PDA

View Full Version : Subject in focus with blurry background



ahazan
02-22-2007, 01:41 PM
I am trying to get the subject-in-focus-with-blurry-background effect. i played with the aperture going to both extremes, and cannot achieve it. I am using a Pentax K100D with the starter 18mm-55mm lens.
Any help is appreciated.
thanks,
Albert

Honest Gaza
02-22-2007, 01:45 PM
Maximum Aperture (smallest f/stop no.)
Relatively close to the subject
Some distance between subject and background

....and I'm also guessing the 55mm focal length will produce better result than the 18mm focal length (for this purpose)

DonSchap
02-22-2007, 02:37 PM
I am trying to get the subject-in-focus-with-blurry-background effect. i played with the aperture going to both extremes, and cannot achieve it. I am using a Pentax K100D with the starter 18mm-55mm lens.
Any help is appreciated.
thanks,
Albert

With your "KIT" lens (18-55mm f/3.5-5.6), you are only going to get the widest aperture (f/3.5) the lens has when it is ranged to 18-24mm ... after that, the aperture starts closing down ... automatically. By the time you are zoomed out to a focal length where you could actually REALIZE your DOF (Depth of Field) desire, the best aperture you can get is f/5.6 ... and that simply isn't going to get it done.

For what you are asking, you really need a 50mm f/1.8 lens. These PRIME lenses are relatively common and cheap (~$75). You will begin to see the desired effect, as you widen your aperture.

You are asking a little much out of the kit when it comes to DOF variations.

Get a PRIME and try it out. ;)

zmikers
02-22-2007, 06:17 PM
Using that lens it is possible to get a blurred background. Maybe not as extreme as with a faster lens but it is possible. Try this. 1) keep the aperature as high as possible (lower f/number). 2) Get as close to your subject as possible. The closer the subject is to the lens then the more blurred the background will be. 3) It would actually be easier to blur the foreground. Try using a subject a bit further away and see waht happens to the bushes, or trees or grass or whatever is in your foreground. But that being said, Don is right, using a faster lens (lower f/numbers) will give you a much shallower depth opf field (more blurred background.

DonSchap
02-22-2007, 07:01 PM
The problem is the "kit" lens is "variable aperture" coupled directly with a wide angle lens. As you zoom up to the higher focal lengths, the DOF just isn't available like it is with a fixed normal lens or a constant aperture zoom.

Now, if you had a 17-50mm f/2.8 or something similar (constant aperture zoom) ... when you got the lens around 40 to 50mm, the f/2.8-blur effect would be very obvious. But at f/5.6, which is 2 full f-stops slower, it's just not going to be there. In fact, it is one of the primary reasons people toss the "kit" lens in preference to the f/2.8 constant aperture lens.

Please study the Depth of Field (http://en.wikipedia.org/wiki/Depth_of_field) (DOF) concept to appreciate this discussion.

Also, these lens-type issues are handled on the lens manufacturing web-sites.

Canon has some definitions and formulas (http://www.usa.canon.com/consumer/controller?act=GlossaryAct&fcategoryid=216&alpha=DEF) that will allow you to calculate what your lens is capable of, but you have to monitor your aperture as you zoom.

You can also by-pass the paperwork, since you are already online and use this really NICE ON-LINE DOF Calculator (http://www.dofmaster.com/dofjs.html)

Here's a good way to deal with the "KIT" lens.

With the lens mounted on the camera, set the camera to Manual or Av mode.
Set the focal range to 18mm
Focus on your subject exactly seven (7) feet away (Measure this if you have to).
With the aperture control, open the aperture up to full (f/3.5)
Keep looking through the veiwfinder and zoom the lens to:

35mm and write down the aperture.
40mm ditto
45mm ditto
50mm ditto
55mm ditto

You now have all the information you will need to determine your DOF with the calculator. In other words, how far around your subject (which was seven feet away) will be in focus and what won't be, at the widest aperture setting you can get with that lens at any particular focal length.

This is a good exercise and you should learn quite a bit with it. :D You can chart all your lenses and know when and if something is OOF (out of focus), just by knowing how far away it is and what settings your are using.

Good luck! :D

sla
02-23-2007, 02:15 AM
I am trying to get the subject-in-focus-with-blurry-background effect. i played with the aperture going to both extremes, and cannot achieve it. I am using a Pentax K100D with the starter 18mm-55mm lens.
Any help is appreciated.
thanks,
Albert
Hi
I am not expert. But I have some thoughts.
If you can't get your effect even with the widest aperture, you are probably relatively too far from your subject. I suggest to set your lens to the widest angle position ant close up. I use an automatic camera and usually have no big problems to have blurry background if it is twice as far as the subject, or farther.
Remember, important is RELATIVE DIFFERENCE between the two distances. If the background is about 1m behind the subject, try to stand closer than 1m from the subject. If 0,5m, come closer to it. And so on. If distances are bigger (like 10meters), difference should be bigger (for instance 1:4, 1:6, 1:10).
That's my experience.
regards
s.

timmciglobal
02-23-2007, 08:22 PM
Your best (and only bet) with that lens is to zoom to long end of the lens and get as close to object as possible while having background as far away from subject as possible.

Tim

toriaj
02-23-2007, 09:37 PM
I have a similar lens, the Nikkor 18-55 mm 3.5-5.6.

With both shots, I was about 12 - 24" from the subject, at F/5.6 and 55 mm. Very minimal PP.

20608


20609
Here, the background was clear across the street.

As you can see, blurring the background is possible, but yields less beautiful bokeh than with higher-end lenses.