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Jun
06

Compact Digital Cameras – ISO, Focal Length, Resolution and Disappointment

Photography is one of my six million hobbies, but sometimes it gets more than its fair share of my time and attention. With no real artistic talent, I focus my photographic energies on the bits and bytes of creating passable digital images.

While my tool of choice is a digital SLR (right now a several generations old and soon to be replaced Canon D60), I often try to mix photography with another hobby/activity, cycling. Of course, it’s difficult to carry an SLR body and lenses while riding, so I pack a compact digital camera when I’m on the road. The problem that this creates is that the more I see in the images produced by digital cameras and the more I understand about what to expect from them, the more disappointed I am with the images captured by compact digital cameras.

For sure, there are always going to be tradeoffs simply by virtue of the size difference between an SLR and a compact. The manifestations of these tradeoffs, however, have a much greater impact on the images produced than most people believe. To be more specific, the key problems of a compact that effect image quality and intent are the compact’s seemingly limitless depth of field (DoF), the amount of digital noise (or grain for those of you still using film terminology), and softness (slight blurriness) that can be seen in captured images.

Soft Images

Of these problems, softness is the least related to the application of digital technology in the camera. While all digital cameras produce somewhat soft images, it’s the size of compact cameras (digital or film) that cause even softer pictures to be produced. This is an artifact of the size of the lens. Generally speaking, the smaller the camera, the smaller its lens and the smaller the diameter of the lens, the more that light diffracts around the border of the lens. It’s just what light wants to do.  The smaller the aperture, the more diffraction and the more light gets deviated from hitting the sensor (or film) perpendicular to its plane.

This is something to consider when buying a compact camera. Always look for the largest diameter lens available when you have a choice (e.g. if you can’t decide between two cameras, you may want to use the size of the lens to help make your choice). It’s also something to consider when you’re shooting pictures. In general, you want to keep the aperture of the lens as wide as possible (letting in the most light) by selecting the lowest f-number that works for a shot. As many compact cameras become increasingly automatic, though, this is often not a choice.

Image softness is certainly not a major problem and it’s one that’s somewhat fixable by post-processing your pictures in photo editing software – something that wasn’t an easy option with film.  If you’ve always shot with a compact, you’re unlikely to see any major difference in softness with a digital compact.  If you’re looking at improving your pictures, though, keep in mind that this is an important factor in your final result.

Depth of Field

You may consider Depth of Field an opportunity rather than a problem.  The greater the DoF, the more of the foreground and background of the image you take will be in focus.  If you’re into taking sweeping panoramas or you like taking shots where subjects are scattered throughout your image, you may love the fact that a compact gives you an incredible amount of DoF in virtually every shot you take – almost everything is always in focus. Similarly, if you take action pictures with your subject moving closer to you or further away, big DoF is what you want.  If, however, you want to make your subject stand out, there’s nothing quite like playing with the DoF of the image to capture an eye-popping photo.

In the digital world, the DoF is significantly affected by the size of the image sensor.   At any given aperture opening of the lens (the setting for how much light enters through the lens), DoF is inversely proportional to the image sensor’s diagonal dimension (all other things being equal including focal length).  So, a compact digital camera with a 10mm sensor will have almost three stops more DoF than an SLR with a 27mm sensor at the same aperture setting.  Wow, that’s a mouthful!

What this means is that a camera with a bigger sensor will have less DoF at the same aperture setting.  Of course, you can change the aperture setting to balance things.  This is much easier to do on an SLR than on a compact.

There’s some saying about the value of pictures over words, so lets try this in photos.  The pictures below are taken with two digital cameras – one compact and one SLR.  Both cameras have a resolution of 6 megapixels – virtually identical resolutions (slightly different aspect ratios, though – the compact is 2816X2112 and the SLR is 3072X2048).  The pictures are taken at very similar focal lengths and aperture settings and identical ISO settings.

6MP Compact

6MP D-SLR

Full Image at f4.9 ISO 100

Full Image at f4.5 ISO 100

Zoomed Image – More DoF

Zoomed Image – Less DoF

The zoomed area is outlined in black in the original picture.  You can see that at a very similar crop, the DoF of the compact is far better than that of the SLR.  What you’re seeing is the difference in the sensor size.  Remember, that the sensors on these cameras are the same resolution, but the physical sizes of the sensors are different.

Remember, you may not even think of this as a deficiency in your compact.  For those who want to be more artistic, however, you’ll find it’s very difficult to do with a compact camera because of it’s virtually non-adjustable DoF.

Noise/Grain

Somewhat harder to see in the pictures above is the how much more grainy or noisy the image from the compact is.  With film, the ISO rating represented the sensitivity of the film to light.  The higher the ISO number, the more sensitive the film was.  If you were going to shoot in the dark, you would shoot with high-ISO film.  ISO 400 was very common for indoor shots.  For shooting in the daylight, a lower ISO film was desired.  Perhaps ISO 100.

In the digital world, the sensitivity of the sensor is electronically fixed.  When you use a higher ISO setting, though, amplifiers increase the gain of the image sensor.  Since there is no additional data, the increased gain results in more noise in the picture.  As you can see above and below, at certain resolutions, this is not a problem (note the non-zoomed pictures), but as the picture detail becomes more apparent, like when printing an image in a larger format, the grain becomes quite visible.

The pictures below were shot at ISO 400, a fairly normal medium light, indoor setting.  The full pictures don’t look too bad, but the zoomed pictures show the difference in grain produced by the smaller sensor in the compact digital and the larger sensor in the SLR.  Again, these pictures are at the same resolution.  The smaller sensor creates more noise because the pixels of the sensor are much closer together.  If the signal in one pixel is amplified, it tends to interfere, electronically, with its adjacent pixel sites.  This is what causes noise.  A larger sensor has its pixels further apart and, therefore, each pixel interferes with its adjacent pixels less.

One thing to keep in mind here is that digital image sensors have “native” ISO levels.  That means that the sensor is constructed to have a certain sensitivity to light.  Today, almost all sensors have a native ISO setting of between 100 and 200.  This means that when the ISO setting of the camera is adjusted to a level higher than this, amplification is going to take place and, therefore, there’s going to be noise in the image.  As we can see, the extent of that noise depends greatly on the size of the sensor.

6MP Compact

6MP D-SLR

Full Image at f4.9 ISO 400

Full Image at f4.5 ISO 400

Zoomed Image – More Noise/Grain

Zoomed Image – Less Noise/Grain

This has even a more profound effect at ISO settings of 800 or higher.  Below you see the difference between the two cameras at an ISO setting of 800.

 

6MP Compact

6MP D-SLR

Full Image at f4.9 ISO 800

Full Image at f4.5 ISO 800

Zoomed Image – Loads More Noise/Grain

Zoomed Image – Less Noise/Grain

If you end up printing pictures in a 3X5 format, you may never see the problem.  If you print in larger sizes, however, or view the pictures at a reasonable size on your monitor, the problem becomes very apparent.

Compact cameras have there place and come in very handy.  Their use comes with several trade-offs, though.  Some of these issues can be minimized if you know what is going on.  First, when looking for a compact, try to find one with a larger sensor.  There are many sensor sizes and while there is a some correlation between the size of the camera’s body and the size of its sensor, some manufacturers are putting larger sensors in small camera bodies.  Also during the buying process, make sure that you select a camera with a wider lens diameter.  Each manufacturer has a different philosophy here, so you will be able to find some with larger lenses.

Once you choose your new photographic weapon, consider the choices you can make with camera settings and lighting when you shoot pictures.  Try shooting at the widest aperture possible; maximizing the light you have available to take your shot; and forcing the camera to use lower ISO settings (assuming you have enough light).  Thinking about these adjustments before you take your shot can have a very big positive impact on the quality of the images you produce.

Of course, if you’re not into it.  Switch the camera to its fully-automatic mode and fire away.  You’ll still have a blast looking at the images with family and friends.

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 June 6th, 2006  
 Will  
 Photography  
   
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