Does standing further back & zooming in increase the acceptable focus zone?

paul_b

Senior Member
- Suppose you have your subject at point A, and you and camera are at point B
- Lets invent 2 situations, each with an imaginary distance between points A & B:
- Situation 1: It has 10 meters between point A & B and NO camera zoom
- Situation 2: It has 20 meters between point A & B with the camera zoomed in to get a same composition as situation 1
- In both situations the photographer has pre manually focused on point A
- Now lets say in both situations a subject runs through point A and then a further 1 meter towards point B causing a loss of focusing accuracy.
- In theory: in situation 1 the focusing point is then 10% out, ie 1 meter error from a distance of 10 meters
- In situation 2 the focusing point is only 5% out, ie 1 meter error from a distance of 20 meters.

Therefore I'm imagining the focusing area would be less critical if you stood further back and zoomed in more.

Does that makes any sense? Would I be right in my assumption?
 

480sparky

Senior Member
If you're referring to depth of field, no. Moving back and zooming in (or moving closer and zooming out) will not change it unless you change the aperture.
 

WayneF

Senior Member
Therefore I'm imagining the focusing area would be less critical if you stood further back and zoomed in more.

Almost, but not necessarily exactly equal DOF.

A 2x greater distance increases DOF.
For same subject size, that requires 2x longer focal length, which decreases DOF.

But the math formula is complicated, and we cannot assume exactly equal and opposite effects.
But it is close.

There is an old rule of thumb that says any two pictures showing the same subject size will have the same depth of field. And again, pretty close, however, perspective of where you stand to do this changes things, which means it sort of depends on which depth into the picture we examine. But to a superficial glance, it seems very true.
 

BackdoorArts

Senior Member
Nothing seems right in this thread. I've reread it a few times and I just can't wrap my head around it, so I'm just going to try and explain it fresh.

When moving back from 10 meters to 20 meters and zooming you're increasing the focal length of the lens. With all other things being equal (i.e. aperture), the DoF at the subject will have decreased. Not only this, the overall composition will have changed even if the size and position of the subject is identical.

Take a look at this and you can see both the narrowing of the DoF and the change in composition as the photographer steps back and zooms in.

Exploring How Focal Length Affects Images
 
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WayneF

Senior Member
Nothing seems right in this thread. I've reread it a few times and I just can't wrap my head around it, so I'm just going to try and explain it fresh.

When moving back from 10 meters to 20 meters and zooming you're increasing the focal length of the lens. With all other things being equal (i.e. aperture), the DoF at the subject will have decreased. Not only this, the overall composition will have changed even if the size and position of the subject is identical.

Take a look at this and you can see both the narrowing of the DoF and the change in composition as the photographer steps back and zooms in.

Exploring How Focal Length Affects Images


I'll try to help then.

Yes, where we stand does affect the perspective of the overall scene, from the different viewpoint of where we stand to look. This makes discussing images with the "same subject size" be vague and iffy. :)

But computed DOF depends on four factors:
numeric f/stop number
focal length
focused distance
and sensor size.

You are ignoring and leaving out distance. But (for same size subject) if we use a 2x longer lens, then we have to stand back 2x farther, which are opposite effects, largely canceling each other out, DOF wise (same subject size too). All other things are not equal.

Check any DOF calculator, a good one is
Online Depth of Field Calculator

At the defaults at which it first comes up, it shows the DOF at 10 feet has an overall DOF range of 6.61 feet.

Change 10 feet to 20 feet and DOF range changes to 38.5 feet (a very large difference).

But then also change the default 55mm focal length to 110 mm (for same subject size), and the range is back to 6.15 feet.

Not precisely opposite and equal (degree will vary with the numbers), but very nearly so. The percentage in front and in back are still more different than the sum of the range. The hyperfocal distance is quite different. So it's pretty hard to argue "same DOF", but it is vaguely ballpark close, might satisfy a quick check in some cases.

And yes, of course the perspective due to where we stand is different too.

Focal length matters.

Distance also matters. One of four factors that matter.
 
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Stoshowicz

Senior Member
Is this correct.... that , While the girls size in the frame stays constant , the bridge certainly appears to be clearer for the short focal length, but its not actually, it just 'looks' sharper and farther away in the short focal length presentation and so You cant actually see the rivets in either photo?
SO In the longer focal length image , its as if you cropped the bridge blew it up and superimposed a relatively smaller girl on it. This being the effect of the narrowed field of view? So if you DO want to see the rivets , one needs to narrow the aperture and change the actual real world depth of that which is focus.
 
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WayneF

Senior Member
So if you DO want to see the rivets , one needs to narrow the aperture and change the actual real world depth of that which is focus.

That which you call focus is called Depth of Field (the distance zone in which the scene appears sharp to us). Narrow Aperture does increase DOF, as does shorter focal length, greater focus distance, or smaller camera sensor. These last three each alone all cause a smaller subject image. It appears sharper because it is a wider view, and thus shows smaller objects (if seen at the same enlargement), so we can't actually see the blur. Enlarge it more and we see more.

It is Not rocket science. If you show the image objects smaller, you cannot see the blur as well, and will imagine it is sharper. But enlarge it more, then the opposite.

In any picture, focus has only ONE value, focus is at only one distance, period. But scenes have more depth, both behind and in front of this point of focus. All those other distances are simply NOT in focus. There is only one point of focus.

But the photo may or may not still look acceptable "focus".

A star seen in a telescope is a point source, it has no actual diameter seen. If we assume imaginary point sources in our photo, tiny dimensionless points, being not in focus makes that infinitesimal point appear larger, seen as an unfocused blob, but larger. This visible blob diameter seen in the lens can be computed, and is called Circle Of Confusion (CoC, see Wikipedia).

Given a specified degree of viewing magnification (standard viewing for DOF is assumed to be an 8x10 inch print viewed at 10 inches), then too-small CoC cannot be seen as blobs (seen still as a point). But larger CoC we can see as a blurred image, not sharp. What our human eye can see there is judged to be a limit for CoC (again, enlarge it more, and we can see more).

Then Depth of Field (DOF) computes the zone or distance range that will compute the CoC large enough to be visible (at the standard viewing magnification), and we call that distance range zone DOF. Scene parts within that DOF zone are imagined sharp, and those outside it are seen as blurred. It is relative however, and a very gradual change with distance. There is still only one distance in focus, and areas slightly outside or slightly inside this DOF zone are hardly different.

DOF calculations are simply a mathematical limit of the numbers. Saying for example, if our CoC limit is 0.03 mm, then 0.02999 is inside it, and 0.03001 is outside of it, but there is no actual visible difference between them at that point. But DOF draws a line there (assuming the standard viewing magnification). And certainly 0.01 is much better, and 0.05 is much worse.

So regarding DOF, definitions of CoC diameter that our eye can see is just someones supposed judgement call (100 years ago), and our actual viewing enlargement is always arbitrary too, so we ought NOT to expect exact numbers from DOF. But it is a good guide. The concept is much more valuable than any specific numbers from it. We certainly should know how to increase or decrease it (and position it), as desired.

If you want some point to be very sharp, then focus there (like the eye in a portrait). If you want a larger range to be pretty much sharp, almost, then you might place focus at some compromise distance to enhance the full range of interest.

See Wikiipedia

Circle of Confusion

Depth of Field.
 
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rocketman122

Senior Member
- Suppose you have your subject at point A, and you and camera are at point B
- Lets invent 2 situations, each with an imaginary distance between points A & B:
- Situation 1: It has 10 meters between point A & B and NO camera zoom
- Situation 2: It has 20 meters between point A & B with the camera zoomed in to get a same composition as situation 1
- In both situations the photographer has pre manually focused on point A
- Now lets say in both situations a subject runs through point A and then a further 1 meter towards point B causing a loss of focusing accuracy.
- In theory: in situation 1 the focusing point is then 10% out, ie 1 meter error from a distance of 10 meters
- In situation 2 the focusing point is only 5% out, ie 1 meter error from a distance of 20 meters.

Therefore I'm imagining the focusing area would be less critical if you stood further back and zoomed in more.

Does that makes any sense? Would I be right in my assumption?

first off. no it doesnt but will answer according to what I made out of it. the further you go back while zooming in, the focus is even more critical. BUT! if youre talking about the same size subject in frame.

@Waynef. It is Not rocket science - really? a guy posts a simple question and you always answer with nasa calculations. your reply confused me more than his question. dahell man..hahaha
 

Stoshowicz

Senior Member
That which you call focus is called Depth of Field (the distance zone in which the scene appears sharp to us). Narrow Aperture does increase DOF, as does shorter focal length, greater focus distance, or smaller camera sensor. These last three each alone all cause a smaller subject image. It appears sharper because it is a wider view, and thus shows smaller objects (if seen at the same enlargement), so we can't actually see the blur. Enlarge it more and we see more.

It is Not rocket science. If you show the image objects smaller, you cannot see the blur as well, and will imagine it is sharper. But enlarge it more, then the opposite.

In any picture, focus has only ONE value, focus is at only one distance, period. But scenes have more depth, both behind and in front of this point of focus. All those other distances are simply NOT in focus. There is only one point of focus.

But the photo may or may not still look acceptable "focus".

A star seen in a telescope is a point source, it has no actual diameter seen. If we assume imaginary point sources in our photo, tiny dimensionless points, being not in focus makes that infinitesimal point appear larger, seen as an unfocused blob, but larger. This visible blob diameter seen in the lens can be computed, and is called Circle Of Confusion (CoC, see Wikipedia).

Given a specified degree of viewing magnification (standard viewing for DOF is assumed to be an 8x10 inch print viewed at 10 inches), then too-small CoC cannot be seen as blobs (seen still as a point). But larger CoC we can see as a blurred image, not sharp. What our human eye can see there is judged to be a limit for CoC (again, enlarge it more, and we can see more).

Then Depth of Field (DOF) computes the zone or distance range that will compute the CoC large enough to be visible (at the standard viewing magnification), and we call that distance range zone DOF. Scene parts within that DOF zone are imagined sharp, and those outside it are seen as blurred. It is relative however, and a very gradual change with distance. There is still only one distance in focus, and areas slightly outside or slightly inside this DOF zone are hardly different.

DOF calculations are simply a mathematical limit of the numbers. Saying for example, if our CoC limit is 0.03 mm, then 0.02999 is inside it, and 0.03001 is outside of it, but there is no actual visible difference between them at that point. But DOF draws a line there (assuming the standard viewing magnification). And certainly 0.01 is much better, and 0.05 is much worse.

So regarding DOF, definitions of CoC diameter that our eye can see is just someones supposed judgement call (100 years ago), and our actual viewing enlargement is always arbitrary too, so we ought NOT to expect exact numbers from DOF. But it is a good guide. The concept is much more valuable than any specific numbers from it. We certainly should know how to increase or decrease it (and position it), as desired.

If you want some point to be very sharp, then focus there (like the eye in a portrait). If you want a larger range to be pretty much sharp, almost, then you might place focus at some compromise distance to enhance the full range of interest.

See Wikiipedia

Circle of Confusion

Depth of Field.

So the answer was 'yes' , thanks.
 

Stoshowicz

Senior Member
No, the answer is 'no'.

If you move, then zoom to the same crop of the scene as you originally had, you will have the same DOF. This assumes you use the same aperture in both shots.
I didnt say anything that doesnt fit with either Waynes Or your statement , why are you trying to disagree?
Its very annoying. The girl is held at the same relative size magnification in my example , you cant both move and zoom in with a longer focal length ,,,, and not move at the same time.

What I said /...
one needs to narrow the aperture and change the actual real world depth of that which is (in ) focus.

What you said...
you will have the same DOF. This assumes you use the same aperture in both shots


DOnt worry about it- really ! its not rocket science.
If I wanted to pick nits , Id say that in a three dimensional world with a curved lens and flat sensor, there is no single distance that is actually in focus its a hypothetical focus distance , but I would consider that childish.
 
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480sparky

Senior Member
I didnt say anything that doesnt fit with either Waynes Or your statement , why are you trying to disagree?
Its very annoying. The girl is held at the same relative size magnification in my example , you cant both move and zoom in with a longer focal length ,,,, and not move at the same time.
DOnt worry about it- really !


I guess I'm totally confused as to what exactly you are asking then.

Let's go back to your first post:

- Suppose you have your subject at point A, and you and camera are at point B
- Lets invent 2 situations, each with an imaginary distance between points A & B:
- Situation 1: It has 10 meters between point A & B and NO camera zoom
- Situation 2: It has 20 meters between point A & B with the camera zoomed in to get a same composition as situation 1
- In both situations the photographer has pre manually focused on point A
- Now lets say in both situations a subject runs through point A and then a further 1 meter towards point B causing a loss of focusing accuracy.
- In theory: in situation 1 the focusing point is then 10% out, ie 1 meter error from a distance of 10 meters
- In situation 2 the focusing point is only 5% out, ie 1 meter error from a distance of 20 meters.

Therefore I'm imagining the focusing area would be less critical if you stood further back and zoomed in more.

Does that makes any sense? Would I be right in my assumption?


You state in situation 2 that you zoom in to obtain the same composition as you had in situation 1. If that's the case, then the DOF in both situations will be identical (unless you changed your aperture).
 

WayneF

Senior Member
first off. no it doesnt but will answer according to what I made out of it. the further you go back while zooming in, the focus is even more critical. BUT! if youre talking about the same size subject in frame.

@Waynef. It is Not rocket science - really? a guy posts a simple question and you always answer with nasa calculations. your reply confused me more than his question. dahell man..hahaha

I don't know what to tell you. I guess block me on your Ignore List if it is of no use to you. I always thought questions wanted actual factual answers, not just some wild notions. :) Circle of Confusion is simply how it works, which has been known for nearly 200 years. I'd call it a basic. :) Not First Day stuff, but a basic about DOF.

Actually, the rocket science comment was instead agreeing with Stoshowicz that a wider lens does show things smaller, which does make it look sharper, since the blurred detail is far too small to see. That hardly seems like rocket science to me. :)


I frankly think the photos of the girl on the bridge are NOT achieved by standing back at greater distance. The article never stated that there was any standing back. It would have been a very appreciable distance, worthy of comment. And because it is a standard fact that standing back and zooming in to same subject size is said to "appear to be" about the same depth of field. I say "appear to" which that would, but there are details that dispute the preciseness of it. Nevertheless, "same size has same DOF" is an old notion that it is certainly ball park true. It's very easy to find it said. Sparky is saying it. :)

But this bridge DOF stuff does not come close to what real work would show if standing back for same size. There had to be a little (14 to 200mm), but it was NOT the goal. Cropping achieved the same size in that example. There is no offsetting DOF increase due to standing back. The depth of field (the ground level rivets) is clear evidence it is done by cropping, not by standing back.

Judging the field height by roughly guessing the girls height, standing back would require:

12mm 7 feet height at 5.3 feet. - DOF calculates 2.1' in front, 10' behind (using f/4 as stated)
35mm 6 feet height at 13.4 feet. - DOF calculates 2.1' in front, 3' behind
100mm 6 feet height at 38 feet. - DOF calculates 2.3' in front, 2.6' behind
200mm 6 feet height at 77 feet. - DOF calculates 2.4' in front, 2.6' behind

Then achieving same size with the greater distance increasing the DOF, and a longer lens decreasing DOF, then we would have seen approximately similar DOF. But this obviously did not happen. Not even close.

To the authors credit, he never says "standing back", he only mentions zooming. That was a false assumption made in this thread, assuming this example was "standing back". Clearly it is not. It does not claim it.

So then lens focal length does what was claimed and shown, without any opposite compensation due to standing back farther. However then the cropping introduces a significant enlargement factor difference, which is the confusion factor here. DOF is about as much about enlargement factor as any of the conventional factors. In practice, you may want to stand back more instead of cropping, to keep your image pixels.


Anyway, if it HAD BEEN standing back with a longer lens to achieve the same subject height, then DOF ought to be roughly similar. That was not the case in the example shown here.
 
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paul_b

Senior Member
It's all gone a bit advanced for me, lol

In short, if I've understood you all correctly:

If using the same camera, lens & aperture, the depth of field (focusing sweet area) remains roughly the same if you do these 2 things:
1) take a photo of point A from point B
2) then stand a lot further back and zooming in to point A to get the same frame as 1)

In conclusion, there's no focusing advantage to be had by standing further back and zooming in (your depth of field remains roughly the same).
 

BackdoorArts

Senior Member
I always thought questions wanted actual factual answers, not just some wild notions.

This is Nikonites. Don't confuse opinion and speculation by offering up facts, and for godsake never point out speculation when you see it - there are folks here that have torches and pitchforks at the ready.
 

Danno

Senior Member
I always thought questions wanted actual factual answers, not just some wild notions. :) Circle of Confusion is simply how it works, which has been known for nearly 200 years. I'd call it a basic. :) Not First Day stuff, but a basic about DOF.


Wayne, I will not claim to understand all that you documented here... it is just part of the condition my condition is in... But I did look at it as it relates to the output of the different lens I have and the results and It helped me understand the settings and things I need to consider more carefully when composing a shot, especially with my longer lens. Please do not ask me to explain... it REALLY does make my head hurt, but I do appreciate the help. I had kind of skipped over this post the other day and saw it this morning and found your remarks helpful in spite of my dain bramage :cheerful:.
 

Stoshowicz

Senior Member
I guess I'm totally confused as to what exactly you are asking then.

Let's go back to your first post:




You state in situation 2 that you zoom in to obtain the same composition as you had in situation 1. If that's the case, then the DOF in both situations will be identical (unless you changed your aperture).
That other post is Pauls , I was just checking myself, Wayne , as I read it, confirmed in a longhand way. I figure he didn't care for my wording, so he proceeded in his own.
 

Woodyg3

Senior Member
Contributor
I think this whole thing has gotten way more complicated than it needs to. @paul_b I bet you weren't expecting quite this response to your question. For practical purposes the simple answer is that the depth of field will be very close to the same, but you will change the perspective of your shot, and so the composition will be significantly different.

:)
 
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