Digital Camera Buyer’s Guide: Compact Point and Shoot

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I’ve never owned a camera (besides what is in my cell phone and laptop). I want to take pictures at parties and post them online or email them to my friends.

A: a compact point and shoot camera is likely the best option- everything is automatic, all you need to do is point the camera where you want and press the button. Because this camera does everything for you, you give up having control over most or all of the parameters discussed below. Most likely, you will either outgrow this camera or need to replace it in about 1 or 2 years.

One important thing to look for in an entry-level camera is ‘ease of use’. Chances are, you would rather take images rather than fiddle with camera settings. For example, are the buttons and dials easy to find and set? Is the LCD display sharp and bright enough to see in sunlight? Other priorities may include the ability to take a picture quickly (to catch moving objects like animals and small children), fast startup or turn-on times, portability, etc. These cameras produce very acceptable images that can achieve a professional-quality 4″ x 6″ print. We will have a lot more to say about resolution and print size later. At this level of performance, the sensor and the lens do not limit the image resolution- usually the automatic gain control and noise reduction do.

At this level, you don’t need to know any detailed imaging theory. However, we present some basic definitions needed to interpret the specifications on a camera and lens- these concepts will be refined in later sections. Then, we discuss what is inside the camera.

Basic definitions: A camera lens will have some numerical specification, for example ’18-200mm f/3.5-5.6′, and the sensor will have some size specification, either a number of pixels (8 MP) or the diagonal dimension of the sensor (e.g. 1/1.7″). For the lens, the numbers ’18-200mm’ specify the rear focal length of the lens (in this case, a zoom lens with varying focal length between 18 and 200 millimeters). The rear focal length of a lens is not the distance between the lens and the sensor. There may be two sets of focal lengths- the actual and ’35mm equivalent’, and this is discussed more fully below. The term ‘f/3.5-5.6’ is the maximum f-number available for the lens (in this case, the maximum f-number varies with focal length). The f-number is defined as f/D, where f is the rear focal length and D the diameter of the entrance pupil (defined below).

As you will see, the focal length and f-number are essential quantities to understand. For now it’s sufficient to know that the focal length is a measure of the angular magnification of the image and with the f-number, is used to calculate the depth of field (DOF- the distance over which near and far objects are in focus). The f-number relates to a number of image metrics besides DOF, including the optical resolution and the amount of light passing through the lens.

The f-number is usually controlled by varying the size of an aperture within the lens body. However, the size of that aperture is not ‘D’, above- D is the diameter of the entrance pupil. The entrance pupil is the projection of the aperture into object space. Put another way, when you look into a lens and see the aperture, what you really see is the aperture projected through the lens; since you are looking into the front of the lens, what you see is really the entrance pupil. The distinction may not appear significant, but in some cases (zoom and fisheye/ultrawide lenses and panoramic photography) it is.

Digital camera sensors are often specified in terms of the diagonal length (like televisions), and just like televisions, the aspect ratio of the sensor may be 4:3, 3:2, or 16:9. The size of the sensor controls the field of view and also limits the maximum useful size an image can be enlarged. Because the image size is fixed, many imaging properties of lenses are interrelated: field of view and focal length, for example. Because there are no standard specifications for digital sensors, the practice of quoting 35mm equivalent focal lengths is done to provide a rational means to compare different sized sensors.

What makes a digital camera special? A digital camera differs from a film in only two significant respects. First, and most obvious, a chemical emulsion (film) has been replaced by an array of electronic light detectors (pixels)- the sensor. A second key difference is what happens when you press the button to take a picture. In manual cameras, not much happens- a mechanical shutter opens for a set amount of time, exposing the film, and then closes. Digital cameras do a lot of things when that button is depressed, including light metering and focusing, all of which take time- this is “shutter lag” and is why the camera doesn’t take a picture immediately. This can be a hassle when trying to photograph moving objects as they may move out of the frame (or focus) before the image is acquired.

The ‘half-press‘ is a technique that becomes very useful with these cameras for a couple of reasons. First, it lets the camera get ready to take an image (light metering, autofocus, etc) while you compose the shot. Second, many cameras will allow you to ‘hold’ the setting, reposition the camera, and then take a shot without changing the setting. This can be useful if you want something off-center to be in focus, for example. Another example is dealing with scenes with large variations in brightness- bright skies and shadowed valleys. Mastering the ‘half press’ can also alleviate a lot of lag time problems.

Bayer filter: the pixels only detect the total amount of light incident; they do not distinguish colors. In order to generate a color image, sensor companies coat the sensor with an array of color filters, and the particular pattern has been standardized to a ‘Bayer filter’: Every other pixel sees green, and the other pixels alternate between red and blue. One important result from this is that the final image (say a color Jpeg file) has been produced by interpolating between pixels in order to appear that each image pixel has full color information. RAW images consist of the actual individual pixels and are used in more advanced photography, because each pixel retains its original identity and the photographer/print shop has more control over the final color print.

Flash: Usually, these cameras have an automatic flash, so make sure you know how to turn it off: flash photography is often forbidden in museums, churches, etc. One important property of the flash is that closer objects will appear brighter than more distant ones, from the inverse-square law. This can result in a close person appearing bleached white, while the wall behind them is black. Also, because the flash is (relatively) low power, there is a limited useful range- the flash may not be able to effectively illuminate people more than a dozen feet away. Another issue related to flash photography is the “red-eye effect”: light enters the subject’s eye, bounces off the fundus, and back out of the eye- the red color comes from the blood vessels in the choroid. Because the problem is so common and pervasive, some cameras have a ‘red eye reduction’ mechanism which often consists of a series of bright flashes designed to contract the subject’s pupil; make sure your subject doesn’t think the picture was taken during those flashes and walk away.

Gain: These cameras almost always have a high f-number lens. This provides a large depth of field (most of your objects will be in focus) and combined with the small sensor size, produce little aberration. If the aperture is fixed, the amount of light at the sensor is controlled by the shutter speed (or if there is no shutter, the integration time). Often there will be automatic gain correction (AGC) to control the sensor output levels to prevent bleaching (overexposure or white pixels) and underexposure- black pixels. AGC can introduce a lot of noise in your images- even indoor lighting, without a flash, can tax the gain level of your camera. Because of this, camera manufacturers add noise reduction schemes that result in a loss of fine detail.

Dynamic Range: the dynamic range of an image, like any other signal, represents the amount of (intensity) variation that can be resolved. In a well-exposed image, there are not too many underexposed black pixels or overexposed white pixels. Visual studies have shown that human vision cannot distinguish more than 256 discrete grey levels: this is why computer displays are either 8-bit monochrome or 24-bit RGB color. Images with less than 8 bits (24 bit color) dynamic range appear ‘posterized’, while images with more than 8 (24) bits of dynamic range must be re-scaled prior to display.

Sensors can have more then 8 bits of dynamic range- there are 16-bit sensors on the market- but increasing the number of bits does not automatically increase the maximum signal to noise ratio that can be supported. Images with more than 8 (24) bits allow more flexibility in post-processing to adjust brightness, contrast, and color balance (discussed below).

Optical zoom vs digital zoom: Some of these camera lenses have zoom capability. Optical zoom means the lens has a variable focal length. Digital zoom is nothing more than digitally magnifying the original image- just like you can do on your computer. Digital zoom is rarely useful, and is usually combined with optical zoom to make the customer think the camera can do more than it really can. Optical zoom, by contrast, does represent a real increase in magnification and resolvable detail.

White balance– the color of objects depends on the spectral distribution of illumination. For example, sunlit objects appear different if instead lit indoors under fluorescent lights. Film photographers used filters (gels) to compensate for or enhance this effect. Electronic cameras can do this without a filter by weighting the different colored pixels differently. Taking a picture of a white piece of paper using different white balance settings can be instructive. Often, indoor images have an orange cast to them: since fluorescent lights emphasize blue colors, the camera tries to compensate by boosting the orange tint.

Live preview – this is simply using the LCD to display what the image would look like, in real time. Different effects can be previewed prior to imaging.

Image stabilization – any relative motion between the sensor and subject while an image is acquired will result in Ômotion blurÕ. While this can be used to emphasize isolated elements of an image (moving wheels, for example), a uniformly blurred image is unappealing. There should be some image stabilization- at this level, manufacturers implement it electronically- but it will help your images appear ‘sharp’.

Shooting modes – Often, cameras will have a number of “modes”: nighttime, landscape, people, cloudy weather, etc. These represent the manufacturer’s opinion about the best fine-tunings for the camera parameters in particular lighting situations, and may or may not be useful to you. One popular mode is ‘panoramic’: multiple images are (later) combined to produce an image covering a field of view much larger than a single image. While there are some techniques to properly rotate the camera, the manufacturer’s software is usually very forgiving of alignment errors- images will be stretched and distorted slightly to produce a seamless transition.

Video is mentioned here only because some sort of video capability is becoming a standard feature. This sticky does not address issues specific to video other than general optical concepts that apply.




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