Understanding Exposure September 10, 2007

Sunny 16 & BeyondThe extremes of brightness that one encounters in the natural world are not that varied. For this reason there is the so-called Sunny 16 rule.

This says that on the brightest day normally encountered the proper exposure is roughly the reciprocal of the film speed at f/16. Thus, if you are shooting ISO 200 film then the exposure will be 1/250 second @ f/16. This is the same whether you’re in Himalayas or Kerala , mid-summer or mid-winter. From the extremes of a sunny day outdoors down to typical indoor room lighting covers a range of about 10 stops. With the exception of seldom encountered situations like fireworks, cityscapes and moonlight scenes these 10 stops encompass every lighting situation you are ever likely to encounter. Only on the ski slopes or at the beach will you need to stop down one more stop beyond Sunny 16 because of reflections off the snow and sand.

So, that being the case, why is exposure so difficult? Most people should have no problem in recognising 10 different light levels, shouldn’t they?

The Eye’s Autoexposure Unfortunately (or fortunately, depending on your point of view), the human eye and brain have a superb autoexposure mechanism built in. This means that once your eyes have adjusted to the current lighting situation, and without clues as to what is causing the light level encountered, it is almost impossible to tell how bright things are on a relative basis. As long as the light level lies somewhere within that 10 stop range for most people it all appears the same.This is why light meters, whether built-in or handheld, are such vital tools. But before exploring light meters and how best to use them it’s worthwhile to have in ones mind a firm idea of what “proper” exposure settings are for the ten light levels normally encountered. This way you’re not a blind slave to the meter. Let’s assume an F stop of f/8 and a ISO (film speed) of 400.

Here’s what these 10 light levels are and the shutter speed that would be needed.-

• a Sunny day outdoors - 1/2000 sec-
• a hazy bright day - 1/1000 sec-
• a bright cloudy day without shadows - 1/500 sec-
• an overcast day, or open shade on a sunny day - 1/250 sec-
• a heavily overcast day - 1/125 sec- deep shade.
• The woods on an bright overcast day - 1/60 sec-
• just before a thunderstorm or late on a heavily overcast day - 1/30 sec-
• brightly lit store interior - 1/15th sec-
• a well lit stage or sports arena - 1/8th sec
• a well lit home interior - 1/4 sec

Of course you would vary the F stop and shutter speed combinations to whatever would be most appropriate. In the case of a home interior, for example, instead of 1/4 second at f/8 you might choose 1/30 sec at f/2.8. The point is though that these 10 brightness levels represent 95% of the conditions under which we all do our shooting.

What a Meter Does

A light meter does one thing. It tells you what the correct exposure is for 13% Gray. This is approximately the tonality of green grass or concrete. The meters that are built into almost all cameras today are reflective meters. This means that they are measuring the light being reflected off the subject. This is a convenient way to determine proper exposure but there are potential problems because grass and concrete usually aren’t our main subjects. Fortunately, most of time the mixture of objects in a scene: grass, sky, people, trees, rocks and so forth, when averaged together usually are pretty close to an 13% gray. But, because much of the time the things that we photograph are not so conveniently neutral in tonality, manufacturers of TTL metering systems on cameras have had to go to great lengths to design multi-zone patterns and sophisticated software algorithms to enable their metering systems to provide pleasing and accurate exposures.

Digital Photography- The Megapixel Myth September 10, 2007

PIXELS, RESOLUTION and PIXEL COUNT (MEGAPIXELS)

Pixels

Images are made up of little dots called pixels. Pixel stands for PICture ELment. Put enough of them together and you have a  picture. They are arranged horizontally and vertically. Get close enough to your computer screen (or use a magnifier) and you’ll see them.

Resolution (Linear Resolution)

Image Resolution

Resolution is how many pixels you have counted horizontally or vertically when used to describe a stored image. Digital cameras today have between 2,048 and 4,500 pixels horizontally. 3 MP cameras have 2,048 pixels horizontally and 14 MP cameras have 4,500 pixels. They have fewer pixels vertically since the images aren’t as tall as they are wide.

Print Resolution

Resolution is also how many pixels you have per inch or other linear unit when you print on paper. Most prints are made at 200 – 300 pixels per inch (PPI or DPI, dots per inch). This is the image resolution and has nothing to do with the technology by which the print is made. (For instance, inkjet printers’ nozzle sizes are the silly 2880 DPI or other numbers you see. These printer numbers are often used by hucksters to hoodwink and distract you when talking about resolution. These only refer to how the ink is spat out on the paper.)

Screen Resolution

Most computer screens today are about 100 DPI, dots per inch. There isn’t much variation from screen to screen so we rarely discuss this. It’s easy to figure out: most computer screens are about 1,024 x 768 pixels. If your screen is 10″ wide then divides 1,024 by 10 and you have a 102.4 DPI screen. Bigger screens tend to have more pixels, for instance, my 22″ CRT has 1,600 x 1,200 pixels and has a viewing area of 16 x 12.” Yes, laptops with bigger screens tend to have lower linear resolution. No big deal.

Pixel Count, expressed as Megapixels

Pixel Count, expressed as Megapixels, is simply multiplying the number of horizontal pixels by the number of vertical pixels. It’s exactly like calculating area. A 3 MP camera has 2,048 (horizontal) x 1,536 (vertical) pixels, or 3,145,728 pixels. We call this simply 3 MP. Small differences in pixel count, between say 5 MP and 8MP, are unimportant because pixel counts are a square function. It’s exactly like calculating area or square footage. It only takes a 40% increase in linear dimensions to double the pixel count! Doubling pixel count only increases the real, linear resolution by 40%, which is pretty much invisible.

THE MYTH

The megapixel myth was started by camera makers and swallowed hook, line and sinker by camera measurebators. Camera makers use the number of megapixels a camera has to hoodwink you into thinking it has something to do with camera quality. They use it because even a tiny linear resolution increase results in a huge total pixel increase, since the total pixel count varies as the total area of the image, which varies as the square of the linear resolution. In other words, an almost invisible 40% increase in the number of pixels in any one direction results in a doubling of the total number of pixels in the image. Therefore camera makers can always brag about how much better this week’s camera is, with even negligible improvements. This gimmick is used by salespeople and manufacturers to you feel as if your current camera is inadequate and needs to be replaced even if the new cameras each year are only slightly better.One needs about a doubling of linear resolution or film size to make an obvious improvement. This is the same as a quadrupling of megapixels. A simple doubling of megapixels, even if all else remained the same, is very subtle. The factors that matter, like color and sharpening algorithms, are far more significant.

The megapixel myth is also prevalent because men always want a single number by which something’s goodness can be judged.

Unfortunately, it’s all a myth because the number of megapixels (MP) a camera has has very little to do with how the image looks. Even worse, plenty of lower MP cameras can make better images than poorer cameras with more MP.

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