I have come to the conclusion that the most tedious post job is a white-on-white silo cut out. No one will ever be able to convince me otherwise. Well, except for a composite, but I don't do composites. Wouldn't even try. I'd be afraid that I'd end up on PhotoshopDisasters.
A lot of people struggle with getting a clean white background when they shoot silos, so I thought I'd pass along a few pointers that work well for me.
First, and most important, is that many people tend to underexpose the main subject when they first try shooting silos. It's important to remember that if you are relying on your camera's internal meter, that nice bright white background tricks your meter and your camera is going to want to expose it as 18% gray. Just like when you are shooting snow, you need to compensate for this either by spot metering on the subject itself, setting your exposure compensation +1.5 - 2 stops, or by setting your manual exposure accordingly. You may want to bracket a few shots to get the best exposure.
Once you've selected your shot and you're ready for post, the key point to keep in mind is that you need an absolutely spotless true white (255,255,255) background. The point of a silo is that the image should blend seamlessly on a white magazine page. What looks like a pure white background to your eyes probably isn't 100% until you do some work in post.
After processing in ACR and opening in Photoshop CS3 I make a duplicate so I'm not working on my original. I make a curves adjustment layer, then I hold the ALT button down as I move the right slider to the left. Holding down the ALT button puts a black mask over your preview image, and as the highlights blow out to pure white you will see it on the preview. If your main subject has no white and no specular highlights, you can usually blow out most of the background this way. Move the slider to the left until as much background as possible is blown out without blowing out the highlights on your subject. Of course, depending on the particular subject you may be able to mask out an extreme levels adjustment by painting in black on the layer mask. I find that is really dependent on the amount and location of the blown out area. Anyway, as you're making the adjustment pay careful attention to the edges of your subject. It's better to do too little than too much because we can always fine tune in the next step.
Looks pretty good already, doesn't it? But we're not finished, and here's where the tedious work comes in. I mentioned that what looks like pure white to your eyes isn't necessarily so, and I've learned a neat trick to help you see this in an image.
First, depending on your workflow preference either flatten your layers and Control-j or Ctrl-ALT-Shift-E to copy merged and then Control-j. I prefer to keep all of my levels intact on my master file. Next make a Levels adjustment layer and slowly move the left slider to the right. As you do so, the parts of your image that are not pure white will begin to change color and you'll see the areas that need to be cleaned up. Pay careful attention to the areas along the edges of your main subject. Don't worry about how this looks - we're going to delete this layer later and it won't have any effect on your final image. It's just a tool to help you see where you need to clean up your background. You can toggle it on and off as you're working by clicking on the eyeball. Just make sure that the actually work you are doing is on the merged layer below it.
Click on your merged layer (below the Levels adjustment layer you just made) so you can clean it up. I've learned two different ways to clean up the background, and depending on the colors of my main subject I may use either or a combination of both.
The first and simplest is to use Select-Color Range, feather your selection slightly and adjust the levels (move the left slider) until the stray areas have gone to pure white. You MUST pay careful attention to your main subject when using Select-Color Range; if you have areas of white or specular highlights in your main subject they will be chosen too. You most likely don't want to blow these areas out. I can tell you that this method definitely does not work for subjects like brushed silver cellphones! So while this method is the easiest and quickest, I personally find it somewhat clumsy and use it only when there is no white or specular highlights in my main image and even still I find I need to do some touchup work.
The next method is my favorite. I'll either use this method instead of Select-Color Range, or after Select-Color Range to fine tune. Working on your merged layer, select the Dodge tool, set it for highlight and choose an appropriate sized brush. I usually set Exposure around 25% to start. Start dodging over the areas that need to be cleaned up, changing your brush size and exposure level as needed. Use a larger brush to get close to your subject, then a smaller one as you work nearer to the edges.
It is imperative that you work at 100% to clean up your background. Sometimes more depending on how fussy you want to be. It is amazing what you notice at 100% that you would never see on a smaller sized image, and if you've never worked on your photos at 100% before it will be an eye-opening experience. I worked on the above image at 200% with a brush as small as 3 pixels.
When you are finished (take your time and be patient!), delete that top Levels adjustment layer and voila!
The true test is to post your image on a white web page or print it on white paper. Your subject should appear to float on the page with absolutely no evidence of the background.