DOF & Focal Length


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#1
Hi all,

I'm using D90 18-55mm & 55-200mm.
Just wondering how does focal length affects DOF?
When using 18-55mm, i can't seems to have a shallow DOF but 55-200mm is ok.
Set to Aperture priority.
Anyone can advise what's my problem?
 

chiangkxv

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#2
With the same Aperture, longer focal length will give u a shallower dof (more background blur). :D that explains why u get a shallower dof with your 55-200mm.
 

desertstrike

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#3
hmm... to double checking.... get a F1.8 50mm, and no need long focal length rite? ;)
 

Jul 15, 2008
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#4
hmm... to double checking.... get a F1.8 50mm, and no need long focal length rite? ;)
Yes. If you are shooting at f1.8 - f2.8, you will still get a very shallow DOF even though the focal length is only 50mm. Do take note that for most prime lens, shooting at wide open (f1.8 in this case) will result in photos that are a bit soft. If you want very sharp photos, you should normally shoot at 2-stop above the maximum aperture but that will varies from len to len.
 

chiangkxv

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#5
DOF is determined by 3 factors:

1. Focal length (longer -> shallower DOF)
2. f-stop (smaller number -> shallower DOF)
3. Subject-lens distance (nearer -> shallower DOF)

do check out some videos on DOF avail at You-Tube.. :D
 

#6
Hi guys,

Thanks for the many replies.
18-55mm has marking of 3.5-5.6 on the lens
Does that means that the f number is 3.5-5.6?
So the 3.5 would give the shallowest DOF?

Also, anywhere on the web or any book I can read on DOF, focal length, aperture & shutter control?
e.g when to use what aperture & shutter number.
Thanks for all the many seniors here for the patience & guidance to us newbies.
 

2ichigo2

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#7
18-55mm F3.5-5.6
this means that at 18mm the largest aperture u can go is at F3.5.
And at 55mm the largest aperture u can go is at F5.6.

When u see something like this 17-50mm F2.8.
This is a constant aperture lens which is more expensive because at any focal length, the largest aperture can be at F2.8.

Here is one website that provides explaination on the photography terms:
http://www.dpreview.com/learn/?/Glossary/ read and understand more on the exposure
 

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Octarine

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#8
Also, anywhere on the web or any book I can read on DOF, focal length, aperture & shutter control?
e.g when to use what aperture & shutter number.
Thanks for all the many seniors here for the patience & guidance to us newbies.
Look, it's just right on top of all threads in Newbies Corner as a sticky one, called "Photography Notes for Newbies"
 

desertstrike

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#10
Have already read it but that only gives a generally understanding of what they are but not indetail how & when to use them.
i think 2ichigo2 answer u very well already
clear, consise and to the point
 

ahbian

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May 23, 2006
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#11
Have already read it but that only gives a generally understanding of what they are but not indetail how & when to use them.
With countless lighting conditions and the photographer's different intentions of portrayal, it is impossible to give a generic answer on when to use what.

It would be better if you provide a photo that didn't work out, then its easier for us to advise on the adjustments to the settings.
 

#12
Have already read it but that only gives a generally understanding of what they are but not indetail how & when to use them.
I suggest you try this article instead.

http://www.mir.com.my/rb/photography/fototech/apershutter/aperture.htm

http://www.mir.com.my/rb/photography/fototech/htmls/depth.html

His English is not perfect but his explanation of basic camera concepts is quite clear.

As a bonus, he uses his daughter who is quite good looking.... hee hee
:devil:
 

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Octarine

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#13
With countless lighting conditions and the photographer's different intentions of portrayal, it is impossible to give a generic answer on when to use what.
A good starting point is using P mode and see what the camera suggests. Knowing the basics you are able to adjust one parameter (e.g. aperture) to achieve a certain effect (DOF) - the camera will adjust the shutter speed to 'follow'. Adjust the ISO speed if necessary. Check the camera manual about how to set certain parameters and what they mean. Good idea to follow scientific approach: change only one parameter, leave the others untouched and watch the outcome.
 

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