Depth of field - rule of thumb?

i apologise if this question has been answered before, but i couldn't find a "simple" answer (if one exists).

my understanding is that dof is a product of three variables, lens aperture (small eq greater dof), lens focal length (short eq greater dof) and distance from subject (far eq greater dof).

my question is the relationship between focal length and distance from the subject. for a given subject, with a zoom lens you can get more or less the same composition either close-up with short (wide) focal length, or far with long (tele) focal length. in these two cases the greater dof arising from one factor is opposed by decreased dof arising from the other factor.

is there a rough rule of thumb which states which of these cases will generally give the greatest dof, or is it entirely dependent on the exact numbers (focal length:distance) in each case? or is there perhaps one rule of thumb for "near subject" shots (e.g. when the distance from the subject for both wide and tele focal lengths is say less then 2 feet or somesuch), and another when both distances are greater (than say 6 feet or somesuch)? or maybe both factors simply cancel each other out and it makes no difference?!

i hope this question is at least understandable.

thanks,
-keith

... that at the same magnification and f/ratio, the DOF is very nearly the same. You do gain working distance, however. In some cases the longer lenses will close down more (my 100-400 hits f/40!) but you pay for that with diffraction, which softens the image.

There is no way to win ... as someone else has in their tagline, compromise is another word for photography.
KP

Keith Evans said:

i apologise if this question has been answered before, but i
couldn't find a "simple" answer (if one exists).

my understanding is that dof is a product of three variables, lens
aperture (small eq greater dof), lens focal length (short eq
greater dof) and distance from subject (far eq greater dof).

my question is the relationship between focal length and distance
from the subject. for a given subject, with a zoom lens you can get
more or less the same composition either close-up with short (wide)
focal length, or far with long (tele) focal length. in these two
cases the greater dof arising from one factor is opposed by
decreased dof arising from the other factor.

is there a rough rule of thumb which states which of these cases
will generally give the greatest dof, or is it entirely dependent
on the exact numbers (focal length:distance) in each case? or is
there perhaps one rule of thumb for "near subject" shots (e.g. when
the distance from the subject for both wide and tele focal lengths
is say less then 2 feet or somesuch), and another when both
distances are greater (than say 6 feet or somesuch)? or maybe both
factors simply cancel each other out and it makes no difference?!

i hope this question is at least understandable.

thanks,
-keith

Click to expand...

--

A couple of Canon DSLRs, a nice off-white lens and some red-stripe lenses and one with a gold stripe, some misc. accessories including various photon ejectors and paper holders. A ton of film equipment from 35mm to 4 x 5. A minivan and a Fender Stratocaster. A three bedroom ranch on an acre. Also, absolutely no Canon 1200mm f/5.6.

Keith Evans said:

my question is the relationship between focal length and distance
from the subject. for a given subject, with a zoom lens you can get
more or less the same composition either close-up with short (wide)
focal length, or far with long (tele) focal length. in these two
cases the greater dof arising from one factor is opposed by
decreased dof arising from the other factor.

is there a rough rule of thumb which states which of these cases
will generally give the greatest dof, or is it entirely dependent
on the exact numbers (focal length:distance) in each case? or is
there perhaps one rule of thumb for "near subject" shots (e.g. when
the distance from the subject for both wide and tele focal lengths
is say less then 2 feet or somesuch), and another when both
distances are greater (than say 6 feet or somesuch)? or maybe both
factors simply cancel each other out and it makes no difference?!

Click to expand...

I recommend a down-loadable book for you, called "The INs and OUTs of FOCUS" by Harold M. Merklinger. It has been extremely valuable to my photography. The answer to your particular question is this: If you keep the aperture constant, then ignoring diffraction effects, your depth of field will not change provided that you keep the size in your viewfinder of the object you are focusing on the same size (by moving the camera) when you zoom. (Also, of course, the subject must not move relative to its surroundings.)

Merklinger's object-field method (chapter 5) gives an elegant way to see why. This is the link to download the book:

http://www.trenholm.org/hmmerk/download.html

My answer was given using the object-field method, which is not familiar to most photographers. Merklinger calls the traditional approach the subject-field method. Even with the subject-field approach, you get approximately the same answer, provided we are not discussing close-up photography or situations where the hyper-focal distance is small compared to the focusing distance.

When considering diffraction, it is more difficult to be sure of a clear answer to your question, because not everyone agrees on whether large diffraction effects increase or decrease depth of field. By my way of thinking, diffraction decreases the depth of field because less detail is resolved, and this implies that you get more depth of field on the far end with the telephoto when diffraction matters. But again, this requires keeping the aperture constant and keeping the size in the viewfinder of the subject that you are focusing on constant.

Your question should be fun, because I am almost sure that many will disagree with Harold and me.

--
Jerry Fusselman
Chicago

I find that the following post can be quite helpful:

http://www.dpreview.com/forums/read.asp?forum=1019&message=2824952

It is true that DoF roughly goes up by the Square of the Distance to the focus point, Down by the square of the Focal length, and Up linearly with the F-number. Thus while the Distance and Focal length have the "largest effect" (square law) they cancel each other out if you keep the subject the same size by changing the distance AND the focal length at the same time and thus the F-number gives you the DoF control.

Keith Evans said:

i apologise if this question has been answered before, but i
couldn't find a "simple" answer (if one exists).

my understanding is that dof is a product of three variables, lens
aperture (small eq greater dof), lens focal length (short eq
greater dof) and distance from subject (far eq greater dof).

my question is the relationship between focal length and distance
from the subject. for a given subject, with a zoom lens you can get
more or less the same composition either close-up with short (wide)
focal length, or far with long (tele) focal length. in these two
cases the greater dof arising from one factor is opposed by
decreased dof arising from the other factor.

is there a rough rule of thumb which states which of these cases
will generally give the greatest dof, or is it entirely dependent
on the exact numbers (focal length:distance) in each case? or is
there perhaps one rule of thumb for "near subject" shots (e.g. when
the distance from the subject for both wide and tele focal lengths
is say less then 2 feet or somesuch), and another when both
distances are greater (than say 6 feet or somesuch)? or maybe both
factors simply cancel each other out and it makes no difference?!

i hope this question is at least understandable.

thanks,
-keith

Click to expand...

Keith Evans said:

is there a rough rule of thumb which states which of these cases
will generally give the greatest dof, or is it entirely dependent
on the exact numbers (focal length:distance) in each case? or is
there perhaps one rule of thumb for "near subject" shots (e.g. when
the distance from the subject for both wide and tele focal lengths
is say less then 2 feet or somesuch), and another when both
distances are greater (than say 6 feet or somesuch)? or maybe both
factors simply cancel each other out and it makes no difference?!

Click to expand...

Two lenses, set to the same f-number, will have about the same depth of field given the following:

1. The shorter lens is focused at less than about 1/4 the hyperfocal distance for the lens.
2. The image size is held constant.

A detailed discussion of this is at: http://www.dofmaster.com/dof_imagesize.html

Of course, this rule of thumb doesn't tell you what the depth of field is. So, here's a quick way to calculate the near and far distances of depth of field when you know the hyperfocal distance:

Focus distance: H / x
Near distance: H / (x+1)
Far distance: H / (x-1)

Say, for example, the hyperfocal distance is 24 feet, and the focus distance is 1/4 the hyperfocal distance, or 6 feet:

Focus distance = 24 / 4 = 6 feet
Near distance = 24 / 5 = 4.8 feet
Far distance = 24 / 3 = 8 feet

For the same lens focused at 12 feet:

Focus distance = 24 / 2 = 12 feet
Near distance = 24 / 3 = 8 feet
Far distance = 24 / 1 = 24 feet

Or, focused at 8 feet:

Focus distance = 24 / 3 = 8 feet
Near distance = 24 / 4 = 6 feet
Far distance = 24 / 2 = 12 feet

In my opinion, depth of field scales are the only way to quickly calculate depth of field when I'm out shooting pictures. Because useful depth of field scales on lenses are a thing of the past, I wrote software to print depth of field scales for my lenses. It's free for downloading here:

http://www.dofmaster.com

There's also a free depth of field program for Palm devices on the site.

Don

dimage said:

I find that the following post can be quite helpful:

http://www.dpreview.com/forums/read.asp?forum=1019&message=2824952

Click to expand...

Good post by Karl.

dimage said:

It is true that DoF roughly goes up by the Square of the Distance
to the focus point, Down by the square of the Focal length, and Up
linearly with the F-number. Thus while the Distance and Focal
length have the "largest effect" (square law) they cancel each
other out if you keep the subject the same size by changing the
distance AND the focal length at the same time and thus the
F-number gives you the DoF control.

Click to expand...

A lot of times, people equate DoF with blur (less DoF = more blur). While this is the trend, it is not strictly true. With DoF, if you keep the subject the same size in the image by changing the distance and focal length, the two will cancel out. However, the amount of blur in an out-of-focus object will depend on the magnification, of which the longer focal length will have more. So if you take two shots, one at 100mm f/2.8 and another 200mm f/2.8 and keep the subject the same size in both images, the DoF will be the same, but the 200mm's background will look blurrier simply because it's magnified more. Hence one of the reasons longer focal lengths are preferred for portraits.

In addition to all the above, you might keep in mind that dof is relative determined by the enlargement size of the image in the print and the distance at which the print is viewed. What dof calculations are based on is that the eye won't be able to resolve more than 100 points per inch at a viewing distance of 10 inches. Ergo, the greater the enlargement of the "negative" to make a print, the less the dof given the same lens, focus point and f/stop.

In reality, there is no dof. There is a plane in front of the lens (usually a curved plane due to spherical abberation in the lens and a using a flat image plane) and only object points falling on that plane will be in best focus in the image. Any point in front of or behind that plane in the object will be out of focus. So, almost all points in any 3-d object will be out of best focus. The question is, will the eye-brain detect the out-of-focus condition in a given point in the printed image? Case in point is the computer display screen you are looking at right now. We have trained ourselves to not resolve the 72 dots per inch of the displayed image. We hardly ever notive the individual dots. That's why bigger displays require "more resolution", actually more total dots horizontally and vertically. The real resolution of 72 dots per inch remains the same.

The point of all this rambling is that percieved dof also depends on the final viewing size of the (full) image and the viewing distance. Dof calculations emerged in the film era and the numbers (and lens markings) were based on a final 8"x10" full frame print viewed at a distance of 10 inches. Enlarge an image more than this and your depth of field will shrink unless you view the print from a greater distance.

my thanks to all of you that have responded. i have learned much, and certainly the answer to my original question.

but now, of course, a new question comes to mind. given that one could shoot more-or-less the same picture (same subject size etc), at different focal lengths by moving towards or away from the subject, is there a rule of thumb for which would provide the "best" picture?

i have learned that diffraction is greater at longer focal lengths. i have also learned that backgrounds are blurrier at longer focal lengths. it is also of course harder to hold the camera still enough at longer focal lengths. so, if i don't specifically want a blurry background - does this imply that in general shooting closer with a shorter focal length is "best"?

what about lens characteristics - do these vary wildly from lens to lens, or is it generally true that zoom lenses are sharper at one end or the other (or in the middle)?

thanks again,
-keith

A longer lens will result in a "flat" subject, while a shorter lens will result in a lot of "depth" to the image. Simply due to the change in perspective required to get the same FOV from each lens.

Which is best? Depends on what you want.

Keith Evans said:

my thanks to all of you that have responded. i have learned much,
and certainly the answer to my original question.

but now, of course, a new question comes to mind. given that one
could shoot more-or-less the same picture (same subject size etc),
at different focal lengths by moving towards or away from the
subject, is there a rule of thumb for which would provide the
"best" picture?

i have learned that diffraction is greater at longer focal lengths.
i have also learned that backgrounds are blurrier at longer focal
lengths. it is also of course harder to hold the camera still
enough at longer focal lengths. so, if i don't specifically want a
blurry background - does this imply that in general shooting closer
with a shorter focal length is "best"?

what about lens characteristics - do these vary wildly from lens to
lens, or is it generally true that zoom lenses are sharper at one
end or the other (or in the middle)?

thanks again,
-keith

Click to expand...

--
The Unofficial Photographer of The Wilkinsons
http://thewilkinsons.crosswinds.net
Photography -- just another word for compromise

DavidP said:

A longer lens will result in a "flat" subject, while a shorter lens
will result in a lot of "depth" to the image. Simply due to the
change in perspective required to get the same FOV from each lens.

Click to expand...

While "flat" is a term photographers like to use to describe perspective, it isn't very descriptive to the newcomer.

Perspective basically means two same-size objects will look different sizes depending on distance from the camera. A "flat" perspective (achieved with telephotos) means a (say) 1' object far away looks about the same size in the image as a 1' object nearby. A "normal" perspective (achieved with normal lenses) means the size difference in the two objects is about what you see with the unaided eye. A "wide" perspective (achieved with wide angle lenses) means the nearby object looks much bigger than the far object.

The size differences are crucial in portrait photography, where you don't want to exaggerate someone's nose to where it's half the size of their face. Also, the "flat" perspective gets its name from creating the illusion that far objects are closer to near objects than they really are (hence why telephoto lenses are sometimes called "compression" lenses).

Member said:

... So
if you take two shots, one at 100mm f/2.8 and another 200mm f/2.8
and keep the subject the same size in both images, the DoF will be
the same, but the 200mm's background will look blurrier simply
because it's magnified more. Hence one of the reasons longer focal
lengths are preferred for portraits.

Click to expand...

I'm not sure what there is in theory to support this.

In the formula for calculating circle of confusion, if you hold constant the distance between the focus plane and some point behind it, you double the focal length and double the distance between the camera and the focus plane (thus preserving the size of the subject in the frame), then you get the same size circle of confusion. So that point behind the focus plane will translate into the same CoC on the film plane whether you use the 100mm or a 200mm from twice the distance. Only varying the aperture or the distance between the background and the subject will have any effect, varying the focal length and the distance to the subject by corresponding amounts have nullifying effects.

When we talk about magnification in depth of field, we talk about the magnification between the film plane and, for instance, a print being viewed by an observer, e.g. the "enlargement" factor. Obviously this enlargement factor makes "blurriness" more apparent to an observer.

Longer focal lengths are preferred for portraits because they allow a shot to be taken from a greater distance, which provides flatter perspective. A shot of a person from too close up, thus with significant "perspective" effects, exaggerates the parts of the face closer to the camera, leading to the dreaded "big nose in the lens" syndrome.
-harry

If the subject is the same distance in the frame, the perspective will be the same no matter what the lens focal length. The only thing that changes is the amount of subject included in the image.

In other words, shoot an object at ten feet with a wide, normal and tele lens. Crop the photos to show exactly the same amount of the subject and the perspective will be the same.

Keith Evans said:

i have learned that diffraction is greater at longer focal lengths.

Click to expand...

No, diffraction is a function of aperture and magnification. The magnification depends on the size of the digital sensor or film frame, and the size of the print you make from it (and, technically, how far the observer views the print from).

I think somebody mentioned that some longer focal length lenses provide access to smaller apertures (bigger f-numbers), where the effects of diffraction become greater. You'll get comparable softness due to diffraction at a particular f-number, independent of the focal length.

The way typical humans use this information is to either be blissfully unaware of it and somehow manage to lead a perfectly fulfilling life any way, or else to just steer clear of smaller apertures, e.g. smaller than f/22 or f/16 (again, smaller aperture means bigger F number). This is dependent on magnification, so the diffraction that a full-frame film camera sees at f/22 will show up on, say, a D60 at f/14. The D60 will get the same depth of field at f/14 that the film camera got at f/22, so it sort of washes out, you just have to "interpret" an aperture f-number in the context of the format you're shooting.

Keith Evans said:

i have also learned that backgrounds are blurrier at longer focal
lengths.

Click to expand...

If you can quickly unlearn this, you'll be way ahead of most of your peers.

It's a common misconception, but it isn't true. It's better to think of aperture as your way of controlling depth of field, and to remember that smaller subjects lead to narrower depth of field, thus depth of field is a major challenge in macro photography.

Keith Evans said:

it is also of course harder to hold the camera still
enough at longer focal lengths.

Click to expand...

Yes.

Keith Evans said:

so, if i don't specifically want a
blurry background - does this imply that in general shooting closer
with a shorter focal length is "best"?

Click to expand...

It's all about compromises.

For portraits, if you get too close to your subject you'll give them a really big nose (perspective), if you get too far away you'll bump into the wall behind you, or end up having to shout at them from really far away, or end up using a big and heavy lens, or accidentally step backwards off a cliff.

For wide depth of field, you want a small aperture. Small apertures require longer shutter speeds, if your shutter speed is too long you run into blur due to camera shake.

For narrow depth of field, you want wide apertures. Lenses tend to have less sharpness due to aberrations at very wide apertures. Wide aperture lenses are also bigger and heavier and more expensive.

Very wide angle lenses do tend to be less sharp than longer focal lengths, and can sometimes suffer from distortion effects (e.g. straight lines turn into curved lines) or vignetting (the corners of the image are a little darker than the middle).

In general, though, I think your typical humans choose an aperture that leads to the desired depth of field (commonly wide for landscapes and often narrow for portraits) without resulting in a shutter speed that gives undesired blur from camera shake or motion blur, and chooses focal length either to select perspective or to compensate for a limited ability to place the camera (like "I need a really long focal length because they don't let the photographers onto the middle of the football field").
-harry

I don't remember the focal lengths for either of these, but they were both shot wide-open ( at f/2.8 ) and at high ISO ... with an ultrawide lens. Depth of field has almost nothing to do with focal length, and almost everything to do with magnification -- how big your subject is in real life, compared to how big that subject is on the film plane. You can get the same magnification out of any lens, unless your subject is small enough that your lens isn't able to focus it.

( I think a 70-200 can focus about 5 feet from the film plane; the 16-35 can focus to 11 inches from the film plane. If you do the math about angles of view, you can figure out which one can throw the background further out of focus. )

"Do I shoot short or long?" The others have in more technical terms explained that you can produce very similar results with different focal lengths and the same DOF. To answer your question of whether to shoot short or long the answer as I see it is how you want your subject to relate to the background. A long telephoto lens will magnify the background so much that it may appear indistinguishable, in other words your subject will be very isolated from the background. A wider focal length with an upclose subject may make the background dissapear in the distance getting infanently smaller but definately more distinguishable than a long focal length. If you can imagine a person standing if front of a mountain with a long focal length you will see so little of the mountain that you may not be able to determine what the background of the photo is. Same instance with a wide angle lens and the mountain may be so small that it looks like a mound of dirt behind the person. I believe the answer to your question of whether to shoot long or short has to do with how you want your subject to interact with the background. Hope that helps, made sense to me when I wrote it, an hour from now that may not apply
--
http://www.pbase.com/davkrat

Expect failure and you can only be pleasantly surprised.

hm said:

hm said:

... So
if you take two shots, one at 100mm f/2.8 and another 200mm f/2.8
and keep the subject the same size in both images, the DoF will be
the same, but the 200mm's background will look blurrier simply
because it's magnified more. Hence one of the reasons longer focal
lengths are preferred for portraits.

Click to expand...

I'm not sure what there is in theory to support this.

Click to expand...

Rather than work from the theory to try to understand this, just take a look at the photographic evidence:

http://www.wlcastleman.com/equip/reviews/85_100_135/index.htm
http://www.luminous-landscape.com/tutorials/dof2.shtml

As for the theory:

hm said:

In the formula for calculating circle of confusion, if you hold
constant the distance between the focus plane and some point behind
it, you double the focal length and double the distance between the
camera and the focus plane (thus preserving the size of the subject
in the frame), then you get the same size circle of confusion. So
that point behind the focus plane will translate into the same CoC
on the film plane whether you use the 100mm or a 200mm from twice
the distance. Only varying the aperture or the distance between the
background and the subject will have any effect, varying the focal
length and the distance to the subject by corresponding amounts
have nullifying effects.

Click to expand...

The circle of confusion is only relevant to the portion of the image that's in focus. In the out of focus portions, the diameter of a blur circle (the circle formed by an out of focus point light source) depends on distance to the object (not the subject) and the diameter of the entrance pupil (the physical diameter of the front of the lens on most telephotos). The entrance pupil is the focal length divided by the f-ratio, so focal length plays as big a factor as aperture in the blurriness of the out-of-focus sections.

If we changed the distance to the out-of-focus object so it remained the same size with both 100mm and 200mm lenses, then you'd be correct - the amount of blur would be the same. However, assuming the subject (in-focus) to background (out-of-focus) distance remains the same, this would make the subject a different size in both images due to perspective, which makes the comparison pointless.

hm said:

Longer focal lengths are preferred for portraits because they allow
a shot to be taken from a greater distance, which provides flatter
perspective. A shot of a person from too close up, thus with
significant "perspective" effects, exaggerates the parts of the
face closer to the camera, leading to the dreaded "big nose in the
lens" syndrome.

Click to expand...

They also blur the background more.

hm said:

hm said:

i have also learned that backgrounds are blurrier at longer focal
lengths.

Click to expand...

If you can quickly unlearn this, you'll be way ahead of most of
your peers.

Click to expand...

Don't unlearn it - it is true. Just look at the pictures if you don't believe me.

http://www.wlcastleman.com/equip/reviews/85_100_135/index.htm
http://www.luminous-landscape.com/tutorials/dof2.shtml

hm said:

It's a common misconception, but it isn't true. It's better to
think of aperture as your way of controlling depth of field, and to
remember that smaller subjects lead to narrower depth of field,
thus depth of field is a major challenge in macro photography.

Click to expand...

It's not a common misconception. The common misconception is that whatever applies to DoF also applies to bluriness. They are different things.

Member said:

The circle of confusion is only relevant to the portion of the
image that's in focus.

Click to expand...

The circle of confusion is a means to quantify "how blurry is blurry". Except for the focus plane itself, everything is out of focus, so the notion of "in focus" is just the region where the resulting CoC is below some threshold. The notion of CoC has application throughout, though, and can be used to measure how blurred out the background is, even beyond the plane where the CoC surpasses the depth of field threshold.

Member said:

In the out of focus portions, the diameter
of a blur circle (the circle formed by an out of focus point light
source) depends on distance to the object (not the subject) and the
diameter of the entrance pupil (the physical diameter of the front
of the lens on most telephotos).

Click to expand...

The references I've seen calculate the CoC in terms of four factors: the distance to the focus plane, the distance to the "point" behind the focus plane, the focal length, and the aperture.

That all said, I think I'm going to eat some crow, now.

In general, in terms of DoF, doubling the focal length and doubling the distance to the subject results in approximately the same depth of field (approximately). This approximation works pretty well for small f-numbers and reasonably large subject distances relative to the focal length. But as you continue to go further away from the focus plane, as the resulting CoC continues to grow beyond the "acceptable limit" that defines the rearward limit of depth of field, the approximation becomes less accurate, and the CoCs start to diverge a bit with the longer focal length lenses (from further away) creating larger CoCs faster than the smaller focal length lenses (from nearer).

I was interested enough in this to chart this (nerd!). The sample scenario is one where you have a subject with a constant field of view of about 7ft (on the "longer" side), so you shoot from 5 ft with 25mm or 10ft with 50mm or 20 ft with 100mm, etc, keeping the same framing.

If you graph how CoC grows as you look at points that are further and further behind the plane of focus, you get something like this:



So while the different focal lengths hit the DoF barrier at very nearly the same distance (approximately), the longer focal lengths from further away do "blur out" the background beyond the depth of field more quickly.

Assuming I got the CoC equation right.

Of course the real question is "are these differences in blurriness significant to an observer looking at a print?" Interestingly, the Luminous Landscape URL you provided was demonstrating that there is no difference! And in the images provided there, I must admit that there doesn't seem to be significantly visible differences to my eyes. The lesson is probably that only very gross differences in blurriness are really distinguishable, so you shouldn't go calculating very precise levels of blurriness and charting them!
-harry

hm said:

Of course the real question is "are these differences in blurriness
significant to an observer looking at a print?" Interestingly, the
Luminous Landscape URL you provided was demonstrating that there is
no difference!

Click to expand...

The Luminous Landscape site simply demonstrates that there is no difference in blurriness relative to the size of the object . It is correct - if you crop the 17mm's background and blow it up, it looks just as blurry as it does in the 20mm shot.

What I've been saying is that relative to the size of the picture , the background looks blurrier with a longer focal length lens. The longer focal length magnifies the background more, resulting in a blurrier looking background.

hm said:

And in the images provided there, I must admit that
there doesn't seem to be significantly visible differences to my
eyes. The lesson is probably that only very gross differences in
blurriness are really distinguishable, so you shouldn't go
calculating very precise levels of blurriness and charting them!
-harry

Click to expand...

The difference is blindingly obvious to me. You're telling me if you were taking a portrait, it wouldn't matter to you which of the following two you used as your background?

I can sure tell the difference. And one certainly looks blurrier than the other, even though the DOF's are identical (or at least VERY similar).

I'll leave it as an exercise for the reader to figure out which one that is.

Member said:

The difference is blindingly obvious to me. You're telling me if
you were taking a portrait, it wouldn't matter to you which of the
following two you used as your background?

Click to expand...

--
The Unofficial Photographer of The Wilkinsons
http://thewilkinsons.crosswinds.net
Photography -- just another word for compromise