Focal Distance: furthest possible maintaining blurred BG

[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

 

[Loswr]

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:
---
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.
---

 

[Loswr]

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.

 

[Loswr]

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.

Obviously. Misinformed opinion trumps fact every day of the week, in one's own mind if not in the real world.

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

Well said Sir.

Why, thank you! You were, of course, quite correct. Don Quixote notwithstanding, the windmill will remain in this case. I'm out.

 

[PackLight]

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.

Obviously. Misinformed opinion trumps fact every day of the week, in one's own mind if not in the real world.

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

Well said Sir.

Why, thank you! You were, of course, quite correct. Don Quixote notwithstanding, the windmill will remain in this case. I'm out.

Amazingly the entire thread was blurry from the start. Unless the OP had provided some definition of what "isolate" meant. We can discuss at what point equal lenses have equal DOF and similar Blur but unless you can define at what point an item reaches "isolation" it is all meaningless.

Using the common definition of isolate which means "alone" one could say in the picture with the tower that was used as an example the original subject could have not been considered "isolated" as it was still apparent in the background.

 

[risc32]

When i started reading the thread i wasn't understanding the OP. I figured it perhaps just over my head, but the terminology seemed a bit funny. Then i got a better understanding of what he was asking and promptly agreed with neuroanatomist and his side. This might be a hard thing for some to get their head around.

 

[PackLight]

When i started reading the thread i wasn't understanding the OP. I figured it perhaps just over my head, but the terminology seemed a bit funny. Then i got a better understanding of what he was asking and promptly agreed with neuroanatomist and his side. This might be a hard thing for some to get their head around.

Yes Kudos to Neuro for answering an otherwise vague question.

I don't think it is a hard thing for someone to get their "head" around, as much as it is a scenario that would very seldom come in to play. In very few instances would the comparison of two lenses come up like this. I have heard this discussion on a forum at least one other time. But since you have a different FOV for your background you will never be comparing identical pictures. Usually a person thinks about proper framing first.

I suppose if you were big time and had a shoot coming up, knew the location you were going to and what type of bokeh you were shooting for it would help in determining which lenses to take along. Personally I don't have that problem, all my lenses fit in a few bags I just throw them all in and go. Neuro might have a little more difficulty and have to think about how much of his kit to take along.

 

[bycostello]

biggest focal length as close as you can be to the subject

 

[PeterJ]

biggest focal length as close as you can be to the subject

Probably an MP-E 65 then, or would the 180mm be better?

 

[SwissBear]

Well, my question was quite blurry - that might be due to the fact that i'm neither native speaker nor have a vast vocab in this rather specific section of technical english.

But "to get better with my english", what could be a better wording for it?

Thank you again for all the answers. as it seems, this topic isnt quite easily understood

 

[PackLight]

Well, my question was quite blurry - that might be due to the fact that i'm neither native speaker nor have a vast vocab in this rather specific section of technical english.

Could be,

But the answer was for a specific lens, after your clarifications it would be this one;

http://www.lensrentals.com/rent/canon/lenses/normal-range/canon-50mm-f1.0l

 

[moocowe]

I'm probably way off here, but could the TS-E 90mm be a viable alternative to the 85mm f/1.2 for subject isolation since it is closer focusing and can be tilted for selective focus?

I notice the original post says "without tilting", but I'm curious.

 

[AprilForever]

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.

Sorry, but it 404'ed...