Originally posted by rattrap OK, so after a few weeks of tinkering and playing I have only been able to get a blurred background once. Even then it was an accident and now i can't figure out how to do it again.
using a DA2 18-55mm
I'm going to restate some of what's been said, repackaging it, and add a couple of things.
There's more to depth of field than aperture
First, shallow depth of field (which I think is what you mean by "blurred background") is a function of a several things. The three that you should keep in mind most of the time are
- aperture,
- focal length, and
- distance of camera from the focal point on the subject.
People often only mention aperture, but focal length and distance from subject are just as important. You should absolutely NOT think that, for narrowest depth of field possible, you must use your widest aperture (f/3.5) which means, with your lens, going to your widest focal length. This doesn't work
because the wider focal length offsets the larger aperture.
Examples
Every lens has a minimum distance at which it can focus - a number of feet or meters that represents the closest you can get to your subject and focus sharply. I don't know what the minimum distance is for your lens, but let's say it's 3 ft. Now, go here:
Online Depth of Field Calculator
If you're 3 ft from the subject and shooting at 18mm and f/3.5, you'll have about 1.2 ft of depth of field. Maybe that's shallow enough. But if you want even narrower depth of field, don't despair. Zoom to a focal length of 55mm and let the aperture adjust itself naturally to its max at 55mm, which if f/5.6. And don't move the camera or the subject - in other words, stay three feet from the subject. Guess what: your depth of field shrinks to about 0.2 ft!! And if the safe focus distance for your lens is even less than 3 ft, well, you can narrow the depth of field even further by getting even closer.
Let me emphasize my point with one more illustration. Say you had the Pentax 16-45 f/4 lens. This has a fixed f/4 max aperture, in other words, you can get f/4 at 16mm and you can get f/4 at 45mm as well. Now, say you're standing 10 ft away from the focal point on your subject, and you're shooting wide open (at f/4). If your focal length = 16mm, you'll have huge depth of field - almost 200 ft. At a focal length of 16mm, almost everything from about 5 ft away to close to infinity is going to be within the depth of field. Now don't move - stay 10 ft from the subject - and don't change the aperture, either, but simply zoom to 45mm. Your depth of field shrinks by a factor of almost 50, from almost 200 ft to just over 2 ft!
Why do people talk about aperture and depth of field so much?
Why do people so often talk as if aperture were the only thing that mattered to depth of field? Well, I want to admit that this isn't unreasonable. What I've just said is technically true. A longer focal length + a narrower aperture can actually give you NARROWER depth of field than a shorter focal length + a wider aperture. Seems odd, because we tend to think wider aperture = narrower depth of field.
But the reason we think that way is that, when we take photos, we usually aren't just interested in depth of field! We're trying to frame a shot properly. Say you're trying to take a head shot - just a person's head, maybe a bit of shoulder, and some background but not much more than that. You can stand really close and use a wider focal length; but that may distort the person's face. Wider focal lengths used close up tend to make people look, well, wider, not to say fatter. So your choice of focal length might be dictated by the desire NOT to distort the subject. So let's say you decide to use a focal length like 40mm, which gives something close to a "normal" field of view, in other words, it makes the subject's face look normal, the way we perceive it with our eyes. Now that you've decided on your focal length, you set about framing the shot. To get a head into the shot and not much more, shooting with a 40mm focal length, you'll need to stand 3-4 ft away from the point that you want to focus on. And now, when you start to think about depth of field,
the only variable left to deal with is aperture.
Focal length and distance from subject work together to frame the shot, and framing the shot is often - usually - more important than getting a particular depth of field. Once you've figured out your perspective and framed the shot, THEN you worry about the aperture. And that's why we tend to speak as if aperture were the only thing that matters to depth of field.
Summary
Nevertheless, it's important to remember that there's more to depth of field than aperture! Remember focal length and distance from subject as well. It is certainly not the case that f/2.8 = narrow depth of field, not in any absolute sense.
Will