Digital Photography Revealed

Vignetting (V) is a widespread photographic problem. Most lenses and certainly all compact cameras show it. It is most visible at the corners as a radial loss of brightness, due to lowered exposure.Vignetting can be used as an effect to draw attention to the central subject of a picture, but in general it is undesirable. Vignetting actually has two aspects: The common underexposed corners and the bright center, called the hot spot.

There are several causes of vignetting: 1. Mechanical. 2. Optical. 3. Natural. 4. Pixel.

Mechanical vignetting can be caused by three things: too many filters stacked on the lens, too small a lens hood attached to the lens; or secondary lenses. This will reduce the light at the corners and darken them. Depending on the lens aperture, the darkening will be abrupt or gradual. The smaller the aperture, the more abrupt the vignetting.

Optical vignetting is inseparable from the lens itself and is caused partly by the combination of several lens elements, partly by the fact that the lens itself has a considerable length. Optical vignetting is also called artificial vignetting.The length of the lens is a problem because when the lenses are used with a wide open aperture, the edges of the lens will occlude part of the aperture from oblique incoming light, thus shading the edges. Zoom lenses and wide angle lenses are particularly prone to this kind of V. The rear elements of the lens are shaded by the elements in front of them reducing the light radially and causing vignetting. One can often cure optical V by reducing the aperture two or three stops. This kind of vignetting can be reduced by using very large front lens elements and this is often done in wide angle lenses. Lastly, the contrast of the film or sensor plays a part: the stronger the contrast, the more pronounced the vignetting.

Natural vignetting (also called natural light falloff) is a natural light falloff proportional to the angle the light reaches the sensor or film; it is not caused by the lens. Technically the falloff is proportional to the fourth power of the cosine of the angle of the in-falling light on the film or sensor. Lenses in compact cameras are particularly prone to such falloff. So are wide angle lenses. Telephoto lenses show the least falloff. At large apertures both optical and natural vignetting are present. The combined effect is often called illumination falloff or radial density.

Pixel vignetting is of course not relevant for film, but only for digital cameras. It is created because most sensors have an angle dependency of the in-falling light.Light hitting the sensor at a right angle produces a stronger impression than light hitting at an oblique angle, thus the corners become underexposed. Digital cameras often have a built-in compensation for this, used when converting the RAW image data to tiff or jpg. If one works with RAW images, one will have to do post-processing to remove pixel V.

This article is based on this one Vignetting in Photos.

Throughout modern history artists and mathematicians have been fascinated by a special proportion known as the golden ratio, or golden section as it is also called. It has the special feature that if you divide a line into two sections, a larger A and a smaller B, according to the golden ratio, then A is to B as A+B is to A. Numerically it is about 1: 1.618. Artists and architects, even musicians, have used the golden ratio as a basis for their paintings and buildings, or music.

No other number in the history of mathematics has inspired thinkers of all disciplines like the golden ratio. It has inspired men for at least 2.400 years since Pythagoras and Euklid in ancient Greece. Other outstanding thinkers, who have pondered the golden ratio, are Leonardo of Pisa, Johannes Kepler and present day physicist Roger Penrose. It has fascinated biologists, artists, musicians, architects, psychologists and occultists alike. The 12\’th century mathematician Fibonacci came upon what is today known as the Fibonacci sequence: 1, 1, 2, 3, 5, 8, 13, 21, etc. in which each new number is the sum of the two preceding. The further you take this sequence, the closer it comes to the golden ratio. The pentagram is a peculiar figure in that all its line segments stand in a golden ratio relationship with some other segment of the pentagram.

In honor of Phidias, the great Greek sculptor from about 400 BC who used the golden proportion extensively in his sculptures, the golden proportion is now commonly known as Phi, the first letter of Phidias\’ name. The golden ratio has also been known as the divine proportion since 1509, when Luca Pacioli published a three volume book on the golden ratio entitled \”De Divina Proportione\”. Pacioli saw religious significance in the proportion, hence the title of his book. The book was a major influence on artists and architects for hundreds of years.

The modern Swiss architect Le Corbusier is famous for his use of the golden ratio. He saw the ratio and the Fibonacci sequence as representing a mathematical order of the universe, and he described them as: \”rhythms apparent to the eye and clear in their relations with one another. And these rhythms are at the very root of human activities. They resound in man by an organic inevitability, the same fine inevitability which causes the tracing out of the Golden Section by children, old men, savages and the learned.\”

Painters, such as the 17\’th century master Vermeer, have used the golden ratio extensively, so did a modern master like Salvador Dali. Dali adored Vermeer, by the way. The golden ratio and the Fibonacci sequence have also been used by composers. The modern composer Bartok, for example, based the xylophone progression in \”Music for Strings, Percussion and Celeste\” on the Fibonacci sequence 1, 2, 3, 5, 8, 5, 3, 2, 1. Similarly Satie and Debussy are known to have used the golden ratio as a basis for some of their compositions.

One also finds the golden ratio in nature. The arrangement of branches along the stems of plants, for instance, often follows the golden ratio.

The company Power Retouche has a dedicated Photoshop plugin for drawing the Golden Section, Golden Spiral, etc. in Photoshop.

White balance adjustment means to adjust the light in a photo to white. In order to do this you must have a neutral in the image like a gray card or a white sheet of paper. Software can correct white balance, but not all software is equally good.

The first thing you might like to work on, when you begin to edit your images, is white balance. White balance refers to the color of the light and assumes that the best light color is white. Some shots, like sunset or candlelight, do not have white light, but in general an impression of white light is desirable. Some use Photoshop\’s auto levels to set white balance, but that is not the best, since auto levels only sets the darkest areas to black and the brightest to white without considering the mid tones. But what if the lightest pixel in your photo is not white? Or what if you do not have pure black in the photo? (Most pictures have black areas, but the lightest pixels are rarely pure white).

When correcting white balance, the mid tones are the most important and to help set the mid tones correctly one adds a grey card to the photo when taking the picture. A grey card is a sheet of cardboard or plastic of an exact mid tone neutral gray. Ideally one has three cards: a black, a gray and a white. Photoshop\’s levels adjustment panel has three color pickers for picking color: one for white, one for gray and one for black. By clicking the gray color picker on the gray card, one can adjust the mid tones to neutral gray. One can of course only include a gray card in the image if one intends later to crop the photo.

If one does not want a gray card in the image, or if one doesn\’t have a gray card at hand, one can later use dedicated software that analyzes the image and calculates the color of the light and sets it to white. There are draw backs with such software: what if there are no neutral areas in the image to calculate the color of the light from? Some applications do not need a neutral in the picture, but most do to get a good result.

If you work with RAW images, you will have found that RAW converters normally come with a slider for color temperature, which means a slider to adjust the picture cool or warm. But what if the color of the light is greenish as when you have taken a picture in neon light? The cool-warm control is good for regular incandescent light, but not for fluorescent.

Color adjustment controls are rarely good for correcting white balance, because the color correction will not just neutralize the gray card, but will also tone the picture in an undesirable way: usually the blacks become toned or the whites or both. In short one needs some neutrals in an image to set white balance. A white wall or a sheet of white paper will do well; preferably add a gray card for the mid tones.

Source: Whitebalance in Photography.

Chromatic aberration may sound difficult, but it is actually quite easy. Imperfections in the camera lenses create the unwanted colors, that are seen as magenta and blue-green fringes. Chromatic aberration can be created in two ways: 1. The lens does not focus the different colors on the same sensor plane. 2. The different colors produce images of different size. Here we will take an in-depth look at what chromatic aberration is and how to avoid or solve it.

The first thing to grasp is refractive index, so let us briefly explain what that is. Light changes its direction when it passes through a medium like the glass of the lenses. For example light may hit the lens at a 90 degree angle, but leave the lens at an 80 degree angle. Unfortunately different wavelengths of light have different refractive indexes. For example blue might leave the lens at 79 degrees while red might leave at 81 degrees. This difference will create thin magenta fringes known as longitudinal chromatic aberration. Since green is in-between red and blue it is used to focus the lens. Thus the red and blue are slightly out of focus which creates the magenta (red+blue) fringes.

Transverse chromatic aberration arises when light does not reach the lens at 90 degrees, but from a different angle. Here the different colors focus on the same sensor plane, but not at the same spot, thus the red image will be larger than the green and blue image, and blue smaller than red and green.This also creates colored fringes, but now both a magenta and a blue-green one. Chromatic aberration is hard to avoid, since it is in the nature of light, but of course lens manufacturers do their best to eliminate it.

Both types of chromatic aberration produce color fringes, but of a different sort. Longitudinal aberration produces magenta fringes around objects and is distributed uniformly throughout the image. Transverse aberration is absent at the center of the image, but grows in intensity towards the corners. Longitudinal chromatic aberration is most pronounced in wide aperture lenses. It can be minimized by using a small aperture. Transverse chromatic aberration is most pronounced in telephoto lenses. However, lenses can be designed in many ways. So called achromatic lenses have minimal chromatic aberration and are very popular. So called superacromatic and apochromatic lenses virtually eliminate color errors, but they are not common. Chromatic aberration can be observed on film, but is most pronounced on digital images. One explanation is that the sensors are more sensitive to ultraviolet and infrared light, which are at the outer edge of the spectrum where aberration is most pronounced.

Chromatic aberration can be corrected with software. Longitudinal chromatic aberration is somewhat corrected by sharpening the red and blue channels; the green channel is used to focus the image and should be sharp. Transverse chromatic aberration can be satisfactorily corrected by radially enlarging the blue channel image and radially reducing the red channel image.

Purple finging is a different kind of chromatic error. It appears along hard contrast edges when photographing something against a hard back light, or when photographing a light source against a dark background.The purple fringe invades the dark area. We saw that chromatic aberration is a lens error; purple fringes on the other hand are sensor errors. Purple fringing is not a simple geometric error like transverse chromatic aberration, but is an overflow of light from the brightly illuminated sensor to its neighbors; hence it is very difficult to correct with software. Also the original color is usually lost. Software can reduce the color of the purple fringe to a grayish tone. At best the local color is not completely eradicated by the purple fringe and can be reconstructed.

Source: Chromatic aberration 1 and Chromatic aberration 2

Photoshop plugins load into Photoshops Filter menu when Photoshop launches. They add all sorts of extra functions to Photoshop. In the old days Photoshop plugins were just a bunch of weird effects. Recently a number of plugins have been produced, that do sophisticated image retouching that would otherwise be impossible or very time consuming in Photoshop. Photoshop has since begun to offer functions similar to some of the old plugins, like lens correction and proper black-white conversion.

But how does one install plugins into Photoshop, you may ask? Inside the Adobe Photoshop folder, there is a folder called Plug-Ins. All you have to do is place the plugins inside the Plug-Ins folder. Launch Photoshop and the menu Filters will have your plugins listed. If Photoshop was already running, when you installed the plugins, you will have to quit Photoshop and launch Photoshop anew. You can actually install the plugins in a different folder than Photoshop\’s Plug-Ins folder. This is how to set Photoshop up to load plugins from any folder you like:

1. First create the alternative plugins folder where ever you like and call it what ever you want.. 2. Run Photoshop. 3. Open the menu Edit. Move the mouse to the bottom of the Edit menu, to Preferences. Open Preferences. 4. Plug-Ins might be called \”Plug-Ins and Scratch Disk\” depending on your Photoshop version. Go there. 5. Activate Additional Plug-Ins Folder by checking it. 6. Click the button Choose to browse to your desired alternative Plug-Ins folder.

That\’s all there is to it! You can now store all your plugins in this alternative plugins folder. Close the Preferences and quit Photoshop. The plugins in your alternative location will be listed at the bottom of the Filter menu next time you run Photoshop.

In general there are two kinds of plugins: 1. Retouching plugins. 2. Effects plugins. Retouching plugins tend to manipulate what is already in the photo without adding anything new. On the other hand, effects plugins add, well, effects to the photograph. Sharpening, exposure or saturation would be examples of retouching. Effects examples could be lens flare, bokeh or raster. The distinction is not always so easy. What about lens correction? Is that a retouch or an effect? It is a retouch if you correct barreling or pincushion, but if you make a regular image look like a fish eye photo, it is an effect.

Third party plugin were made possible in 1991 when Photoshop introduced the possibility in Photoshop 2. Three years later Joe Ternasky released Filter Factory for writing third party plugins. In 1997 Alex Hunter released Filter Meister as an improvement over Filter Factory and many of today\’s plugins are written in Filter Meister. Ten years after Filter Meister was released, a novel approach to filter development was released as Filter Forge. Filter Forge plugins require Filter Forge to run and they are not stand alone. Currently Filter Meister plugins only support 32bit Photoshop. But Alex Hunter says 64 bit support will come some time in 2013. Filter Meister is only available for the Windows platform.

This article is based on these articles about Photoshop plugins and this Photoshop plugins

Some tips will let you improve your abilities with the camera. Use this information to avoid common photography pitfalls and make sure that you never miss out on the opportunity to capture a special or unique moment on camera.

Your arms should be positioned near your body when you use a camera. Also, make sure you support the camera from the bottom and the sides. This keeps the shaking to a minimum and your shots will be more clear. Putting your hands beneath the lens and camera, instead of having them on top, prevents you from dropping your camera accidentally.

Take the time to read your camera\’s manual from front to back. The reason is because manuals are very thick and inconvenient to carry around. Therefore, most of the time they end up getting stuffed in a drawer or simply thrown in the trash. Instead, take the time necessary to go through it and understand your camera. It can enable you to take better photos, and it will also prevent you from making stupid mistakes.

Built-in flash comes on almost all digital cameras. It can kick in automatically in low-light situations. While this may work for the average photo, you will need an external flash device for more professional photos with more light. To attach an external flash onto your camera, make sure it has a hot shoe on top. Then take it to a camera store, so they can help you pick out a flash that lines up with your camera.

Invest in a memory card with advanced ability to record a large number of images, so that you will not be limited in the quantity of shots you can take at any given time. By increasing the size of the memory card that you use in your camera, you are increasing the number of photos that you can take before changing cards. Having a memory card with a large capacity allows you to experiment with different shots and analyze them later to see which techniques worked best for you.

For a gripping photo, experiment with depth of field. A lower f-stop means that the main subject will be sharply focused in contrast to a blurry background. This is good for taking pictures of people when the person is very close to your camera. The bigger the f-stop number, the bigger the depth of field. This means that if your depth of field is larger, then a larger portion of the picture will appear focused on. You\’d use this technique when shooting landscapes.

When you finally find that perfect moment to snap a shot, make sure not to move at all when you press the shutter. Even hold your breath, if you have to. Even minimal movement can cause your perfect shot to be ruined. Some people agree that it\’s best to stop breathing right before pressing the button, as a way of personally steadying yourself.

One way to foster creativity in your photography is to impose limitations on yourself. Focus on specific concepts to narrow the scope of your photography. Then go and take 100 different photos in the same room or with the same point of view. Working under such limitations will spur you to think creatively and take more experimental photos.

The majority of photographs focus on a subject who is looking directly into the camera. For some unique pictures, try to have your subject look off the camera, have them focus on something outside the field of view of the camera. You might also direct the subject to focus on an object or person within the frame.

Take pictures of small gestures during a wedding. For example, a close up of the brides flower arrangement or some make up sitting on the vanity. It also allows you to catch a few hidden gems of details that might be overlooked when the event is finally underway.

To be a great photographer you must take the time to expand your knowledge on the field. Be patient with the tips that require a prolonged commitment to learning, and enjoy the ones that will instantly make your images stand out. Make sure you are having fun when you are taking photos. By keeping this advice in your mind, you can find success in photography shortly.

The initial step in landscape photography is to determine precisely what it is about a specific landscape that defines it. If taking landscape photographs is a passion of yours then visit the www.LandscapePhotographyTips.org site to uncover some techniques for taking better quality landscape photographs.

Chromatic aberration is an important photographic phenomena to grasp. It might sound difficult, but it is easy enough. It is seen in photos as magenta and blue-green fringes produced by the lenses. It comes in two varieties: 1. The lens does not focus the individual colors on the same sensor plane. 2. The different colors produce images of different size. In the following article we will look in depth at the phenomena of chromatic aberration and how to avoid or solve it.

First we have to understand what refractive index is. When light passes through a medium, for example the glass of the lenses, the angle of the light changes. For example light may hit the lens at a 90 degree angle, but leave the lens at an 80 degree angle. The problem is that the different wavelengths of light have different refractive indexes. For example blue could leave the lens at 79 degrees while red might leave at 81 degrees. This difference will create thin magenta fringes known as longitudinal chromatic aberration. The sensor focuses on the green channel and chromatic aberration causes the blue and red to be slightly out of focus, which creates the combined magenta fringes.

Transverse chromatic aberration arises when light does not reach the lens at 90 degrees, but from a different angle. In this case the different colors focus evenly, but not at the same spot. This causes the red image to be larger than the green and blue, and the blue the smallest of them all.This also creates colored fringes, but now both a magenta and a blue-green one. It is in the interest of lens manufacturers to avoid chromatic aberration, but since it is in the nature of light, it is hard to eliminate.

You get different kinds of fringes for each kind of chromatic aberration. Longitudinal aberration shows as magenta fringes around objects and is distributed uniformly throughout the image. Transverse aberration is absent at the center of the image, but grows in intensity towards the corners. Longitudinal chromatic aberration is most pronounced in wide aperture lenses. It can be reduced by using a small aperture. Transverse chromatic aberration is most pronounced in telephoto lenses. There are numerous lens designs. So called \”achromatic\” lenses have minimal chromatic aberration and are very popular. More rare are the so called \”superachromatic\” and \”apochromatic\” lenses, that virtually eliminate chromatic aberration. Digital images tend to show more chromatic aberration than film for some reason. One explanation is that the sensors are more sensitive to ultraviolet and infrared light, which are at the outer edge of the spectrum where aberration is most pronounced.

Chromatic aberration can be corrected with software. By sharpening the red and blue channels, one can somewhat correct longitudinal chromatic aberration; the green channel is used to focus the image and should be sharp. Transverse chromatic aberration is satisfactorily corrected by radially enlarging the blue channel image and radially reducing the red channel image.

Purple fringing is a different kind of chromatic error. It appears along hard contrast edges when photographing something against a hard back light, or when photographing a light source against a dark background. The purple fringe invades the dark area. We saw that chromatic aberration is a lens error; purple fringes on the other hand are sensor errors. It is very difficult to correct purple fringes with software since it is really an overflow of light from one sensor to the surrounding ones, and is not a simple geometric error like transverse chromatic aberration. Also the real color is usually lost. Software can reduce the color of the purple fringe to a grayish tone. At best the local color is not completely eradicated by the purple fringe and can be reconstructed.

Sources: Chromatic Aberration and Chromatic Aberration

When editing pictures you should first consider white balance and contrast. White balance is normally what you ought to fix first, then contrast.White balance should be addressed before contrast because color contrast can not be set properly if the image has a color cast.

White balance addresses the color of the illumination within the picture and sets white as a goal. White balance software attempts to adjust the tint of the illumination to white and to do that, the software normally needs some neutrals in the picture to calculate the correct filter tone from. The whites can for example be a white wall or a sheet of paper or a dedicated white card. Gray cards are manufactured for the purpose of adding a neutral gray to the photograph.

White balance software usually has both a manual and an automatic mode. The manual mode usually consist of a single temperature slider for adjusting the light cool or warm. This is OK for incandescent light, but not for fluorescent light or mixed light. When converting RAW pictures, one normally has a temperature slider. Some RAW converters also have three color sliders for red, green and blue. Color sliders can somewhat correct fluorescent light and mixed light, but the problem with using color sliders is that the black and the whites usually get a bad tone. Automatic white balance correction usually needs neutrals in the image, like a gray-card or white-card or both. Some applications can dispense with that, but usually neutrals are needed.

There are three kinds of contrast: hue, saturation and brightness. Very few applications have more than a single slider for contrast, that addresses all three kinds of contrast at once. However, a single slider for all three is unsatisfactory since the result is usually over saturated and gaudy. At best the software will have a control for luminance contrast and for color contrast.

The usual way to manipulate contrast is simply by changing the difference between the individual red, green and blue values and the middle value (128); like this: R= (R-128) * contrast + 128; and likewise for the green and blue channel. If the image is very dark or very pale this method will not do. What about very dark or very pale images? In that case you change the algorithm to: R=(R-RAverage)*contrast+RAverage where RAverage is the average red channel value of the image. And so on for G and B. The algorithms are essentially the same since a full brightness range image will have 128 as an average value.

Another problem with contrast adjustment is that not only may the average value not be 128, but the darkest and brightest areas may not be black and white. If that is the case, one should also be able to expand the brightness range to reach black and white. This is essentially what levels adjustment does. One can do this with Photoshop\’s levels adjustment like this: First convert the image to Lab mode, select the L channel only and run auto levels on that. Then convert back to RGB mode.

This article is based on this about white balance and this about contrast

Low Light Capability/ISO Speed

If you\’re likely to grab pictures in low light, when it\’s sunset scenery or simply indoors, good low light effectiveness is important. Most DSLRs can catch photos at ISO 3200 and some even ISO 6400.

Extra Speed

So why are some compact cameras so sluggish and digital SLRs so fast? It\’s simply electronics against mechanics. There is also no shutter lag. This is the amount of time taken between pressing the button and the camera in reality taking the shot.

Image Stabilization

Also known as anti-shake or vibration adjustments, this is often built into the lenses (Nikon, Canon, Panasonic) or into the camera itself (sensor-based, e.g. Sony, Pentax, Olympus). It\’s recognized that lens-based stabilization is fairly better, but it ties you into selecting expensive lenses to get the advantage, sensor-based stabilization works with any lens.

Burst Mode/Frame Rate

Typically an important factor for someone who shoots motion and wildlife, it is the feature to fire off a number of pictures in quick sequence. Just the most regular DSLRs now possess a frame rate of 3 per second but some are much faster. An optimal buffer memory is mandatory, it\’s where pictures are kept prior to getting saved. The bigger the buffer is, the more photos you can take before it fills and the camera falls to a momentary stop. Raw files, are larger and can load up the buffer faster than JPEGs.

Anti-dust

Each time you replace the lens on the DSLR, small dust particles can break into the uncovered lens mount and fall on the sensor, showing awful spots on the photos. Dust is also formed by use of moving components in the camera. Most camera makers have a dust removal and cleaning system for the sensor of some sort, except Nikon, saying dust isn\’t a real concern anyway.

HD video

The increasing acceptance of taking home videos or quite possibly professional movies with DSLR suggests that the occurrence of a high-definition video mode is growing to be more valuable. Many DSLR cameras now feature this functionality, from the starter Nikon D5000 up to the more higher-priced pro level variations.

Customization

Among the strengths of DSLRs is the sheer level of customization they provide. Decide on which dials handle which functions, or choose if you want to capture JPEG, Raw or both at the same time. There are plenty of options you can fine-tune to your hearts desire. And the substantial variety of lenses and accessories to select from.

For more information about DSLR Cameras visit http://forever-camera.weebly.com/my-blog.html.

White balance adjustment means to adjust the illumination in a picture to white. To accomplish this you must have a neutral in the image like a gray card or a white wall. Software can correct white balance, but not all software solutions are equally good.

The first thing you want to take care of, when you begin to edit your pictures, is white balance. White balance refers to the color of the light and assumes that the best light color is white. Some shots, like sunset or candlelight, do not have white light, but in general an impression of white light is desirable. Some use Photoshop\’s auto levels to set white balance, but that is not ideal, since auto levels just sets the darkest pixels to black and the brightest to white without looking at the mid tones. But what if the lightest pixel in your photo is not white? Or what if you do not have pure black in the photo? (Most images have black areas, but the palest pixels are rarely pure white).

When correcting white balance, the mid tones are the most important and to help set the mid tones correctly one adds a grey card to the photo when taking the picture. A grey card is a sheet of cardboard or plastic colored an exact mid tone neutral gray. Ideally one has three cards: a black, a gray and a white. Photoshop\’s levels adjustment panel has three eye droppers for picking color: one for white, one for gray and one for black. By clicking the gray color picker on the gray card, one can adjust the mid tones to neutral gray. One can only include a gray card in the picture if one intends later to crop the image.

If one does not want a gray card in the image, or if one doesn\’t have a gray card at hand, one can later use specialized software that scans the image and calculates the color of the light and sets it to white. There are problems with such applications: what if there are no neutral areas in the picture to calculate the color of the light from? Some applications do not need a neutral in the photo, but most do to get a good result.

If you work with RAW images, you will have discovered that the RAW converters generally come with a control for color temperature, which means a control to adjust the photo cool or warm. But what if the color of the light is greenish as when you have taken a picture in fluorescent light? The cool-warm control is good for regular incandescent light, but not for fluorescent.

Color correction controls are rarely good for correcting white balance, because the color correction will not just neutralize the gray card, but will also color the photo in an undesirable way: usually the blacks get toned or the whites or both. In short one needs some neutrals in an image to set white balance. A white wall or a sheet of white paper will do well; at best add a gray card for the mid tones.

This article is based on this article about White balance and this one White balance