It's Not Just Megapixels

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

By Rob Sheppard

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?" That’s a valid question, one being asked by many photographers. Eager to get the latest and greatest, there’s a quest for megapixels that obscures other important issues about sensors and digital cameras.

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 sensors are of the same type, say, CMOS, they're identical except for the number of pixels. This isn't true in either quantitative or qualitative looks at image files coming from sensors. Sensors are different.

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.