It’s Not Just Megapixels

Image quality in a digital camera is affected by much more than how many pixels fit its sensor


It‚’s Not Just Megapixels  Recently, I received a letter from a young photographer friend asking about the issue of megapixels and digital cameras. He wanted to make a purchase and wondered if a slight increase in megapixels was worth the cost: "If one photograph was taken with a 6-megapixel camera and one with an 8-megapixel camera, when would you see a difference as the photos were enlarged?"

To start, here's a crucial concept: all sensors aren't the same, even if they all produce quality images. There's a misconception that if two sensors have the same megapixels, say, 6, they're essentially the same, and that if two

Once you reach a certain size of sensor, megapixels may not be the most important consideration in looking at a digital camera. Megapixels are a size issue, not necessarily a quality issue. If you're printing 8x10- or even 12x18-inch prints, you wouldn't see any difference between 6- and 8-megapixel cameras based on pixel count alone, and truth be told, you'd see little difference between them and any higher-pixeled cameras. If you started making big prints, say, 20x30 or larger, you'd see additional pixels beginning to matter.

The final quality of any digital image or print is related to a number of interconnected factors. Megapixels are too often used as the gauge when they mainly affect size. Sensor qualities such as tonal range, noise reduction and the ability to deal with certain colors all can affect a photo, but so can the lens used and photographic technique of the photographer. No sensor will deal well with a bad exposure, for example.

Sensor qualities can be difficult to measure, unfortunately. Think of film. I could make a strong case that Kodachrome is absolutely the best slide film for nature photographers and someone else would make an equal case for Fujichrome Velvia; for our shooting needs, we'd both be right.

Camera sensors are a little like that. There are differences among sensors even from the same brand, especially when comparing sensors of older technology versus the latest units. This is one reason why setting the same white balance on two different camera models won't necessarily give matching photos.

It comes down to what you like and need from a photo. For example, Fujifilm has just started shipping its latest digital SLR, the FinePix S3 Pro, with a unique sensor. It has 12 megapixels of photosites (as the individual pixel sensors are called), but only 6 megapixels of resolution. This puts the megapixel question into a whole new realm and definitely highlights how very different sensors can be.

A challenge of digital cameras has long been dealing with highlights. If most sensors acted like film in the way highlights are treated, RAW files might be unnecessary, but they don't, and RAW is one way to ensure that better highlight detail is rendered when needed. The Fujifilm Super CCD SR II sensor takes this a step further in capturing highlight detail. In some early work with the camera, I found this capability most impressive.

But here's the dilemma facing any discussion of sensors. In the past, those who shot Velvia (which in essence has a reduced tonal range) didn't complain about its problem with highlights in contrasty scenes. On the other hand, some photographers only shot print film because of that problem. So if an extended tonal range is helpful to your style of photography, then this is a perfect sensor for you. If you have little use for that extra tonal range, but you need a camera that can give you giant prints, then this sensor does nothing for you. Great product, yes, but only in the hands of someone who can use that technology effectively.

Sensors keep changing. I have no idea what will be the next step in sensor technology, but it doesn't have to be more megapixels to add a real benefit to photographers. Once you reach 6 megapixels, noise tends to be a far more important issue for photographers than simply adding more pixels, yet it's an issue rarely discussed by photographers.

Less noise means cleaner, more attractive shadows, better color and finer tonal gradations. As a general rule, sensors in the most recent cameras have much better noise characteristics than earlier cameras. The Fujifilm FinePix S3 Pro is a good example, both in terms of a sensor with less noise and new capabilities, allowing more exposure (which always will lessen noise in shadows) without blowing out highlights.

Another example is the Canon EOS 20D. Many people wonder if the newest EOS 20D at 8 megapixels is really worth the expense if they already own a 6-megapixel Canon EOS 10D, D60 or Digital Rebel. The additional megapixels aren't a lot and are insignificant with smaller prints.

The EOS 20D features the latest in sensor technology, however. The sensor itself (not just noise-reduction circuits in the camera) produces significantly less noise than any of Canon's earlier 6-megapixel cameras. A simple way to quantify this is to compare ISO settings. The EOS 20D at ISO 400 will match the earlier cameras at ISO 100 in terms of noise. That's a significant improvement that affects more than image quality. It will mean you can use a smaller, lighter, less-expensive, though slower (smaller maximum ƒ-stop) lens for equal results to the more expensive, faster lens.

Noise is one problem with small, compact digital cameras. An 8-megapixel Nikon Coolpix camera won't match the noise results of a Nikon D70, for example. The compacts use very small sensors, which are susceptible to noise. At low ISO settings of 50 to 100, results from both cameras will be close, although the smaller camera will have a bit more noise noticeable when the image is magnified, especially in sky and shadows.

Still, advanced compact digital cameras do remarkable work at ISO 50 to 100, and their size makes them a great companion, even if their sensors don't quite match the digital SLRs in terms of noise. These little cameras even offer some features big SLRs don't, but the noise can mean photos won't enlarge as well as those from cleaner files.

At higher ISO settings with the compacts, you can expect a lot of noise, far more than any digital SLR will have at the same and slightly higher settings. For many people, this means not shooting advanced compact digital cameras at higher ISO settings. It's also a great advantage to the digital SLR.

No matter what camera you have, there are several steps you can take to keep noise to a minimum:

1.Watch your exposure; underexposure will increase noise.
2.Use lower ISO settings.
3.Don't oversharpen areas that emphasize noise, such as sky
and out-of-focus areas (try focusing different parts of the image separately).
4.Increase the threshold level of Unsharp Mask to between 3 and
10 to reduce the effect of sharpening on noise.
5.Sharpen different parts of the photo separately; if noise starts
to become too strong, sharpen that area less (or not at all).
6.Avoid overbrightening dark areas; that will boost noise a lot.
7.Shoot JPEG with compact digital cameras because more noise processing algorithms are used to create those files.

It's important to note that noise isn't all bad. It can be an interesting or creative element of the right scene. Columnist Bob Krist, for example, likes its affect on certain travel scenes, especially when combined with a warming filter or warm white-balance setting. Noise also is an old Photoshop trick used to make soft photos look sharper. If you add noise to a less-than-sharp photo, the viewer sees sharp noise and is tricked into thinking the photo is sharper than it is.

The PCPhoto-Guide to Digital SLRs by editor Rob Sheppard is now in bookstores. His intermediate Photoshop tape, Enlightened Photography: Photoshop by Example, can be found at


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