Hello there!
A rather technical question that bothers me: It seems accepted knowledge that depth of field (dof) is dependent on focal lenght (among other influencing factors).
However, there are some resources on the web that state that dof is "independent" on focal lenght.
These sites state that there just "appears" to be a larger dof with a wide-angle lens or a shorter dof for a tele-lens, but in fact dof is the same.
The point is, due to the laws of physics, only one plane is in exact focus, hence there is no depth of field in the closest sense. In my recent understanding, dof is just an illusion, and this illusion is supported by a wide angle lens because everything gets shrunk and the out-of focus -objekts get so small that our eye cannot recognize them as out of focus. On the opposite, a tele-lens depicts a small area, everything is magnified and the out-of focus objects are noticable.
So if it holds true that in fact there is no dof, dof can never be a function of focal lenght - am I wrong?
Then there is this agreement that dof is a distance within objects appear to be sharp - so there is a virtual dof - Is this dof - albeit virtual - influenced by focal lenght? Or put this way, can a virtual value be influenced by focal lenght.
I hope i can express my self somewhat understandable. I just want to know what is true:
a) It is obsolete to speak of dof because in fact there "IS NO" dof
so there can never be an influence.
b) If one accepts that there is a "virtual dof", this dof then only
"appears" to be influenced by focal lenght, because of the
magnifying or shrinking abilities of a tele/wide-angle - the
illusion is enhanced or degraded by focal lenght.
c) The virtual dof "IS" definitely influenced by focal lenght.
Here are some links that say there's no influence by focal lenght on dof, some of my statements stem from reading these resources:
Theres also something on the luminous-landscape-site and on other sites as well.
The thing why this bothers me so much is the difference in accepted knowledge and the content of some web-resources.
I also had a discussion on a german photography-forum but my persistence in asking made them go crazy and eventually one of the members got offensive. And they disagreed what was stated on these sites.
I just want to know! Whats the truth?
Anyone who can answer this? (I can get technical, I have some basic understanding, but please keep it dummy-like
With kindest Regards,
Dietrich
Posted by Basil Greber on February 3, 2007 at 2:56 PM
I was actually wondering about that, too. I used a DOF calculator offered by one of the lens makers (I think it was Tamron), and found that DOF is indeed focal length independent if you maintain the framing (since you will be standing further back with the longer lens), which somehow contradicted my own experience (or at least what I believe is my experience ).
However, what I think is true is that you get less DOF on full frame cameras (since you'll either be standing closer when using the same lens or will be using a longer lens if you maintain focussing distance) than on crop sensor cameras. In that sense, DOF is dependent on focal length.
Also, DOF is really an illusion. Only one geometrical plane is actually in sharp focus, everything in front and behind will look like in focus, but is actually not (depends on the acceptable circle of confusion and hence on print size/enlargement ratio - but I'm not very sure about what CoC really is, except that it's confusing me a lot ).
Basil, thanks for your input!
Yes thats the point: dof is subjective and the circle of confusion comes into play. Actually, these dof-calculations take a "circle of confusion" of a defined size, dependend on the sensor/film format. So with this respect, dof is dependend on sensor/film-format, and since a circle of confusion is also dependend on how far away one would look at the final image, dof is also influenced by the viewing distance. Still no focal length had come into the "picture".
The thing that I am struggling with is that dof really seems to be a solely virtual/subjective thing- derived from a definition that takes a subjective human perception into account.
So, without anything beeing an actual physical value, there's nothing that can be influenced - only the "perception" can be altered, true? So with that respect I am starting to agree with the statement that dof is not influenced by focal lenght but that this virtual illusion is altered by focal length.
But here's the transition to the saying that dof is influenced by focal lenght.
Maybe I am nitpicking here.
I am still interested in gaining more understanding of this, so please keep some replies coming...
Regards Dietrich
Posted by Mark Stahlke on February 3, 2007 at 4:34 PM
Hi Dietrich,
Here are all the answers to your questions: Introduction to Depth of Field Depth of Field in Depth
Sorry, but these are very technical papers. My advice is to learn to use the DOF preview button on your camera and quit worrying about it.
Cheers,
Posted by David Bostedo on February 3, 2007 at 6:19 PM
Dietrich, first, I don't think it's accepted knowledge at all that DOF is dependent on focal length. I think the writings you refer to, and the question asked here show that. I'm guessing maybe you already read these, but the two main essays on DOF on Luminous Landscape are about the simplest/best I've seen, if you want only semi-technical explanations.
The second one, in particular is good because it shows how you can ignore all the technical aspects and apply the DOF in real life.
And CoC is well named, as I've read a lot and it's still somewhat confusing - basically, it's just a measure of the eyes ability to resolve detail - and therefore plays into whether something appears sharp or out-of-focus. This then helps determine DOF. Here's the way I understand it :
1) There is only one plane parallel to the sensor/film that is actually in sharp focus.
2) The DOF is defined as the distance in front and behind the subject where there is ACCEPTABLE focus.
3) Acceptable focus is in large part determined by the CoC - since if your eye can't resolve the detail, it doesn't matter if it's blurry or not, and it will be in acceptable focus.
4) The DOF for a given aperture doesn't change with focal length - but it does change based on where the lens is focused (based on the distance from your subject). I think this is why all of the DOF calculators ask for a focal length, and distance to subject - and why it SEEMS as though long lenses have less DOF. The second link above shows how you need to move to keep the size of the main object in the image the same in order to maintain a constant DOF.
I hope this helps (and that I don't have this wrong myself)....
Posted by Kent Staubus on February 3, 2007 at 8:25 PM
I've actually made the test shots myself and strongly disagree that lens length has any effect on DOF. My Nikon 20mm has the same DOF as does my Nikon 200mm. The two things that DO affect DOF are (1) image size (2) f-stop.
1) Circle-of-confusion / CoC (or more technically circle-of-least-confusion) is dependent on the resolving power of the lens and not that of the eye. When we refer to CoC for the purpose of determining depth-of-field (DoF), we are actually meaning "maximum-permissible-circle-of-confusion." This is partly dependent on the ability of our eyes to resolve detail and also the amount by which we are going to enlarge the original capture (for example when making a 16x20 print from 4x5 film, you are enlarging 4x times). Note that the maximum-permissible-circle-of-confusion is quite subjective and different people may use different values for it, depending on what sharpness (or lack of) is acceptable to them.
2) Its true that there is only one "real" plane of focus. However, this doesn't have to be parallel to the lens or the capture surface (film or digital sensor). For instance, by using the movements offered by a view-camera, one can select a "real" plane of focus that is at an angle to the film plane.
Posted by David Bostedo on February 4, 2007 at 12:29 AM
Thanks for the corrections Anil. I didn't consider lens movements - that's something I've thought was really cool when I've heard about it, but don't often think about since I've never used anything even as large as full frame 35mm, much less a view camera.
Hello, thanks for all the answers! I am now much more secure and confident to propagate the point that dof is independent on focal length, given all the feedback here. I knew of these largephotography-papers - did not read them because I was too lazy. I know they are technical but i hope I am gonna get it (have a master in technical chemistry, so lets proof if my education is worth anything )
The thing why I was confused so much was not that i did not understand the stuff presented on these i-net-resources. It was more the vehemence of posters on that german board where I tried to clear things up in a fruitful discussion and discuss these dof-issues as i now did here - (with so much more success.)
They just would not accept that the dof-issue might be somewhat different form what they know, and eventually got mad at me because encouraged them to get bit more in depth insead swallowing everything that is accepted knowledge
one of the posters clearly statet the following:
dof depended on:
focal lenght
aperture
focussing-distance/distance from object
how could I argue ?
Thanks for all input here - this cleared things up! I also fully understand why it is called "circle of CONFUSION"
Posted by Steve Foss on February 4, 2007 at 10:01 AM
David:
Thanks for those urls to the Luminous Landscape site. Reading those cleared up a couple questions I'd had, too.
Posted by Dan Borzynski on February 5, 2007 at 9:38 AM
I tend to think of DOF as being a function of two things: image magnification and f-stop. A 20mm lens, with the subject appearing at 1/4 life size on film, and using f11, will have EXACTLY the same DOF as a 600 mm lens at f11 at 1/4 life size on film. Feel free to subsititute any other image magnification or f-stop. It doesn't matter.
What changes immensely from shot to shot is your background control and perspective. A tree in the center of your shot with the 20 mm lens that is 1 mm wide on the dead center of ther frame is suddenly going to be 30 mm wide in the frame of the 600 lens. Instead of getting a thin, nearly sharp tree in the background with the wide angle, you now have this big blob of a tree trunk back there You also get to see wide angle distortions or telecopic compression as well when you switch between these two focal lengths, but neither affects your DOF.
If you are a real math geek (and before anyone complains, I am officially a certified math geek ), run the equations cited backwards. Start with the lens and the magnification desired. Determine the distance needed to obtain that. Pick a number, any number for the circles of confusion. Pick a depth of field. Now plug all of that into the DOF calculations and solve for the f-stop. You'll find that you'll get the same f-stop.
Dan, thanks for your contribution! I 've now read these large-format-photography papers on depth of field, and i now have a quite good understanding of the matter (I read the intro and the in depth article - but only the first 14 pages or so, the rest was of no significance - i think)
However some things remain unsolved...
If you or (others for that matter) have read the papers as well, there are two very important equations derived:
eq. 24 clearly indicates the influence of focal lenght on dof
However eq. 25, with the assumption that focussing-distance (object-distantce) is much smaller than the hyperfocal distance, states that focal lenght has no impact on dof.
So, both cases seem to be valid.... this still confuses me a bit, or lets say, shows that this dof-issue can't be answered without taking other things into account. There are cases in which focal lenght does matter, and then there are cases where it does not.
So i think using a rule of thumb can be dangerous here..
But thanks for the advice with playing around with the equations... Since everything is derived, and no scary integrals or other analysis-stuff appears in these papers, I think I can manage this task. I am definately no math-geek, but I am a officially certified chemistry-geek - and to get to this point, i had to go through some painful math-examinations...
Regards Dietrich
Posted by Dan Borzynski on February 5, 2007 at 2:11 PM
Chemistry, hmmm. Almost as bad as non-associative algebras. Yeah, I served my time in chemistry labs before defecting to mathematics.
From having looked at that a long time ago (mostly off of memory and quickly looking at the articles to refresh), I *think* equation 24 is the technically correct one, in that it tells you exactly what is going on with DOF, focal length and image magnification. If you are doing high magnification work, beyond the 1:1 range, the difference between the object distance and the hyperfocal distance is for practical purposes, nil. Things then begin to get weird.
Equation 25 is what most people will see in the real world under normal photographic conditions. Photographing at twice life size is not normal! There is a significant difference between the hyperfocal distance and the object distance, at which point that term,especially once squared, becomes 0 quickly. What you will see in prints or on film, with the same image magnification and f-stop, is essentially the same DOF. Technically, yes, there is the some slight difference, but it is nothing your eyes will ever pick up on. (My opinion/guess: The error introduced by the approximation will most likely be cancelled by limitations on the optics and shooting technique)
I'm perfectly willing to be corrected here, but I think that is what is going on.
In short, under normal shooting and printing conditions, DOF really only practically depends on image magnification and f-stop.
Dan, thanks again for bringing me closer to the core! And cool that someone with a strong technical background is willing to step down from the high levels and uses a language that is understandable for mortals
Well some questions still float around: As far as I know - especially for wide-angle landscape work- they say: "focus your lens at the hyperfocal distance and every thing from near to infinity will be sharp".
Ok, now, since focussing at the hyperfocal distance means that equation 25 does not hold true because I dont focus shorter than the hyperfocal distance, right? So then still 24 has to apply?
"DOF really only practically depends on image magnification and f-stop"
Do you mean image-magnification by using a tele-lens or by enlarging a recorded image to its final viewing size?
If the tele-lens is meant , as far as i get it, it can really be said that due to the enlargement - (the magnifying effect of the tele-lens) - the blur-spot(Coc) gets enlarged as well and that is what appears to be a poor depth of field, right?
I hope no one is getting angry at me...I promise i will shoot more and ask less...
Thanks Dan for answering this...
Regards Dietrich
Posted by Dan Borzynski on February 5, 2007 at 4:00 PM
The first question I haven't given too much thought too, certainly not calculating out by hand. Let me think about that, but offhand, I would say equation 25 applies, since landscape work lies far outside of the realm of macro and the hyperfocal distance is significantly different from the subject distance, which is where the approximation comes from.
Your second question is much easier. Yes, as long as you don't change the circles of confusion. I'll get back to this tonight, as I have a meeting to run off to. In short, the answer comes down to, assuming perfect optics/scanning/printing/darkroom and avoiding macro work, there is no practical difference in DOF, assuming the same degree of magnification (which is DIFFERENT from final print size or recording medium), regardless of how you get there.
Ok thanks again, NOW i got it....but do i look forward to another reply...
Thanks,
Dietrich
Posted by Dan Borzynski on February 5, 2007 at 8:46 PM
I had a beautiful argument explaning the magnification, but realized it was circular. So, when all else fails, go back to the definitions.
Keep in mind through all of this that I am assuming that the image magnification is **NOT** changing. Without that, everything below is wrong. We are also not talking about macro work here, with magnifications greater than 1:1. Just to be safe, let's say no more than 1/10 life size on film.
Go back to the first paper cited by Mark, Intro to Depth of Field, and look at equations 2 and 3. "far distance" - "near distance" is the depth of field. This equation also includes the focal length of the lens. If the focal length changes from 20mm to 200 mm, the near and far distances will be 1/10 as large. What we wind up with is a 1/10 factors out of everything, and cancels the 200mm to 20 mm and we wind up where we started. Pick any focal length pair and the same thing happens.
What, then, are we left with? The circle of confusion determines the f-stop needed, or the f-stop determines the COC produced. The focal length doesn't enter into the argument, as long as magnification does not change. Again, if we change the magnification, all bets are off.
Now for the issue of the print vs focal length. What is the role of the circles of confusion? It is essentially telling us how small of a blur spot will still look sharp to our eyes, in the final print. Aha! Once we decide what constitutes acceptably sharp in the final print, our f-stop is determined for us.
Note that the focal length of the lens never came into play here, since in the above, a longer focal length was cancelled by the proportional change in near and far image distances. We always wound up back at the same starting shorter focal length. But again, we had to assume that the magnification did not change!
Actually, we normally do not sit there in the field and ask ourselves" Hmmm, I think I want to print this at 11x14, and I want a 0.1mm COC, have everything from 8 feet to infinity sharp and the magnification will be 1/200 life size. That of course means f##. DUH!!" No, instead the logic is I want this sharp from 8 feet to infinity, according to my hyperfocal chart with this lens, that means f-##.
Interesting point there. I added the phrase "with this lens." Remember, a 20 mm lens has 1/10 the magnification of a 200 mm lens. So if I wanted to accomplish the same thing with a 200, I would want everything sharp from 80 feet to infinity and after walking 72 feet backwards, so 8 feet away is now 80 feet away. surprise!, f-## again. But the images will have a different look to them, because the 200 has a narrower field of view (go back to the tree example in my first post).
Posted by Dan Borzynski on February 5, 2007 at 8:56 PM
I can't answer your hyperfocal question here at home since I seem to be getting an error when I open that file. I'll try printing it at work tomorrow and go from there.
Hmmm, one more thing. I tend to be a bit of a macro specialist, so I tend to think in terms of magnification. Not as much in this is 3.2 times life size, but its effects on images. The math behind it all is interesting, but other than creating a hyperfocal table or for scientific reproduction, niether of which applies to me, I pretty much ignore it. My personal philosophy is if in doubt, stop down. I have gotten nailed a couple of times by diffraction once I reach the realms of f64-f96, but that is another story.
Dan, I thanks so much for your effort! The important point here is cleary the same or constant magnification. I did "dare" to email Jeff Conrad (author of the two papers) and he basically explained the same - magnification has to be kept constant. This issue was not entirely clear to me - and I think thats where the confusion comes from. People just change lenses to change the composion, without moving - magnification is then changed and dof is changed...
Thanks to all, learned a lot!
Regards Dietrich
Discussion
18 repliesA rather technical question that bothers me: It seems accepted knowledge that depth of field (dof) is dependent on focal lenght (among other influencing factors).
However, there are some resources on the web that state that dof is "independent" on focal lenght.
These sites state that there just "appears" to be a larger dof with a wide-angle lens or a shorter dof for a tele-lens, but in fact dof is the same.
The point is, due to the laws of physics, only one plane is in exact focus, hence there is no depth of field in the closest sense. In my recent understanding, dof is just an illusion, and this illusion is supported by a wide angle lens because everything gets shrunk and the out-of focus -objekts get so small that our eye cannot recognize them as out of focus. On the opposite, a tele-lens depicts a small area, everything is magnified and the out-of focus objects are noticable.
So if it holds true that in fact there is no dof, dof can never be a function of focal lenght - am I wrong?
Then there is this agreement that dof is a distance within objects appear to be sharp - so there is a virtual dof - Is this dof - albeit virtual - influenced by focal lenght? Or put this way, can a virtual value be influenced by focal lenght.
I hope i can express my self somewhat understandable. I just want to know what is true:
a) It is obsolete to speak of dof because in fact there "IS NO" dof
so there can never be an influence.
b) If one accepts that there is a "virtual dof", this dof then only
"appears" to be influenced by focal lenght, because of the
magnifying or shrinking abilities of a tele/wide-angle - the
illusion is enhanced or degraded by focal lenght.
c) The virtual dof "IS" definitely influenced by focal lenght.
Here are some links that say there's no influence by focal lenght on dof, some of my statements stem from reading these resources:
http://en.wikipedia.org/wiki/Depth_of_field
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm
Theres also something on the luminous-landscape-site and on other sites as well.
The thing why this bothers me so much is the difference in accepted knowledge and the content of some web-resources.
I also had a discussion on a german photography-forum but my persistence in asking made them go crazy and eventually one of the members got offensive. And they disagreed what was stated on these sites.
I just want to know! Whats the truth?
Anyone who can answer this? (I can get technical, I have some basic understanding, but please keep it dummy-like
With kindest Regards,
Dietrich
However, what I think is true is that you get less DOF on full frame cameras (since you'll either be standing closer when using the same lens or will be using a longer lens if you maintain focussing distance) than on crop sensor cameras. In that sense, DOF is dependent on focal length.
Also, DOF is really an illusion. Only one geometrical plane is actually in sharp focus, everything in front and behind will look like in focus, but is actually not (depends on the acceptable circle of confusion and hence on print size/enlargement ratio - but I'm not very sure about what CoC really is, except that it's confusing me a lot
Basil
Yes thats the point: dof is subjective and the circle of confusion comes into play. Actually, these dof-calculations take a "circle of confusion" of a defined size, dependend on the sensor/film format. So with this respect, dof is dependend on sensor/film-format, and since a circle of confusion is also dependend on how far away one would look at the final image, dof is also influenced by the viewing distance. Still no focal length had come into the "picture".
The thing that I am struggling with is that dof really seems to be a solely virtual/subjective thing- derived from a definition that takes a subjective human perception into account.
So, without anything beeing an actual physical value, there's nothing that can be influenced - only the "perception" can be altered, true? So with that respect I am starting to agree with the statement that dof is not influenced by focal lenght but that this virtual illusion is altered by focal length.
But here's the transition to the saying that dof is influenced by focal lenght.
Maybe I am nitpicking here.
I am still interested in gaining more understanding of this, so please keep some replies coming...
Regards Dietrich
Here are all the answers to your questions:
Introduction to Depth of Field
Depth of Field in Depth
Sorry, but these are very technical papers. My advice is to learn to use the DOF preview button on your camera and quit worrying about it.
Cheers,
http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml
http://www.luminous-landscape.com/tutorials/dof2.shtml
The second one, in particular is good because it shows how you can ignore all the technical aspects and apply the DOF in real life.
And CoC is well named, as I've read a lot and it's still somewhat confusing - basically, it's just a measure of the eyes ability to resolve detail - and therefore plays into whether something appears sharp or out-of-focus. This then helps determine DOF. Here's the way I understand it :
1) There is only one plane parallel to the sensor/film that is actually in sharp focus.
2) The DOF is defined as the distance in front and behind the subject where there is ACCEPTABLE focus.
3) Acceptable focus is in large part determined by the CoC - since if your eye can't resolve the detail, it doesn't matter if it's blurry or not, and it will be in acceptable focus.
4) The DOF for a given aperture doesn't change with focal length - but it does change based on where the lens is focused (based on the distance from your subject). I think this is why all of the DOF calculators ask for a focal length, and distance to subject - and why it SEEMS as though long lenses have less DOF. The second link above shows how you need to move to keep the size of the main object in the image the same in order to maintain a constant DOF.
I hope this helps (and that I don't have this wrong myself)....
Kent in SD
1) Circle-of-confusion / CoC (or more technically circle-of-least-confusion) is dependent on the resolving power of the lens and not that of the eye. When we refer to CoC for the purpose of determining depth-of-field (DoF), we are actually meaning "maximum-permissible-circle-of-confusion." This is partly dependent on the ability of our eyes to resolve detail and also the amount by which we are going to enlarge the original capture (for example when making a 16x20 print from 4x5 film, you are enlarging 4x times). Note that the maximum-permissible-circle-of-confusion is quite subjective and different people may use different values for it, depending on what sharpness (or lack of) is acceptable to them.
2) Its true that there is only one "real" plane of focus. However, this doesn't have to be parallel to the lens or the capture surface (film or digital sensor). For instance, by using the movements offered by a view-camera, one can select a "real" plane of focus that is at an angle to the film plane.
The thing why I was confused so much was not that i did not understand the stuff presented on these i-net-resources. It was more the vehemence of posters on that german board where I tried to clear things up in a fruitful discussion and discuss these dof-issues as i now did here - (with so much more success.)
They just would not accept that the dof-issue might be somewhat different form what they know, and eventually got mad at me because encouraged them to get bit more in depth insead swallowing everything that is accepted knowledge
one of the posters clearly statet the following:
dof depended on:
focal lenght
aperture
focussing-distance/distance from object
how could I argue ?
Thanks for all input here - this cleared things up! I also fully understand why it is called "circle of CONFUSION"
Thanks for those urls to the Luminous Landscape site. Reading those cleared up a couple questions I'd had, too.
What changes immensely from shot to shot is your background control and perspective. A tree in the center of your shot with the 20 mm lens that is 1 mm wide on the dead center of ther frame is suddenly going to be 30 mm wide in the frame of the 600 lens. Instead of getting a thin, nearly sharp tree in the background with the wide angle, you now have this big blob of a tree trunk back there You also get to see wide angle distortions or telecopic compression as well when you switch between these two focal lengths, but neither affects your DOF.
If you are a real math geek (and before anyone complains, I am officially a certified math geek
However some things remain unsolved...
If you or (others for that matter) have read the papers as well, there are two very important equations derived:
eq. 24 clearly indicates the influence of focal lenght on dof
However eq. 25, with the assumption that focussing-distance (object-distantce) is much smaller than the hyperfocal distance, states that focal lenght has no impact on dof.
So, both cases seem to be valid.... this still confuses me a bit, or lets say, shows that this dof-issue can't be answered without taking other things into account. There are cases in which focal lenght does matter, and then there are cases where it does not.
So i think using a rule of thumb can be dangerous here..
But thanks for the advice with playing around with the equations... Since everything is derived, and no scary integrals or other analysis-stuff appears in these papers, I think I can manage this task. I am definately no math-geek, but I am a officially certified chemistry-geek - and to get to this point, i had to go through some painful math-examinations...
Regards Dietrich
From having looked at that a long time ago (mostly off of memory and quickly looking at the articles to refresh), I *think* equation 24 is the technically correct one, in that it tells you exactly what is going on with DOF, focal length and image magnification. If you are doing high magnification work, beyond the 1:1 range, the difference between the object distance and the hyperfocal distance is for practical purposes, nil. Things then begin to get weird.
Equation 25 is what most people will see in the real world under normal photographic conditions. Photographing at twice life size is not normal! There is a significant difference between the hyperfocal distance and the object distance, at which point that term,especially once squared, becomes 0 quickly. What you will see in prints or on film, with the same image magnification and f-stop, is essentially the same DOF. Technically, yes, there is the some slight difference, but it is nothing your eyes will ever pick up on. (My opinion/guess: The error introduced by the approximation will most likely be cancelled by limitations on the optics and shooting technique)
I'm perfectly willing to be corrected here, but I think that is what is going on.
In short, under normal shooting and printing conditions, DOF really only practically depends on image magnification and f-stop.
Well some questions still float around: As far as I know - especially for wide-angle landscape work- they say: "focus your lens at the hyperfocal distance and every thing from near to infinity will be sharp".
Ok, now, since focussing at the hyperfocal distance means that equation 25 does not hold true because I dont focus shorter than the hyperfocal distance, right? So then still 24 has to apply?
"DOF really only practically depends on image magnification and f-stop"
Do you mean image-magnification by using a tele-lens or by enlarging a recorded image to its final viewing size?
If the tele-lens is meant , as far as i get it, it can really be said that due to the enlargement - (the magnifying effect of the tele-lens) - the blur-spot(Coc) gets enlarged as well and that is what appears to be a poor depth of field, right?
I hope no one is getting angry at me...I promise i will shoot more and ask less...
Thanks Dan for answering this...
Regards Dietrich
Your second question is much easier. Yes, as long as you don't change the circles of confusion. I'll get back to this tonight, as I have a meeting to run off to. In short, the answer comes down to, assuming perfect optics/scanning/printing/darkroom and avoiding macro work, there is no practical difference in DOF, assuming the same degree of magnification (which is DIFFERENT from final print size or recording medium), regardless of how you get there.
Thanks,
Dietrich
Keep in mind through all of this that I am assuming that the image magnification is **NOT** changing. Without that, everything below is wrong. We are also not talking about macro work here, with magnifications greater than 1:1. Just to be safe, let's say no more than 1/10 life size on film.
Go back to the first paper cited by Mark, Intro to Depth of Field, and look at equations 2 and 3. "far distance" - "near distance" is the depth of field. This equation also includes the focal length of the lens. If the focal length changes from 20mm to 200 mm, the near and far distances will be 1/10 as large. What we wind up with is a 1/10 factors out of everything, and cancels the 200mm to 20 mm and we wind up where we started. Pick any focal length pair and the same thing happens.
What, then, are we left with? The circle of confusion determines the f-stop needed, or the f-stop determines the COC produced. The focal length doesn't enter into the argument, as long as magnification does not change. Again, if we change the magnification, all bets are off.
Now for the issue of the print vs focal length. What is the role of the circles of confusion? It is essentially telling us how small of a blur spot will still look sharp to our eyes, in the final print. Aha! Once we decide what constitutes acceptably sharp in the final print, our f-stop is determined for us.
Note that the focal length of the lens never came into play here, since in the above, a longer focal length was cancelled by the proportional change in near and far image distances. We always wound up back at the same starting shorter focal length. But again, we had to assume that the magnification did not change!
Actually, we normally do not sit there in the field and ask ourselves" Hmmm, I think I want to print this at 11x14, and I want a 0.1mm COC, have everything from 8 feet to infinity sharp and the magnification will be 1/200 life size. That of course means f##. DUH!!" No, instead the logic is I want this sharp from 8 feet to infinity, according to my hyperfocal chart with this lens, that means f-##.
Interesting point there. I added the phrase "with this lens." Remember, a 20 mm lens has 1/10 the magnification of a 200 mm lens. So if I wanted to accomplish the same thing with a 200, I would want everything sharp from 80 feet to infinity and after walking 72 feet backwards, so 8 feet away is now 80 feet away. surprise!, f-## again. But the images will have a different look to them, because the 200 has a narrower field of view (go back to the tree example in my first post).
Hmmm, one more thing. I tend to be a bit of a macro specialist, so I tend to think in terms of magnification. Not as much in this is 3.2 times life size, but its effects on images. The math behind it all is interesting, but other than creating a hyperfocal table or for scientific reproduction, niether of which applies to me, I pretty much ignore it. My personal philosophy is if in doubt, stop down. I have gotten nailed a couple of times by diffraction once I reach the realms of f64-f96, but that is another story.
Thanks to all, learned a lot!
Regards Dietrich