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FORUMS Photography Talk by Genre General Photography Talk 
Thread started 03 Jul 2013 (Wednesday) 15:47
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backgrund blur - same aperture, different fl, different positions

 
Romax12
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Jul 03, 2013 15:47 |  #1

hey guys,

i got the 70-200 f2.8 is, and today I was trying it with some portraits.
I wonder what will give me the most background blur/ subject seperation:
examples:
1. me--------------->subject--------------->some trees.
2. ...............me--------------->subject--------------->some trees.


lets say that the first example was me standing 20 feet away from my subject with 200mm
and the second example was me getting closer to my subject, but reducing focal length for about 135mm.
which settings will give me the most blurry background?


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--- EF-S 18-135 f3.5-5.6 IS --- EF 70-200 f2.8 L IS usm ---
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sandpiper
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Jul 03, 2013 16:38 |  #2

Well, if you are framing the subject the same, and using the same aperture, the DOF is going to be essentially the same either way. As your subject to trees distance is essentially the same, going by those examples, I doubt you would see a difference.

I would go with a third option, using 135mm in the same available space. Instead of moving closer to the subject, move the subject closer to you:

1. me--------------->subject--------------->some trees.
2. ...............me--------------->subject--------------->some trees.

3. me--------------->subject--------------->some trees

Get more distance between the subject and the trees, relative to the distance between you and the subject, for the focal length you choose.




  
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JeffreyG
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Jul 03, 2013 21:27 |  #3

For equal framing and equal aperture, the DOF will be the same.

The easiest way for me to explain this is as follows:

Imagine taking a portrait at 50mm and f/2.8 with a distance of 5 feet from the camera to the subject.

Now you can take a shot with a 200mm lens at f/2.8 with a distance of 20 feet from the camera to the subject.

Shot 1 and 2 will have the same framing
Shot 1 and 2 will have the same DOF
The perspective between the two shots will be quite different
Shot 2 will have a much blurrier background despite having the same DOF as shot 1


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goldboughtrue
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Jul 04, 2013 23:30 |  #4
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Is it possible to go outside and test your options and then see which one you like best? Just be sure to write down exactly what you're doing for each shot so you don't forget when comparing them.


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sandpiper
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Jul 05, 2013 06:21 |  #5

goldboughtrue wrote in post #16092515 (external link)
Is it possible to go outside and test your options and then see which one you like best? Just be sure to write down exactly what you're doing for each shot so you don't forget when comparing them.

This is, naturally, the best course of action.

The OP will learn far more about DOF control, and controlling background blur, by going out and experimenting and seeing the results. Answers in a forum will never give you the experience of feeling the distances involved and seeing the results. That way you learn to be an instinctive photographer, just doing what needs doing to get the result you want, because you have experience behind you.




  
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ride5000
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Jul 05, 2013 06:52 |  #6

the answer to the question depends on the distance between subject and background.


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Jul 05, 2013 12:23 |  #7

ride5000 wrote in post #16093041 (external link)
the answer to the question depends on the distance between subject and background.

^
...but since we are dealing with identical f/stop in this case (for example f/2.8), the longer focal length blurs more, regardless of background distance. The 'it depends' happens when we compare different FL with different aperture, and the shorter FL can be more blurred for 'near background' but not 'far background'

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backgrund blur - same aperture, different fl, different positions
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