# Focal Distance: furthest possible maintaining blurred BG



## SwissBear (Nov 23, 2012)

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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## Ryan708 (Nov 23, 2012)

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


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## neuroanatomist (Nov 23, 2012)

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 (Nov 25, 2012)

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 (Nov 25, 2012)

SwissBear said:


> 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 (Nov 25, 2012)

Ryan708 said:


> 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.)


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## TrumpetPower! (Nov 25, 2012)

PavelR said:


> Ryan708 said:
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> > My guess would be the EF-800mm F/5.6
> ...



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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## Meh (Nov 25, 2012)

PavelR said:


> Ryan708 said:
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> > My guess would be the EF-800mm F/5.6
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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.


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## Meh (Nov 25, 2012)

neuroanatomist said:


> Meh said:
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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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## PackLight (Nov 25, 2012)

SwissBear said:


> 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".
> 
> ...



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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## neuroanatomist (Nov 25, 2012)

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 (Nov 25, 2012)

Of course, the size of the object is not the only variable - the other is the lens.


neuroanatomist said:


> ...
> 
> 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.
> 
> ...



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 (Nov 25, 2012)

SwissBear said:


> 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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## PavelR (Nov 25, 2012)

Meh said:


> PavelR said:
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> > Ryan708 said:
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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.


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## PavelR (Nov 25, 2012)

TrumpetPower! said:


> PavelR said:
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> > Ryan708 said:
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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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## PavelR (Nov 25, 2012)

privatebydesign said:


> ---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...


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## PavelR (Nov 25, 2012)

privatebydesign said:


> 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).


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## dr croubie (Nov 25, 2012)

SwissBear said:


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



Short answer: this one


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## neuroanatomist (Nov 25, 2012)

PavelR said:


> privatebydesign said:
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> > 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.
> ...



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.


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## PavelR (Nov 25, 2012)

neuroanatomist said:


> 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"?


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## Meh (Nov 25, 2012)

PavelR said:


> privatebydesign said:
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> > 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.
> ...



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


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## PavelR (Nov 25, 2012)

Meh said:


> PavelR said:
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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.


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## PavelR (Nov 25, 2012)

privatebydesign said:


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


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## neuroanatomist (Nov 25, 2012)

PavelR said:


> neuroanatomist said:
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> > 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.
> ...


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.


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## Meh (Nov 25, 2012)

PavelR said:


> Meh said:
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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.


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## Meh (Nov 25, 2012)

neuroanatomist said:


> PavelR said:
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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).


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## PavelR (Nov 25, 2012)

neuroanatomist said:


> 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.
> 
> ...


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.
---


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## neuroanatomist (Nov 25, 2012)

PavelR said:


> 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. 



Meh said:


> 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.


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## Meh (Nov 25, 2012)

neuroanatomist said:


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



Well said Sir.


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## PavelR (Nov 25, 2012)

neuroanatomist said:


> PavelR said:
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> > 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.
> ...


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.


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## neuroanatomist (Nov 25, 2012)

PavelR said:


> 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. 



Meh said:


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



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


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## PackLight (Nov 26, 2012)

neuroanatomist said:


> PavelR said:
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> > I meant the same framing, of course.
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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.


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## risc32 (Nov 26, 2012)

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.


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## PackLight (Nov 26, 2012)

risc32 said:


> 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.


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## bycostello (Nov 26, 2012)

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


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## PeterJ (Nov 26, 2012)

bycostello said:


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


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


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## SwissBear (Nov 26, 2012)

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


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## PackLight (Nov 26, 2012)

SwissBear said:


> 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


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## moocowe (Nov 26, 2012)

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.


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## AprilForever (Nov 26, 2012)

neuroanatomist said:


> PavelR said:
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Sorry, but it 404'ed...


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## rpt (Nov 26, 2012)

AprilForever said:


> neuroanatomist said:
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Take the period out of the url and fire again.
http://toothwalker.org/optics/dof.html


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## AprilForever (Nov 26, 2012)

rpt said:


> AprilForever said:
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Thanks!


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## neuroanatomist (Nov 26, 2012)

rpt said:


> Take the period out of the url and fire again.
> http://toothwalker.org/optics/dof.html



Apologies. You can thank my iPhone's automatic punctuation...


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## Pinchers of Peril (Nov 26, 2012)

Just use a lensbaby and have all the control in the world... allbeit slightly softer images


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## yogi (Nov 26, 2012)

Im feeling dizzy and everything is getting blurry. Kidding aside, interesting subject & links. One thing you could do is use the lenses/cameras available to you and experiment on your own, unless you are trying to determine what lens you might be interested in buying to get the desired effect.


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## RobT (Nov 26, 2012)

I thought about making a new topic for this question, but its very relevant to whats going on here.

Is compression based on focal length constant per lens or does it change based on sensor size?

For example, would an 85mm lens on a crop body visually maintain the same compression when put on a FF camera even though the focal length would appear shorter?


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## dr croubie (Nov 26, 2012)

RobT said:


> For example, would an 85mm lens on a crop body visually maintain the same compression when put on a FF camera even though the focal length would appear shorter?



a) If you take a head-shot photo with 85mm on aps-c, then swap the lens to an FF body and take the same photo *from the same spot*, you'll get a wider field of view and the head will be smaller within the frame. Then if you crop the second photo, you'll end up with *exactly* the same photo as the aps-c (except less MP).

b) If you take a head-shot photo with 85mm on aps-c, then swap the lens to an FF body and *move closer so the head is the same size in the frame*, then perspective will change and you'll get more 'wide-angle' or less 'compression' or whatever you want to call it.


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## RobT (Nov 26, 2012)

Thank you!

That answers my question exactly.


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