Thought experiment with depth of field...want to help?

arpaagent · 09-16-2009, 07:07 PM

Ok,

So I think I understand how depth of field works at a pretty decent level. Lets disregard the effect that format (sensor size) has on this discussion, because that's not what I'm curious about. I know you can have a smaller DOF for the same field of view on larger formats, but lets end it there

.

So here's what I know from a photographer's perspective: to get more blurred background of the same subject at the same magnification, either use longer lens at the same aperture (have to step back to maintain magnification) or use the same focal length and larger (smaller number) aperture, or some combination of both. Lets not worry about the case of a smaller focal length and bigger aperture...

Ok, so then I want to look at a case study with 2 lenses, a 200mm f2.8 and a 50mm f2.8, both wide open. Consider shooting a portrait with these 2 lenses, both with the same magnification. So we will step back to 4x the distance for the 200mm lens. From experience, correct me if I am wrong, but the background of the output with the 200mm will appear blurrier (more out of focus). If you disagree please say so.

Alright, taken the above as common fact (i hope), I'm a bit confused by what some depth of field calculators (Online Depth of Field Calculator) tell me. Basically, if I shoot with the 200/2.8 at 20 feet, it tells me that my depth of field is 0.33ft. And if I shoot with the 50/2.8 at 5 feet, it tells me that my depth of field is 0.33ft. So I will get the same depth of field in both shots according to the DOF calculator, and both will have the same subject magnification.

But somehow the backgrounds appear more blurred with the 200mm, maybe by enlarging the background through perspective compression. But I'm curious how this perspective compression doesn't come back and reduce the DOF even further. Is there some non-linear factor that I'm not taking into account when trying to think about this? It just doesn't make sense to me how I can have the same DOF on my subject, but have a blurrier background.

Calling those who have taken a course in optics, or just your general theorizers and discussionists!

discuss!

arpaagent · 09-16-2009, 08:14 PM

Ok,

not sure if anyone cares, but I did a little bit of google research to try and help clear this up for me. And it did. Wikipedia has some details in the Depth of Field article in the section "Foreground and Background Blur." Also, an awesome article that talks about this in even more detail and thought-provoking is Toothwalker Depth of Field Article.

Also, I didn't know this, but some macro lenses (dependent on the lens design) can actually have more DOF than macro lenses of shorter focal length! Just read the toothwalker article and find the section "DOF at constant magnification III." Maybe this helps explain why the A*/FA*200mm f4 Macro lenses are simply amazing. I always would have assumed that you get less DOF for the longer lenses, but it can in fact be the opposite!!

branphlake · 09-16-2009, 08:30 PM

I read through this post (before your second post) an hour ago, but was afraid to reply in fear of sounding like an idiot

.

I'll read through the articles, but my initial thought to your post was that I've always been told that longer focal lengths "smash" the photos more, redering a 2-d effect. I think for this reason, as well, you get a more out-of-focus looking background. We also have to take into account lens construction, etc, right? At some point, we are comparing apples to oranges.

Loved your post. Thanks for nerding out!

newarts · 09-16-2009, 10:56 PM

There is a difference between depth of field and how blurry a background object looks. Here's an example:

The photos on the left have the same DOF (same f-stop) but the red car in the background looks more "out of focus" in the top photo. Actually, if the image of the red car in the lower photo is magnified to be the same size as that in the top photo, it looks the same! The perspective effect which reduces the apparent size of the red car also changes our perception of of how "out of focus" it is.

A point of light becomes a disk as it goes out of focus. The absolute size of this disk increases with distance in front of or behind the plane of focus. However, the perspective effect simultaneously decreases the apparent size of the out-of-focus disk with distance behind the plane of focus to a degree that depends on the ratio of the distance from the camera to the subject and the distance from the subject to the background.

The net effect of these effects is that a long lens makes out of focus background objects the same distance behind the subject look larger hence we perceive them as being more out of focus.

Dave

PS another way to think of the long-lens effect on subject isolation is that with a long lens there are fewer items in the background and their edges are fuzzier, hence the background looks less "busy"; from a mathematical basis, there are fewer high frequency components in the background of the long lens photo competing with our visual attention. Imagine a backdrop with randomly sized polka dots painted on it; would out-of focus photos look different depending on what focal length lens you used?

Last edited by newarts; 09-16-2009 at 11:16 PM.

DavidWasch · 09-17-2009, 02:30 AM

Originally posted by newarts

Actually, if the image of the red car in the lower photo is magnified to be the same size as that in the top photo, it looks the same! The perspective effect which reduces the apparent size of the red car also changes our perception of of how "out of focus" it is.

A point of light becomes a disk as it goes out of focus. The absolute size of this disk increases with distance in front of or behind the plane of focus. However, the perspective effect simultaneously decreases the apparent size of the out-of-focus disk with distance behind the plane of focus to a degree that depends on the ratio of the distance from the camera to the subject and the distance from the subject to the background.

Thanks Dave; your clear example and description really brought this topic together for me. This forum spoils me; I'd normally have to spend a lot more time looking stuff up and puzzling stuff out. The resources here are incredible.

-David

geauxpez · 09-17-2009, 06:07 AM

Tamron has a tool that allows you to experiment with different distances, focal lengths and apertures. Type in the distance, then play with the sliders and watch how the image changes.

Cheers,

Adam

reeftool · 09-17-2009, 10:54 AM

Many of these discussions leave my head spinning and I mostly choose a lens based on my experience of what has worked for me in the past instead of looking at charts. It seems logical to me that the background would be different on the 2 different lenses. Newarts example shows it well. The statue is in focus in both shots but the distance is different for both the statue and the background. The long lens will have a much narrower DOF than the shorter lens. While both have an out of focus background, the long lens produces a much more pronounced blur. Also the background for the 100mm lens shot is much further away than than the 28 mm and we shouldn't expect to see the same amount of blur.

arpaagent · 09-17-2009, 06:14 PM

Originally posted by reeftool

Many of these discussions leave my head spinning and I mostly choose a lens based on my experience of what has worked for me in the past instead of looking at charts. It seems logical to me that the background would be different on the 2 different lenses. Newarts example shows it well. The statue is in focus in both shots but the distance is different for both the statue and the background. The long lens will have a much narrower DOF than the shorter lens. While both have an out of focus background, the long lens produces a much more pronounced blur. Also the background for the 100mm lens shot is much further away than than the 28 mm and we shouldn't expect to see the same amount of blur.

But that's the thing, the lenses have the same DOF. The long lens does not have a narrower DOF (until you get close to hyperfocal distance), but only makes the blur appear bigger due to perspective compression.

arbib · 09-17-2009, 07:12 PM

DOF Principles....
As all have said...The DOF is the same when you maintain a constant objects magnification ratio with different lenses on the same size sensor. But, a wider lens will include more background, but not more DOF -- if you maintain a constant objects magnification ratio. So, A telephoto lens can help isolate your subject from the background and include less background too.

reeftool · 09-18-2009, 07:08 AM

Yes, both have the same or similar depth of field as it is obvious that the backgrounds of both the photos are out of focus. The longer lens appears much more out of focus in the background. What we are seeing is the difference in bokeh between the 2 lenses. That is also the reason that fast lenses in the 75-100 mm range are so sought after for portrait shooters. Often the background disappears altogether.

09-16-2009, 07:07 PM	#1
arpaagent Veteran Member Join Date: Sep 2008 Location: Raleigh, NC Photos: Gallery Posts: 803	Thought experiment with depth of field...want to help? Ok, So I think I understand how depth of field works at a pretty decent level. Lets disregard the effect that format (sensor size) has on this discussion, because that's not what I'm curious about. I know you can have a smaller DOF for the same field of view on larger formats, but lets end it there . So here's what I know from a photographer's perspective: to get more blurred background of the same subject at the same magnification, either use longer lens at the same aperture (have to step back to maintain magnification) or use the same focal length and larger (smaller number) aperture, or some combination of both. Lets not worry about the case of a smaller focal length and bigger aperture... Ok, so then I want to look at a case study with 2 lenses, a 200mm f2.8 and a 50mm f2.8, both wide open. Consider shooting a portrait with these 2 lenses, both with the same magnification. So we will step back to 4x the distance for the 200mm lens. From experience, correct me if I am wrong, but the background of the output with the 200mm will appear blurrier (more out of focus). If you disagree please say so. Alright, taken the above as common fact (i hope), I'm a bit confused by what some depth of field calculators (Online Depth of Field Calculator) tell me. Basically, if I shoot with the 200/2.8 at 20 feet, it tells me that my depth of field is 0.33ft. And if I shoot with the 50/2.8 at 5 feet, it tells me that my depth of field is 0.33ft. So I will get the same depth of field in both shots according to the DOF calculator, and both will have the same subject magnification. But somehow the backgrounds appear more blurred with the 200mm, maybe by enlarging the background through perspective compression. But I'm curious how this perspective compression doesn't come back and reduce the DOF even further. Is there some non-linear factor that I'm not taking into account when trying to think about this? It just doesn't make sense to me how I can have the same DOF on my subject, but have a blurrier background. Calling those who have taken a course in optics, or just your general theorizers and discussionists! discuss!

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09-16-2009, 08:14 PM	#2
arpaagent Veteran Member Join Date: Sep 2008 Location: Raleigh, NC Photos: Gallery Posts: 803 Original Poster	Ok, not sure if anyone cares, but I did a little bit of google research to try and help clear this up for me. And it did. Wikipedia has some details in the Depth of Field article in the section "Foreground and Background Blur." Also, an awesome article that talks about this in even more detail and thought-provoking is Toothwalker Depth of Field Article. Also, I didn't know this, but some macro lenses (dependent on the lens design) can actually have more DOF than macro lenses of shorter focal length! Just read the toothwalker article and find the section "DOF at constant magnification III." Maybe this helps explain why the A/FA200mm f4 Macro lenses are simply amazing. I always would have assumed that you get less DOF for the longer lenses, but it can in fact be the opposite!!

09-16-2009, 08:30 PM	#3
branphlake Forum Member Join Date: Jun 2008 Posts: 84	I read through this post (before your second post) an hour ago, but was afraid to reply in fear of sounding like an idiot . I'll read through the articles, but my initial thought to your post was that I've always been told that longer focal lengths "smash" the photos more, redering a 2-d effect. I think for this reason, as well, you get a more out-of-focus looking background. We also have to take into account lens construction, etc, right? At some point, we are comparing apples to oranges. Loved your post. Thanks for nerding out!

09-16-2009, 10:56 PM	#4
newarts Inactive Account Join Date: Dec 2008 Location: Ames, Iowa, USA Photos: Albums Posts: 2,965	There is a difference between depth of field and how blurry a background object looks. Here's an example: The photos on the left have the same DOF (same f-stop) but the red car in the background looks more "out of focus" in the top photo. Actually, if the image of the red car in the lower photo is magnified to be the same size as that in the top photo, it looks the same! The perspective effect which reduces the apparent size of the red car also changes our perception of of how "out of focus" it is. A point of light becomes a disk as it goes out of focus. The absolute size of this disk increases with distance in front of or behind the plane of focus. However, the perspective effect simultaneously decreases the apparent size of the out-of-focus disk with distance behind the plane of focus to a degree that depends on the ratio of the distance from the camera to the subject and the distance from the subject to the background. The net effect of these effects is that a long lens makes out of focus background objects the same distance behind the subject look larger hence we perceive them as being more out of focus. Dave PS another way to think of the long-lens effect on subject isolation is that with a long lens there are fewer items in the background and their edges are fuzzier, hence the background looks less "busy"; from a mathematical basis, there are fewer high frequency components in the background of the long lens photo competing with our visual attention. Imagine a backdrop with randomly sized polka dots painted on it; would out-of focus photos look different depending on what focal length lens you used? Last edited by newarts; 09-16-2009 at 11:16 PM.

09-17-2009, 02:30 AM	#5
DavidWasch Senior Member Join Date: Jul 2009 Location: Connecticut, USA Photos: Gallery Posts: 167	Originally posted by newarts Actually, if the image of the red car in the lower photo is magnified to be the same size as that in the top photo, it looks the same! The perspective effect which reduces the apparent size of the red car also changes our perception of of how "out of focus" it is. A point of light becomes a disk as it goes out of focus. The absolute size of this disk increases with distance in front of or behind the plane of focus. However, the perspective effect simultaneously decreases the apparent size of the out-of-focus disk with distance behind the plane of focus to a degree that depends on the ratio of the distance from the camera to the subject and the distance from the subject to the background. Thanks Dave; your clear example and description really brought this topic together for me. This forum spoils me; I'd normally have to spend a lot more time looking stuff up and puzzling stuff out. The resources here are incredible. -David

09-17-2009, 06:07 AM	#6
geauxpez Veteran Member Join Date: Jul 2008 Location: Atlanta Photos: Gallery Posts: 1,196	Tamron has a tool that allows you to experiment with different distances, focal lengths and apertures. Type in the distance, then play with the sliders and watch how the image changes. Cheers, Adam

09-17-2009, 10:54 AM	#7
reeftool Pentaxian Join Date: Dec 2007 Location: Upstate New York Photos: Gallery \| Albums Posts: 9,553	Many of these discussions leave my head spinning and I mostly choose a lens based on my experience of what has worked for me in the past instead of looking at charts. It seems logical to me that the background would be different on the 2 different lenses. Newarts example shows it well. The statue is in focus in both shots but the distance is different for both the statue and the background. The long lens will have a much narrower DOF than the shorter lens. While both have an out of focus background, the long lens produces a much more pronounced blur. Also the background for the 100mm lens shot is much further away than than the 28 mm and we shouldn't expect to see the same amount of blur.

09-17-2009, 06:14 PM	#8
arpaagent Veteran Member Join Date: Sep 2008 Location: Raleigh, NC Photos: Gallery Posts: 803 Original Poster	Originally posted by reeftool Many of these discussions leave my head spinning and I mostly choose a lens based on my experience of what has worked for me in the past instead of looking at charts. It seems logical to me that the background would be different on the 2 different lenses. Newarts example shows it well. The statue is in focus in both shots but the distance is different for both the statue and the background. The long lens will have a much narrower DOF than the shorter lens. While both have an out of focus background, the long lens produces a much more pronounced blur. Also the background for the 100mm lens shot is much further away than than the 28 mm and we shouldn't expect to see the same amount of blur. But that's the thing, the lenses have the same DOF. The long lens does not have a narrower DOF (until you get close to hyperfocal distance), but only makes the blur appear bigger due to perspective compression.

09-17-2009, 07:12 PM	#9
arbib Veteran Member Join Date: Apr 2009 Location: Camby, Indiana Photos: Gallery Posts: 379	DOF Principles.... As all have said...The DOF is the same when you maintain a constant objects magnification ratio with different lenses on the same size sensor. But, a wider lens will include more background, but not more DOF -- if you maintain a constant objects magnification ratio. So, A telephoto lens can help isolate your subject from the background and include less background too.

09-18-2009, 07:08 AM	#10
reeftool Pentaxian Join Date: Dec 2007 Location: Upstate New York Photos: Gallery \| Albums Posts: 9,553	Yes, both have the same or similar depth of field as it is obvious that the backgrounds of both the photos are out of focus. The longer lens appears much more out of focus in the background. What we are seeing is the difference in bokeh between the 2 lenses. That is also the reason that fast lenses in the 75-100 mm range are so sought after for portrait shooters. Often the background disappears altogether.