Gear Talk > Lenses

Focal Distance: furthest possible maintaining blurred BG

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PavelR:

--- Quote from: Ryan708 on November 23, 2012, 08:58:01 AM ---My guess would be the EF-800mm F/5.6

--- End quote ---
My too.
I use/d 85/1.4, 135/2, 200/2, 300/2.8 and the most blurred background I've got with longer FL.
FL does affect the isolation of the subject more than open [wide] aperture.
(Using the same framing of a human body / face.)

TrumpetPower!:

--- Quote from: PavelR on November 25, 2012, 10:03:18 AM ---
--- Quote from: Ryan708 on November 23, 2012, 08:58:01 AM ---My guess would be the EF-800mm F/5.6

--- End quote ---
My too.
I use/d 85/1.4, 135/2, 200/2, 300/2.8 and the most blurred background I've got with longer FL.
FL does affect the isolation of the subject more than open [wide] aperture.
(Using the same framing of a human body / face.)

--- End quote ---

If the two of you are guessing based on the maximum physical size of the aperture, then the winner is actually the 600 f/4. The 400 f/2.8 and the 800 f/5.6 are tied for second.

800 / 5.6 = 142.857
600 / 4 = 150
400 / 2.8 = 142.857

If you're going off focal length, then the 600 with a 1.4x teleconverter would beat the 800 without one.

At the same time, there's really not any significant difference in actual aperture size between them, just a small fraction of a stop: stop the 600 down to f/4.5 (a third of a stop, one click of the wheel) and it's now got a 133mm physical aperture.

If you were trying to decide between the three, your decision should be made primarily on focal length needs and you shouldn't give any consideration to the physical aperture. Indeed, they might all actually be the same, what with rounding and all: 600 / 142.857 = 4.2, which is closer to f/4 than f/4.5.

Cheers,

b&

Meh:

--- Quote from: PavelR on November 25, 2012, 10:03:18 AM ---
--- Quote from: Ryan708 on November 23, 2012, 08:58:01 AM ---My guess would be the EF-800mm F/5.6

--- End quote ---
FL does affect the isolation of the subject more than open [wide] aperture.

--- End quote ---

Not quite.  Contrary to popular opinion, Depth of Field is not affected by focal length only the physical size of the aperture (when maintaining subject height).  To maintain subject height in the image the subject distance must be increased as you increase focal length (in other words, you have to step back when using a longer lens).  Now, greater subject distance increases depth of field but a longer lens with the same f-number (eg. f/2.8 ) has a physically larger aperture which decreases depth of field.  It turns out the the increase of DoF due to increases subject distance is almost exactly offset by the decrease in DoF due to the physically larger aperture with the result that focal length does not affect DoF.

Meh:

--- Quote from: neuroanatomist on November 25, 2012, 11:37:28 AM ---
--- Quote from: Meh on November 25, 2012, 11:28:30 AM ---
--- Quote from: PavelR on November 25, 2012, 10:03:18 AM ---
--- Quote from: Ryan708 on November 23, 2012, 08:58:01 AM ---My guess would be the EF-800mm F/5.6

--- End quote ---
FL does affect the isolation of the subject more than open [wide] aperture.

--- End quote ---

Not quite.  Contrary to popular opinion, Depth of Field is not affected by focal length only the physical size of the aperture (when maintaining subject height).  To maintain subject height in the image the subject distance must be increased as you increase focal length (in other words, you have to step back when using a longer lens).  Now, greater subject distance increases depth of field but a longer lens with the same f-number (eg. f/2.8 ) has a physically larger aperture which decreases depth of field.  It turns out the the increase of DoF due to increases subject distance is almost exactly offset by the decrease in DoF due to the physically larger aperture with the result that focal length does not affect DoF.

--- End quote ---

As I stated...magnification and aperture, and with the subject filling the frame, just aperture (meaning f/number, in this case).

--- End quote ---

Yes, you did.  But your succinct and precise statement would not be immediately obvious to someone who didn't understand the implications and/or subscribed to the widely held belief that longer lenses have narrower DoF.   Case in point being the comment I responded to which was posted after your comment.

PackLight:

--- Quote from: SwissBear on November 25, 2012, 07:29:28 AM ---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...

--- End quote ---

No, not really. You are wanting to focus a 36mm stick and provide separation ( blur,   DOF, or whatever you want to call it) with the the lens focused at 80 percent to infinity. Are we to assume that the subject will placed at a distance 80  percent to infinity?

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