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Author Topic: Focal Distance: furthest possible maintaining blurred BG  (Read 8410 times)

SwissBear

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Focal Distance: furthest possible maintaining blurred BG
« on: November 23, 2012, 08:43:24 AM »
Hi!
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.
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Focal Distance: furthest possible maintaining blurred BG
« on: November 23, 2012, 08:43:24 AM »

Ryan708

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #1 on: November 23, 2012, 08:58:01 AM »
My guess would be the EF-800mm F/5.6
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neuroanatomist

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #2 on: November 23, 2012, 09:30:09 AM »
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?
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SwissBear

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #3 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...
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neuroanatomist

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #4 on: November 25, 2012, 08:55:56 AM »
I hope this makes clearer what i want to know...

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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PavelR

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #5 on: November 25, 2012, 10:03:18 AM »
My guess would be the EF-800mm F/5.6
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!

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #6 on: November 25, 2012, 10:17:00 AM »
My guess would be the EF-800mm F/5.6
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.)

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&

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #6 on: November 25, 2012, 10:17:00 AM »

Meh

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #7 on: November 25, 2012, 11:28:30 AM »
My guess would be the EF-800mm F/5.6
FL does affect the isolation of the subject more than open [wide] aperture.

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.
« Last Edit: November 25, 2012, 11:34:42 AM by Meh »

Meh

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #8 on: November 25, 2012, 11:46:02 AM »
My guess would be the EF-800mm F/5.6
FL does affect the isolation of the subject more than open [wide] aperture.

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.

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

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.

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #9 on: November 25, 2012, 11:55:47 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...

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?

neuroanatomist

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #10 on: November 25, 2012, 12:02:30 PM »
Fair enough.  ;)

My succinct and precise statement is a good general rule, although it relies on a couple of assumptions which break down in some scenarios.

Here's a good read: http://toothwalker.org/optics/dof.html
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SwissBear

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #11 on: November 25, 2012, 01:41:09 PM »
Of course, the size of the object is not the only variable - the other is the lens.

...

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. 

...

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

This actually answers my question and also makes sense.
So, Rule of Thumb: the faster the Lens, the bigger an object can be maintaining some blurred background

Now for the "why": I enjoy OOF and i want to know the limitations/dependencies of it. (and i like numbers - might be the case why i study maths)

Thank you for all the answers :)

Now that the test parameters are fixed, whats the maximum working distance with a 85f/1.2 maintaining mackground blur?
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neuroanatomist

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #12 on: November 25, 2012, 02:16:08 PM »
Now that the test parameters are fixed, whats the maximum working distance with a 85f/1.2 maintaining mackground blur?


That one is going to depend on the physical separation between subject and background.

If you run Windows, you might try Bob Atkins background blur calculator:

http://www.bobatkins.com/photography/technical/bokeh_background_blur.html
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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #12 on: November 25, 2012, 02:16:08 PM »

PavelR

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #13 on: November 25, 2012, 02:40:18 PM »
My guess would be the EF-800mm F/5.6
FL does affect the isolation of the subject more than open [wide] aperture.

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.
I do not know math behind the scene, but you hardly find the real aperture size of any lens with selected F stop. Thus the real aperture size is not attribute I can use to choose lens to shoot with. I know subject isolation ~ DOF of my lenses and longer FL isolate the subject more than shorter; even if I need to step back to maintain the framing & prolong focus distance.
« Last Edit: November 25, 2012, 04:42:23 PM by PavelR »

PavelR

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #14 on: November 25, 2012, 02:59:13 PM »
My guess would be the EF-800mm F/5.6
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.)

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&
Hmm, than the winner should be Sigma 200-500/2.8.
I did not take into account TC because it also blur subject :-D
(I do not have the experience of a difference between magnification by crop of a sensor [narrower DOF] and  prolonged focus distance [wider DOF]. I have only crop bodies 1.5x and 1.3x.)

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Re: Focal Distance: furthest possible maintaining blurred BG
« Reply #14 on: November 25, 2012, 02:59:13 PM »