Saturday, March 8, 2008

All about Exposure

Today I'm going to give a basic overview on how a camera controls exposure, that is, how much light is let into the camera. This can get a bit complicated but I'm going to try to explain everything in a simple way that will give you the basic knowledge you need to improve your photos and let you move away from "auto" mode with confidence.

There are three basic elements that control exposure.

1. Aperture
2. Shutter Speed
3. ISO

I'll discuss each one separately first, then in the end talk about how all three work together. I see a lot of people on sites like Etsy and Ebay who do their own product photography that have trouble with exposure - mostly with dark, underexposed photos so I think this is going to help those people a lot.


Out of the three things that control exposure, Aperture is probably the most confusing to most people. But if you learn what it is and how to use it, you will be able to have more control over how your photos look, and you'll be able to take much more professional looking pictures.

Put simply, Aperture is the size of the opening in a lens when you take a picture. Well, what does that mean? This is one of the ways your camera (or you) determines how much light reaches the sensor. If the hole only opens up a little bit, less light enters the camera. If the hole opens up wider, more light enters.

Aperture is measured in F-stops. These numbers, such as F/2.8, F/5.6, F/22, etc simply denote the size of the hole that opens when you press the shutter and take a picture. Each time you change the aperture by one F-stop, you are either doubling or halving the amount of light that reaches the sensor.

So here is where it can be a bit confusing. If you look at these two F-stop numbers, which would you think lets in more light: F/2.8 or F/16? Actually, the smaller number is a bigger hole and let's in more light. Here is one of my shoddy charts to help you get an idea. Keep in mind this is not an exact diagram, it's just meant to help you visualize

As you can see, as you go from F22 to F2.8 the hole in the lens opens more and more to let in more and more light. This is one of those things that you just have to remember. "The smaller the number, the more light" F2.8 gives you more light than F4, which gives more light than F5.6, etc.

Aperture not only helps control the amount of light, it also controls what we call depth of field. That is how much of your photo is or is not in focus. But we'll save that for it's own discussion. For now, let's just focus on exposure.


The shutter speed is how long the shutter stays open to allow light into the camera. This is a much more simple concept than Aperture. Shutter speed is measure in seconds, then in fractions of seconds. The longer the shutter stays open, the more light enters the camera. Changing the shutter by 1 setting doubles or halves the amount of light entering the camera.

So, for example, a shutter speed of 1/500 of a second lets in half the light of a shutter speed of 1/250th of a second.

Faster shutter speeds are required when you want to freeze action, such as sports, flying birds, etc, or even when you are handholding your camera for product photography. If your shutter speed is too slow, it is impossible for you to hold the camera still enough for a clear picture.

This is one of the things that often goes wrong for amateur photographers. If your pics are often blurry, chances are your shutter speed is too slow without you knowing it, and you are getting movement. The shutter opens, you move and create blur, then it finally closes. If you are hand-holding your camera and it's not on a tripod, you need to use faster shutter speeds to freeze the shot even if you're a bit shaky.


ISO is simply your camera's sensitivity to light. ISO used to be controlled by the film you bought, as different films were more or less sensitive to light. Remember buying ISO 200 or ISO 400 film? Well now it's all programmable on the camera, and ISO can be changed from shot to shot.

Easier to understand than aperture, ISO is pretty straightforward with the numbering.

ISO is measured in numbers which differ a bit with each camera. Some cameras give you more ISO options than others, but basically you only have to know that as the number increases, the camera is more sensitive to the light. Going from less sensitive to highly sensitive would look like this:

100,200, 320, 400, 500, 640, 800, 1000, 1250, 1320, 1600, 3200

Most cameras will start at either 100 or 200 and might go to 800, especially point and shoot models. When there is a ton of light, such as outdoors in the sun at the beach, you don't need to have a very sensitive sensor. So ISO 100 or 200 still gives you good shutter speeds. If you're in a really dark room and your ISO is set to 100, chances are your camera would need a LONG shutter speed to have enough light to expose the picture correctly. That is why without a flash, most people's photos in a dark room are totally blurry.


So now you know you have three elements to work with that control how light or dark your pictures are going to appear. Don't be concerned with the exact numbers as all cameras will be a bit different if you're talking about a dslr or point and shoot. Just remember the concept for each element and you'll be fine.

So how do we use ISO, aperture and shutter together? Well the first thing to understand and remember is that they are all dependent on one another. If you change one, it effects the other. For now, let's leave the ISO out of it because you won't change that as often as the other two.

Remember that each time you change either the shutter speed or the aperture by 1 unit, the light entering the camera either doubles or is cut in half. So let's take a hypothetical example:

Suppose you set your camera on Auto and take a picture, and the photo looks perfect at F8 and 1/250th of a second. If you changed the aperture from F8 to F5.6 (the next aperture setting), what would happen? Well, twice as much light enters the camera, and the photo would be overexposed. So to compensate and make the photo look perfect again, we increase the shutter speed to 1/500, cutting the light in half. Now you're back to good exposure.

To wrap this up and keep it as simple as possible:

If your photos are too dark : Use a bigger aperture (F2.8, F4 instead of F16 or F22), Reduce your shutter speed, or increase the ISO number.

if your photos are too bright: Use a smaller aperture ( F16, F22 instead of F4, F2.8), Increase your shutter speed, or decrease the ISO number.

I know that was a handful and maybe you have more questions now than when you first read this. Over the next week or so I"m going to talk about each element in detail - shutter speed, aperture and ISO so you will have it all mastered soon.

I"ll also get into a lot more detail about product photography tips, using different colored backgrounds and how that can throw off your camera's metering system, tips on lighting, etc.


Huage said...

I agree with your views. Well, do share with the rest of us more on your expertise.

Tim said...

Your blog is great.

Nnairda's said...

Thank you so much for sharing this! It's greatly appreciated!

ChrisY said...

Thank you so much to share this information.It is useful to many especially the beginners.