Chromakey or Green Screen Photography, an Introduction


Table of Contents

I.    Introduction

II.    What and Why of Chromakey

III.    Studio and Camera Setup

IV.    Selection and Replacement

 

I.  Introduction

This paper provides a simple introduction to the use of chromakey backgrounds.  It describes a little about what chromakey is, why it's used, how it's used, and simple directions to get you started.  It expects the reader to have basic skills in using computers, printers, Windows and Photoshop software, and their cameras.

This paper covers only one simple method for making selections and replacing backgrounds while referring the reader to other sources for more methods and details.
 

II.  What and Why of Chromakey

Chromakey (a.k.a. Green Screen) is a technique that makes it easier to replace one background in your photo with another one.  That is, you can take a studio photo of someone in front of a chromakey background and quickly make it look like that person is in a different place and time.  You see this all the time on television, especially the talk shows, where the background looks like New York City as seen from a skyscraper or some other scene that can't possibly be in the studio.  These backgrounds are placed in the videos by expensive computers and software.

For us non-commercial photographers we can do something similar either with expensive software or with the software that we have.  This paper will be describing how it can be done in Photoshop 5 (which is what I have) but it can also be done in many other common software packages.

Chromakey means that the background is all one color and therefore easier for you and your computer to distinguish between the background and the subject.  Chromakey is typically associated with a green screen but often a Blue Screen is used when green might cause confusion between the subject and the color green.  In fact, a chromakey background can be any color.  For some subjects a black background will do just as well as a chromakey, but not as often.

Not every colored background can be considered "chromakey."  Most colored backgrounds actually have a wide range of color frequencies that they reflect making it harder for the computer to distinguish between the background and the subject.  "Chromakey" backgrounds are specially manufactured to reflect in a very narrow range of color frequencies to reduce the chances of mistaking the background with the subject.

Chromakey backgrounds can be easily purchased as either cloth or paper and in a wide range of sizes.  They can be purchased as folded fabric (easy for shipping), on rolls, and on flexible/collapsible frames such as those used in car windshield shades.  They can also be simulated by shining lights through colored gels onto a white background so that the background appears to be chromakey.

 

III.  Studio and Camera Setup for Chromakey

With chromakey photography the subject is lighted as you would for a normal background with the exception that you may want to coordinate the lighting of the subject with that of the background you want to insert.  For example, if you light your subject as if sunlight were coming from the right side of the photo but in the background the sunlight is obviously coming from the left then the viewer will see that the background has been switched and may consider it of poor quality.  On the other hand, many professional photos taken outdoors have the subject shaded from the sun and lighted by strobes and reflectors making it obvious that the subject was not lighted naturally.  This is so common that we've come to accept these as attractive and artistic.

The chromakey background, on the other hand, requires some special lighting.  Often we direct a light onto a background directly behind the subject to give them a halo effect.  With chromakey we want the entire background evenly lit to make it easier for the computer to distinguish what is chromakey and what is not.  Uneven brightness in the chromakey background would make this more difficult.  The lights used for this should be aimed so as to shine on the background and not on the subject.  The subject should also be much better lit than the background.

The subject should be at least 6 feet, preferably more, in front of the chromakey background to reduce the amount of green reflection at the edges of the subject.  This is often a problem with faces and with reflective fabrics such as whites and satins.  For that reason you may want to be very selective of what your subject wears.

Hair, especially long hair, is often loose, curly, or frizzy at the outer edges.  This allows the green to show between the individual hairs making it more time consuming to remove.  There is software that can easily remove the green from the hair but it can be expensive.  There are techniques described on YouTube and other media sources that describe how to do this in Photoshop and other common software packages, but they're too detailed for this brief introduction.  When starting out you may want to provide your subject with a head scarf or hat or have them fix their hair so that it lays smooth.

For chromakey the camera adjustments are the same as for other studio photography.  Just make sure that your subject is well within the boundaries of the background.  Most experts recommend that you always use Custom White Balance and shoot in RAW, though this is not universal and people who use point and shoot or phone cameras may not have that capability.  For my setup alternatives are Auto White Balance or Daylight.

My setup at the February 28, 2012, workshop uses inexpensive PVC pipes for a framework, shop lights for much of the lighting, and SRS Studio lighting stands and bulb holders.  The four bulbs used to light the chromakey background are GE Daylight (5,000 k), 40W (equivelant-e) CFL bulbs.  The hairlight is a 100W (e) bulb.  The main light is a 500W (e) 5,500 k bulb while the fill light is a 300W (e) 5,500 k bulb, both purchased from Samys.  The difference in bulb temperature is minor, in this case, because both the lights on the subjects face are the same temperature.  The hair light is slightly warmer but not likely to be noticable.

 

IV.  Selection and Replacement

As stated, chromakey doesn't replace the background it merely makes it easier to replace the background.  The first half of this process is to "select" the chromakey background and remove it.  The second half is placing your subject onto a new background.

In going through online hints, YouTube, and my "Photoshop for Dummies" book I've come to the conclusion that there are many, many ways to select the background.  Some of these are using the Eraser Tool, the Magic Wand tool, and using masks.  The more you know about "selecting" the easier these will be to understand and the better you will be able to separate your subject from the background.  However, this introduction will only give you a quick and dirty start.  After that you're on you own to get more complex and accurate.

 

Making Your Selection Using Photoshop's Magic Wand.

The first thing you want to do is open your photo then save it as a .psd file so that you don't lose your original.  If the lighting between your subject and chromakey background is distinct then you can use the Magic Wand.  Add a Duplicate Layer and turn off the view of the Background layer.  Click the Magic Wand anywhere on the background and it should select the entire background.  If it misses parts of the background then you can set your selection mode to additive and click in a part that was not selected.  You may also want to select Refine Edge and adjust the Feather to about 6-8 pixels to see if you like it.  When you are satisfied then press the Delete key on your keyboard and the background should disappear.  Looking closely you will notice that there will likely be some background color on the edges of your subject.  Cleaning up those edges is too complex for this introduction but there are many sources available to help you do this.

Your New Background

Next, under Select choose Inverse which will select your subject as opposed to your background, which should now be gone.  Now under Edit select Copy.  You now have a copy of your subject that you can place on your background photo.  Open your new background photo and add a Layer using New and Layer...  In that layer select Edit then Paste.  If you want to change the size or position of your subject then select Edit, Transform, and Scale.  You can move your subject with the Move Tool.  You can also edit your subject and/or background for brightness, contrast, and all the other adjustments.

When you are finished save your newly modified photos as .psd files with different names so that you don't lose your originals and so you can make adjustments later.  You can now flatten your image (select Layer) and save it as a .jpg for printing and e-mailing.

To improve your skill at using chromakey backgrounds you can find many text and video based tutorials on the internet.  A Google (Yahoo, or other) search for the keywords "chromakey," "green screen," or "blue screen" will net you many sources.  There's even some "free" green screen software listed but I have no experience with any of these.  Most comprehensive books on Photoshop and other software will also show you how to make precise selections as well as other ways of making composite images.

When selecting your background be aware of copyrights.  Some images on the internet are free but most are copyrighted and have restrictions on their use.  Playing around with them at home isn't likely to get you into trouble but if you post a photo online, show it in a competition, or try to sell it with someone else's material in it without express permission you could get yourself sued.  Plus it's more fun to use your own backgrounds.

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