a VERY basic lens question

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tikigod19

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Hi all,

I'm a video guy who's considering a DSLR for video as well as photography. I've researched cameras and am in the process of deciding (probably a 550d, 7d, or even 5d II, depending on what the panasonic GH2 is like) however I'm confused when it comes to lenses...

I've been recommended to get the 18-55/2.8 however there are obviously a lot of cheaper f3.5 lenses.

Now, if I stood say 5m away from a subject and zoomed into say 40mm at f2.8, would I get a better depth of field than if I was zoomed into 70mm on an f3.5 lens? I'll certainly be getting a prime with it but need this very basic question answered before I can start weighing up my more general purpose lens which I'd like to go quite wide, but doesnt need a huge zoom.

I'm aware that the sensor crop on the 550d and 7d will also come into play but if someone could answer the above question regarding simply getting further away and zooming in more to create a blurry background that would be great!
 
The bigger the f number the more depth of field you have.

e.g. f8 has more depth of field than f2.8.


I'm sure some more technical person can explain it better will be a long shortly, but that's my quicky answer
 
Unfortunately, depth of field is governed by aperture, focal length AND subject distance (obviously, you'd need to be further back for the same framing on the 70mm as the 40mm...). Have a play with an online DoF calculator such as: http://www.dofmaster.com/dofjs.html
 
Consider comparing apples with apples I think.

If you had an item at a fixed distance, with the following two lenses on the same camera.
50mm prime, at f/1.8
or the more expensive
50mm prime, at f/1.4

Then your depth of field on the 1.8 will be deeper (i.e. acceptable focus far point minus acceptable focus near point will be larger) than the 1.4 lens.
However, the 1.4 lens set at 1.8 will be identical.
At 1.4, there will be more light hitting the sensor, so lower ISO or 'faster' 'shutter' speed could be achieved.

Now, if the subject was focused further away, /on the same lens and settings/, the depth of field of both lenses would increase.

If you were to use a lens like the 28-135, and force the aperture to be f/5.6 (for example),
At 40mm, the deepth would be larger than at 120mm


If you are shooting films and want the blurry background, it is worth trying to arrange the subjects such that the background is just further away. Having a couple say 2 ft from a wall is not going to look amazing unless you get to a really small aperture. For some reason, to me at least, movies are just slightly less blurry than standard shots. Also, consider that on some of the longer lenses (70-300mm if that is a consideration), that there is a minimum focusing distance (1.5m on that lens) which can make things more difficult (depending on whether you stage your movies, or whether they are off-the-cuff and can't be reshot because focus couldn't happen)
 
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thanks to all for your very helpful answers.
The depth of field calculator was particularly helpful and has shown me that a wider prime lens will not get the same shallow depth of field as say a 50mm if positioned the same distance from the subject. Just what I wanted to find out.

Thanks again
 
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