# Thread: DOF and f number

1. ## DOF and f number

can a F3.5 camera give a shallow DOF say, 35mm with the object being at 1-2 meter away?

my friend says that his digicam with F2.8 cant give shallow effect, so i need to go for faster lens.

isnt this incorrect? thanks

2. Hi Bordeaux78,

The depth-of-field is not solely determined by the aperture width (F stop) but also by the Focal Length. I believe your friend digicam has a very short focal length, that's why he/she cannot get a shallow DOF effect at F2.8.

3. To add to that, I believe that digicams have a much larger DOF than film cameras, even at the same f-stop and focal length.

4. 3 factors for DOF
1) focal length - longer length, lesser depth
2) Aperture - smaller aperture, more depth
3) object distance - the nearer, the lesser depth..

there is a formula somewhere...

5. http://www.wrotniak.net/photo/dof/

Surf over to this URL to get the answer to your qns. BTW, the bit about the 5 stop rule isn't too 'objective' IMO, so take with a pinch of salt.

The formula is located at the bottom of the page.
BTW, there are more than 3 factors that affect DOF.

6. Originally posted by Lennier

The formula is located at the bottom of the page.
BTW, there are more than 3 factors that affect DOF.
the formular is as complicated as A maths.

7. Like what skf said, those are the 3 rules to remember. In addition, most digital P&S will never get good background blur. For example, the G1 has a focal length of 7-21 mm (equivalent to 35-105 in 35mm). Even at f/2.8 at the longest, your background will never be nicely blurred out.

Regards,
JK

Actual aperture size = focal length / f-stop value

hence the notation f/2.8, f/5.6 etc. Comparing a lens with, say 15mm and a lens with 150mm, with the same f-stop value, the 15mm will have a much smaller actual aperture opening which would result in deeper DOF.

In the case of digital camera, eg. the G1, since the lens is 7-21mm, at 21mm (105mm equivalent at 35mm) its aperture opening is still much smaller then its 35mm equivalent at the same f-stop, resulting in the deeper DOF.

9. wow thanks a lot for the info guys

so a 50mm prime lens can achieve shorter DOF than a 35mm one...in general..rite? also a faster lens will give shorter DOF at its max aperture as compare to a slower one at the same focal length....rite?

thanks...

chris

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