Lens Guides

A Bit about Bokeh, Background Blur and Selective Focus


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.

Selective focus at a wide aperture gives shallow depth of field for background separation and cool holiday light effects photography. SIgma 35mm F1.4 DG HSM at 1/320 F1.4 ISO 100 on Canon Rebel T3i.

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.

Every lens, even the widest ultrawides and fisheyes, are capable of isolating the subject from the background. Here these two seed pods are mere inches from the lens of the Sigma 8-16mm ultrawide zoom. There’s a sweeping field of view, and a softer, but totally recognizable background, at F4.5 at 1/1000 at ISO 100 on  Reb T3i.
Here, with the Sigma 19mm F2.8 EX DN wide open at F2.8 on an Olympus E-PL2 for an equivalent 38mm focal length on the four-thirds sensor, the background is a bit more abstract, but still very recognizable as a pond.
And here is the same scene shot at F2.8 with the Sigma 30mm F2.8 EX DN on the same E-PL2 camera. Notice the differneces in the depth of field and field of view here, despite the very similar composition.
Now, interestingly, compare this shot to the one directly above. This is made with the Sigma 18-250mm F3.5-63 DC Macro OS HSM at 43mm at F5 on the Rebel T3i, which is an APS-C sensor. There’s a relationship between sensor size and depth of field.
Now this shot was made with the Sigma 70mm F2.8 EX DG HSM at about 1:2.5 magnification on the Canon EOS 5D, a full-frame camera. notice how the background is even more defocused due to the very close focusing and large sensor. In fact, at F2.8, only the edge of the seed pod is on the focal plane.
And finally, here’s the same composition captured at 1:2.9 macro on the Sigma 18-250mm DC Macro OS HSM at 250mm. The depth of field is extremely shallow even at a wide-open F6.3, and the small angle of view renders the background a few yards from the subject incredibly abstract.

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.

Here is a variation on the first image. Our focal distance is now about ten feet to the nearer tin soldier, we are stopped down to F4.5. The increased distance to subject, and smaller aperture significantly increases the depth of field. Shot again with the Sigma 35mm F1.4 DG HSM on the Reb T3i.

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.

Here with the Sigma 35mm F1.4 DG HSM we are close-focused to mere inches from the lens on the foreground fir branches. Every single orange circle of light is an identical small bulb, but the distance from the focal point changes the effect. Sigma 35mm F1.4 DG HSM 1/500 F1.4 ISO 100 on the Reb T3i.
And depth of field increases as the aperture grows smaller, even when focal distance remains constant. The top image is at F1.4, the middle at F4.5 and the bottom at F10, all focused on the same point with the Sigma 35mm F1.4 DG HSM and the Sigma 35mm F1.4.

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!

On a cloudy day in New York, the light at 1:30PM was flat enough to capture both the detail in the foreground statue and the tiny lights in the big tree on the opposite side of the Skating Rink rendered as soft circles thanks to a wide F1.4 aperture at 1/400 on the Sigma 35mm F1.4 DG HSM on the Rebel T3i. Day or night, make the most of the season’s lights!

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!

For these shots, I set up my tripod and composed the scene through the LCD of the Olympus E-PL2 and Sigma 19mm F2.8 EX DN at F2.8, so what I saw on-screen was what would be captured to the sensor since this is widest this lens goes. The exposure was set at 1/40 based on test shots for the tree and alleyway lighting. I, the foreground subject, was lit by the pop-up flash, dialed back 2/3. The great distance between me and the tree ensures that there’s no subject/background flash contamination to ruin the effect. Sigma 19mm F2.8 EX DN on the Olympus E-PL2 1/40 F2.8 ISO 250.
And here, notice how stopping down to F4 scales down the size of the light-circles on the tree. And also notice how much darker the background details are with the same 1/40 exposure as the last image. Sigma 19mm F2.8 EX DN on the Olympus E-PL2 1/40 F2.8 ISO 250.
Now, here’s another variation on this scene, captured through the new Sigma 35mm F1.4 DG HSM stopped down to F1.4 to ensure my whole body is in sharp focus at such a close subject-to-lens distance. I was able to position myself into the focal plane by employing Live View and the pivoting LCD viewfinder. How does this composition and blur effect impact you differently than the above ones, if at all? Sigma 35mm F1.4 DG HSM 1/50 F1.4 ISO 100 on the Reb T3i. Shoe-mount flash with diffuser panel down, set to -2/3
And here is the same scene stopped down to F9.0, with a longer shutter speed to keep a similar background exposure. Is this lesser degree of background blur more of less pleasing to you than the previous image? If you were framing your shot, which would you choose of these two? Sigma 35mm F1.4 DG HSM 1/6 F9.0 ISO 100 on Reb T3i. Shoe-mount flash with diffuser panel down, set to -2/3.

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.

Learn more about how aperture and F-Stop choice affects the feel of a photograph.

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