Focal Length - Depth of Field?

[zcar21]

Depth of field: the distance between the nearest and farthest part with relatively shap detail. What is sharp for some may not be as sharp for somebody else, specially if you change the size and enlarge it. But I'll stick to my statement.

Focal length does not affect depth of field.

LOL, ok. Only when your subject magnification remains the same and the camera to subject distance changes to keep that magnification the same.

This is not usually the case. Let me ask you this, when doing portait work do you keep moving back everytime you swap to a lens of a longer focal length? Or do you keep a relative distance (10-15 feet) and change lens based on how you want to frame your next exposure?

When your camera to subject distance remains constant, while using a constant aperture, the focal length you choose WILL greatly effect your DOF. Period.

If you have compared the two pictures after enlarging one of them I'll might believe it, but my eyes tell it's just a different size.

 

[EchoingWhisper]

Depth of field: the distance between the nearest and farthest part with relatively shap detail. What is sharp for some may not be as sharp for somebody else, specially if you change the size and enlarge it. But I'll stick to my statement.

Focal length does not affect depth of field.

What is sharp may not be sharp for somebody else - exactly, the higher resolution, the more likely you are gonna perceive the lack of sharpness, which means smaller COC = higher resolution = less depth of field.

 

[EchoingWhisper]

Most calculation of COC is based on the d(iagonal)/1500, which isn't really accurate nowadays, because there wasn't such thing as sensor resolution last time and there wasn't any good lens resolution tests like DxOMark.

 

[Ysarex]

Not done. If you take the same photo with a full frame versus a crop sensor camera you'll have different DOF between the two.

Joe

It's the same, only you get more of the sides (it's not actually wider), because the subject reflected on the sensor is the same size at the same distance and same focal length. But again, in practical application, a larger sensor will get less DOF, cause' you move.

When I said "same photo" I meant the exact same content in each. So in the case of a portrait, the same subject framing using either camera. Sorry I wasn't more clear. In that case the smaller sensor camera will reproduce the subject at a smaller magnification so that at the same f/stop, the smaller sensor camera will produce a photo with more DOF.

You don't have to move to frame the subject the same way with different sensor size cameras. You can put say a 5D and a 7D side by side to take a photo and have both cameras frame the subject identically. The lens focal lengths in this case will be different.

Again we look to magnification as the active agent. With a smaller sensor everything is photographed at smaller magnifications. The circle of confusion for the smaller sensor is smaller and DOF is greater.

Joe

Here's the math using the DOFMaster calculator. I've checked their math in the past and it's good.

I selected a full frame 35mm camera and a 4/3 camera (2x crop factor). The subject is 4 feet wide. From the same distance both cameras are taking the exact same photo -- the content is identical (allowing any proportional variance). The DOF difference is due to the magnification reduction that results from using a smaller sensor.

Joe

 

[jake337]

Depth of field: the distance between the nearest and farthest part with relatively shap detail. What is sharp for some may not be as sharp for somebody else, specially if you change the size and enlarge it. But I'll stick to my statement.

Focal length does not affect depth of field.

LOL, ok. Only when your subject magnification remains the same and the camera to subject distance changes to keep that magnification the same.

This is not usually the case. Let me ask you this, when doing portait work do you keep moving back everytime you swap to a lens of a longer focal length? Or do you keep a relative distance (10-15 feet) and change lens based on how you want to frame your next exposure?

When your camera to subject distance remains constant, while using a constant aperture, the focal length you choose WILL greatly effect your DOF. Period.

If you have compared the two pictures after enlarging one of them I'll might believe it, but my eyes tell it's just a different size.

See now I'm confused. So is it the magnification? When I read it, the statement seems to contradict itself. I need a nap.

Understanding Depth of Field in Photography
"On the other hand, when standing in the same place and focusing on a subject at the same distance, a longer focal length lens will have a shallower depth of field (even though the pictures will show something entirely different). This is more representative of everyday use, but is an effect due to higher magnification, not focal length."

But to get that higher magnification, you need a longer focal length lens.....

 

[Ysarex]

LOL, ok. Only when your subject magnification remains the same and the camera to subject distance changes to keep that magnification the same.

This is not usually the case. Let me ask you this, when doing portait work do you keep moving back everytime you swap to a lens of a longer focal length? Or do you keep a relative distance (10-15 feet) and change lens based on how you want to frame your next exposure?

When your camera to subject distance remains constant, while using a constant aperture, the focal length you choose WILL greatly effect your DOF. Period.

If you have compared the two pictures after enlarging one of them I'll might believe it, but my eyes tell it's just a different size.

See now I'm confused. So is it the magnification?

Yes, it's the magnification.

When I read it, the statement seems to contradict itself. I need a nap.

Understanding Depth of Field in Photography
"On the other hand, when standing in the same place and focusing on a subject at the same distance, a longer focal length lens will have a shallower depth of field (even though the pictures will show something entirely different). This is more representative of everyday use, but is an effect due to higher magnification, not focal length."

But to get that higher magnification, you need a longer focal length lens.....

To get the higher magnification you can use a longer lens or you can get closer or you can use a larger sensor or you can use any combination of the three. In the end it's the magnification. What you don't want to do is think that it's a fixed property of the lens. The reason for the confusion is this: Many photographers first learn this equation:

DOF = f/stop + focal length. Short lenses have more DOF and long lenses have less -- fair enough.

Eventually they learn this equation:

DOF = f/stop + focal length + subject distance. But they have that idea holding on from before about short and long lenses which, although not wrong, can cause confusion.

Hopefully they eventually learn this equation:

DOF = f/stop + focal length + subject distance + sensor size. But since they're artists they're now hopelessly confused and they fail to apply one of the foundation rules of algebra: simplify to solve the equation.

Magnification = (focal length + subject distance + sensor size) and therefore:

DOF = f/stop + magnification.

==================================

Next you get somebody come along (zicar21) who realizes that if you hold magnification constant, which by the way gives you the same content and that makes sense, then focal length and subject size drop out of the equation and hey look!! same DOF from two different focal lengths! What I first learned about short and long lenses isn't true!!

Not so fast. It's a bit more complicated than that. You've got landscapes at infinity for one. It's just silly to say focal length doesn't effect DOF when shooting landscapes because you can't physically manipulate a 200mm lens to produce the same magnification as a 20mm with infinity in the photo.

It's still more complicated, because as I noted earlier in this thread, you've got to factor in the spatial relationship between camera to subject and subject to background distance.

And it's still more complicated because you'll next want to factor in the DOF distribution around the focal point.

================================

Fact: Focal length plays a roll in determining DOF.
Fact: With infinity in the photo it's a fact that a shorter lens will have a closer hyperfocal distance.
Fact: Without infinity in the photo and the magnification held constant, changing lens focal length will not significantly effect the total DOF.
Fact: Lens focal length will most certainly alter the DOF distribution as well as factor into the spatial relationship between camera/subject and subject/background distances and that will effect how blurry the background appears regardless of the total DOF.

OK now?

Joe

 

[EchoingWhisper]

LOL, ok. Only when your subject magnification remains the same and the camera to subject distance changes to keep that magnification the same.

This is not usually the case. Let me ask you this, when doing portait work do you keep moving back everytime you swap to a lens of a longer focal length? Or do you keep a relative distance (10-15 feet) and change lens based on how you want to frame your next exposure?

When your camera to subject distance remains constant, while using a constant aperture, the focal length you choose WILL greatly effect your DOF. Period.

If you have compared the two pictures after enlarging one of them I'll might believe it, but my eyes tell it's just a different size.

See now I'm confused. So is it the magnification? When I read it, the statement seems to contradict itself. I need a nap.

Understanding Depth of Field in Photography
"On the other hand, when standing in the same place and focusing on a subject at the same distance, a longer focal length lens will have a shallower depth of field (even though the pictures will show something entirely different). This is more representative of everyday use, but is an effect due to higher magnification, not focal length."

But to get that higher magnification, you need a longer focal length lens.....

Not really, you could walk closer to get that higher magnification.

 

[jake337]

If you have compared the two pictures after enlarging one of them I'll might believe it, but my eyes tell it's just a different size.

See now I'm confused. So is it the magnification?

Yes, it's the magnification.

When I read it, the statement seems to contradict itself. I need a nap.

Understanding Depth of Field in Photography
"On the other hand, when standing in the same place and focusing on a subject at the same distance, a longer focal length lens will have a shallower depth of field (even though the pictures will show something entirely different). This is more representative of everyday use, but is an effect due to higher magnification, not focal length."

But to get that higher magnification, you need a longer focal length lens.....

To get the higher magnification you can use a longer lens or you can get closer or you can use a larger sensor or you can use any combination of the three. In the end it's the magnification. What you don't want to do is think that it's a fixed property of the lens. The reason for the confusion is this: Many photographers first learn this equation:

DOF = f/stop + focal length. Short lenses have more DOF and long lenses have less -- fair enough.

Eventually they learn this equation:

DOF = f/stop + focal length + subject distance. But they have that idea holding on from before about short and long lenses which, although not wrong, can cause confusion.

Hopefully they eventually learn this equation:

DOF = f/stop + focal length + subject distance + sensor size. But since they're artists they're now hopelessly confused and they fail to apply one of the foundation rules of algebra: simplify to solve the equation.

Magnification = (focal length + subject distance + sensor size) and therefore:

DOF = f/stop + magnification.

==================================

Next you get somebody come along (zicar21) who realizes that if you hold magnification constant, which by the way gives you the same content and that makes sense, then focal length and subject size drop out of the equation and hey look!! same DOF from two different focal lengths! What I first learned about short and long lenses isn't true!!

Not so fast. It's a bit more complicated than that. You've got landscapes at infinity for one. It's just silly to say focal length doesn't effect DOF when shooting landscapes because you can't physically manipulate a 200mm lens to produce the same magnification as a 20mm with infinity in the photo.

It's still more complicated, because as I noted earlier in this thread, you've got to factor in the spatial relationship between camera to subject and subject to background distance.

And it's still more complicated because you'll next want to factor in the DOF distribution around the focal point.

================================

Fact: Focal length plays a roll in determining DOF.
Fact: With infinity in the photo it's a fact that a shorter lens will have a closer hyperfocal distance.
Fact: Without infinity in the photo and the magnification held constant, changing lens focal length will not significantly effect the total DOF.
Fact: Lens focal length will most certainly alter the DOF distribution as well as factor into the spatial relationship between camera/subject and subject/background distances and that will effect how blurry the background appears regardless of the total DOF.

OK now?

Joe

Perfect! Thanks for the clarification in defintion. Everything you stateed was in my brain, just not properly defined if that makes sense.

 

[EchoingWhisper]

It's the same, only you get more of the sides (it's not actually wider), because the subject reflected on the sensor is the same size at the same distance and same focal length. But again, in practical application, a larger sensor will get less DOF, cause' you move.

When I said "same photo" I meant the exact same content in each. So in the case of a portrait, the same subject framing using either camera. Sorry I wasn't more clear. In that case the smaller sensor camera will reproduce the subject at a smaller magnification so that at the same f/stop, the smaller sensor camera will produce a photo with more DOF.

You don't have to move to frame the subject the same way with different sensor size cameras. You can put say a 5D and a 7D side by side to take a photo and have both cameras frame the subject identically. The lens focal lengths in this case will be different.

Again we look to magnification as the active agent. With a smaller sensor everything is photographed at smaller magnifications. The circle of confusion for the smaller sensor is smaller and DOF is greater.

Joe

Here's the math using the DOFMaster calculator. I've checked their math in the past and it's good.

I selected a full frame 35mm camera and a 4/3 camera (2x crop factor). The subject is 4 feet wide. From the same distance both cameras are taking the exact same photo -- the content is identical (allowing any proportional variance). The DOF difference is due to the magnification reduction that results from using a smaller sensor.

Joe

Sensor size got nothing to do with magnification, it deals with field of view, not magnification. Does a 1:1 macro lens turn into a 2:1 macro lens when fitted on a DX body? No.

 

[Ysarex]

Sensor size got nothing to do with magnification, it deals with field of view, not magnification. Does a 1:1 macro lens turn into a 2:1 macro lens when fitted on a DX body? No.

Put a US quarter on the table in front of you.

Now photograph that quarter so that it completely fills the frame edge to edge. Start with a 4x5 camera and work you way down to a micro 4/3s.

Your photo of the quarter taken with a 4x5 camera will be larger than life-size as a quarter is no where near 4 inches wide.

Your photo of the quarter taken with a 70mm camera will also be larger than life-size since a quarter is also less than 2.25 inches wide.

Your photo of the quarter taken with a 35mm camera will be almost exactly life-size since a quarter is just about 24mm wide.

Your photo of the quarter taken with a micro 4/3s camera will be less than life-size.

Note that you can't photograph a quarter at 1:1 using a micro 4/3s camera and include the entire quarter in the photo. The sensor is physically smaller than a quarter and so to include the entire quarter in the frame the sensor size physically forces you to reduce magnification -- and you'll have more DOF as a result.

Repeat process using Grand Tetons, 6 story building, 5.5 foot person, the family dog and the same holds true. To record the same subject on a smaller sensor you have to reduce magnification and as a result you get more DOF.

Joe

 

[EchoingWhisper]

Sensor size got nothing to do with magnification, it deals with field of view, not magnification. Does a 1:1 macro lens turn into a 2:1 macro lens when fitted on a DX body? No.

Put a US quarter on the table in front of you.

Now photograph that quarter so that it completely fills the frame edge to edge. Start with a 4x5 camera and work you way down to a micro 4/3s.

Your photo of the quarter taken with a 4x5 camera will be larger than life-size as a quarter is no where near 4 inches wide.

Your photo of the quarter taken with a 70mm camera will also be larger than life-size since a quarter is also less than 2.25 inches wide.

Your photo of the quarter taken with a 35mm camera will be almost exactly life-size since a quarter is just about 24mm wide.

Your photo of the quarter taken with a micro 4/3s camera will be less than life-size.

Note that you can't photograph a quarter at 1:1 using a micro 4/3s camera and include the entire quarter in the photo. The sensor is physically smaller than a quarter and so to include the entire quarter in the frame the sensor size physically forces you to reduce magnification -- and you'll have more DOF as a result.

Repeat process using Grand Tetons, 6 story building, 5.5 foot person, the family dog and the same holds true. To record the same subject on a smaller sensor you have to reduce magnification and as a result you get more DOF.

Joe

Maybe you don't know what is magnification eh? I remember Overread telling me something about it. Magnification is subject size to fill the frame divided by sensor size. To fill the frame of all those sensors, you have to change your distance to the subject, so... the magnification got to do with you moving (focus distance) and not the sensor size.

 

[zcar21]

I did the test and looked at looked at some pictures to compare. Ysarex is right.

There is too much crap on the internet confusing people. Gotta be more careful.

 

[Ysarex]

Sensor size got nothing to do with magnification, it deals with field of view, not magnification. Does a 1:1 macro lens turn into a 2:1 macro lens when fitted on a DX body? No.

Put a US quarter on the table in front of you.

Now photograph that quarter so that it completely fills the frame edge to edge. Start with a 4x5 camera and work you way down to a micro 4/3s.

Your photo of the quarter taken with a 4x5 camera will be larger than life-size as a quarter is no where near 4 inches wide.

Your photo of the quarter taken with a 70mm camera will also be larger than life-size since a quarter is also less than 2.25 inches wide.

Your photo of the quarter taken with a 35mm camera will be almost exactly life-size since a quarter is just about 24mm wide.

Your photo of the quarter taken with a micro 4/3s camera will be less than life-size.

Note that you can't photograph a quarter at 1:1 using a micro 4/3s camera and include the entire quarter in the photo. The sensor is physically smaller than a quarter and so to include the entire quarter in the frame the sensor size physically forces you to reduce magnification -- and you'll have more DOF as a result.

Repeat process using Grand Tetons, 6 story building, 5.5 foot person, the family dog and the same holds true. To record the same subject on a smaller sensor you have to reduce magnification and as a result you get more DOF.

Joe

Maybe you don't know what is magnification eh? I remember Overread telling me something about it. Magnification is subject size to fill the frame divided by sensor size. To fill the frame of all those sensors, you have to change your distance to the subject, so... the magnification got to do with you moving (focus distance) and not the sensor size.

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

 

[EchoingWhisper]

Put a US quarter on the table in front of you.

Now photograph that quarter so that it completely fills the frame edge to edge. Start with a 4x5 camera and work you way down to a micro 4/3s.

Your photo of the quarter taken with a 4x5 camera will be larger than life-size as a quarter is no where near 4 inches wide.

Your photo of the quarter taken with a 70mm camera will also be larger than life-size since a quarter is also less than 2.25 inches wide.

Your photo of the quarter taken with a 35mm camera will be almost exactly life-size since a quarter is just about 24mm wide.

Your photo of the quarter taken with a micro 4/3s camera will be less than life-size.

Note that you can't photograph a quarter at 1:1 using a micro 4/3s camera and include the entire quarter in the photo. The sensor is physically smaller than a quarter and so to include the entire quarter in the frame the sensor size physically forces you to reduce magnification -- and you'll have more DOF as a result.

Repeat process using Grand Tetons, 6 story building, 5.5 foot person, the family dog and the same holds true. To record the same subject on a smaller sensor you have to reduce magnification and as a result you get more DOF.

Joe

Maybe you don't know what is magnification eh? I remember Overread telling me something about it. Magnification is subject size to fill the frame divided by sensor size. To fill the frame of all those sensors, you have to change your distance to the subject, so... the magnification got to do with you moving (focus distance) and not the sensor size.

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.

 

[Ysarex]

Maybe you don't know what is magnification eh? I remember Overread telling me something about it. Magnification is subject size to fill the frame divided by sensor size. To fill the frame of all those sensors, you have to change your distance to the subject, so... the magnification got to do with you moving (focus distance) and not the sensor size.

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