Knitwear Photography Pt. 3: Exposure

Last week I talked about lenses. You guys left me some excellent feedback, so instead of talking about lighting and reflectors this week, I'm going to talk about the finer points in getting the right exposure. When showing off yarn and garments, the goal is to get a balanced photo with even lighting. In your arsenal, you have the f stops (size of the aperture), the shutter speed, and the ISO.

The f stop, or focal ratio, affects the amount of light that comes in as well as the depth of field. The smaller the f stop number, the larger the aperture is, the shorter the depth of field is. This means that a camera set to f/1.4 will have a lot of light come in, but the trade off is that only a thin slice of the photo will be in focus. Often times this is ideal if you want to have much of the photo out of focus. But if you need to get a whole skein into focus a smaller aperture needs to be used, such as f/9. Typically, any f stop over f/8 will allow the entire photo to be in focus.

In most cases, you'll be setting the aperture before the shutter speed in order to determine the depth of field, or how much of the photo is in focus. 

The shutter speed determines how long the light is allowed to hit the sensor. The numbers represent length of time the shutter is open. 1/100 means light hits the sensor at one hundredth of a second and 1/1000 means light hits the sensor at a thousandth of a second. Typically, you won't need a tripod for any shutter speed above 1/60. Lower than 1/60 and you'll need a tripod or to put the camera on a hard surface. Sometimes, despite using a tripod, your images can come out shaky at lower shutter speeds. This is because you are shaking the camera just by pressing the button. There are two ways to fix this. With film cameras, a shutter release cord was used. But with my digital camera, I set the self timer instead.

In most cases, you'll choose your f stop, then you'll match a shutter speed to it in order to get the light balanced. But you can get some interesting effects by changing the shutter speed. In the photos below there is water running in the background. The faster shutter speeds are able to freeze the water droplets whereas the slower shutter speeds allow the water droplets to look more like streaks. If you've ever seen a photo of the ocean where the water looks milky, the photo was taken at a slower shutter speed, usually in the range of 1/8.

Lets say that you have set your aperture and shutter speed to capture a photo in a particular way, but the photo is still coming out too light or dark. You can change the ISO. ISO stands for International Standard for Organization. Like the shutter speed, the numbers also refer to how fast light is captured by the camera. ISO 640 means light is being captured at 1/640 of a second. But unlike the shutter speed, it does not determine how long the shutter is open. Originally it referred to the grain of the film. Silver crystals in film would capture an image when exposed to light. The smaller the crystals, the finer the grain, the more light was needed to get a correct exposure. Bigger crystals could capture light faster, but the resulting image would look grainy because you were seeing the actual crystals.
These days, most DSLR cameras can capture clear pictures at high ISO readings because the sensors are so sensative. But at very high ISO numbers, such as 6400, you can start seeing graininess in the photo because bigger portions of the sensor are needed to capture the light faster. In most cases, it is hardly noticeable until you blow up the image. As for me, I often leave my camera at ISO 640. I find it gives me a clear image with more leeway in choosing the aperture and shutter speed.

And now for the bit I've been looking forward to. I'm assigning you all homework. You mission, should you choose to accept it, is to shoot a series of images using what you've learned from this post.

1. Shoot the same picture using three different f stops. (f stop suggestions: f/1.4, f/4, f/8)

2. Shoot the same picture using three different shutter speeds. Be sure there is something moving the background, such as leaves. (Shutter speed suggestions: 1/8, 1/60, 1/250)

3. Take the same image, but set your camera at different ISOs. Determine at what point your images start becoming grainy. Start at the highest setting and work your way down.

Next week I will be talking about using a reflector and how different weather can affect your images. And again, if you have any suggestions for this series, please do share it with me.