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10-19-2009, 02:01 PM   #1
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depth of field

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

10-19-2009, 02:39 PM   #2
Damn Brit
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The wider the aperture (low number), the shallower the DOF (how much is in focus).
Distance to subject also makes a difference depending on how wide an aperture your lens has. Focal length of lens also has an affect, a 24mm will have a deeper DOF than a 50mm.

If you lens is as wide as f/2 for example, you can get quite close to your subject.

Here's a link to a DOF calculator that might help you - Online Depth of Field Calculator

An example that I gave someone the other day is -

Three people are standing in a line diagonal to each other.
Focus on the first person at f/2, the first person will be in focus the others out of focus.
Focus on the first person at f/4, the first two people will be in focus.
Focus on the first person at f/8, all three people will be in focus.
Note, this is just an example for explanatory purposes, in the real world, distance to subject and distance between subject as well as focal length of lens would have to be taken into consideration.
10-19-2009, 06:21 PM   #3
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Welcome to the forums rattrap!
I see it as your fist post!
If you want to consistently produce the effect you want then always zoom to 50mm and only use the widest aperture (3.5).
This is using the DA 18-55mm.
The explanation was already given by Damn Brit.
Mine is the clutter free procedure..hehe
10-19-2009, 06:28 PM   #4
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Welcome Rattrap,
The DA 18-55's biggest aperture (smallest f number) is 3.5 at 18mm, but 5.6 at 55mm.

If you zoom to 55mm and set aperture (the Av number) to 5.6, then get as close as you can to the subject you want to capture (without taking away the background), focus in on your close subject and take the shot, you should see the background blurry.

Enjoy the discoveries, and keep at it.

10-20-2009, 02:54 PM   #5
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QuoteOriginally posted by Ash Quote
Welcome Rattrap,
The DA 18-55's biggest aperture (smallest f number) is 3.5 at 18mm, but 5.6 at 55mm.

If you zoom to 55mm and set aperture (the Av number) to 5.6, then get as close as you can to the subject you want to capture (without taking away the background), focus in on your close subject and take the shot, you should see the background blurry.

Enjoy the discoveries, and keep at it.
..oh yeah, you can't use 3.5 for the 55mm end..duh!
Sorry about that, just blabbed out from the top of my head.
10-22-2009, 11:10 AM   #6
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And for more, you can check out these links... and a google shall bring up useful stuff too...

Understanding Depth of Field in Photography

DOF

Effective use of hyperfocal focusing for wide angle landscape photos
10-24-2009, 07:25 AM   #7
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If you check the exif data on that shot you took that gave you the result you were looking for it should give you a pretty good idea on how to do it again. Depth of field and bokeh as its is refered to isn't exactly the same. A shorter lens like the 18-55 kit lens won't blur out the background quite the same way a longer lens will. A faster lens like the 16-50 will also produce that effect readily shooting wide open. You can get it with the kit lens as you have found that you can. Just make sure you have the lens wide open. Thats f/5.6 on the kit at 55mm, not exactly the aperture to produce extremely blurry bokeh but you can also add some in PP. Some folks call that cheating but sometimes you just have to use the tools at hand.
10-28-2009, 07:00 PM   #8
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Here is a decent "bokeh" shot I took with the Kit lens. I was VERY close to the flower, less than a foot away. This was at F8 also, so you don't necessarily NEED a small aperture to get good bokeh. The flowers in the background were about 20 feet away, so even at F8, I was able to get some nice bokeh.



10-28-2009, 10:26 PM   #9
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QuoteOriginally posted by rattrap Quote
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
  1. aperture,
  2. focal length, and
  3. 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
10-28-2009, 11:20 PM   #10
Ash
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We can always rely on you Will for the complete explanation.
Thanks mate - you're an asset to the forum.
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