We have experienced the increase of the megapixels on
digital cameras during the past few years, I still remember when 0.5 megapixels was the
largest image size we could find meanwhile nowadays we can find cameras with
24.3 megapixels and the megapixels will continue to increase as the camera
companies keep telling users that largest megapixels translate in better image
quality. Personally, as an amateur photographer and researcher in the field of
image processing, I think that most of the time an image with more than 6
megapixels is a waste of memory and camera resources.
Let me start explaining the reasoning camera makers use to
convince user that more megapixels is better: Printing quality. As you know, a
good printing quality is achieved when the printing resolution is equal or
higher than 300 PPI (pixels per inch) and, therefore, if you want to print a
large image with good quality you would need to have a large image, for example
with a 2 megapixels image the largest print size at 300 PPI would be of 14.7 cm
x 9.7 cm (5.8” x 3.8”). You can do the math yourself, but in the page of
Imagine 123 you will find a table
of the image size and printing sizes you may have. The camera makers tell
users that with larger pixels they won’t just be able to print in larger format
but also they will obtain more detailed photographs since you will have more
pixels to represent the objects in the image. I don’t say this claim is
completely false, but you need to consider other aspects that aren’t as
straightforward as the concept “bigger is better” and this discussion has been
in the air since some years ago as you can see in this cnet news note from 2007.
If we accept as a fact that most photography enthusiasts
don’t print their photos in large format, then the camera makers just have the
detail in the image as the only reason to offer users more and more megapixels
every day. But, it is really true that more megapixels are synonym of more
detail? My answer is yes for just few cases but most of the times is a big no.
Let me explain you my reasons:
First we need to consider the sensor of a digital camera, it
is an array of light sensitive elements and each pixel will correspond to a
small area of the sensor, meaning that the information in each pixel is the sum
of the light arriving through the lenses into the pixel area. Now, if we keep
the size of the sensor constant and we increase the megapixels the resulting
pixel size will be reduced and therefore less light will arrive to each pixel
increasing the effects of electrical noise in the sensor degrading not just the
sensitivity to finer tonal gradations but also the quality of the image in dim
conditions. As an example, I took two different photographs using my camera
with 6 megapixels (2816 x 2112 pixels) and a 7.18 mm sensor and one of the
cameras of the HORUS system with just
1 megapixel (1024 x 768 pixels) but a 8 mm sensor, i.e., more than twice larger
pixels. You can see how there is more noise in the image captured with the 6
megapixels camera despite the fact that there are more pixels to represent the
same object. You can see the complete
pictures in my blog.
My camera |
HORUS system camera |
The noise is not a problem in highly illuminated scenes, that’s one of the few cases were bigger is better, but for dim conditions the camera makers try to solve the problem using clever image processing methods, for example increasing the gain of the light sensor and using filtering algorithms to reduce the noise, most of the times reducing also the image size. As you can imagine, the image processing will end up with an altered image and for purists this could be a downside of using cameras with large megapixels.
At the end, maybe professional photographers will fully exploit the advantages of large images, but we must keep in mind that the image quality is not completely determined by the megapixels of it, we also must take into account the camera’s optics (lenses) and especially the sensor’s size and sensitivity and, therefore, we shouldn’t trick ourselves into the “bigger is better” mantra of most of the camera makers and sellers.