But what about energy levels? In the previous two sources, light was produced when an electron changed energy levels. Is that true in this case? Yes, the light you see from a filament is also produced by electrons transitioning between energy levels. The difference between a filament and a fluorescent bulb is that the filament is a solid. In solid materials, atoms interact with other atoms to slightly change nearby atom’s energy levels. The result is that you have many different atoms with many different energy levels. There is such a variety of energy levels that you get all possible transitions and all possible colors of light (once you have enough energy). When you combine all the possible colors of light, the human eye perceives this as white light.

Lens­es with an angle of view of 35° or nar­row­er are con­sid­ered long-focus lens­es. This trans­lates to a focal length of about 70 mm and greater on full-frame cam­eras, and about 45 mm and longer on APS‑C cam­eras. It’s com­mon for pho­tog­ra­phers to (incor­rect­ly) refer to long-focus lens­es as “tele­pho­to” lens­es. A true tele­pho­to lens is one whose indi­cat­ed focal length is longer than the phys­i­cal length of its body. Due to this ubiq­ui­tous mis­use of the word, there exists a fur­ther clas­si­fi­ca­tion of long-focus lens­es whose angle of view is 10° or nar­row­er called “super tele­pho­to” lens­es (equal to or greater than 250 mm on full-frame cam­eras and 165 mm on APS‑C cam­eras). For­tu­nate­ly, super tele­pho­to lens­es are more often than not actu­al tele­pho­to designs. A great exam­ple is the Canon EF 800 mm f/5.6L IS USM Lens, which is only 461 mm long.

If you’re into math—and who isn’t?—the gen­er­al for­mu­la for cal­cu­lat­ing the angle of view when you know the focal length and the sen­sor size is:

The incandescent light might seem like the simplest light to explain. If you examine one carefully, you can see that there is not much to look it. Basically, it is just a wire inside a glass container. If you want to get a little more complicated, inside the glass bulb there are two wires that support a much tinier wire in between them---the tiny wire is called the filament.

A zoom lens allows pho­tog­ra­phers to vary its effec­tive focal length through a spec­i­fied range, which alters the angle of view and mag­ni­fi­ca­tion of the image. Zoom lens­es are described by stat­ing their focal length range from the short­est to longest, such as 24–70 mm and 70–200 mm. The focal length range of a zoom lens direct­ly cor­re­lates to its zoom ratio, which is derived by divid­ing the longest focal length by the short­est. Both of the lens­es above have a zoom ratio of approx­i­mate­ly 2.9x, or 2.9:1. The zoom ratio also describes the amount of sub­ject mag­ni­fi­ca­tion a sin­gle lens can achieve across its avail­able focal length range.

Due to their abil­i­ty to mag­ni­fy dis­tance objects, long-focus lens­es present pho­tog­ra­phers with many uses. They are almost uni­ver­sal­ly laud­ed for por­trai­ture because their nar­row angle of view allows for a high­er mag­ni­fi­ca­tion of the sub­ject from con­ven­tion­al­ly more pleas­ing per­spec­tives. As a rule of thumb, a desir­able focal length for a por­trait lens starts at twice the nor­mal focal length for the cam­era sys­tem (about 85 mm for full-frame and 56 mm for APS‑C).

Is there a downside? Right now, the only downside is that they are a bit more expensive for larger applications. The price for these devices seems to be dropping fast though. Soon we might be using LED lights much more than we did in the past.

Light illuminationunit

Here is the basic operating principle. When you run electric current through a wire, it gets hot. The filament is just a wire that gets so hot that it glows. It’s that simple. But then, why the glass? The glass bulb serves one primary function---keep the air out. When a hot filament comes in contact with air, it will burn and melt. With a melted filament, you no longer have a working bulb.

Types ofilluminationpdf

Another problem with the fluorescent lamp is the lifespan. If you crack the glass tube, the gas will escape and the light won’t work. The ballast can also fail and the elements inside the bulb eventually wear out. They don’t last forever.

For instance, on full-frame cam­eras, whose image sen­sors mea­sure 36×24 mm, the diag­o­nal length is approx­i­mate­ly 43 mm, and yet, the 50 mm lens is con­ven­tion­al­ly con­sid­ered nor­mal. On APS‑C cam­eras (24 × 16 mm), whose diag­o­nal spans about 28 mm, a 35 mm focal length is regard­ed as nor­mal pri­mar­i­ly because its angle of view is sim­i­lar to the 50 mm lens on the full-frame for­mat. There­fore, nor­mal focal lengths will dif­fer as a func­tion of the camera’s image sen­sor size. In fact, as you con­tin­ue read­ing, keep in mind that descrip­tive terms such as “ultra-wide,” “short,” “long,” et cetera, implic­it­ly refer to the angle of view of a lens.

There is one thing in common to all light production methods. They all deal with electrons changing energy levels. When we think of energy for macroscopic objects, we imagine that they could have any particular energy level. I can throw a tennis ball so that it has 10 Joules of kinetic energy or 10.1 Joules or any value in between. This isn’t exactly true and as we look at smaller and smaller things, it’s obviously not true. An electron in some type of system can only have certain energy levels.

If the LED is a relatively new method for creating artificial light, why am I starting with this one first? In terms of physics, I think the LED might be the easiest to explain. Now wait, don’t get me wrong. The LED is still complicated---but it might still be the easiest device to explain.

Light illuminationlevel

For any giv­en cam­era sys­tem, nor­mal lens­es are gen­er­al­ly the “fastest” avail­able. Adjec­tives such as “fast” and “slow” always describe lens speed, which refers to a lens’ max­i­mum aper­ture open­ing. For instance, a lens with a ƒ/2 or larg­er aper­ture is gen­er­al­ly con­sid­ered fast; a lens with a ƒ/5.6 or small­er aper­ture is deemed to be slow. How is speed rel­e­vant to aper­ture? Recall the reci­procity law: larg­er aper­tures per­mit more light into the cam­era, there­by allow­ing you to use faster shut­ter speeds, and vice ver­sa.

Let’s look at a similar light first---the neon lamp. Start with a glass tube that is filled with neon gas. Now apply a large voltage across the ends of the tube. The electric potential difference inside the tube will cause free electrons to accelerate and collide with the neon atoms. On collision, these electrons can excite electrons in the neon to higher energy levels. When the excited electrons in the neon atoms come back down to lower energy levels, they produce light.

Go outside on a bright and sunny day. Take a look at a flower or a tree. You can see that flower because light from the sun travels all the way to the flower. When the light reflects off the flower it then travels to your eye and you can see the flower. Remove the light from the sun and you just see blackness. Even at night humans can see things---but there has to be some type of light reflecting off objects to see. Sunlight reflected off the surface of the moon provides a surprising amount of light for most outdoor activities at night.

A “nor­mal” lens is defined as one whose focal length is equal to the approx­i­mate diag­o­nal length of a camera’s image sen­sor. In prac­tice, such lens­es tend to fall into a range of slight­ly longer focal lengths that are claimed to pos­sess an angle of view com­pa­ra­ble to that of the human eye’s cone of visu­al atten­tion, which is about 55°.

You’ve used LED lights for quite some time. They are in your infrared (IR) remote control for your TV. They are the light source for the flash on your smartphone camera. There’s even a good chance that LED lights are used to make your computer screen visible. The LED started seeing real uses in the 1960s and today they are everywhere.

The angle of view describes the breadth, or how much, of a scene is cap­tured by the lens and pro­ject­ed onto your camera’s image sen­sor. It’s expressed in degrees of arc and mea­sured diag­o­nal­ly along the image sen­sor. Thus, the angle of view of any lens of a giv­en focal length will change depend­ing on the size of the cam­er­a’s image sen­sor. For exam­ple, a 50 mm lens has a wide angle of view on a medi­um for­mat cam­era, a nor­mal angle of view on a full-frame cam­era, a nar­row­er angle of view on an APS‑C cam­era, and a nar­row angle of view on a Micro Four-Thirds cam­era.

That’s it. The four ways humans make light. Yes, these explanations are not complete. In order to really understand light, you would probably need to take several undergraduate courses in physics. Well, that would at least get you started.

Light illuminationtest

If you go inside a building at night, you might not be able to use the moonlight. In that case you need some artificial light source to see. Since this is the International Year of Light, let me go over the four common methods for creating artificial light along with the basic physics that makes them work.

What does this have to do with the LED? The LED is a solid state device. This means that the process is not governed by a typical chemical reaction or mechanical method. The solid state device is a combination of two different semiconductor materials in which electrons can move about at different energy levels due to the periodic nature of the material. This produces an energy gap for electrons in the system. Yes, when electrons transition across this energy gap, they produce light, light of a particular color.

Where does the light come from? In the burning process, there is something other than carbon dioxide produced. It is generally called soot---but it is basically unburned pieces of material. This extra material gets caught in the along with hot air and rises above the combustion area. Since the soot is hot, it produces light in the visible spectrum just like a lightbulb filament or a hot stove eye.

A true zoom lens, known as a par­fo­cal lens, main­tains a set focus dis­tance across its entire focal length range. In the days before dig­i­tal photography—before elec­tron­ic aut­o­fo­cus, even—it was com­mon prac­tice to focus a zoom lens at its longest focal length before tak­ing the pic­ture at the desired (if dif­fer­ent) focal length. This tech­nique is no longer pos­si­ble because con­tem­po­rary vari­able focal length lens­es designed for pho­tog­ra­phy are almost exclu­sive­ly var­i­fo­cal lens­es, which do not main­tain set focus across their zoom range. In prac­tice, most pho­tog­ra­phers do not know the dif­fer­ence because the aut­o­fo­cus algo­rithms in their cam­eras com­pen­sate for the slight vari­a­tions.

A prime or fixed focal length lens has a set focal length that can­not be changed. There are sev­er­al crit­i­cal dif­fer­ences between prime and zoom lens­es that you should know. Prime lens­es are gen­er­al­ly small­er, faster, and have bet­ter opti­cal char­ac­ter­is­tics than zoom lens­es. Despite this, pho­tog­ra­phers fre­quent­ly opt to shoot with zoom lens­es because of their con­ve­nience: a sin­gle lens can replace sev­er­al of the most pop­u­lar focal length prime lens­es. This is espe­cial­ly impor­tant when you’d pre­fer to pack light, such as dur­ing a trip or a hike.

Light illuminationcalculation

But why aren’t these fluorescent lamps (and compact fluorescents) as good as an LED light? There are a couple of disadvantages. First, in order to excite the gas you need a high voltage applied to the tube. To get this high voltage, a fluorescent lamp uses an electromagnetic ballast that take the normal household voltage and ramp it up to a higher level. This ramp up process isn’t perfect and produces heat in the process which means the lamp isn’t as energy efficient as the LED.

Light illuminationNagoya

In pho­tog­ra­phy, the most essen­tial char­ac­ter­is­tic of a lens is its focal length, which is a mea­sure­ment that describes how much of the scene in front of you can be cap­tured by the cam­era. Tech­ni­cal­ly, the focal length is the dis­tance between the sec­ondary prin­ci­pal point (com­mon­ly and incor­rect­ly called the opti­cal cen­tre) and the rear focal point, where sub­jects at infin­i­ty come into focus. The focal length of a lens deter­mines two inter­re­lat­ed char­ac­ter­is­tics: mag­ni­fi­ca­tion and angle of view.

Here is a simple experiment. Turn on the stove in your kitchen, but don’t put a pot on it. Very soon, the eye of the stove will become hot (don’t touch it). As the temperature continues to increase, you will eventually see the eye glowing red. This is exactly what happens with the filament in the bulb. It is so hot that it doesn’t glow red, but yellow-white.

Modern fluorescent bulbs produce appropriate colors and don’t flicker as much as older bulbs. This makes them an excellent replacement for the older incandescent bulbs.

Early LED lights produced only infrared light (light with frequencies that humans can’t see). After that, we started creating red and then green LEDs. Finally, a blue LED was created (but carefully combining different semiconductors). With the blue LED you get two things. First, you can use red, green, and blue (RGB) lights to make video displays. Second, using blue LEDs and some other tricks you can make a white looking LED that can be used for lights.

The focal length of a lens deter­mines its mag­ni­fy­ing pow­er, which is the appar­ent size of your sub­ject as pro­ject­ed onto the focal plane where your image sen­sor resides. A longer focal length cor­re­sponds to greater mag­ni­fy­ing pow­er and a larg­er ren­di­tion of your sub­ject, and vice ver­sa.

Is fire more efficient than an incandescent lightbulb? Well, it is difficult to compare the two lighting methods. One runs on electricity and the other runs on carbon-based material (with no electricity). Of course the fire still produces lots of heat which may or may not be a good thing (depending on what you are using it for). The other issue with fire is that it produces carbon dioxide which isn’t really a good thing to have too much of. Oh, sometimes fire gets other things so hot that they also start interacting with the oxygen. Sometimes these other burning things are important things like your house. So overall, fire is nice but we can do better.

There are two types of wide-angle lens­es, rec­ti­lin­ear and fish­eye (some­times termed curvi­lin­ear). The vast major­i­ty of wide-angle lens—and oth­er focal lengths, too—are rec­ti­lin­ear. These types of lens­es are designed to ren­der the straight ele­ments found in a scene as straight lines on the pro­ject­ed image. Despite this, wide-angle rec­ti­lin­ear lens­es cause ren­dered objects to pro­gres­sive­ly stretch and enlarge as they approach the edges of the frame. In pho­tog­ra­phy, all fish­eye lens­es are ultra wide-angle lens­es that pro­duce images fea­tur­ing strong con­vex cur­va­ture. Fish­eye lens­es ren­der the straight ele­ments of a scene with a strong cur­va­ture about the cen­tre of the frame (the lens axis). The effect is sim­i­lar to look­ing through a door’s peep­hole, or the con­vex safe­ty mir­rors com­mon­ly placed at the blind cor­ners of indoor park­ing lots and hos­pi­tal cor­ri­dors. Only straight lines that inter­sect with the lens axis will be ren­dered as straight in images cap­tured by fish­eye lens­es.

Every atom has its own unique energy levels. This means that different gasses would produce different colors corresponding to the different energy levels. Neon has that classic red-orange. If you are excite a gas of mercury vapor, you get a different color (from different energy levels). These gases don’t just create one color of light, instead they make many different colors that correspond to different energy level transitions. You can see the individual colors by looking through a diffraction grating (a slide with many tiny lines on it). This is what that would look like for both the neon and mercury vapor.

What makes the LED light so great? First, they can be very small and robust. If you don’t run too much current through them, they last a very long time and they don’t break just by shaking them. Second, the LED light doesn’t get very hot when it is on. The less energy that goes into heating the device means that more energy goes to light. LED lights are much more energy efficient that other devices.

Let’s look at the simplest case---the hydrogen atom which consists of just a proton and an electron. At its lowest energy level, the electron is at an energy level of -13.6 eV (electron volts is a unit of energy). If the electron moves to the next higher energy level, it would be at -3.4 eV (which is indeed higher than -13.6 eV. The electron in hydrogen can NOT be at an energy level between -13.6 and -3.4 eV. That’s just the way it is.

Fluorescent lights have been around for quite some time. They began to be popular for office and industrial settings in the 1950s, but now they are in most homes. Now we also have the compact fluorescent. As you can guess, this is just a fluorescent bulb that is small enough to fit in the sockets of traditional incandescent lights. But how do they work?

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Sub­ject size is direct­ly pro­por­tion­al to the focal length of the lens. For exam­ple, if you pho­to­graph a soc­cer play­er kick­ing a ball, then switch to a lens that is twice the focal length of the first, the ren­dered size of every ele­ment in your image, from the per­son to the ball, will be dou­bled in size along the lin­ear dimen­sions.

The rela­tion­ship between the angle of view and a lens’s focal length is rough­ly inverse­ly pro­por­tion­al from 50mm and up on a full-frame cam­era. How­ev­er, as the focal length grows increas­ing­ly short­er than 50mm, that rough pro­por­tion­al­i­ty breaks down, and the rate of change in the angle of view slows. For exam­ple, the change in angle of view from 100mm to 50mm is more pro­nounced than the change from 28mm to 14mm.

Illuminationlighting meaning

That seems like a complete explanation, but why do hot things produce light? It turns out that all solids produce light. Yes, it’s true. Your pencil produces light. The apple on the countertop produces light. These everyday things produce light, but they produce light that you can’t see---light with wavelengths longer than the wavelength of red light. We call this light infrared. If you take an object and slowly increase its temperature, it will produce different wavelength of light. When it gets hot enough, the light will be in the visible range.

It’s impor­tant to rec­og­nize that the con­ve­nience and flex­i­bil­i­ty of zoom lens­es can inspire lazy pho­tog­ra­phy. The ease of chang­ing the angle of view encour­ages pho­tog­ra­phers to set­tle on com­po­si­tions that are good-enough, instead of seek­ing out bet­ter per­spec­tives and gain­ing a deep­er under­stand­ing of their sub­jects. What­ev­er lens you have, be it zoom or prime, it’s vital for the devel­op­ment of good pho­tog­ra­phy to con­sid­er your sub­ject from sev­er­al per­spec­tives by walk­ing towards, step­ping away, and cir­cling around them.

In pho­tog­ra­phy, the term macro refers to extreme close-ups. Macro lens­es are nor­mal to long-focus lens­es capa­ble of focus­ing on extreme­ly close sub­jects, there­by ren­der­ing large repro­duc­tions. The mag­ni­fi­ca­tion ratio or mag­ni­fi­ca­tion fac­tor is the size of the sub­ject pro­ject­ed onto the image sen­sor in com­par­i­son to its actu­al size. A macro lens’ mag­ni­fi­ca­tion ratio is cal­cu­lat­ed at its clos­est focus­ing dis­tance. A true macro lens is capa­ble of achiev­ing a mag­ni­fi­ca­tion ratio of 1:1 or high­er. Lens­es with mag­ni­fi­ca­tion ratios from 2:1 to 10:1 are called super macro. Ratios over 10:1 cross over into the field of microscopy. When shop­ping for a macro lens, keep in mind that in the con­text of kit lens­es and point-and-shoot cam­eras, some man­u­fac­tur­ers use the macro moniker as mar­ket­ing short­hand for “close-up pho­tog­ra­phy.” These prod­ucts do not achieve 1:1 mag­ni­fi­ca­tion ratios. When in doubt, check the tech­ni­cal spec­i­fi­ca­tions.

In gen­er­al, a short focal length—or short focus, or “wide-angle”—lens is one whose angle of view is 65° or greater. Recall from above that angle of view is deter­mined by both focal length and image sen­sor size, which means that what qual­i­fies as “short” is pred­i­cat­ed upon a camera’s image sen­sor for­mat. There­fore, on full-frame cam­eras, the thresh­old for wide-angle lens­es is 35 mm or less, and on APS‑C cam­eras, it’s 23 mm or less. Lens­es with an angle of view of 85° or greater are called “ultra wide-angle,” which is about 24 mm or less on full-frame and 16mm or less on APS‑C cam­eras.

Although the incandescent lightbulb is simple to make, it’s not the best device for light. The problem is that the bulb makes light by getting very hot. Very hot means very bad and wasted energy. Most of the energy you get from an incandescent bulb goes straight into thermal energy that you don’t want (unless you are using the lightbulb to heat things up).

LEDilluminationlights

Beyond por­trai­ture, long-focus lens­es are use­ful for iso­lat­ing sub­jects in busy and crowd­ed envi­ron­ments. Pho­to­jour­nal­ists, wed­ding, and sports pho­tog­ra­phers exploit this abil­i­ty reg­u­lar­ly. Due to their mag­ni­fy­ing pow­er, super tele­pho­to lens­es are a main­stay for wildlife and nature pho­tog­ra­phers. Last­ly, long-focus lens­es are fre­quent­ly used by land­scape pho­tog­ra­phers to cap­ture dis­tant vis­tas or to iso­late a fea­ture from its sur­round­ings.

Wide-angle lens­es rep­re­sent the only prac­ti­cal method of cap­tur­ing a scene whose essen­tial ele­ments would oth­er­wise fall out­side the angle of view of a nor­mal lens. Con­ven­tion­al sub­jects of ultra wide-angle lens­es include archi­tec­ture (espe­cial­ly inte­ri­ors), land­scapes, seascapes, cityscapes, astropho­tog­ra­phy, and the entire domain of under­wa­ter pho­tog­ra­phy. Wide-angle lens­es are often used for pho­to­jour­nal­ism, street pho­tog­ra­phy, auto­mo­tive, some sports, and niche por­trai­ture.

You might think that fire is the simplest of all lighting sources. Yes, it is simple to create and simple to control. However, it is not so simple to explain. Much of the organic matter we see (like wood and coal and oil) contain carbon that is bound to other molecules. It turns out that this carbon can also make very strong chemical bonds with oxygen to form carbon dioxide. Although it takes some energy to pull a carbon away from its other bonds, the formation of carbon dioxide also produces extra energy. And this is the basic idea behind fire. With a little bit of starting energy, you can turn organic carbon and oxygen into carbon dioxide.

The con­stant angle of view of a prime lens forces this type of experimentation—“zooming with your feet”—because the oth­er options are either bad pic­tures or no pic­tures. Fur­ther­more, restrict­ing your­self to a sin­gle focal length for an extend­ed peri­od of time acquaints you to its angle of view and allows you to visu­al­ize a com­po­si­tion before rais­ing the cam­era to your face.

It’s impor­tant to under­stand that the degree to which the focal length mag­ni­fies an object does not depend on your cam­era or the size of its image sen­sor. Assum­ing a fixed sub­ject and sub­ject dis­tance, every lens of the same focal length will project an image of your sub­ject at the same scale. For exam­ple, if a 35 mm lens casts a 1.2 cm image of a per­son, that image will remain 1.2 cm high regard­less of your camera’s sen­sor for­mat. How­ev­er, on a Micro Four Thirds for­mat cam­era, the image of that per­son will fill the height of the frame, where­as it will occu­py half the height of a full-frame image sen­sor, and about one-third the height of a medi­um for­mat image sen­sor. As you progress from a small­er sen­sor to a larg­er one, the 1.2 cm high pro­jec­tion of the per­son remains unchanged, but it occu­pies a small­er part of the total frame. There­fore, although the absolute size of the image will stay con­stant across vary­ing image sen­sor for­mats, its size in pro­por­tion to each image sen­sor for­mat will be dif­fer­ent.

Clearly we don’t want to use neon lamps for normal lighting. It’s just not the right color. Mercury vapor seems closer, but not quite right. Here is the trick for fluorescent lamps---fluorescence. Fluorescence is the process through which a material absorbs a particular wavelength (color) of light and re-emits a color with a longer wavelength. In the case of a fluorescent bulb, there is a coating on the inside of the glass that absorbs ultraviolet light (which you can’t normally see) and re-emits it as visible light. Yes, it’s a complicated process, but that’s how it works.

As you have learned in the sec­tion on aper­tures and f‑numbers, “an increase in focal length decreas­es the inten­si­ty of light reach­ing the image sen­sor.” This rela­tion­ship is most obvi­ous in zoom lens­es. A “vari­able” aper­ture zoom lens is a lens whose max­i­mum aper­ture becomes small­er with increased focal length. These types of zoom lens­es are sim­ple to spot because they list a max­i­mum aper­ture range instead of a sin­gle num­ber. The range spec­i­fies the max­i­mum aper­ture for the short­est and longest focal lengths of the zoom range. Vari­able aper­ture lens­es are the most com­mon type of zoom lens. A con­stant aper­ture or “fixed” aper­ture zoom lens is one whose max­i­mum aper­ture remains con­stant across the entire zoom range. Fixed aper­ture lens­es are typ­i­cal­ly more mas­sive and more expen­sive than their vari­able aper­ture coun­ter­parts. They are also more straight­for­ward to work with when prac­tic­ing man­u­al expo­sure at the max­i­mum aper­ture since no com­pen­sa­tion for lost light is required dur­ing zoom­ing.

But what does this have to do with light? It turns out that when an electron makes the transition from a higher energy level to a lower energy level, it produces light. Also, the frequency of this light is proportional to the energy change. Humans perceive different frequencies of light (in the narrow spectrum of all electromagnetic waves) as different colors of light.