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03-07-2012, 02:52 PM   #1
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DOF

can somebody explain to me how to get a decent exposure with a sharp center and a blurry background (lithe DOF) with my K5 and 18-135DA. I don't get it right. Thanx

03-07-2012, 02:59 PM   #2
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Use the longer end of your zoom wide open at F5.6 ! It will work better if your subject is close and foreground further back.
Unfortunately F5.6 is not great for this but its poss to get reasonable results.
You need a fast lens to do it well. Genraly at least an F2.8.
03-07-2012, 03:00 PM   #3
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Hi Cons:

When you say a "Sharp center" I am going to assume you mean a sharp SUBJECT. In order to get the best you can, you are going to have to shoot wide open (smallest aperture number/largest opening). With an aperture range at the bottom end of f3.5 to f5.6, you will need all of it to get some shallow DoF. Pick your focus point more toward the front of your subject so you can maximize the softness of a shallow DoF as much as possible. Finally, shoot at focal lengths above 70mm or so, again to maximize the shallow DoF. The longer the lens, the less DoF you will have. Focal lengths of 35 and lower have extreme amounts of DoF, so much so that on some lenses, even at f5.6, you almost do not need to focus them (except for extreme close-ups). See if these tips help you out.

Regards,
03-07-2012, 03:23 PM   #4
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Thanx. Indeed i mean Subject. I was already afraid that this all purpose lens is not the right tool for what i want. So if i understand well i have to use a aperture range of 1.8 or 2.0 etc or a more further away focal length. The combination of both is very expensive. So for a better DOF i have to invest in a second lens or replace this one. Food for thoughts!

03-07-2012, 03:39 PM   #5
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C:

If you use the lens you have, but stick in the 70mm focal length or above AND shoot wide open, then you should start getting some useful DoF. If you shoot close focus (e.g. macro) your DoF will be even less. The nice thing about digital is that you can play and experiment as much as you want with no real cost (unlike the film days). Also, to help you understand what's happening use the DoF preview switch on the camera (the on / off switch, push in the direction of "ON" to activate this... see your manual).

Getting new lenses (glass) is also an option and depending on what you want to do, may not need to break the bank! Look for a nice 50mm f1.7 or f2 in Pentax a mount. You lose auto focus, but that is about it. You have similar options, but as a construction friend of mine says, "there is no such thing as construction problems, only money problems".

Regards,
03-07-2012, 04:41 PM   #6
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The IQ of the 18-135 leaves a lot to be desired. I think you would get more joy from a tamron 17-50 0r 24-70 2.8
A huge boost in image quality and pretty nice bokeh at a reasonable price.
03-07-2012, 07:11 PM   #7
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QuoteOriginally posted by westmill Quote
The IQ of the 18-135 leaves a lot to be desired. I think you would get more joy from a tamron 17-50 0r 24-70 2.8
A huge boost in image quality and pretty nice bokeh at a reasonable price.
But since you don't have that on hand, I would suggest this as well

QuoteOriginally posted by westmill Quote
Use the longer end of your zoom wide open at F5.6 ! It will work better if your subject is close and foreground further back.
Unfortunately F5.6 is not great for this but its poss to get reasonable results.
You need a fast lens to do it well. Genraly at least an F2.8.
Get the subject as close as you can to the minimum focus distance of the lens and try to work in an area without too many things behind your subject and you may get a better result.
03-07-2012, 08:43 PM   #8
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Here are the DOF guidelines:

For thinner DOF use a longer focal length and/or wider aperture and/or closer lens-subject distance.

For thicker DOF use a shorter focal length and/or tighter aperture and/or further lens-subject distance.

So a fast long 135/2.5 lens at 2m has MUCH thinner DOF than a slow wide 21/5.6 lens at 10m.

There are ways to trick a slow lens into producing thin DOF. Simplest is to use a cheap +1 dioptre closeup adapter, the kind that comes in sets of +1+2+4 dpt for under US$20 new, cheaper used. With a +1dpt optic screwed into any host lens, focus range narrows to about 20-40in / 50-100cm and DOF gets pretty thin, even with an f/5.6 lens. This approach leaves you with the auto functions (focus and aperture) but diminishes IQ slightly, mostly at the image edges. But you don't care about the edges, right?

A cleaner but clumsier trick is to add the thinnest possible extension, such as a cheap macro tube set (under US$10 new). But since your DA lens lacks an aperture ring, you'd need an expensive A-type tube set. Hay, it's only money, eh? I use this approach mostly with M42 lenses that lack all automation anyway. Just a little extension can pull-in the focus range significantly, with no loss of IQ.

03-07-2012, 10:28 PM   #9
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QuoteOriginally posted by RioRico Quote
Here are the DOF guidelines:

For thinner DOF use a longer focal length and/or wider aperture and/or closer lens-subject distance.

For thicker DOF use a shorter focal length and/or tighter aperture and/or further lens-subject distance.

So a fast long 135/2.5 lens at 2m has MUCH thinner DOF than a slow wide 21/5.6 lens at 10m.

There are ways to trick a slow lens into producing thin DOF. Simplest is to use a cheap +1 dioptre closeup adapter, the kind that comes in sets of +1+2+4 dpt for under US$20 new, cheaper used. With a +1dpt optic screwed into any host lens, focus range narrows to about 20-40in / 50-100cm and DOF gets pretty thin, even with an f/5.6 lens. This approach leaves you with the auto functions (focus and aperture) but diminishes IQ slightly, mostly at the image edges. But you don't care about the edges, right?

A cleaner but clumsier trick is to add the thinnest possible extension, such as a cheap macro tube set (under US$10 new). But since your DA lens lacks an aperture ring, you'd need an expensive A-type tube set. Hay, it's only money, eh? I use this approach mostly with M42 lenses that lack all automation anyway. Just a little extension can pull-in the focus range significantly, with no loss of IQ.
Great tips! Another option is simply use Photoshop to create that boked blur
03-07-2012, 11:25 PM   #10
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QuoteOriginally posted by ducdao Quote
Great tips! Another option is simply use Photoshop to create that boked blur
Every example of that that I've seen looks very fake. It's much better to shoot properly in the style one is going for.

@OP, you don't need a really fast lens, if you can set up your shot how you want. A fast lens will be more versatile though, and they're often a better quality. Have the subject very close to the lens with a more distant background, and you can get good subject isolation even at f./8 or smaller.

This I took at f./11 with my 70-300mm Sigma zoom (a cheapie). It would have been better with an extra bit of DoF on the subject, but it gets the point across that smaller apertures can work for this. The dragonfly was at the minimum focusing distance of the lens, or very close to it. The background is made up of yellow flowers and various weeds, perhaps 20 feet away.

03-08-2012, 03:11 AM   #11
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QuoteOriginally posted by consigliere Quote
I was already afraid that this all purpose lens is not the right tool for what i want.
That depends on what you want But I doubt your statement is true.

Go to Online Depth of Field Calculator and fill in the numbers for the 18-135
Focal length 135mm, aperture f/5.6 and 40 cm subject distance and your DOF is from about 1mm in front of the subject to 1mm behind the subject. How much shallower do you want it?

Focal length 18mm, aperture f/3.6 (3.5 is not an option in the program) and 40cm subject distance and your DOF is from about 31mm in front to 37mm behind; not as impressive maybe, but still good enough for plenty situations (e.g. flower 'closeup' with blurred bushes in the background).
03-08-2012, 10:07 AM   #12
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I thought of this question early on, but did not post it, what is the subject you are shooting? This will make a difference in the amount of DoF you can obtain (e.g., more with close-ups than with landscapes).

Regards,
03-08-2012, 10:08 AM   #13
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QuoteOriginally posted by sterretje Quote
How much shallower do you want it?
On an 18mm+ lens, almost any aperture setting below the diffraction limit is going to have pretty thin DOF when the subject is under 50cm distant. (But I have a real hard time getting OOF areas at any distance when using the Zenitar 16/2.8.)

Closing the focus range is a sure way to thin the DOF substantially. That's why I'll do portrait or other close shoots with an M42 lens in the 50-90mm range using a cheap safe thick-flange no-infinity-focus M42-PK adapter and maybe a thin macro tube, because for close work I don't care about infinity.
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