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Old Mar 10, 2007, 9:09 AM   #1
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I have a K100D with the 18-55 kit lens. It is my first dSLR and the firstcamera I have owned thatI am able tomanipulate DOF with. I am finding that to get any DOF, I have to zoom all the way to 55mm and the subject has to have atleast 3 feet behind them for the background to be blurred out. I understand that to create a shallow DOF, you want to open up the aperatureand at 55mm on this lens the aperature can only open to f5.6. When I zoom out, I can open up the aperature more, but then I don't really get much DOF at all. Does this mean thatfocal length alsois a factor in DOF?
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Old Mar 10, 2007, 9:37 AM   #2
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There are several factors that contribute to DOF:
  • Sensor size - smaller the sensor, deeper the DOF - thus the reason digicams don't do well at producing shallow dof[/*]
  • aperture - as you mentioned[/*]
  • focal length[/*]
  • distance to subject
[/*]
These are the factors that control DOF. Additionally, while the DOF remains the same, the distance between your subject and the background will control how much the background is blurred. For example, assume 4 feet of DOF - a tree 3 feet behind your subject will be in focus but 30 feet behind it will be blurred. In both cases the DOF is still 4 feet.

Here is a site with a nice calculator so you can see the affects aperture, focal length and distance to subject have on dof:

http://www.dofmaster.com/dofjs.html


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Old Mar 10, 2007, 10:23 AM   #3
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Frame tighter (go for a head shot versus a full length shot) for a better chance of getting the look you want (and as JohnG mentioned, put more distance between your subject and the background).

You're probably better off leaving it zoomed in (even though you only have f/5.6 available), since the different distance to your subject using a longer focal length for the same framing gives you a perspective that compresses the background more (which can help with the illusion of a shallower depth of field).

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Old Mar 10, 2007, 10:04 PM   #4
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With f/5.6 at 55mm, your not going to get a lot of DoF to play with.

Maybe you should consider another lens. For portraits, the Pentax 55mm f/1.4 would be a good choice for the K100D, and it's reasonably priced. There's also the Pentax 35mm f/2.0, 28-105mm f/3.2-4.5, and 100mm f/3.5, the Sigma 50mm f/2.8, 105mm f/2.8, 70mm f/2.8, and24-70mm f/2.8, and the Tamron 90mm f/2.8, and 28-75mm f/2.8.

Any of those will give you a lot more (less?) DoF for portraits.
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Old Mar 11, 2007, 5:52 AM   #5
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Thanks for your suggestions guys!

JohnG, I like that DOF site you suggested, thanks!

JimC, I have found that framing tighter does help out alot.

TCav, I'm looking into getting the Tamron 28-75 f2.8.



Still, I am very impressed with the capabilities of this camera over my Canon A520 PS camera. Its amazing what I can do with this camera that I couldn't do before.
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Old Mar 27, 2007, 4:50 AM   #6
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Contriver wrote:
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I have a K100D with the 18-55 kit lens.* It is my first dSLR and the first*camera I have owned that*I am able to*manipulate DOF with.* I am finding that to get any DOF, I have to zoom all the way to 55mm and the subject has to have atleast 3 feet behind them for the background to be blurred out.* I understand that to create a shallow DOF, you want to open up the aperature*and at 55mm on this lens the aperature can only open to f5.6.* When I zoom out, I can open up the aperature more, but then I don't really get much DOF at all.* Does this mean that*focal length also*is a factor in DOF?
I understand the dilemma. If you find that you are not satisfied with the results you are getting with your lens, there is another fix. When in Photo Shop or a similar program go to the filter menu and choose "Blur", "Smart Blur" or "Lens Blur". When in that filter, draw the area around the subject that you wish NOT to effect and then select "Inverse" so the background is selected. Play with the sliders till you get the desired effect. I have often used this when I have shot with an extreme wide angle lens, where pretty well every thing is in focus. Even though virtual, it can be quite convincing.

http://www.olaphoto.com/zoom/benjamin-kanarek.php
Here is an example::idea:
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Old Mar 28, 2007, 12:36 PM   #7
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dont think a 75mm lens would be much different to the 50mm lens, this was shot yesterday with my 150mm, birds where about 10 foot away

Gary


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Old Mar 28, 2007, 7:56 PM   #8
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Reanimator wrote:
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dont think a 75mm lens would be much different to the 50mm lens, this was shot yesterday with my 150mm, birds where about 10 foot away

Gary

Actually there is a lot of difference especially as the lens is twice as bright going as wide as f2.8. Lets usea subject 3m away as an example, the 55mm f5.6 lens is going to give a dof of 0.67m where as at 75mm f5.6 this is reduced to 0.35m. If you go to 75mm f2.8 then this is reduced to only 0.18m. The reason your shot does not have a very narrow dof is that it was shot at f7.1.
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Old Mar 29, 2007, 11:30 AM   #9
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The 50 1:1.4 lenses on Ebay are hard to get sometimes as they are wanted by a lot of folks. The 55 1:1.8 lenses are a little easier but for most of those you'll need the M42 to K-mount adapter and use them in manual mode. Great glass though.

Try this.......http://cgi.ebay.com/Pentax-Normal-SM...QQcmdZViewItem


Or this one...http://cgi.ebay.com/Pentax-FA-50mm-f...QQcmdZViewItem

Or this...http://cgi.ebay.com/Pentax-SMC-FA-50...QQcmdZViewItem

These are Auto focus and K-mounts but they do cost more than the old Glass!

Here is an older one with the screw mount but looks good...http://cgi.ebay.com/M42-MOUNT-1-1-8-...QQcmdZViewItem


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