How can I get dramatic shallow DOF with a kit lens?
I'm currently using an entry-level DSLR with 18-55mm lens and having great trouble trying to creating a blurred background effect in my photos. Now matter what aperture or shutter speed I choose, I'm not able to get it.
There's a general question about How can I maximize the "blurry background, sharp subject" (bokeh) effect?, but what specific things can I do to get this effect with an entry-level lens?
I was going to add to the answers, but not much point. Bob Atkins has a fairly lengthy article on the subject here: http://www.bobatkins.com/photography/technical/bokeh.html and it's worth a read.
Given that gear, you'll get maximum background blur by zooming in to 55mm, going aperture priority mode (Av) and setting the aperture to 5.6 which is the maximum aperture on that lens at that focal length.
You have a combination of a crop sensor (less visible bokeh) combined with a lens that maxes out at f/3.5 at the wide end (less obvious bokeh than lenses which go to a wider aperture). That lens has a variable aperture, so as you zoom closer in towards 55mm your widest aperture will be further limited but as seen in other answers, this is less significant than other factors.
If you're not satisfied with the amount of background blur, unfortunately this is one of those cases where different gear is the answer.
Yes, a lens at f/1.8 will give you quite a bit more out-of-focus areas, aka bokeh
I'm not convinced you'll get any more blur at 18mm f/3.5 than you will at 55mm f/5.6 seeing as the physical aperture remains the same size. I'd like to see some comparison images.
The perspective changes and magnification from the longer focal length coupled with no change in minimum focus distance will almost certainly make it easier to create more out of focus areas on
Bear in mind that going to a shorter focal length (18mm) increases depth of field (i.e. decreases likelihood of getting bokeh) versus a longer focal length -- I'm not sure this would mix with the (ratio-wise) aperture change, so I agree with Matt Grumm that it'd be nice to see comparisons (ideally with at least 3 images -- one at 55, and two at 18: same focus distance (camera-to-object) for one, same subject-in-frame size for another). While ahockley certainly has some things correct in this answer, I think Matt's more accurately reflects the main issues at hand.
You'll get a narrow depth of field at 55mm (with f/5.6) than at 18mm (with f/3.5). http://www.dofmaster.com/dofjs.html
@MattGrum - what do you mean when you say that the physical aperture is the same size at f/3.5 and f/5.6? Aren't those two different aperture sizes?
What I mean is the solid round hole in the lens through which light passes doesn't grow or shrink when you zoom, hence you're not gaining anything by zooming out and using f/3.5 The relationship between the apparent aperture and focal length (the f-stop) does change. In fact the apparent size of the aperture increases when you zoom in, meaning using the 18mm end is the worst thing you could do for bokeh!
To elaborate on Matt's comment, remember that the f-stop is a mathematical expression giving you the effective aperture size. Divide focal length (f) by relative aperture (3.5 or 5.6) to get the real size of your aperture. 18mm/3.5 = 5.14mm, whereas 55mm/5.6 = 9.82mm.
@David: looks like your images clearly demonstrate that 55mm at f/5.6 gives more background blur than 18mm at f/3.5. I removed them from the question since they're really an _answer_, not a question; if you visit here again, consider actually providing a new answer to your own question and linking the images there.
f-number is not what is critical here, entrance pupil (the so-called *effective aperture*) size is. The e.p. for 55mm @ f/5.6 is 9.8mm wide. The e.p. for 18mm @ f/3.5 is only 5.14mm wide. That's why 55mm @ f/5.6 gives more blur than 18mm @ f/3.5.