Focal length / DOF question

T

tylerrad

Active member
I'm trying to find out what focal length lens I'll need to rigidly separate background and foreground elements in an image -- i.e. to render the in-focus foreground subject as one plane, and flatten (and render completely out of focus) the background, so it seems almost as if the image is comprised of two pieces of paper set against one another. I've provided a screenshot to help illustrate what I'm looking for (you know the movie): http://img510.imageshack.us/img510/7994/mdnw.jpg

Obviously, I'll need a long lens, but how long? 105mm, 135mm -- or are we talking something like 300mm? If I got a 105mm lens, shot wide open (f/2.5 or 2.8), would that achieve the desired effect? How similar would that shot be to a 300mm lens shot at f/4 (or f/2.8) -- light and everything else being equal?

Curious to hear people's thoughts. I'd like to buy a shorter focal length lens if it will work, because I know I'd never use a 300mm or 200mm lens.

(Cheap) lens suggestions are welcome, too.
 
You don't have to have a long lens to get the effect shown in your example. A 50mm or even a 35mm in the f/2.0 to 1.4 range will work quite nicely.

Here is a website that you can input the focal length of your lens, f/stop and focus distance and it will tell you the total DOF. This can be very handy in a number of scenarios. You can even get the info on your iphone for free.
www.DOFMaster.com
 
You most certainly would not need a super-telephoto lens to pull this off; it's standard depth-of-field practice.

Three things affect depth-of-field:

1) Aperture size (f-stop)
2) Lens focal length
3) Camera-to-subject distance

In the screengrab you posted, the lens is probably opened up wide (2.8 and wider) and the camera is very close to the talent.

A simple 50mm lens under f2 could pull this off. Get close to your subject and open up the lens and you will achieve razor-thin focus on the subject and blur the background beyond recognition (obviously, camera-to-background distance plays a part in the level of blur, but that still falls under #3).

You would not need a telephoto lens, although it would only save you from having to move the camera close to the subject.

Hope that helps.
 
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