Focal Distance: furthest possible maintaining blurred BG

[PavelR]

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 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
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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

[neuroanatomist]

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"?

[Meh]

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).

You're confusing what is in the background vs. how much that background is out of focus.

[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).

You're confusing what is in the background vs. how much that background is out of focus.

It is not confusion of those characteristics, but sum of all that make final subject isolation, which OP was asking about. There is only one place (distance from camera) which is in the perfect focus, all other things at other distances are blurred, but if you use longer focused distance or bigger F number or smaller FL the blur is smaller ~ even invisible on the final picture...
Another example: shooting straight en face with 50/2.8 both eyes are sharp pretty the same, but shot with 300/2.8 = it is easy to say which eye was focused on.

[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"?

Background blur is already well defined, you are not using it in its common context. As I said, the towers are both blurred exactly the same, below is an image where I have made the tower the same size from the 200 and 400 shots, they are exactly the same.

 You are not referring to how blurred an object is, you are referring to how big objects at different distances are in relation to each other. That is perspective, not depth of field. For a same sized subject and aperture a longer lens shot from further away will give you a bigger apparent background object, hence the illusion of it being more blurred.

You did the operation which affects DOF - you changed magnification - look at: http://en.wikipedia.org/wiki/Depth_of_field

[neuroanatomist]

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"?

If you crop just the tower from both images, and view them at the same size on the screen, they will look identical.  That means the blur is identical. 

Examples from http://toothwalker.org/optics/dof.html.

100mm f/4:


28mm f/4:


The car to the right of the subject has more blur with the 100mm lens than with the 28mm lens, right? 

 

Wrong. Same blur. That's the case even in your extreme example.

I'm not saying it looks the same, the tower and car do look more blurred with the longer focal lengths. Like I said, it's good illusion.

[Meh]

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).

You're confusing what is in the background vs. how much that background is out of focus.

It is not confusion of those characteristics, but sum of all that make final subject isolation, which OP was asking about. There is only one place (distance from camera) which is in the perfect focus, all other things at other distances are blurred, but if you use longer focused distance or bigger F number or smaller FL the blur is smaller ~ even invisible on the final picture...
Another example: shooting straight en face with 50/2.8 both eyes are sharp pretty the same, but shot with 300/2.8 = it is easy to say which eye was focused on.

Clearly, you are highly confident in your belief and seem not able to accept the very well reasoned explanations that have been provided.  So, carry on and enjoy.

[Meh]

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"?

If you crop just the tower from both images, and view them at the same size on the screen, they will look identical.  That means the blur is identical. 

Examples from http://toothwalker.org/optics/dof.html.

100mm f/4:


28mm f/4:


The car to the right of the subject has more blur with the 100mm lens than with the 28mm lens, right? 

 

Wrong. Same blur. That's the case even in your extreme example.

I'm not saying it looks the same, the tower and car do look more blurred with the longer focal lengths. Like I said, it's good illusion.

It always has and always will be difficult to convince people that what they think they see is not in fact true (even with explanations and examples).

[PavelR]

If you crop just the tower from both images, and view them at the same size on the screen, they will look identical.  That means the blur is identical. 

Examples from http://toothwalker.org/optics/dof.html.

100mm f/4:


28mm f/4:


The car to the right of the subject has more blur with the 100mm lens than with the 28mm lens, right? 

 

Wrong. Same blur. That's the case even in your extreme example.

I'm not saying it looks the same, the tower and car do look more blurred with the longer focal lengths. Like I said, it's good illusion.

Thx for the link - there is deep explanation we all agree with (I suppose).
And our different view on the last posts is nicely summed up by paragraph:
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This is true when we speak of the absolute blur. The absolute blur is given by the blur disk diameter of a point in the background, such as the highlight reflections off the cars in the street. However, when we speak of the relative blur we must relate the blur disk size to the "image magnification" of the background. And that magnification is larger with the 100-mm lens too. As a matter of fact, the relative blur of the backgrounds is identical.
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[neuroanatomist]

Another example: shooting straight en face with 50/2.8 both eyes are sharp pretty the same, but shot with 300/2.8 = it is easy to say which eye was focused on.

Sure it is, if you are shooting at the same distance - in that case, if the face fills the 300mm frame, it's only a small portion of the 50mm shot, and if it fills the 50mm frame then your 300mm shot is only showing the eyes (or would be, if you weren't closer than the MFD of the lens). But if you're 6 times further away at 300mm compared to 50mm, so the framing is the same, then the DoF is the same. 

It always has and always will be difficult to convince people that what they think they see is not in fact true (even with explanations and examples).

Through the filter of our own misconceptions, everyone else's statements are part of the blurry background.

[Meh]

Through the filter of our own misconceptions, everyone else's statements are part of the blurry background.

Well said Sir.

[PavelR]

Another example: shooting straight en face with 50/2.8 both eyes are sharp pretty the same, but shot with 300/2.8 = it is easy to say which eye was focused on.

Sure it is, if you are shooting at the same distance - in that case, if the face fills the 300mm frame, it's only a small portion of the 50mm shot, and if it fills the 50mm frame then your 300mm shot is only showing the eyes (or would be, if you weren't closer than the MFD of the lens). But if you're 6 times further away at 300mm compared to 50mm, so the framing is the same, then the DoF is the same. 

It always has and always will be difficult to convince people that what they think they see is not in fact true (even with explanations and examples).

Through the filter of our own misconceptions, everyone else's statements are part of the blurry background.

I meant the same framing, of course.

My opinion was determined by looking on my photos, thus your simplified theories are not able to reverse my mind.