# Does Focal Length affect DOF?



## jamesino (Aug 21, 2008)

I've searched the forums for several old posts about this issue but have not found a satisfactory answer.

Unarguably, aperture and focal distance (distance between camera and subject) affects the DOF. 

Many sites, including this one http://www.cambridgeincolour.com/tutorials/depth-of-field.htm,
claim that a longer focal length only appears to decrease the DOF than a shorter focal length, but if the subject was to occupy the same fraction of the viewfinder, the DOF would be the same.

Obviously, this would mean getting closer with the wide angle or moving farther away with the telephoto. But by doing so, you have changed one critical factor- the focal distance. You are now no longer the same distance away from the subject and since we know for sure that the DOF is affected by the focal distance, does this mean the the focal length DOES have an impact on the DOF?


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## photogincollege (Aug 22, 2008)

Yes, focal length does in fact have an affect on dof, the longer the focal length, less dof.


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## Helen B (Aug 22, 2008)

Graystar said:


> ...So the practical (and qualified) answer is when subject size and aperture are maintained, DOF remains the same for any given focal length.



One of the qualifications is that this only applies as long as all of the lenses in the comparison are being focused well in front of their hyperfocal distance. As the focal distance approaches the hyperfocal distance of the shorter lens, a shorter lens will have greater depth of field than a longer lens at the same subject magnification and f-number.

Best,
Helen


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## Garbz (Aug 22, 2008)

here's a visual example which I think shows pretty well what is going on.

http://thephotoforum.com/forum/showpost.php?p=1220311&postcount=14

but then again I posted it so I'm kinda biased on that opinion


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## ksmattfish (Aug 22, 2008)

jamesino said:


> Many sites, including this one http://www.cambridgeincolour.com/tutorials/depth-of-field.htm,
> claim that a longer focal length only appears to decrease the DOF than a shorter focal length, but if the subject was to occupy the same fraction of the viewfinder, the DOF would be the same.



Here's exactly what it says:

"If the subject occupies the same fraction of the viewfinder (constant magnification) for both a wide angle and a telephoto lens, the total depth of field is virtually* constant with focal length!  *This would of course require you to either get much closer with a wide angle lens or much further with a telephoto lens...*"

Focusing distance also effects DOF.  If you only change focal length longer focal lengths have less DOF.  If you move closer to your subject DOF is decreased.  So a long focal length coupled with a long focusing distance can have the same DOF as a short focal length and a short focusing distance.   

DOF gets pretty technical, and much of it goes over my head, but in the real world if you were to change any one of the following aspects without changing the others DOF is affected, at least if you are making normal prints and not doing some radical cropping/enlarging.  If you crop the wide angle shot to the angle of view of the telephoto shot then DOF may appear to be similar, but that's not usually how people do it.  If you change one aspect that decreases DOF, and another that increases DOF, then it may even out.

Increases DOF:  shorter focal length, longer focusing distance, smaller aperture, smaller format size.

Decreases DOF:  longer focal length, shorter focusing distance, larger aperture, larger format.

Take your digital camera out and take test shots only changing one variable at a time.  The results are obvious.


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## Village Idiot (Aug 22, 2008)

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


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