Focal Length - Depth of Field?

[Ysarex]

I think I see the problem here.

You're probably reacting to my earlier post where I said DOF = f/stop + magnification and I noted that magnification was focal length + subject distance + sensor size.

My fault there for being sloppy; although the point is still correct. The role sensor size plays isn't in determining magnification, but rather in limiting magnification relative to the size of the object being photographed. So DOF = f/stop + (recording) magnification.

Joe

 

[zcar21]

In other words magnification is height of the object divided into height of its image.

 

[zcar21]

This is what got me confused about focal length. Read it if you have time to waste.
Background blur vs focal length, absolute aperture and relative aperture

 

[EchoingWhisper]

In this context, magnification is the size of the image of the object (directly on the sensor or film) relative to the actual size of the object. At a magnification of X1 (1:1 reproduction -- same thing) the size of the object and the size of the image of the object are the same. You can photograph an entire quarter using a full frame sensor camera at a magnification of X1. You can't photograph an entire quarter at a magnification of X1 with any crop sensor camera. To photograph the whole quarter with a crop sensor camera you'll have a magnification less than X1 and with less magnification you'll get more DOF.

Joe

You're still not getting the point. Do the maths and you'll understand.

I did that math over 30 years ago.

I think you're hung up here on the definition of magnification. I'm not saying that magnification is calculated using sensor size. Magnification is calculated using focal length and lens/subject distance.

Magnification = focal length/(subject distance - focal length).

My point is this: To photograph the same exact object that has a fixed physical size a smaller sensor camera is forced to use a smaller magnification factor. As a result you get more DOF. Smaller sensor cameras must use smaller magnification factors to take the same photos as larger sensor cameras.

You may have a big head, but odds are it's not bigger than 250mm x 200mm. That means we could photograph your head with an 8x10 sheet film camera, include your entire head in the photo, and achieve a magnification factor of X1. The image of your head would be the same size as your head. Take a photo of someone's head, include their entire head in the photo, and use a camera with a micro 4/3s sensor. That sensor is 18mm X 13mm. You won't be able to use an X1 magnification factor. If they have an average size head your magnification factor will be more like X.08 AND, you'll get more DOF. Yep, I just did the math (again) and yes I understand.

Joe

Smaller sensor cameras must use smaller magnification factors to take the same photos as larger sensor cameras is correct, but I'm just trying to say magnification doesn't change due to sensor size, a 1 cm long rice will still be 1 cm on a larger sensor.

 

[Derrel]

>SNIP>>As for a link proving anything; I can direct you to a link that proves Obama is a Muslin, George Bush was a space alien and we never really landed a man on the moon.

Okay, where can I see this Obama muslin link? Is he's a muslin, he would make a fantastic photography backdrop! I'd love to order the Old Master's Obama muslin backdrop in both brown and blue shades!

 

[Ysarex]

You're still not getting the point. Do the maths and you'll understand.

I did that math over 30 years ago.

I think you're hung up here on the definition of magnification. I'm not saying that magnification is calculated using sensor size. Magnification is calculated using focal length and lens/subject distance.

Magnification = focal length/(subject distance - focal length).

My point is this: To photograph the same exact object that has a fixed physical size a smaller sensor camera is forced to use a smaller magnification factor. As a result you get more DOF. Smaller sensor cameras must use smaller magnification factors to take the same photos as larger sensor cameras.

You may have a big head, but odds are it's not bigger than 250mm x 200mm. That means we could photograph your head with an 8x10 sheet film camera, include your entire head in the photo, and achieve a magnification factor of X1. The image of your head would be the same size as your head. Take a photo of someone's head, include their entire head in the photo, and use a camera with a micro 4/3s sensor. That sensor is 18mm X 13mm. You won't be able to use an X1 magnification factor. If they have an average size head your magnification factor will be more like X.08 AND, you'll get more DOF. Yep, I just did the math (again) and yes I understand.

Joe

Smaller sensor cameras must use smaller magnification factors to take the same photos as larger sensor cameras is correct, but I'm just trying to say magnification doesn't change due to sensor size, a 1 cm long rice will still be 1 cm on a larger sensor.

Right -- we were talking past each other there for awhile.

Joe

 

[Ysarex]

>SNIP>>As for a link proving anything; I can direct you to a link that proves Obama is a Muslin, George Bush was a space alien and we never really landed a man on the moon.

Okay, where can I see this Obama muslin link? Is he's a muslin, he would make a fantastic photography backdrop! I'd love to order the Old Master's Obama muslin backdrop in both brown and blue shades!

This is entirely off topic but: You know I just finished a drive to Nashville and back and I spent a couple hours listening to American Family Radio on the way down. It was quite an unsettling experience and I swear three different people called in over the course of a one hour program and commented that Obama was trying to destroy America because he was a Muslum -- woah!

Joe

 

[Derrel]

Depth of field has proven to be a very difficult topic to discuss through forum posts. The Online Photographer blog did a week-long discussion of the factors at play. It's a difficult subject to discuss in part because there are SO MANY PEOPLE who have only a partial understanding of the subject. And a small subset of those people have written web-based articles about the subject, often spewing MIS-information about the subject. That mis-information is then repeated ad nauseum, all across the web. Over the last decade, with the web becoming a prominent place to discuss photographic issues, all too often often we see improper terms, poor examples, and examples that are shown purporting to show "truth", but which do not take into account the way depth of field behaves across the full spectrum of distances.

Depth of field at close distances and high image magnification is VERY, very shallow.In the macro- and close-up ranges, the focal length of the lens is nowhere near as large a factor as is the IMAGE MAGNIFICATION. Weird

Depth of field increases somewhat as camera-to-subject distances get longer than what I call the "close-up range". Then depth of field begins to increase at an EXTRAORDINARY rate as the focusing distance approaches the hyperfocal distance of whatever lens/sensor or lens/film SIZE combination a person is shooting with. As one can see by looking at depth of field charts or markings on lenses that still have them (!) there comes a distance at which depth of field at smaller apertures is QUITE deep, and even at wide apertures like f/4, at long ranges, depth of field can be measured in thousands of meters or more...extending from as close as say,a stone's throw away, and extending out to literally, Infinity.

In practical terms, depth of field issues can be grouped into the four types of scenarios. 1)macro-/extreme close-up 2) close-range 3) medium-distance, and 4) long-range.

One odd thing about depth of field is that the charts and tables can delineate the DOF in distance units of feet and meters quite nicely, but the way the human visual system perceives DOF is not so clear-cut. It's easy to discuss DOF in a quantitative (measured) sense, but much more-difficult to discuss it in a qualitative sense. Just exactly "how out of focus the background seems to be" is something that the on-line DOF calculators and lens-barrel charts are not good at conveying. The same holds true in my opinion for the way many people tend to dismiss the depth of field "look" or "rendering" differences between small-format captures, like 1.5x APS-C d-slr photos, and those shot on say, medium-format rollfilm cameras. One of the things I see the most with small-format images is that many times background that is said to be out of focus is actually QUITE recognizable, despite the depth of field calculator's assurances that the background is "outside" of the DOF zone...Lewis Collard's article mentions this in his summary, where he states:"

• Smaller formats mean less background blur: Given any relative aperture and any given framing of a subject, a larger film format will result in more background blur. This is because given any subject distance, background distance and relative aperture, increasing the focal length results in a disproportionate growth of the defocus blur circle." [http://lewiscollard.com/technical/background-blur/]

Again, this is a tough subject to write about...very challenging at times...and even then, when writing about DOF, it takes only one small misunderstanding, or one mis-statement, and an entire line of thinking can be brought down, or arguments caused,etc.,etc..

Background blur is another fun sub-issue. Take a look here: Bokeh and Background Blur Calculator- Bob Atkins Photography

 

[JClishe]

Depth of field has proven to be a very difficult topic to discuss through forum posts. The Online Photographer blog did a week-long discussion of the factors at play. It's a difficult subject to discuss in part because there are SO MANY PEOPLE who have only a partial understanding of the subject. And a small subset of those people have written web-based articles about the subject, often spewing MIS-information about the subject. That mis-information is then repeated ad nauseum, all across the web. Over the last decade, with the web becoming a prominent place to discuss photographic issues, all too often often we see improper terms, poor examples, and examples that are shown purporting to show "truth", but which do not take into account the way depth of field behaves across the full spectrum of distances.

Depth of field at close distances and high image magnification is VERY, very shallow.In the macro- and close-up ranges, the focal length of the lens is nowhere near as large a factor as is the IMAGE MAGNIFICATION. Weird

Depth of field increases somewhat as camera-to-subject distances get longer than what I call the "close-up range". Then depth of field begins to increase at an EXTRAORDINARY rate as the focusing distance approaches the hyperfocal distance of whatever lens/sensor or lens/film SIZE combination a person is shooting with. As one can see by looking at depth of field charts or markings on lenses that still have them (!) there comes a distance at which depth of field at smaller apertures is QUITE deep, and even at wide apertures like f/4, at long ranges, depth of field can be measured in thousands of meters or more...extending from as close as say,a stone's throw away, and extending out to literally, Infinity.

In practical terms, depth of field issues can be grouped into the four types of scenarios. 1)macro-/extreme close-up 2) close-range 3) medium-distance, and 4) long-range.

One odd thing about depth of field is that the charts and tables can delineate the DOF in distance units of feet and meters quite nicely, but the way the human visual system perceives DOF is not so clear-cut. It's easy to discuss DOF in a quantitative (measured) sense, but much more-difficult to discuss it in a qualitative sense. Just exactly "how out of focus the background seems to be" is something that the on-line DOF calculators and lens-barrel charts are not good at conveying. The same holds true in my opinion for the way many people tend to dismiss the depth of field "look" or "rendering" differences between small-format captures, like 1.5x APS-C d-slr photos, and those shot on say, medium-format rollfilm cameras. One of the things I see the most with small-format images is that many times background that is said to be out of focus is actually QUITE recognizable, despite the depth of field calculator's assurances that the background is "outside" of the DOF zone...Lewis Collard's article mentions this in his summary, where he states:"

• Smaller formats mean less background blur: Given any relative aperture and any given framing of a subject, a larger film format will result in more background blur. This is because given any subject distance, background distance and relative aperture, increasing the focal length results in a disproportionate growth of the defocus blur circle." [http://lewiscollard.com/technical/background-blur/]

Again, this is a tough subject to write about...very challenging at times...and even then, when writing about DOF, it takes only one small misunderstanding, or one mis-statement, and an entire line of thinking can be brought down, or arguments caused,etc.,etc..

Background blur is another fun sub-issue. Take a look here: Bokeh and Background Blur Calculator- Bob Atkins Photography

Best post in this thread.

 

[Shoal]

Hehehe... Beavis, he said CoC.

lol. Seriously though, this is a great thread. Thanks for starting this one

 

[KmH]

This is what got me confused about focal length. Read it if you have time to waste.
Background blur vs focal length, absolute aperture and relative aperture

No ****. That guy is full of it. :cyclops:

Nice graphs though.