Lightroom offers a few tools that make it easy to isolate a subject with a light or dark background. Here's how I isolated my subjects using Adobe Lightroom.
SETTINGS: 1/250 sec at f/3.2, ISO 640, 70mm (70-200mm)
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The photos I'm editing here already have a very dark or light background. While it is possible to use these techniques to isolate a subject if they don't already have a light or dark background, it will be easier to accomplish in Adobe Photoshop. If your image does not already have a mostly light or dark background, consider using the Lasso tools in Photoshop or hire someone on Fiverr.com to do it for you.
I planned on making this photo black and white to remove the distraction of the colors in dad's clothing. To accomplish this you can reduce Saturation to 0 or near 0 or click the "B & W" button on the HSL panel. While clicking "B & W" is faster, sometimes it's nice to leave a hint of color, so try reducing Saturation first.
To get a gritty look, I increased clarity +29. At this time you can also make any other adjustments you like in the Basic panel. For this type of look, I often decrease Highlights and increase Shadows.
I find that it's easiest to isolate the subject when the background is not completely black, so I adjusted the Tone Curve to give the photo a matte look by removing the darkest black from the image. To do this, first click the line about a 1/4 of the way up from the bottom right (the dot to the right of the arrow below). This will add a point to the tone curve to anchor the mid and lighter tones. Then click and drag the leftmost point on the Tone Curve upward. You can drag it as high as you like depending on the look you want.
While you can use radial or graduated filters to help darken or lighten the background, I prefer the precision of the adjustment brush. To darken the image, click the Adjustment Brush icon and select Burn (Darken). Then move the brush's Exposure setting to the far left. Test the brush on the image. If this isn't as dark as you want, also reduce the Highlights and Shadows. You can use the Undo feature any time if you don't like your brush stroke.
Set your brush feathering to a moderate setting. This will allow you to get close to your subject without accidentally darkening them too much. Select an brush size that is large enough to allow you to quickly darken the background without brushing over your subject. If you want Lightroom to help you avoid painting on your subject, select "Auto Mask" before painting. Auto Mask will only apply your adjustments to colors that are similar to the ones you start painting on.
Use the brush to darken the background around your subject. Once you have darkened the background as much as you can with a larger size brush, reduce the brush size and get the tighter corners and edges closer to your subject. Continue reducing the brush size and darkening the background around your subject until all parts of the background are dark.
Here's what my brush overlay ended up looking like. To view your brush overlay, click "O" while using the Adjustment Brush.
Once you are happy with your image, it's time to Export! I export my photos at the largest size unless I need them smaller for internet or blog in which case I usually limit size to something smaller than 700 or 800 pixels.
I used a similar technique to edit the photo below.
SETTINGS: 1/100 sec at f/5.0, ISO 400
First, I used the Clone Heal tool to cover up some of the wrinkled sheets around baby Sully. Then I used the Dodge (Lighten) Adjustment Brush to lighten the bed sheets around him. Instead of lowering the exposure like I did with the Burn Adjustment Brush, I raised it. This lightened and blended the white sheets around him. Lastly, I made some tweaks to the Basic panel to get the look I wanted which is a little darker than clean white.
Here is a dog photo where I used the same concept as the first photo in this post. However, this time I kept pure black and didn't remove the black for the matte style.
SETTINGS: 1/180 sec at f/11, ISO 100
What questions do you have about this technique for darkening or lightening the photo background?