Usar una tarjeta gris al 18%, para mejorar el balance de color en las fotos

A ver si entendí bien… Bueno, es que el artículo desmitifica una costumbre que venía teniendo desde que empecé a hacer video analógico, (y luego, fotos y video digitales), que es, antes de comenzar las tomas, in situ, tomar el tono de blancos con una pared blanca lisa, o simplemente, con una hoja de papel en blanco. Esto, permite capturar la correcta temperatura de color, según la fuente lumínica, que puede de otra forma hacer que los resultados tiendan al rojo o al azul (con una luz mas cálida o fría respectivamente, si la temperatura tomada es diferente a la de referencia de la cámara (o si el modo auto no tiene la fortuna de acertar). Obviamente, cuando me refiero a temperatura, es temperatura de color.

Es decir, me dispongo a sacar por ejemplo, una foto digital color, ok, prendo la cámara y seteo los blancos ayudado por una superficie blanca lisa iluminada de forma pareja, comparando el resultado, o bien por las preconfiguraciones que traiga la cámara (diferentes temperaturas de tubos fluorescentes, luz de sol directa, nublado, lámpara de tungsteno, etc… O bien, lo seteo a mano (a ojo). Esto lo aplico salvo cuando voy a usar flash, en cuyo caso, el Balance de blancos lo dejo en Auto, ya que no tengo forma de medir la incidencia que producirá el flash + la luz ambiente de antemano, y si bien podría hacerlo por ensayo y error, por la cámara que tengo y el uso que le doy, personalmente no justifico dichas pruebas.

Bueno, este artículo barre esta costumbre que traigo encima desde hace 20 años…

Lo que propone el autor, es en vez de usar el blanco como referencia, para un resultado más preciso, es usar una hoja impresa con un gris al 18%, ya que el blanco, dependiendo del tipo de exposición usado, no es una referencia tan exacta para tener una correcta lectura de los colores en la foto tomada. Ésto lo justifica básicamente diciendo que la “tarjeta gris”, es una herramienta de balance neutral, ya que en vez de medir sobre absolutos, como blanco o negro, el balance neutral determina el mejor ajuste sobre el promedio de la luz, incidente sobre el promedio de la toma y ésto, permitirá que cualquier soft de retoque, posteriormente, sepa exactamente que niveles de color hay en la imágen, ya que por ejemplo, las sobreexposiciones de otra forma, quedan completamente blancas, quitando toda información de color de esas zonas.

Como interactúa este resultado con Photoshop? Abrimos la imágen tomada de la hoja gris (tomada preferentemente con un sujeto sosteniendo la hoja a la altura de su cabeza), previamente balanceando la cámara con este método, y simplemente, en Imágen –> Ajustes –> Niveles, hacemos click en el cuentagotas central, tomando una muestra del gris de la foto. Grabamos el ajuste, y listo, cargamos el ajuste que predeterminamos sobre cualquier otra foto tomada en las mismas condiciones de luz para hacer el balance efectivo.

Probar para creer! :)

Use an 18% Gray Card for Better Color Balance in Your Photos

If you've ever relied on your camera's white balancing algorithms you know how imperfect they can be, but you're not out of luck. Getting accurate color balance with just about any camera is pretty easy with an 18% gray card.

A Gray Card for Staged Photos

You might think it makes more sense to balance the white in your images, given that the term we use most often is "white balance," but since we're looking for all-around color accuracy the best balancer is gray. Why? It's the average tone and it's neutral. If you're sampling the white for color balance you're just sampling the high end of the spectrum (or pure white, if your photo is overexposed). In fact, when your camera is white balancing it's (generally) looking for a neutral gray area. The use of the 18% gray card is basically to tell your camera, "look, the neutral gray is over here!" Technology blog Tested explains how to use a gray card for a portrait photo:

Place the gray card where the subject will be, so the light hitting the gray card is the same as the light hitting the subject. If shooting a portrait, have the subject hold the gray card in front of their face for a test shot. When you process your photos later in Photoshop, look at the test shot. Enter the Image > Adjustments > Levels menu, and click on the middle eyedrop icon. Save the level adjustment, and then load it in every other photo under those lighting conditions. You've just color corrected your shots with a gray card.

 

A Gray Card for Everyday Photos

While it's always best to have a reference shot with the gray card in your photos if you want to edit them later, a gray card can help you out for your everyday shots as well. If you're staying in one general location, say for a family barbecue or someone's birthday party, you can use a gray card to manually set the white balance of your camera. How this will work will vary from camera to camera, but generally you'll find this option wherever you'll find white balance settings. From there, all you really have to do is make sure the card fills up most of the frame as you tell your camera to white balance based on what it's currently looking at.

If lighting conditions stay generally the same during the day, manually setting your camera's white balance with a gray card should get you better, more accurate color for all your shots. Just remember you'll need to rebalance every time you move locations, or turn automatic white balancing back on if you're feeling lazy.

 

Making a Gray Card

While you can pick up an 18% gray card at most photography supply stores, you're basically buying a piece of gray board. The benefit of buying a card is that you know you're getting exactly 18% gray. If your photos aren't going on the cover of a magazine and you just want better accuracy in general, you can print out a gray card from your computer. If you just pick a middle-of-the-road gray and print it out, it'll be a good start. Here's an easy way to do this:

1. An easy way to do this is open up Photoshop (or any image editor that can handle layers) and make a new document that's sized at 8.5" x 11" and has a white background.
2. Make a new layer and fill it with black.
3. Reduce the opacity of that layer to 50%.
4. Print.

If your printer has a color profile, you may want to switch to that before printing for more accurate results. I did this with a cheap laser printer, however, and it worked really well. My gray card was uneven and pretty horrible in general, but I still ended up with better and more accurate color than the camera's automatic white balance. A proper gray card is definitely better, but when you need something quick you can get by with even this fairly inaccurate method.

Of course, if you want to make a really accurate gray card you should go for it. There's a great explanation of finding 18% gray on the photo.net forums that'll help you get there.

Send an email to Adam Dachis, the author of this post, at adachis@lifehacker.com.

Fuentes: http://lifehacker.com/5659324/use-an-18-gray-card-for-better-color-balance-in-your-photos - http://www.tested.com/news/how-to-shoot-photos-using-a-gray-card-for-proper-color-balance/1109/