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The CRYPT Mag

Time delay on Digital Cameras


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Digital cameras are becoming more and more affordable each week and many computer users are purchasing one.  These cameras are excellent, allowing you to download photos or video clips to your computer, edit them, print them or save them.

In the main, digital cameras are easy to use ...Point and click just like any other camera. However there is one slight drawback with using digital cameras.

Action Photographs.

Imagine you want to take a photo at ... say .. A motor race!  With a normal camera it is no problem, however most digital cameras have a slight time delay .. that is a delay between the time you click the button and when the camera actually takes the photo.  Normally this means that you photo is almost a second behind what you actually wanted to record .. in moter sport a second is a long time .. You will probably end up with a photograph showing the car boot disappearing out of the side of the shot.

So why does digital cameras have this time delay and how can you overcome it.

Shutter lag on digital cameras is caused by three major components of the picture taking process.

1.)  The camera's focusing system

2.)  The camera's image processing and compression system.

3.)  The speed at which information is processed in the brain and along the neuronal network of the human body.  This is commonly referred to as reflex time.

Before a camera can take a picture it must be satisfied that it  (the camera)  has done an adequate job of focusing the lens on the subject.  This phase of the operation is almost always the biggest consumer of time in the photo shooting process.  And, good news, it can be minimized.

The solution for the majority of users... Pre-focus.

Almost all digital cameras have a two stage shutter release button.  You press this button down half way and the camera goes into its auto-focus and exposure control routine.   Many cameras now come equipped with LEDs around the optical viewfinder that glow different colors to indicate a successful or failed attempt to auto focus and set exposure control.

So,  The cars are racing around the track and coming into range for your shot,  Point the camera at your selected car, hold down the shutter release half way until you get some kind of indication that the camera has successfully focused the shot.   Now maintain the half depressed position on the shutter release and the camera will remain focused and ready to "immediately" take the picture.  Pan the camera to follow the action and maintain a nice composition of the action until the critical moment when the car is in shot.  Then, depress the shutter release fully to take the picture.   The delay between full depression of the shutter release and the photo being taken is almost always under 1/10th of a second.

The word of warning here is that if your subject moves toward or away from the camera the focal distance will change and since you have locked your camera's focus on the initial position, the resulting photo will be slightly out of focus.   If the car is moving to the left or right and maintaining the same approximate distance from the camera, this will not be an issue.

An advanced tip will work in brightly lit (daylight) situations.  If you have a moderately advanced camera, set the focusing system to manual and focus the lens on infinity (landscape).   Set the exposure control system to use a high F stop (F8 or higher).  The camera will then automatically adjust the exposure by changing the shutter speed.  A higher F stop results in a photo with a greater depth of field.  Most cameras in this category will start focusing on infinity between 15 and 30 feet.   And with the increased depth of field resulting in the higher F stop this may be reduced to 5 to 10 feet.  So, everything from that range to infinity is in focus.  Because the camera is in manual focus mode, no focus adjustment is made prior to taking the picture so the much faster exposure control is the only adjustment made while depressing the shutter release.   In most cameras this is so fast it may be considered a non issue.  When your camera is set in this mode you do not need to depress the shutter release button half way and hold it until the right moment comes to take the picture.  Just press the shutter release button all the way down immediately to take an in focus picture with minimal shutter delay.  One word of warning, while using this method, the camera may select a very slow shutter speed for a correct exposure.   If you are shooting moving objects right down to the relatively slight movement of someone casually waving their arm about in a picture, that movement may become blurred.  The blurring will be an artifact of motion not incorrect focus.

2.)  The camera's image processing and compression system.

This process mostly affects the taking of the NEXT picture but it can seem to have an impact on how fast you can snap off photos.  After the image has been captured your camera has a multi step process to move it from the image sensor in the camera to the digital memory medium  (flash card)  for storage.

First,   it makes color corrections.   You tell it how to make these corrections prior to picture taking by setting the white balance.  The camera will examine each and every CCD element on the photo sensor and compensate for the correct color balance by adding and subtracting small variances to each red, green and blue component of the CCD to get the right balance.  If you have a 3 megapixel camera, that means your camera will have to do 9 million calculations.   how come so high.   Well, image sensors work by capturing three channels of color, red, green and blue.  They analyze this data and create one composite pixel with all three values included.

Second, just about all digital cameras perform sharpening to some degree.   This is an analytical process where edges are detected and the contrast is boosted along those edges.

Part two of the image processing system is compression.  Remember how it takes three CCD sensors to create one pixel ?  Well, here is more info.  Digital cameras generally operate at either 12 or 14 bits per each of the CCD sensors.   Since there is no 12 or 14 bit file format, the sensors are usually padded with zeros to bring them up to a full 16 bit size.  This information can then be easily broken down into two 8 bit words  (Bytes)   which is the standard size for file systems in cameras and on computers.  So ... A RAW or unprocessed image in a 3 megapixel camera's memory takes up 18 million bytes  (two bytes per CCD element).

The camera reduces this somewhat by converting the RAW data into pixel data.  Essentially it calculates a color value for each of the 3 million pixels.  On most digital cameras in the point and shoot category, the camera will convert the 16 bit per CCD element value into an 8 bit value.   So, each pixel has 3 bytes of data to describe its red, green and blue components.  This process results in a file size of 9 megabytes.   This is way too big for most folks to deal with.   The expense of carrying flash memory cards would be prohibitive.  So, point and shoot cameras will apply JPEG compression to this image.   and this takes time.

Since it takes time for this post capture processing, if you try and snap off a second picture of another car, you are going to be confronted by two slow downs to the capture of the this picture.

Focusing will still be a major issue and secondarily, if your camera is still figuring out how to compensate for color correction it will wait until this job has been finished before it can take the next picture.

3.)   The speed at which information is processed in the brain and along the neuronal network of the human body.  This is commonly referred to as reflex time.

As for the third component of shutter lag,  the human element, practice is the only solution.   When photons from the racing car arrive at your retina things start to slow down considerably.  Neurons pass the information via the optic nerve to the back of your brain where you process the image information.  You deduce that the car is in the correct position and send the signal to your finger to depress the button all the way down to take the shot.   A maddeningly slow sequence of neuron firing escapes from your brain, follows the nerves along your spinal cord to the branch that leads out to your finger, moves out of your spinal cord, across the trunk of your body to your arm, down the arm, into the hand, and finally reaches the bunch of muscles that control the movement of your index finger.  Then, the muscles "slowly" contract to force your finger to depress the shutter release.

Remember the human body is far more complex than your digital camera.

The solution?  get in some practice. Take picture of cars on the warming up lap.   Then immediately review the picture and note how far the car has moved in the resulting picture.   Most sports photographers become experts at reading body language of players/cars they are photographing and soon start timing their shots just prior to the jump, catch, kick or whatever so that their reflex delay has been anticipated and a good shot is the result.  Of course,  they also have professional digital cameras with rapid multi-shot capability and negligible delay characteristics and they use these features of their cameras to enhance their ability to capture the event.






RIYAN Productions

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