Wednesday, 19 May 2010

Camera Sensors and the Megapixel Myth

A topical subject -- I've just had my sensor professionally cleaned, which led me to research the difference between dust on the sensor, dust in the viewing optics and dust on the lens... which in turn led me to research the specifics of what a sensor is, the different types of sensor you get, and the difference in quality from one to the next.


If you peer through the camera viewfinder and see dust, then the dust is NOT on your sensor. It will, 99% of the time, be on the reflex mirror, focusing screen or inside the viewfinder itself.

If you consider the diagram above, you can see that any dust on the illustrated path of light will show on the viewfinder. Generally speaking, while this is annoying, it's not the end of the world and won't show up on your images.

If you are seeing little dust spots on your image, it is probably dust on your sensor. Technically speaking it could be on your lens, but it would take a sizeable bit of dust on the lens to show in the final image, due to the way the focusing works. With every lens there's a minimum distance that it will focus at and anything closer gets blurrier and blurrier - well imagine how blurry a tiny dust spec would be right on the front lens element, and you get why it's unlikely to be a dusty lens if you can see it on the final image.

So, dust on the sensor. If you can see it on the viewfinder it won't show on the image - similarly if it's on the sensor you won't see it on the viewfinder. To check for dust on your sensor, mount a lens which can be set to 50mm or longer, close the aperture right up (f/22 or thereabouts), and shoot either a uniform blue sky, or a neutral white/grey surface like a wall. You might be able to see the spots on the LCD screen, or just pull the image off and look at it on your computer. It should be easy enough to spot -- if not, then you clearly have nothing to worry about. :)

Your camera's sensor is the equivalent to the film in film photography. It's the bit that is exposed to the light to generate the image (to continue the analogy further, if the sensor is akin to the film, a RAW image is akin to the negative, and the final JPEG is akin to the print). As such the sensor is incredibly sensitive and really if anything permanent happens to it, your camera is FUBAR.

Cleaning the sensor is a delicate matter. I googled and googled the phrase "sensor cleaning" to hell when my camera had dust, and most more experienced photographers will say it's something you can do yourself as long as you're careful and purchase the right tools. DISCLAIMER: You MUST get specialist tools to clean your sensor and you MUST follow the right procedures to avoid permanent damage. The general procedure involves first using a rocket blower to blast any loose bits of dust off the sensor -- once this is done, you can use a wet-clean technique to remove any bits which are stuck on. It's really important that you use the right tools for this - the right cleaning solution, the right swab -- if you mess up you could cause permanent damage to your sensor.

I could go into all the details but let's face it -- I am a klutz at the best of times. The other night, for example, I dropped a totally NOT hot casserole lid onto the worktop (didn't break thank god), then less than five seconds later, dropped my iPod on the kitchen floor. So when I realised I had dust on my sensor, I sent my camera off to be cleaned by a professional. It wasn't that expensive, and even though I was without my camera for three days, as someone who would have almost definitely ruined the sensor had I cleaned it myself, it was well worth it.

Sensor Size, Crop -vs- Full Frame, and Mega-Pixels

This fantastic post on the Photography Forum explains it much better than I can, PLEASE read it through, but I'll attempt to give my own summary perspective on it a little too.

Basically put, the larger your sensor is, the more light will fall on it. If there is not enough light, the sensor signal must work harder to detect what little light is there, which results in a "noisy" or "grainy" shot. You can manually boost the amount of light that hits your sensor by opening the aperture (a faster lens will help with this), slowing the shutter speed, or upping the ISO, which will make the sensor more sensitive to light -- but will still result in digital noise.

A full-frame camera will contain a sensor which is the equivalent size to a 35mm camera. As a brief aside I'll explain what that means to your field of view. If you're ever looking at lenses and see a phrase like "50mm on a crop sensor, which equates to 80mm in 35mm format", what they're referring to is the effective field of view; a shot taken at 50mm focal length on a crop sensor camera will produce the same result as a shot taken at 80mm on a full-frame camera. So you can extrapolate that to mean that 50mm on a full-frame camera will give a wider field of view.

Anyway, back to the point. The benefit of a full-frame camera is a wider field of view and better low-light performance. The flipside of that is how crazy-expensive they are. Nikon's biggest full-frame model, the D3X, retails at around £5000, whereas even the most expensive crop sensor cameras will usually come in less than £1000.

Now, on to what's known as the MEGAPIXEL MYTH. This is one that I was extremely gratified to read about, as it is kind of annoying hearing people bleat on about how many megapixels their camera has as if that makes it better - something I felt even before I was interested in photography.

A few weeks ago a friend of ours was considering switching away from iPhone to another phone model, and gave the reason that the camera that came with it had 14 megapixels. I still pride myself to this day that I was able to dissuade him from this before I even read up about the megapixel myth, purely by following the logic through in my own mind.

Enough boasting from me. The megapixel myth basically states a fallacy that more MP = better image quality. This is not necessarily true! My DSLR has a modest 10.2MP. Heck, Nikon have a full-frame camera model, the D3S, which has just 12.1MP. The truth is, a smaller sensor has a "sweet spot" when it comes to MP resolution, and if you try and cram more MP into a smaller sensor, you're going to end up compromising the image quality by creating sometimes-unacceptable levels of digital noise. So to Dan I said, it doesn't matter if your phone has a 14MP camera, the sensor is way too small to produce a decent-quality image.

Megapixels do matter to an extent, and especially in two particular areas: cropping and large-scale printing. If you're going to crop a huge amount out of a shot, more MP will allow you to do so and maintain image quality when the shot is scaled up to a bigger size. Similarly, if you're going to take a shot that's meant for, say, a huge wall poster or a billboard, I would imagine the MP count would be important there too.

There's a great article from the New York Times which explains the Megapixel Myth and also some genuine tests that were done to disprove it.

I hope this post has helped you to understand your camera's sensor a little better. Needless to say, the bigger sensor you can afford, the better the image quality will be, and megapixel size is NOT the effective comparison tool you think it is when considering which camera to buy next.


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