4.
If you have a dark room then you can increase the ISO? Well yes and no it depends on what you want, as so much does. The problem with increasing the ISO is the noise that comes with it. Now I don’t particularly have an issue with noise. Some people will say that it is the bain of their lives, some people don’t. This comes down to a choice you have to make. For my part, I use a tripod and shoot at a low ISO. However, at a recent wedding, the light inside the building needed a higher ISO as I did not want to use flash. This being the case the noise was dealt with using other means.
If you want some more info on the above Mike Brown does some fantastic Video tutorials the technical ones can be found HERE and are well worth a look.
My Instagram is HERE