Megapixels and cropping

click photo to enlarge

How many megapixels does a photographer need? That question is as difficult and as pointless to answer precisely as, "Which is the best camera?" It all depends on your requirements in terms of image size and the type of photography you undertake.

If you sell photographs to print publications it's quite difficult to argue that you need more than 8 megapixels in a suitable camera with a good lens.. Why 8? Well, a 10 inches by 8inches photograph printed at 300dpi requires 7.2 megapixels. If you bear in mind that some magazines print at 240dpi and that most single page magazine images are less than 10X8, then 8 megapixels is clearly enough. (You'll have worked out that 5 megapixels is more than enough at 240dpi.) If, quite understandably, you want to allow for reasonable cropping then 12 megapixels is absolutely fine. If you aspire to cover the odd two page spread then 16 megapixels will do it comfortably. The number of megapixels that manufacturers offer us continues to increase, but as the above shows, unless you have a particular requirement to print large, then 8, 12 or 16 is plenty. Moreover, as print slowly gives way to screens in their many guises those numbers become significantly more generous and allow much heavier cropping.

If it isn't maximum detail in very large size prints that is used to justify high megapixel counts then it's the ability to crop and stillretain a large file. There are purists who scorn cropping, seeing virtue in composing using the camera's frame and sloppiness in composing after exposure by trimming bits off the four sides of the image. I have no qualms about cropping. I see no intrinsic worth in the offered aspect ratios of 3:2, 4:3, 1:1 or 16:9. I do have preferences, and some suit a particular subject - say portraiture - but generally speaking they all have their uses. But to find compositions in the world that perfectly fit these proportions is not always easy and can be an exercise in futility. So I think that cropping has its place.

Today's photograph is one that is double cropped. If I had a tilt-shift lens it would be only single cropped. The Broadway Theatre on Broadway in Peterborough is quite a large building, probably a former 1930s cinema, with a recent, rather good, big, glazed extension on the main elevation. Even standing on the opposite side of the road with my widest lens at 17mm I couldn't photograph it with the correct verticals that I wanted for an architectural interpretation of the building. So, I placed it in the frame with plenty of space around it and took my shot. Then I corrected the verticals and cropped the surround (smaller photo above). However, that composition wasn't as good - I think - as the further crop (main photo above) that gives greater emphasis to the people that I had deliberately included. My second crop changes the character of the photograph from architectural to a bit more of a "street" or "urban" shot, and makes an image that I like better. It's not the way a purist would shoot, but I see no problem with it. The original shot was 21 megapixels: the second crop took it to 15, and the final crop to 9 megapixels.

photograph and text © Tony Boughen

Photo 1
Camera: Canon
Mode: Aperture Priority
Focal Length: 17mm
F No: f8
Shutter Speed: 1/500 sec
ISO: 100
Exposure Compensation:  -0.67 EV
Image Stabilisation: On 

The megapixels wars and dynamic range

click photo to enlarge
The rise in the megapixel count of compact cameras has become ridiculous. Models are routinely being sold boasting sensors with 14MP, and consumers are buying them thinking that they offer better image quality than the models with a lower number. Manufacturers know this isn't the case. Enthusiast and professional photographers do too. But, the camera makers, years ago, embarked on the "more is better" method of selling new models and seem unable (or unwilling) to depart from it. This has also started to infect DSLRs: witness the concerns over the Canon 50D (15MP) and the 7D (18MP).

Anyone who is confused by all this needs to know that the main factors in image quality are the lens (just as it always has been) and the processor that works on the image in the camera, but that possibly the most important factor is the density of pixels on the sensor. The lens on an inexpensive camera is usually fairly basic, but given that it's small is usually adequate. In-camera processors improve year on year, so that the images from this year's 10MP camera are likely to be better than the equivalent sensor/MP size of three years ago. Now, what about pixel density? Here are some examples from cameras available today:

Samsung TL240 - 14.1MP - 50MP/sq.cm.
Canon Powershot S90 - 10MP - 23MP/sq.cm.
Olympus E510 DSLR - 10MP - 4.1MP/sq.cm.
Pentax K-7 DSLR - 14.6MP - 4.0MP/sq.cm.
Nikon D3S DSLR - 12.1MP - 1.4MP/sq.cm.
Go to this page of DPReview to find out the pixel density of your camera.

The best image quality in that list will come from the Nikon (the one with the biggest sensor) and the worst from the camera with the highest megapixel count, the Samsung (which has the smallest sensor.) I could have chosen 5 different cameras and, by and large, the image quality would have been directly related to the sensor size and pixel density. Of course, when people ignore the megapixel count they tend to buy within a price range, and that's when choice and comparisons become more complicated. However, the truth is that usable detail, dynamic range, visual noise and all the other factors that make for a good image is closely related to the sensor size and pixel density, not the number of megapixels. In fact, most compact consumer digital cameras would today be producing better images if the manufacturers had stopped at 8MP.

Today's photograph was taken with my LX3 (10MP, 24MP/sq.cm.) and illustrates the second theme in today's "reflection." I'd trade dynamic range for megapixels any day. This shot was difficult for the camera because of the dark shadows of the street near the bright, sunlit clouds. I had to set the EV to -1.33 to control the highlights, and that introduced quite a bit more noise into the shadows which I had to clean up afterwards at the expense of detail. If digital camera manufacturers put less effort into increasing the number of megapixels and more into enabling their products to record the detail in both bright highlights and darker shadows, then images would be sharper and closer to what our eye/brain sees. Cameras that could do this would be easier to expose, and, most helpful of all, images would require less post-processing.

photograph & text (c) T. Boughen

Camera: Lumix LX3
Mode: Aperture Priority
Focal Length: 7.9mm (37mm/35mm equiv.)
F No: f5
Shutter Speed: 1/800
ISO: 80
Exposure Compensation: -1.33 EV
Image Stabilisation: On