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Monthly Archives: June 2017

Know the Reasons Why Camera Auto Focus Works and Sometimes Fail

Automatic focus though has its limitations. For example sometimes one might want to produce photos which are a bit fuzzy as an artistic expression. Also the auto focus implementation has its limitations and in some scenarios it might fail. One example is using a high end SLR camera with a passive auto focus system to take a picture of blue skies. In most cases the camera will move its motor back and forth and will eventually give up and fail to focus.

To better use the auto focus system it would help to understand high it actually works. Although implementations can vary we can divide them all into two categories: passive and active. Most pocket cameras use the cheaper passive method while high end professional cameras use either the active or a combination of both.

Passive auto focus:

Passive auto focus can be perceived as imitating how we set the focus manually. The camera defines one or more regions in the picture (usually they are marked as rectangles on the viewfinder or the LCD). The camera then analyzes the picture seen through those regions and calculates a Focus Level number. The camera then tries to move its lenses back and forth as it recalculates the Focus Level. The camera looks for a position where the Focus Level is the highest. For that point if the Focus Level is above a predefined threshold the camera would define this region of the photo as being in focus.

The Focus Level can be calculated in many ways. The common attribute of all calculations is figuring out how much Contrast is there in the photo. Although not in the scope of this article one way to calculate such a number is by running the photo through a high frequency filter – this is based on the fact that high contrast is associated with high frequencies.

Active auto focus:

Active auto focus works by measuring the distance between the camera and the object in the picture. Technically if you knew the exact distance to the object you are taking a picture of you could set the lens to the exact focus position. The active focus system shoots a beam of invisible light, usually infrared, at the object at the center of the picture and measures the distance to that object. Based on that distance the focus is set.

Combined auto focus:

Some high end cameras combine both systems. The camera will pick the right system for the specific scenario or will cross check and use both at the same time. The photographer can also decide manually to use one of the two options. For example when shooting blue skies the camera can try to use the active system and measure the distance. Since the distance is infinite the camera can set the focus and skip the passive focus. In other cases when the distance is not infinite the camera can use the active system to put the lens in approximately the right position and then use the passive system for fine tuning. In dark scenarios the camera can opt to use the active system since the passive one will not work.

So why doesn’t the auto focus work all the time?

Even with all the electronics and computing power in the camera there will always be scenarios where the camera auto focus fails. Failure can be when the camera can not focus and the picture is fuzzy or sometimes when the picture is actually in focus but the camera “thinks” that it is not.

What causes such cases? The list is long but here are just a few examples:

– Taking low light pictures: The passive auto focus system needs to “see” the picture in order to work and in low light scenarios this is not possible. Some systems use a series of flashes to overcome this limitation but this solution fails many times. An active system can measure the distance to the object in such scenarios but will fail if the object is not in the center of the picture or if there are a few objects at different distances.

– Active systems can fail with objects that tend to absorb the infrared beam they are using. Some materials absorb infrared beams and will cause the active system to measure the wrong distance. In some scenarios other infrared sources such as candles and open flame fires can render the active system useless.

– Low contrast objects such as white walls or blue skies. The passive auto focus relies on the fact that the Focus Level changes significantly when moving the lens back and forth. This allows the camera to settle on the right focus position. The Focus Level of low contrast objects does not change much and fails the passive system.

Knowing how the auto focus system works helps a photographer understand why sometimes the camera can not focus. In such scenarios the photographer can look for other solutions. Sometimes the photographer will have to use the manual focus. In other cases focusing on another object in the picture that is in the same distance but easier to focus on and locking the focus on that object will solve the problem.


About Pixels

To understand pixels, one must first remember the good old days when you took pictures on film and let the development labs worry about pixels. But, they didn’t really have to, because the exposed negative just captured the image that was reflected through the lens. It was all there and the only thing that affected quality was the size of the negative. The bigger the negative was, the larger the picture that could be produced. When the world went digital, the pixel was invented. So what is a pixel? If you divided a picture in hundreds of horizontal and vertical lines, you would actually have a large number of little squares that when put together, created the picture. Each square is a pixel. So, when a digital camera says it has 1-megapixel capabilities, it means that the total number of horizontal and vertical squares produced when a picture is taken equals 1 megapixel (1,000,000 pixels).

A 3 megapixel camera produces a picture that has 3 million pixels in its makeup. If all the pictures were the same size in each camera, it would be easy to understand why a camera that produces more pixels would be a better quality camera. Here is where the camera makers are not forthcoming with information. What they don’t tell you is that as far as I can tell, every digital camera produces pictures that have 72 pixels per inch per picture both horizontal and vertical. The only difference is that a 3 megapixel camera produces a larger picture than a 1 megapixel camera and a 5 megapixel camera produces an even larger picture yet. The reason I say “as far as I can tell” is that I have not tried all cameras or studied the information on all cameras. There may be some out there different, but they probably are not in the amateur photographer’s price range.

Conclusion: For video uses only, your camera purchase decision should be based on features other than the number of pixels.


Get Better Digital Photos with These Tips

Be Prepared

Keep all your photography equipment ready for use. Collect everything you’ll need into one place. A camera bag is ideal, because it keeps all your stuff together and lets you carry it all with you. Everything in its place. A good camera bag will let you organize a miniature tripod, extra battereis, memory cards, etc. – even a plastic bag or waterproof housing to protect your camera in wet weather.

Hold your Camera Steady

Blurry photos are almost always the result of camera movement. Just your own unsteadiness, causes your camera to shake enough to blur your pictures.

So steady yourself and your camera before you take the shot.

Plant your feet firmly on the ground and tuck your elbows in close to your sides. Instead of using the LCD viewer, steady your camera against your forehead and frame the shot using your camera’s viewfinder. You can also steady your upper body by leaning against a wall or a tree. Or totally eliminate any camera movement by using a tripod.

Once you’re all set, gently press the shutter release in one motion. Pressing the shutter release too hard could jerk the camera downward.

Get Closer

One difference in “snapshots” and really great photos is the composition of the shot. Unless you’re shooting an outdoor landscape, you can improve most photos just by getting closer to your subject. Depending on the situation, you can physically move closer to your subject, or use the zoom feature on your camera for the same effect. Try to get within a few feet of your subject so you eliminate most of the background. You’ll like the results.

Take more Pictures

Even professionals take loads of shots of the same subject – to get just a few that they will use. With a digital camera, you can delete the images you don’t like, and only print the winners – so don’t hesitate to take several shots of the same subject. Change the angle of the shot. Get a little closer. Adjust the lighting.

Why not fill the entire memory card with pictures of your kid at the pool, or your daughter in her cap and gown? The more pictures you take, the better the odds that you’ll get a few shots that will really thrill you.

Vary the Lighting

Using natural light will give better skin tones when photographing people, so try not to use the flash if you don’t have to. Outdoor daylight shots are easy, but you’ll have to be a little more creative when shooting indoors. Try using the light coming in from a window for warmer tones than you would get using the flash.

Experiment with natural lighting. You can get stronger shadows by moving your subject closer to a window, and turning your subject can create more dramatic shadows.

Eliminate Red-Eye

Red-eye is the result of light passing through your subject’s eye and reflecting back. You’ll get it more often when using your flash, just because the light from the flash isn’t as diffused as natural light. So the first tip for eliminating red-eye is simply to avoid using your flash when you don’t absolutely have to.

Another way to reduce red-eye is to have your subject look anywhere but at the camera. This reduces red-eye because any reflection isn’t directed back at your camera lens.

If you have to use the flash, some digital cameras have a built-in feature to automatically remove red-eye. Use it.

Go for Candid

Instead of posing two (or more) people looking directly at the camera, get a shot of them interacting with one another. Even two people having a conversation is more interesting than having them stand next to each other facing the camera. Some of the best professional portraits have the subject captured deep in thought, with their attention focused inward, rather than on the camera lens.

It makes a more interesting shot. Your portrait will look more natural – less posed.

Create a Scene

Putting your subject in the center of a photo is just boring. You’ll get a much more pleasing result if you place your subject off center when you frame the shot.

This is a truly professional technique. Place your subject so that they occupy 1/3 to 1/2 of the total composition, but NOT at the exact center of the frame. Capture an interesting background object in the rest of the frame.