The holiday season is a great time to make photos with gorgeously softened backgrounds and shallow depth of field for painterly, creative, and festive results. Indoors and out, the Christmas trees bedecked with tiny lights, Menorahs, and decidedly secular seasonal displays that brighten the world each December present the perfect opportunity to experiment, explore and create. While we’ll obviously be featuring a number of Sigma lenses in this how-to, much of the information here is universal.
We do have to start with a bit of the technicals to get to the creative applications, but we’ll try to keep keep it as short, and as easy to grasp as possible.
Every lens for SLRs (and compact mirrorless, too) that Sigma produces, from the widest wide angle to the longest supertelephoto, can be focused to isolate the subject from the background (and foreground, for that matter.) But the degree of recognition of background elements is dependent upon a number of factors: focal length, focal distance, aperture, sensor size, and subject to background distance.
And conversely, every lens, from the widest wide angle to the longest supertele, can capture a shot where pretty much everything appears to be in sharp focus, without any background separation. This is called infinity focus, or the hyperfocal distance. And this, too is dependent on the same factors as above: focal length, focal distance, aperture, sensor size, and subject to background distance.
The whole key to achieving background separation is to ensure you are not over the hyperfocal limit. There are many ways to determine hyperfocal distance for a given lens at a given focal distance and aperture combo on your camera, including online calculators, pocket charts and sliders, iOs apps, and my favorite method, hands-on field experimentation and dead reckoning combined with experience.
Now, let’s talk for a minute about kites and huts. (Hopefully the previous statement will make a lot more sense by the end of this section!) In optical physics models, the plane of critical focus and depth of field range is a kite-shaped quadrilateral, with the plane of critical focus intersecting the two corners of the quadrilateral parallel to the focal plane. As apertures get smaller and focal distance increases the two sides of the kite behind the focal plane grow ever more elongated. Here’s a Flash-based interactive demonstration from the Stanford University website to visualize this.
And at a certain point, parallax takes over and the two legs of the quadrilateral kite behind the subject meet at an infinitely distant vanishing point that in effect, turns the kite into a five sided inverted hut, similar to a very elongated home plate in baseball. And once at the hyperfocal distance, everything from one third in front of the subject to the farthest horizon is captured crisply enough to effectively eliminate the selective focus effect completely.
Shortest focal distance and overall focal range before infinity varies from lens to lens. An ultrawide lens may have a focal range from a few inches to a few feet before infinity, while supertelephotos may have close- and distant focus lengths best measured in horse-racing terms. But with whatever lens, once it goes hyperfocal, there’s little apparent separation of subject and background, and that’s what’s to be avoided here–you want to make sure those two back legs of the kite meet a point much closer than the end of the universe to soften and blur that background to a slight or serious degree.
And especially when there are tons of tiny lights in the background, these can be rendered quite lovely for a suggestion of holiday celebrations, since all bright points in an image that are off even slightly off the focal plane are captured not as what we’d recognize as points, but as circles of light in a three dimensional space, so these circles can overlap, and be drawn to the chip as ovals and ellipses depending on where in the field of view and angle of view they are captured. Remember, creative photography is all about hands-on experimentation, and that’s honestly always the best way to absorb and adapt what I’m describing here.
It’s easiest to understand and visualize this effect on any lens at its widest aperture, because what you see through the viewfinder or LCD is how the image will be captured to the sensor. And as you change your focal distance, focal lengths, and position in regards to little lights in the background you can see how it all happens. And as you can see here, it’s possible to catch these lights both day and night!
One of the challenges for this type of image is to think in terms of background first, then subject. For example, if you want a soft-focused tree with lots of lights to have a certain feel to it, and a certain spot in the composition, you’ve got to determine all of that first. Then once that’s as you’ve envisioned, it’s a matter of placing the subject where they’ll be in sharp focus, and balancing the exposure for both the background and subject exposures. This is much easier outside, and when there’s a nice space between the foreground subject and background lights!
Indoors, there are two big challenges. First is making sure that there is sufficient subject-to-background separation to get the blur effect you desire. And second is to make sure that any added flash exposure doesn’t totally overpower the ambient lights of the trees. So my best advice is that indoors and out, metered manual exposure off the background/ambient light with a touch of fill-flash for the subject is the way to go. Every light display is different, and the best way to nail the exposure is to experiment, review, adjust, and try again and you’ll find what works for you. And whenever possible, do your exposure experimenting and be all ready to roll before gathering the family for the portrait! Trust me on this one.