Comparing SLR cameras is a very complicated affair. In order to know why a certain camera is better than another, one has to understand the technology behind it and how it influences the final image you want to create. Apart from image quality, there are other factors like ergonomics, build quality etc that will influence your buying decision, but those are easy to know and understand, so we will not discuss those here.
This post aims to help you understand these things:
- Will this post be useful for you or not?
- Basic definitions and terminology
- The Megapixel Myth
- Sensor size, pixel density and image quality
Will this post be useful for you or not?
This entire discussion is also based on the assumptions:
- You are passionate about image quality (If not, most of what I say beyond this point is not needed)
- You want to take the pain to know how, why, and which SLRs give great image quality when compared to point and shoot cameras.
- You have used a point and shoot camera before and now want to upgrade to SLRs.
Basic definitions and terminology
Let me cover the basic definitions first:
Point and Shoot (P&S) camera: a compact camera with which you can usually just do just that - point and shoot. Wikipedia has a nice topic: http://en.wikipedia.org/wiki/Point-and-shoot_camera.
SLR (Single Lens Reflex): These cameras are called so due to what happened in the film era. These were the cameras which had just one lens through which you compose the image you are going to shoot. These were considered an improvement from cameras which had two lenses, one for the image that the photographer saw and one for the film. In an SLR, what you see is what gets recorded as the image. But when digital came, even point and shoots capture the image that you see but without a mirror mechanism inside. Wikipedia for more info on how an SLR works: http://en.wikipedia.org/wiki/Single-lens_reflex_camera.
SLRs usually (always?) have interchangeable lenses. But with digital, all these distinguishing factors are getting blurred. For example, the Sony NEX-5 has interchangeable lenses and it is now part of a growing class of hybrid cameras which are a little SLR-like and also have the simplicity and compactness of a P&S. But you get the general idea.
Some people ask me if my SLR is 'digital'. If it weren't a digital SLR (DSLR), it would have been a film SLR. Very, very, very, few people still use a film SLR and film SLRs are almost extinct. So, when I say SLR, it means a DSLR unless I specify otherwise. When digital SLRs were relatively new, it was important to distinguish them as film and digital, which I think is now absolutely unnecessary. Film is officially dead (Kodak retires Kodachrome film: http://www.kodak.com/eknec/PageQuerier.jhtml?pq-path=2709&gpcid=0900688a80b4e692&pq-locale=en_US&_requestid=14093).
Now that we have covered some of the basics, here are some factors that one needs to consider:
The Megapixel Myth
One of the most hyped and yet one of the most misleading part of a camera's specifications is the Megapixels. More megapixels does not mean better image quality. It is the famous megapixel myth. Marketers want you to believe that more is better. I could easily print 5x7" prints with a 3.2 megapixel camera (these days, it is hard to find anything lower than 6 megapixels). You would need more megapixels only if you want to do a lot of cropping, or if you want to print poster size, or shoot very detailed landscapes.
Sensor size, pixel density, and image quality
The two most important parts of a camera that affect image quality are the sensor and the lens.
Among SLRs, the ones which have larger sensors (also called full-frame SLRs) like the Canon 5D Mark II have a marginally better image quality owing to the pixels being less crammed up. This results in lower sensor noise and hence better image quality in low light situations.
If you studied electronics, you must be familiar with circuit noise which is similar to (or the same as) sensor noise in a camera. So find out if you want an SLR which has a a four thirds sensor, an APS-C sensor, or a full-frame sensor, in increasing order of general price, image quality, and low light performance. You can google plenty of full-size image samples from each camera in various situations and compare them. But remember, this is just a general trend but some recent sensor technology is changing and creating exceptions (I will come back to this later). So to be technically correct, I can say, with the same sensor technology, the price, image quality, and low light performance of the sensor will usually increase along with sensor size.
Now that we know this, if you look at the sensor size chart, some of the smallest sensors go into a P&S. Most P&Ss therefore have more or less very low image quality compared to an SLR if the sensors were made using the same technology.
Now look at the example diagram:
If these sensors had only 4 pixels and were exposed to the same light, you can see that each pixel has less and less light falling on it as the sensor becomes smaller. This means that the larger sensor which has more light falling on each pixel will create a stronger electrical signal. Circuit noise is constant for a given technology. This means that the smallest sensor will have a weaker signal to contend with a lot of noise compared to the larger sensor. If you see photographs from a Canon 5D Mark II and from a Canon IXUS 100 IS point and shoot camera, you can actually see the huge difference in image quality.
A very good example of an unlikely great camera was the Nikon D700 which was just 12 megapixels but had a full-frame sensor. Owing to the fewer pixels spread over a larger area, it turned out to be one of the best cameras out there in terms of low light, high ISO performance.
Will talk more about other things you need to consider when buying an SLR in the next part!