Focal Distance: furthest possible maintaining blurred BG

[SwissBear]

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.

 

[Ryan708]

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

 

[Loswr]

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?

 

[SwissBear]

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...

 

[Loswr]

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?

 

[PavelR]

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!]

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&

 

[Meh]

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.

 

[Meh]

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.

 

[PackLight]

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?

 

[Loswr]

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

 

[SwissBear]

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?

 

[Loswr]

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

 

[PavelR]

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.

 

[PavelR]

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.)

 

[PavelR]

---CUT---
This means, as has been said, that 600 f4 has exactly the same dof as a 17mm @ f4 if the subject is the same size in both frames. The images look quite different, but that is because of perspective, this series of images might help. http://www.luminous-landscape.com/tutorials/dof2.shtml
---CUT---

The images of FL 200mm and 400mm in the link prove my point of view - blur of tower is much bigger at 400mm.
+ I did lots of shots within the 50mm-300mm range and I know there is no way to get any comparable background blur if it is used 50/2.8 and 200/2.8 for the shots with the same framing...

 

[PavelR]

No, look again, the actual blur of the tower is the same, BUT the tower is much bigger with the 400, you are looking at the effects of perspective in those shots NOT depth of field.

Yes - the tower is bigger and blurred part is bigger too - thus better blur in my eyes.
In the extreme smallness, the tower will be one pixel width -> perfectly "in focus" = the worst background blur possible, for me...
Opposite extreme: the tower fills whole background = one gray color around the subject = the most blurred background (little boring thus I do not call this "the best" though still good example of the biggest isolation).

 

[dr croubie]

Hi!
I was wondering with which lens (on FF/crop) one gets the largest object still isolated from the background.

Short answer: this one

 

[Loswr]

No, look again, the actual blur of the tower is the same, BUT the tower is much bigger with the 400, you are looking at the effects of perspective in those shots NOT depth of field.

Yes - the tower is bigger and blurred part is bigger too - thus better blur in my eyes.

Exactly. Your eyes are being fooled. The blur is the same. That it doesn't look that way is an illusion. But...it's a good illusion.

 

[PavelR]

Exactly. Your eyes are being fooled. The blur is the same. That it doesn't look that way is an illusion. But...it's a good illusion.

I think that we need to define "background blur" now, because it does not look like the same COC at 200mm and 400mm.
And what about my extreme examples in the previous post? Do you still call it: "my eyes are fooled"?