Focal length, focus distance, and depth of field

[Derrel]

What this kitchen example shows is that at close range, a small stuffed animal can be made to span the frame, and the background can be blurred, when 1) the object being photographed is small in size and the lens angle of view is narrow and 2) when the focused distance is realtively close and WELL short of approaching the hyperfocal focusing distance.

Robert, to show how skewed this stuffed animal and 18-55mm lens example is, I'd like to ask you to take your camera to work at the auto dealership, and photograph a car in side-view, so that the entire driver's side of a car is shown in the frame, using the 18-55mm zoom lens, and a lens aperture of f/5.6. At the shooting distances required, we will have a VERY,very different set of example photographs to look at. I'd like to see a full side viw at 18mm and one done at 55mm, and then look at how blurry the background in THOSE photographs is.

 

[robertwsimpson]

That is something that I will have to test and get back to you on. Clearly, it requires more space than the confines of my office at work.

An interesting experiment I plan to do is to take different pics at different focal lengths, same FOV, but same apertures (which means different f-ratios). For example, same subject taken with all these setups:
50mm at f/2.0
100mm at f/4.0
200mm at f/8.0
All of these shots would have 25mm of physical aperture. As long as the background was more than double the subject distance in all cases, and the subject was framed the same, the background blur should theoretically be the same in all shots, although the DOF will be obviously quite different.

I really enjoy this stuff. Understanding why things happen is the key to making it happen in your photographs.

That is something that I find hard to believe... I anxiously await your results!

 

[robertwsimpson]

What this kitchen example shows is that at close range, a small stuffed animal can be made to span the frame, and the background can be blurred, when 1) the object being photographed is small in size and the lens angle of view is narrow and 2) when the focused distance is realtively close and WELL short of approaching the hyperfocal focusing distance.

Robert, to show how skewed this stuffed animal and 18-55mm lens example is, I'd like to ask you to take your camera to work at the auto dealership, and photograph a car in side-view, so that the entire driver's side of a car is shown in the frame, using the 18-55mm zoom lens, and a lens aperture of f/5.6. At the shooting distances required, we will have a VERY,very different set of example photographs to look at. I'd like to see a full side viw at 18mm and one done at 55mm, and then look at how blurry the background in THOSE photographs is.

I'm not interested in testing that. What I wanted to test is exactly what I tested. I intentionally picked a subject that I could fit to the frame at nearly minimum focus distance. This is where the smallest DoF is achievable. I'm confused as to what you're getting at. We all know that the blurring is more slight when the subject (no matter what size) is closer to infinite focus. In fact, I have a whole paragraph in my OP explaining this, as well as a graph that represents it, well, graphically. Maybe I am not getting what you are trying to say, but it appears to me that you are repeatedly explaining stuff to me that I already know.

 

[Stosh]

That is something that I find hard to believe... I anxiously await your results!

No experiments real soon. Have to wait until the 2-3 feet of snow melt lol.

 

[Gaerek]

I apologize, I misread what was being discussed. It was a bit early for me, and not totally awake when I posted. Carry on! Interesting discussion, for sure.

 

[Derrel]

I'm not interested in testing that. What I wanted to test is exactly what I tested. I intentionally picked a subject that I could fit to the frame at nearly minimum focus distance. This is where the smallest DoF is achievable. I'm confused as to what you're getting at. We all know that the blurring is more slight when the subject (no matter what size) is closer to infinite focus. In fact, I have a whole paragraph in my OP explaining this, as well as a graph that represents it, well, graphically. Maybe I am not getting what you are trying to say, but it appears to me that you are repeatedly explaining stuff to me that I already know.

Robert, you began with this statement: "There seems to be a lot of confusion about how focal length (the mm part of what your lens is called) and focus distance (how far away from the front lens element you are focusing) affect a photo's depth of field" Sorry, but that statement is flat-out wrong: focusing distance has been calculated from the ***focal plane*** Since Day One, and ***not*** from the front of the lens. There is even a small circle with a line scribed through it, located immediately to the left of the hot shoe on most Canon cameras,showing the focal plane. On a 200mm macro lens, calculating the DOF from the front of the LENS would be a joke. So, your understanding of the most basic aspect of DOF starts off with an error of fact, and a misunderstanding. So, frankly, that alone made me suspicious of how much you really "know" about this technical subject.

Then your transitioned into this gem: "First, Depth of Field has far more to do with focus distance than focal length. Let me explain. Everyone is familiar with the focus graph. If not, it looks something like this:" [graph]

Well, let's plug in some REAL examples,and see how your words actually square with the facts. What you stated as a fact, as a principle, does not square with the real world. Online Depth of Field Calculator

55mm lens, focused at 15 feet,aperture f/16. DOF near limit 10.3 feet, far limit 27.5 feet, total DOF band is 17.2 feet deep. In front of subject 4.7 feet or 27%, behind subject 12.5 feet or 73%. Hyperfocal distance for 55mm lens on 1.6x Canon 32.8 feet.

200mm lens, focused at 15 feet, aperture f/16. DOF near limit, 14.5 feet, far limit 15.5 feet, total DOF band, 1.0 feet deep. In front of subject 0.48 feet or 48%, behind subject 0.52 feet (52%). Hyperfocal distance for a 200mm lens on 1.6x Canon, 423.3 feet.

So, your words are clearly wrong. Erroneous. You wrote, "First, Depth of Field has far more to do with focus distance than focal length. Let me explain." Okay---then please explain, correctly this time, what you meant to say. Because as it is obvious, at the identical focus distance of 15 feet, the depth of field from a 55mm lens stopped down to f/16 produces a deep, expansive band of depth of field---the total depth of field with the 55mm lens focused at 15 feet is 17.2 feet deep from front to back. Now, with a 200mm lens, also set to a 15 foot focusing distance, the total depth of field with the lens stopped down to f/16 is 1.0 feet deep from front top back.

So, when you wrote, ""First, Depth of Field has far more to do with focus distance than focal length.Let me explain." what you wrote was wrong. If you wish to give us a dissertation on how to calculate depth of field, then begin 1) with the correct location from which DOF is calculated, and then please use some of the on-line resources to check the veracity of your second premise, which as the numbers have proven here, was simply incorrect. There is a reason peer review is called for in serious academic work, and so when other people try and correct your mis-statements and oversimplifications, I think perhaps you ought to consider that what you "know" might not be provable, and that what "you know" and what you stated as facts in this discussion began with two big errors, right off the bat.

 

[robertwsimpson]

That is something that I find hard to believe... I anxiously await your results!

No experiments real soon. Have to wait until the 2-3 feet of snow melt lol.

BLAST! Let me know when you actually do that, because I would be interested to see what you come up with.

I apologize, I misread what was being discussed. It was a bit early for me, and not totally awake when I posted. Carry on! Interesting discussion, for sure.

no worries. I love discussing stuff like this, since theoretical and real world application seem only loosely related with each other sometimes. It's fun to see if you can make real world results show what theoretical calculations say should be happening.

 

[robertwsimpson]

Aw man, if you put someone on your ignore list, it still shows that they've made an inane comment in your thread? Lame.

 

[Stosh]

I'm not interested in testing that. What I wanted to test is exactly what I tested. I intentionally picked a subject that I could fit to the frame at nearly minimum focus distance. This is where the smallest DoF is achievable. I'm confused as to what you're getting at. We all know that the blurring is more slight when the subject (no matter what size) is closer to infinite focus. In fact, I have a whole paragraph in my OP explaining this, as well as a graph that represents it, well, graphically. Maybe I am not getting what you are trying to say, but it appears to me that you are repeatedly explaining stuff to me that I already know.

Robert, you began with this statement: "There seems to be a lot of confusion about how focal length (the mm part of what your lens is called) and focus distance (how far away from the front lens element you are focusing) affect a photo's depth of field" Sorry, but that statement is flat-out wrong: focusing distance has been calculated from the ***focal plane*** Since Day One, and ***not*** from the front of the lens. There is even a small circle with a line scribed through it, located immediately to the left of the hot shoe on most Canon cameras,showing the focal plane. On a 200mm macro lens, calculating the DOF from the front of the LENS would be a joke. So, your understanding of the most basic aspect of DOF starts off with an error of fact, and a misunderstanding. So, frankly, that alone made me suspicious of how much you really "know" about this technical subject.

Then your transitioned into this gem: "First, Depth of Field has far more to do with focus distance than focal length. Let me explain. Everyone is familiar with the focus graph. If not, it looks something like this:" [graph]

Well, let's plug in some REAL examples,and see how your words actually square with the facts. What you stated as a fact, as a principle, does not square with the real world. Online Depth of Field Calculator

55mm lens, focused at 15 feet,aperture f/16. DOF near limit 10.3 feet, far limit 27.5 feet, total DOF band is 17.2 feet deep. In front of subject 4.7 feet or 27%, behind subject 12.5 feet or 73%. Hyperfocal distance for 55mm lens on 1.6x Canon 32.8 feet.

200mm lens, focused at 15 feet, aperture f/16. DOF near limit, 14.5 feet, far limit 15.5 feet, total DOF band, 1.0 feet deep. In front of subject 0.48 feet or 48%, behind subject 0.52 feet (52%). Hyperfocal distance for a 200mm lens on 1.6x Canon, 423.3 feet.

So, your words are clearly wrong. Erroneous. You wrote, "First, Depth of Field has far more to do with focus distance than focal length. Let me explain." Okay---then please explain, correctly this time, what you meant to say. Because as it is obvious, at the identical focus distance of 15 feet, the depth of field from a 55mm lens stopped down to f/16 produces a deep, expansive band of depth of field---the total depth of field with the 55mm lens focused at 15 feet is 17.2 feet deep from front to back. Now, with a 200mm lens, also set to a 15 foot focusing distance, the total depth of field with the lens stopped down to f/16 is 1.0 feet deep from front top back.

So, when you wrote, ""First, Depth of Field has far more to do with focus distance than focal length.Let me explain." what you wrote was wrong. If you wish to give us a dissertation on how to calculate depth of field, then begin 1) with the correct location from which DOF is calculated, and then please use some of the on-line resources to check the veracity of your second premise, which as the numbers have proven here, was simply incorrect. There is a reason peer review is called for in serious academic work, and so when other people try and correct your mis-statements and oversimplifications, I think perhaps you ought to consider that what you "know" might not be provable, and that what "you know" and what you stated as facts in this discussion began with two big errors, right off the bat.

Woah guys, take it easy. Although technically Derrel's correct in what he wrote, I gave Robert some leeway, based on the rest of his post, that he actually did understand DOF, but was ignoring background blur. The argument about focal distance to front of lens instead of focal plane is definitely correct, but irrelevant in most cases. Knowing that Robert specified that the FOV be the same for his experiment, I knew that he didn't mean it how you interpreted it. He meant that you can achieve out of focus backgrounds if you choose a distance that's close to minimum focus, then go from there with your other lenses to achieve the same FOV.

 

[SoonerBJJ]

Aw man, if you put someone on your ignore list, it still shows that they've made an inane comment in your thread? Lame.

Just curious... were you trying to ignore Derrel's last post?

 

[robertwsimpson]

Aw man, if you put someone on your ignore list, it still shows that they've made an inane comment in your thread? Lame.

Just curious... were you trying to ignore Derrel's last post?

Yes, I find most of what he says useless to me. (no offense, Derrel.)

I'd rather just not even know it's there.

 

[robertwsimpson]

I'm not interested in testing that. What I wanted to test is exactly what I tested. I intentionally picked a subject that I could fit to the frame at nearly minimum focus distance. This is where the smallest DoF is achievable. I'm confused as to what you're getting at. We all know that the blurring is more slight when the subject (no matter what size) is closer to infinite focus. In fact, I have a whole paragraph in my OP explaining this, as well as a graph that represents it, well, graphically. Maybe I am not getting what you are trying to say, but it appears to me that you are repeatedly explaining stuff to me that I already know.

Robert, you began with this statement: "There seems to be a lot of confusion about how focal length (the mm part of what your lens is called) and focus distance (how far away from the front lens element you are focusing) affect a photo's depth of field" Sorry, but that statement is flat-out wrong: focusing distance has been calculated from the ***focal plane*** Since Day One, and ***not*** from the front of the lens. There is even a small circle with a line scribed through it, located immediately to the left of the hot shoe on most Canon cameras,showing the focal plane. On a 200mm macro lens, calculating the DOF from the front of the LENS would be a joke. So, your understanding of the most basic aspect of DOF starts off with an error of fact, and a misunderstanding. So, frankly, that alone made me suspicious of how much you really "know" about this technical subject.

Then your transitioned into this gem: "First, Depth of Field has far more to do with focus distance than focal length. Let me explain. Everyone is familiar with the focus graph. If not, it looks something like this:" [graph]

Well, let's plug in some REAL examples,and see how your words actually square with the facts. What you stated as a fact, as a principle, does not square with the real world. Online Depth of Field Calculator

55mm lens, focused at 15 feet,aperture f/16. DOF near limit 10.3 feet, far limit 27.5 feet, total DOF band is 17.2 feet deep. In front of subject 4.7 feet or 27%, behind subject 12.5 feet or 73%. Hyperfocal distance for 55mm lens on 1.6x Canon 32.8 feet.

200mm lens, focused at 15 feet, aperture f/16. DOF near limit, 14.5 feet, far limit 15.5 feet, total DOF band, 1.0 feet deep. In front of subject 0.48 feet or 48%, behind subject 0.52 feet (52%). Hyperfocal distance for a 200mm lens on 1.6x Canon, 423.3 feet.

So, your words are clearly wrong. Erroneous. You wrote, "First, Depth of Field has far more to do with focus distance than focal length. Let me explain." Okay---then please explain, correctly this time, what you meant to say. Because as it is obvious, at the identical focus distance of 15 feet, the depth of field from a 55mm lens stopped down to f/16 produces a deep, expansive band of depth of field---the total depth of field with the 55mm lens focused at 15 feet is 17.2 feet deep from front to back. Now, with a 200mm lens, also set to a 15 foot focusing distance, the total depth of field with the lens stopped down to f/16 is 1.0 feet deep from front top back.

So, when you wrote, ""First, Depth of Field has far more to do with focus distance than focal length.Let me explain." what you wrote was wrong. If you wish to give us a dissertation on how to calculate depth of field, then begin 1) with the correct location from which DOF is calculated, and then please use some of the on-line resources to check the veracity of your second premise, which as the numbers have proven here, was simply incorrect. There is a reason peer review is called for in serious academic work, and so when other people try and correct your mis-statements and oversimplifications, I think perhaps you ought to consider that what you "know" might not be provable, and that what "you know" and what you stated as facts in this discussion began with two big errors, right off the bat.

Woah guys, take it easy. Although technically Derrel's correct in what he wrote, I gave Robert some leeway, based on the rest of his post, that he actually did understand DOF, but was ignoring background blur. The argument about focal distance to front of lens instead of focal plane is definitely correct, but irrelevant in most cases. Knowing that Robert specified that the FOV be the same for his experiment, I knew that he didn't mean it how you interpreted it. He meant that you can achieve out of focus backgrounds if you choose a distance that's close to minimum focus, then go from there with your other lenses to achieve the same FOV.

Exactly. If you're trying to take a photo of a particular object, then it doesn't matter that changing the focal length at a particular distance from the subject makes the DoF smaller, because now your subject is not correctly in the frame anymore.

 

[robertwsimpson]

I apologize to everyone for not knowing the correct verbiage to convey my point. I didn't realize that exact wording was so important.

 

[SoonerBJJ]

I apologize to everyone for not knowing the correct verbiage to convey my point. I didn't realize that exact wording was so important.

If you are presuming to educate others then using the correct verbiage IS important.

You say you are open to learning and wanted this to be an open discussion yet you "ignore" the participants who question your lesson? Hmmm... note to self.

 

[robertwsimpson]

I apologize to everyone for not knowing the correct verbiage to convey my point. I didn't realize that exact wording was so important.

If you are presuming to educate others then using the correct verbiage IS important.

You say you are open to learning and wanted this to be an open discussion yet you "ignore" the participants who question your lesson? Hmmm... note to self.

You're questioning the person when you should be questioning the data. Unfortunately, there isn't much to question, as I have shown concrete examples of what I am asserting.

I am learning the correct words, and I am discussing this with people in this thread. I am not going to read comments that are disrespectful or sarcastic, and I am doing my best not to type comments that are that way. I'll thank you to do the same.