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Focal Distance: furthest possible maintaining blurred BG

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I was wondering with which lens (on FF/crop) one gets the largest object still isolated from the background.
It is probably safe to define "still isolated" as "if near focus is 0% and infinity is 100%, putting the focus to 80% (maybe 90% for macro lenses) still isolates an object at focal distance".

Of course in a single shot (no pano) with a canon camera and without any tilting, post-processing or other manipulations.

What I don't mean is the distance from the camera, but the size of object fitting the conditions above.

My guess would be the EF-800mm F/5.6

Too many independent variables here, making the question meaningless.  What object - a grain of rice, a person's face, or the Empire State Building?  What focal length?  What subject distance?  What physical distance from subject to background?

The size of the Object is the variable. For a clear definiton, it can be considered as a stick which is framed that it has 100% Image width / long side of the sensor.

The subject distance is maximised in terms of "still isolated" (as mentioned above) from the background. The Background must therefore be at "infinity".

"With the Lens XY it's possible to isolate an object with a maximum size of Z"

I hope this makes clearer what i want to know...


--- Quote from: SwissBear on November 25, 2012, 07:29:28 AM ---I hope this makes clearer what i want to know...

--- End quote ---

Sorry, still not clear (to me, that is).

The size of the object cannot be the only dependent variable. 

You state the background is at infinity.  If that's true (for example, the stick with only clear, blue sky behind it), then the lens would be determined by the length of the stick relative to your distance from it), because the subject is already physically isolated.

But...perhaps this will help: at a basic level, OOF blur is determined by magnification and aperture (ignoring exceptions like near 1:1 macro and fisheye lenses).  In your scenario, you're filling the frame with the subject, therefore magnification is not varying with focal length, although distance to subject will increase with increasing focal length to fully frame the stick.  In that case, the determinant of OOF blur amount will be aperture. Thus, the 'best' current Canon lenses will be the 50/1.2 and 85/1.2. 

So, that's the most OOF blur...but will it isolate the subject from the background?  Depends on how far the background is from the subject.  Also, a longer lens has a narrower angle of view, meaning less background in the image, and that might make it easier to find a patch of background with less detail.

Like I stated, still too many variables.

Let's back up a step - why is this important to you?  What are you trying to accomplish?


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