Focal Length - Depth of Field?

[jake337]

You're right, aperture and distance are the only two factors affecting depth of field. People get confused because at lower focal lenght objects become smaller and appear in focus, at longer focal length objects become larger and blurry objects look even more blurry.

I was confused myself until I rethought the whole thing.

I mostly use f stops 4-8, so the main aspect affecting depth of field in my case is distance to subject.

Nope peolple still aren't understanding this.

Aperture, Focal Length and Distance to subject ALL affect DOF

BUT, in the Pratical Application, Where the foreground subject is framed the same, Aperture is the only thing that affects DOF
Focal Length doesn't because we move
Distance to Subject doesn't because we change focal Lengths

I don't agree with your statement of Practical Application. At least for portraiture, I'm using different focal lengths for different framing. I'm not gonna take a full body with my 50mm lens, swap to 135mm, then back up to achieve the same framing. I'm switching focal lengths for different framing. 50mm for full body, 85mm for 3/4 to chest up, and 100mm for neck and shoulders.

Ideally I would be using:
50mm for full body
85mm for 3/4
105mm for 1/2
135mm for chest up
200mm for shoulder up

 

[jake337]

The link dao provided proves that focal length doesn't affect dof. This is true provided that magnification stays the same, which is not always the case. the link gryhonslair99 provided shows interesting things to notice about dof, even though dof stays constant. At very short focal length almost 2/3 of the depth of field is in the rear of the focal point, 50mm or longer the difference begins to even out.

I would say it is most often not the case. I wouldn't go out and take an exposure at each focal length, keeping the same magnification of my subject, then go home and choose the perspective I like best. I would plan out the perspective I am seeking beforehand, then choose a lens which best suits that perspective.

 

[Ysarex]

The link dao provided proves that focal length doesn't affect dof. This is true provided that magnification stays the same, which is not always the case. the link gryhonslair99 provided shows interesting things to notice about dof, even though dof stays constant. At very short focal length almost 2/3 of the depth of field is in the rear of the focal point, 50mm or longer the difference begins to even out.

You seem to be quite confused. 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. (Not to say Dao's link contains bad info.)

Read what you just wrote. You start with this: "The link dao provided proves that focal length doesn't affect dof." That's a clear factual statement. Then you say this: "This is true provided that magnification stays the same, which is not always the case." In this second statement you're saying your first statement is wrong.

Next you say this: "the link gryhonslair99 provided shows interesting things to notice about dof, even though dof stays constant." What is that dangling clause, "even though dof stays constant," modifying? Are you trying to attach that as a qualifier to the beginning of the next sentence?

And your last assertion: "At very short focal length almost 2/3 of the depth of field is in the rear of the focal point, 50mm or longer the difference begins to even out." That's simply not accurate. It is correct that DOF tends to distribute unevenly around the focus plane, but not the way you describe it here. You are groping in the right direction however. And if in fact you are correct that with shorter focal lengths the uneven distribution of DOF is more uneven than with longer focal lengths (that is correct) then you're providing evidence to prove your first statement false.

Joe

 

[Ysarex]

Aperture, focal length, and distance to subject affect DoF. Additionally, distance between subject and background affect bokeh. Done and done.

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

 

[Ysarex]

You're right, aperture and distance are the only two factors affecting depth of field. People get confused because at lower focal lenght objects become smaller and appear in focus, at longer focal length objects become larger and blurry objects look even more blurry.

I was confused myself until I rethought the whole thing.

I mostly use f stops 4-8, so the main aspect affecting depth of field in my case is distance to subject.

Nope peolple still aren't understanding this.

Aperture, Focal Length and Distance to subject ALL affect DOF

BUT, in the Pratical Application, Where the foreground subject is framed the same, Aperture is the only thing that affects DOF
Focal Length doesn't because we move
Distance to Subject doesn't because we change focal Lengths

I don't agree with your statement of Practical Application. At least for portraiture, I'm using different focal lengths for different framing. I'm not gonna take a full body with my 50mm lens, swap to 135mm, then back up to achieve the same framing. I'm switching focal lengths for different framing. 50mm for full body, 85mm for 3/4 to chest up, and 100mm for neck and shoulders.

Ideally I would be using:
50mm for full body
85mm for 3/4
105mm for 1/2
135mm for chest up
200mm for shoulder up

So you're not suggesting that Rephargotohp's assertion is inaccurate, only that it doesn't resonate with the way you like to work taking portraits. You'd agree then in fact that his assertion is basically accurate; that with magnification held constant, a focal length change will have a negligible effect on DOF and only aperture can effect a DOF change.

It's worth noting that the OP didn't qualify the original question "as applied to portrait photography." So if you instead considered Rephargotohp's practical application in the context of say product illustration, it may in fact be spot on and resonate perfectly with the way a product photographer might work photographing say cosmetics.

And so then in the context of general photography as asked by the OP, you'd agree that Rephargotohp is accurately describing the phenomena in question. OK.

Joe

 

[EchoingWhisper]

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

Circle of confusion doesn't depend on sensor size, it depends on lens resolution, camera resolution and AA filter. You are confused. At the same distance, the same aperture, the same focal length, the same lens, any sensor size, the DOF is the same, Bokeh is also the same. I don't know how changing to DX will 'increase' depth of field. Maybe in practical application yes, but in your argument... sorry I have to say No.

 

[o hey tyler]

Actually, I am pretty sure CoC does depend on sensor size, or frame size.

 

[EchoingWhisper]

Actually, I am pretty sure CoC does depend on sensor size, or frame size.

Do you realise that larger sensor often have larger pixels?

 

[Ysarex]

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

Circle of confusion doesn't depend on sensor size, it depends on lens resolution, camera resolution and AA filter. You are confused. At the same distance, the same aperture, the same focal length, the same lens, any sensor size, the DOF is the same, Bokeh is also the same. I don't know how changing to DX will 'increase' depth of field. Maybe in practical application yes, but in your argument... sorry I have to say No.

First of all, in practical application to calculate DOF, standard circles of confusion are based on the format (sensor/film size) with smaller sensors being assigned smaller circles of confusion. You can see that's the case in this chart at DOFMaster:

Circles of Confusion for Digital Cameras

You're not hearing what I said. You said this: "At the same distance, the same aperture, the same focal length, the same lens, any sensor size, the DOF is the same..." I completely agree. You are right and I've never asserted otherwise.

I'm saying this: Use different sensor size cameras to make the same photograph -- content in each photo is exactly the same. So if it's a tight portrait it begins 2 inches above the subject's head and extends down to the second button of his her shirt/blouse, both cameras produce the same content. To do that they would have to use different focal length lenses from the same distance or be at different distances if they used the same lens.

Because of the decreased magnification from the smaller sensor camera that photo will have more DOF at the same f/stop.

I'm rarely confused.

Joe

 

[EchoingWhisper]

Also, the larger the circle of confusion, the higher the depth of field.

 

[EchoingWhisper]

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

Circle of confusion doesn't depend on sensor size, it depends on lens resolution, camera resolution and AA filter. You are confused. At the same distance, the same aperture, the same focal length, the same lens, any sensor size, the DOF is the same, Bokeh is also the same. I don't know how changing to DX will 'increase' depth of field. Maybe in practical application yes, but in your argument... sorry I have to say No.

First of all, in practical application to calculate DOF, standard circles of confusion are based on the format (sensor/film size) with smaller sensors being assigned smaller circles of confusion. You can see that's the case in this chart at DOFMaster:

Circles of Confusion for Digital Cameras

You're not hearing what I said. You said this: "At the same distance, the same aperture, the same focal length, the same lens, any sensor size, the DOF is the same..." I completely agree. You are right and I've never asserted otherwise.

I'm saying this: Use different sensor size cameras to make the same photograph -- content in each photo is exactly the same. So if it's a tight portrait it begins 2 inches above the subject's head and extends down to the second button of his her shirt/blouse, both cameras produce the same content. To do that they would have to use different focal length lenses from the same distance or be at different distances if they used the same lens.

Because of the decreased magnification from the smaller sensor camera that photo will have more DOF at the same f/stop.

I'm rarely confused.

Joe

During the film days, yes, because pixel size at the same ISO is normally the same, but not in the digital.

 

[Ysarex]

Circle of confusion doesn't depend on sensor size, it depends on lens resolution, camera resolution and AA filter. You are confused. At the same distance, the same aperture, the same focal length, the same lens, any sensor size, the DOF is the same, Bokeh is also the same. I don't know how changing to DX will 'increase' depth of field. Maybe in practical application yes, but in your argument... sorry I have to say No.

First of all, in practical application to calculate DOF, standard circles of confusion are based on the format (sensor/film size) with smaller sensors being assigned smaller circles of confusion. You can see that's the case in this chart at DOFMaster:

Circles of Confusion for Digital Cameras

You're not hearing what I said. You said this: "At the same distance, the same aperture, the same focal length, the same lens, any sensor size, the DOF is the same..." I completely agree. You are right and I've never asserted otherwise.

I'm saying this: Use different sensor size cameras to make the same photograph -- content in each photo is exactly the same. So if it's a tight portrait it begins 2 inches above the subject's head and extends down to the second button of his her shirt/blouse, both cameras produce the same content. To do that they would have to use different focal length lenses from the same distance or be at different distances if they used the same lens.

Because of the decreased magnification from the smaller sensor camera that photo will have more DOF at the same f/stop.

I'm rarely confused.

Joe

During the film days, yes, because pixel size at the same ISO is normally the same, but not in the digital.

Don't believe me -- run the math. Use a DOF calculator and run some simulations and then believe the math.

Joe

 

[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.

 

[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.

 

[zcar21]

Actually, I am pretty sure CoC does depend on sensor size, or frame size.

Agree.