Lenses come in two basic flavors: positive and negative. A positive lens will converge light toward
a point on the far side of the lens and a negative lens will diverge light that same light. If the light
is coming from a long way - ie. the rays from that object are parallel to each other or very nearly
parallel - the light will focus at the focal point of a positive lens. This is the situation of burning
ants with a magnifying glass. A 100mm lens will focus parallel rays of light 100mm from the center
of the lens and a 25mm lens will focus those same rays 25mm from the center of the lens.
a point on the far side of the lens and a negative lens will diverge light that same light. If the light
is coming from a long way - ie. the rays from that object are parallel to each other or very nearly
parallel - the light will focus at the focal point of a positive lens. This is the situation of burning
ants with a magnifying glass. A 100mm lens will focus parallel rays of light 100mm from the center
of the lens and a 25mm lens will focus those same rays 25mm from the center of the lens.
Since a negative lens diverges light, it will not form an image on the far side like a positive lens
will. The diverging rays, if you track them backward they will converge on the front side of the
lens, but will not form a real image. These lenses will have a negative focal length.
will. The diverging rays, if you track them backward they will converge on the front side of the
lens, but will not form a real image. These lenses will have a negative focal length.
Focal length can also be measured in diopters (1000/focal length in mm). A 100mm lens can also
be described as a +10 diopter lens. Shorter focal length lenses will have a higher diopter value
and will bend light more than a longer focal length lens.
The focal length can be measured from either side of the lens - the lens has a front focal length
and a back focal length. A 25mm lens is a 25mm lens whether I look at it from the front or the
back. I can flip the magnifying glass over and still burn the same ant at the same distance.
The end result is that the focal length represents the distance that an object at infinity will be
focused behind the lens. The focal length is a very important number and determines many
aspects of macro photography such as extension, magnification, working distance and will
regularly show up in the coming installments.
be described as a +10 diopter lens. Shorter focal length lenses will have a higher diopter value
and will bend light more than a longer focal length lens.
The focal length can be measured from either side of the lens - the lens has a front focal length
and a back focal length. A 25mm lens is a 25mm lens whether I look at it from the front or the
back. I can flip the magnifying glass over and still burn the same ant at the same distance.
The end result is that the focal length represents the distance that an object at infinity will be
focused behind the lens. The focal length is a very important number and determines many
aspects of macro photography such as extension, magnification, working distance and will
regularly show up in the coming installments.
Lenses work by bending light. Light changes direction when it crosses the interface between two
different substances - such as air/glass. This bending of light is called refraction. Refraction is
quantified by Snell's Law (Wikipedia:refraction). Refraction can be seen in real life by putting a
pen into a glass of water. If you look closely, it will appear that the pen changes direction slightly
when it enters the water.
different substances - such as air/glass. This bending of light is called refraction. Refraction is
quantified by Snell's Law (Wikipedia:refraction). Refraction can be seen in real life by putting a
pen into a glass of water. If you look closely, it will appear that the pen changes direction slightly
when it enters the water.
| Refraction / Simple lenses |