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Long Exposure Photography

Long Exposure Photography Choosing a Shutter Speed

Long Exposure means holding your shutter open for a long period of time. There are a couple of reasons you might want to do this:

First: You may be attempting to capture light trails.
Second: You may be attempting to capture an image with little light.

In both cases a long shutter speed is needed but how we determine the settings to use will differ.







Light Trails

The easiest long exposure photo to start with is light trails. Light trail effects can come from many different sources. It could be fireworks, headlights and taillights from cars, writing your name in the sky with a flashlight, etc.


Long exposure images can be achieved with light trails using pretty much any mode setting the camera is equipped with as long as the shutter speed is not forced to a fast setting. Most cameras when in full auto mode will force the shutter speed to 1/60th of a second and turn on the flash. This shutter speed is too fast so avoid using automatic mode. The best mode to use is full Manual mode where the photographer selects the shutter speed, aperture and ISO manually.
insert image of manual setting

bulb setting Shutter Speed & Bulb
The shutter speed must be long enough to allow the moving light to complete it's cycle. For fireworks this could be about 4 seconds. For a car driving down a windy road this could be 20 seconds. It all depends on how long it takes for your light to move.

The shutter speed on most cameras will go as slow as 30 seconds. Beyond that you need to use a setting called bulb. In this mode, the shutter will stay open as long as you hold the shutter release down. To enter bulb mode on some cameras you simply dial the shutter speed as slow as it can go while in Manual mode. The last setting will display "BULB" on the screen. Some other cameras have a bulb setting on the mode dial. Refer to the camera's owner manual.

If you do not know in advance how much time the light trail will need you can use bulb mode and control the shutter time manually. Simply keep the shutter open until the light is done doing whatever it is that it is doing. The background exposure will not be consistent due to the variable shutter time but in situations where the background is not important it may not matter.

It has to be dark! Long shutter speeds will over expose the entire image if there is too much light. A neutral density filter can help darken the image but even with a filter in place, the image can become over exposed if the surroundings are too bright. This is when it is most important to choose the time of day with the proper lighting. If it's too dark, the background will be black or extremely under exposed. If it's too light, the background will be over exposed. It is important to wait for the perfect light for the perfect exposure.

During the exposure you cannot touch the camera. Even the slightest touch can cause the camera to move enough to blur your image. So in bulb mode you must use a remote control device that utilizes bulb functionality.
insert image of remote unit

Choosing an Aperture
Choosing an aperture is a little more challenging. Remember, the light in your image is moving which means it is only being exposed during the time it is in one place. As it moves past a point it is no longer being exposed at that particular point. For example, if the shutter speed is set at 15 seconds the moving light may only be in one place for a second or less. That means that the shutter speed for the light may only be a half a second. A slow light may be over exposed but a fast light may be under exposed. Under exposed lights will be dark and not very vivid where over exposed lights will wash out and lose their color while becoming blotchy. Use the aperture to control the exposure of the lights in the photo. Also, the brighter the light the more exposed it will be. You may have to take a few practice shots before you get the exposure correct.

Remember that aperture also controls depth of field. Most of the time lights are bright enough to allow small apertures but if getting the correct exposure means a problem with depth of field the last available option for controlling the exposure is to adjust the ISO.

ISO
Sometimes the shutter speed and aperture settings need to stay put. A particular depth of field may be required or perhaps the sharpest image is desired or a star burst effect may be desired. In that case the aperture needs to stay fixed. And if the shutter speed also needs to be at a particular setting then the only other way to control the exposure is by adjusting the ISO. The most important thing to understand about ISO is that the higher the ISO, the more noise will be interjected into the image. Also, the higher the ISO the less dynamic range will be captured. The best solution is to keep the ISO as low as possible.

Diffraction
If you read my tutorial on depth of field then you probably remember that the smaller the aperture, the larger the depth of field. If you want sharp images, use a small aperture. But there is something more every photographer needs to know. As light passes through the iris in a lens some strange things happen. The light bends. This is known as diffraction. If you shine a light on an object in a perfectly dark room it will cast a shadow. If light travels in a straight line, then why isn't the edge of the shadow sharp? Why is it fuzzy? That's because light bends around objects that it hits. When light travels through the iris of a camera it hits the edges of the aperture and bends. The smaller the hole, the more the light bends. This has two unique side effects. The first is that the image will be slightly fuzzy. It loses its over-all sharpness. It also creates a star burst effect around lights. If you can live with a slightly softer image, the star burst effect can be very appealing in your photo.

Below are examples of three different aperture settings. The diagram shows light entering your lens, passing through the aperture and hitting the light sensitive CCD. The image below the diagram shows the star burst effect due to diffraction.

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More coming soon...