Everything in focus versus selective focus.
iPhones versus fast aperture lenses.
Ansel verses Leonardo?
I would bet most people buy their first serious camera because they want the blurry background that makes a shot look “professional”. Ironically, photographers historically have gone to great extremes to get everything as in-focus as possible. I guess we always desire the outcome that is most difficult to achieve.
How to blur the background
You can get a blurry background even with a compact point and shoot camera by using the macro mode and/or getting your subject as close to the camera as possible, and the background as far away as possible. The camera to subject and subject to background distances are critical
For the same camera to subject distance, the wider the aperture (lower f number) the blurrier the background
The larger the image sensor, the blurrier the background because the camera to subject distance changes to maintain the same subject size
Smart phone software can try to fake the blurred background (e.g. “portrait mode”), but it often easy to spot because focus is based on what the software considers to be the main subject, not the physics of relative distance from the lens
When not to blur the background
Ansel Adams and some of his peers formed a group called f64 - an aperture of f64 was needed to get most things in a landscape photograph in focus, even when cleverly using the Scheimpflug principle. With the large format cameras he was using, blurring out the background was so easy it was considered a nuisance. With landscapes, the long shutter speeds required to use these narrow apertures were not a problem. Ansel liked large format for the ability to develop one large negative at a time, but a smaller negative would have made the camera to subject distance larger and consequently had more in focus.
So why did Ansel not use 35mm (or micro 4/3 if it were today) to get more in focus? Because moving the camera changes perspective, and a normal large format lens would have to be replaced with a super-wide small format lens with all the distortions that go with it. In reality, the biggest reason was the size of the negative. Film grain is the same physical size on all formats, so a larger negative dwarfs the grain providing increased apparent image quality.
You probably want as much of the image in focus as possible when:
you have a great composition with a defined subject and no distracting elements
documenting a scene which might be scrutinized closely by the viewer (think press images and landscapes)
printing large and you want to avoid large blurry areas of uninteresting space
f8 on a 35mm camera, along with all elements at a relatively similar distance to the lens, keeps most of the image in focus
But blur can really help you out sometimes
What do Da Vinci and wedding photographers have in common? They think backgrounds should have less detail than the subject. Leonardo says atmospheric perspective naturally removed detail form the backgrounds, and a lot of his portraits had a landscape background.
The human eye only sees a small portion of the world in focus at any one time, but our eyes scan and send all these details to memory, creating the illusion that more is in focus at once than there really are.
Modern portrait photographers blur the background for different reasons. The main reason is to isolate the subject, or as an artistic decision (it can make a scene look dreamy). But caution; often this is to make up for other failures:
lack of tonal contrast between subject to background
poor composition
busy or ugly backgrounds
or lack of sufficient light
f1.4 on a 35mm camera, with a subject relatively close and the background relatively far away blurs distracting background elements. Notice how the color of the background plant remains a distraction regardless of how much it is blurred.
No doubt - shooting wide open can get you out of all these bad situations at a pinch.
Artistic decisions aside, should we brute-force these problems away, or think like a proper photographer and use good technique and problem solving skills? Intentionality is key.
You thoughts are welcome in the comments below.