Focal Length - Depth of Field?

[Ysarex]

Focal length does play a role in DOF, but many (most) photographers are confused about it.

Think of it this way: DOF is a function of f/stop and magnification. At a constant magnification you can change focal lengths without having much effect on DOF. Magnification is a function of focal length, distance to subject and sensor size. Because an immediate and obvious result of changing focal lengths (without moving) is a magnification change it became very easy to conclude that the DOF change that resulted was due to the focal length change -- fair enough, but then the next step was to start claiming that long lenses have inherently less DOF and short lenses inherently more -- now we're getting into possible trouble. If you have a multi-format camera for example that can switch between 4x5 sheet and 6x7 roll is your 90mm lens wide angle or normal -- different DOF from the same lens?

Consider this real story. I'm standing behind the counter in a camera store where I sell the pro gear. In walks a photographer wannabe who has done some weddings and wants to move up to illustration. He has a print in his hands (show off) of open end wrenches shot with the camera at a 45 degree angle. Says he's not getting the DOF he needs from the 80mm (medium format film) and maybe it's time to break the piggy and get the 50mm. That 50mm is serious money and so I say, "Aren't you going to just move in closer with the 50mm so you have the same crop?" "Yeah," he says. "Then the DOF will be the same," I say (and I was right). Since he was the photographer and I was just a sales clerk he put me in my place and explained DOF to me. I thanked him and sold him the lens.

There's always infinity. Although the above story illustrates that magnification and not focal length is the active factor you have to come back to the reality that every focal length has a unique relationship with infinity. As such, at any given f/stop, the hyperfocal distance in a landscape is going to keep moving closer to the camera as the lens focal length shortens. Magnification is likewise being reduced (see bold above), but it is nonsense to suggest to a landscape photographer if s/he just backs up far enough with the 300mm the DOF will be the same as with the 30mm. In photography there's a limit to infinity (always wanted to say that). So it is also reasonable to say that practically speaking very wide lenses typically produce photos with broad DOF and very long lenses typically produce photos with shallow DOF.

Joe

 

[EchoingWhisper]

Focal length does play a role in DOF, but many (most) photographers are confused about it.

Think of it this way: DOF is a function of f/stop and magnification. At a constant magnification you can change focal lengths without having much effect on DOF. Magnification is a function of focal length, distance to subject and sensor size. Because an immediate and obvious result of changing focal lengths (without moving) is a magnification change it became very easy to conclude that the DOF change that resulted was due to the focal length change -- fair enough, but then the next step was to start claiming that long lenses have inherently less DOF and short lenses inherently more -- now we're getting into possible trouble. If you have a multi-format camera for example that can switch between 4x5 sheet and 6x7 roll is your 90mm lens wide angle or normal -- different DOF from the same lens?

Consider this real story. I'm standing behind the counter in a camera store where I sell the pro gear. In walks a photographer wannabe who has done some weddings and wants to move up to illustration. He has a print in his hands (show off) of open end wrenches shot with the camera at a 45 degree angle. Says he's not getting the DOF he needs from the 80mm (medium format film) and maybe it's time to break the piggy and get the 50mm. That 50mm is serious money and so I say, "Aren't you going to just move in closer with the 50mm so you have the same crop?" "Yeah," he says. "Then the DOF will be the same," I say (and I was right). Since he was the photographer and I was just a sales clerk he put me in my place and explained DOF to me. I thanked him and sold him the lens.

There's always infinity. Although the above story illustrates that magnification and not focal length is the active factor you have to come back to the reality that every focal length has a unique relationship with infinity. As such, at any given f/stop, the hyperfocal distance in a landscape is going to keep moving closer to the camera as the lens focal length shortens. Magnification is likewise being reduced (see bold above), but it is nonsense to suggest to a landscape photographer if s/he just backs up far enough with the 300mm the DOF will be the same as with the 30mm. In photography there's a limit to infinity (always wanted to say that). So it is also reasonable to say that practically speaking very wide lenses typically produce photos with broad DOF and very long lenses typically produce photos with shallow DOF.

Joe

Yep, I understand what you are saying, as long as the depth of field doesn't reach infinity, at the same format, any focal length will have the same depth of field. But once you are reaching infinity, short lenses start to have advantage in depth of field. In short, use short lens in landscape, for any other uses, use any focal length you want, as long as you get the Bokeh you want and get near enough to the subject.

 

[Ysarex]

Focal length does play a role in DOF, but many (most) photographers are confused about it.

Think of it this way: DOF is a function of f/stop and magnification. At a constant magnification you can change focal lengths without having much effect on DOF. Magnification is a function of focal length, distance to subject and sensor size. Because an immediate and obvious result of changing focal lengths (without moving) is a magnification change it became very easy to conclude that the DOF change that resulted was due to the focal length change -- fair enough, but then the next step was to start claiming that long lenses have inherently less DOF and short lenses inherently more -- now we're getting into possible trouble. If you have a multi-format camera for example that can switch between 4x5 sheet and 6x7 roll is your 90mm lens wide angle or normal -- different DOF from the same lens?

Consider this real story. I'm standing behind the counter in a camera store where I sell the pro gear. In walks a photographer wannabe who has done some weddings and wants to move up to illustration. He has a print in his hands (show off) of open end wrenches shot with the camera at a 45 degree angle. Says he's not getting the DOF he needs from the 80mm (medium format film) and maybe it's time to break the piggy and get the 50mm. That 50mm is serious money and so I say, "Aren't you going to just move in closer with the 50mm so you have the same crop?" "Yeah," he says. "Then the DOF will be the same," I say (and I was right). Since he was the photographer and I was just a sales clerk he put me in my place and explained DOF to me. I thanked him and sold him the lens.

There's always infinity. Although the above story illustrates that magnification and not focal length is the active factor you have to come back to the reality that every focal length has a unique relationship with infinity. As such, at any given f/stop, the hyperfocal distance in a landscape is going to keep moving closer to the camera as the lens focal length shortens. Magnification is likewise being reduced (see bold above), but it is nonsense to suggest to a landscape photographer if s/he just backs up far enough with the 300mm the DOF will be the same as with the 30mm. In photography there's a limit to infinity (always wanted to say that). So it is also reasonable to say that practically speaking very wide lenses typically produce photos with broad DOF and very long lenses typically produce photos with shallow DOF.

Joe

Yep, I understand what you are saying, as long as the depth of field doesn't reach infinity, at the same format, any focal length will have the same depth of field.

With magnification held constant -- same framing for the subject.

But once you are reaching infinity, short lenses start to have advantage in depth of field....

It's still a simplified explanation, but yes!

The next factor you want to fold into the equation is the spatial relationship between the camera, subject and background. Think of it in terms of significant distances. An extreme example: The sun is 93 million miles from Earth give or take. Exposure at sea level is X and if you climb to the top of Mt. Whitney exposure is still X. That 15,000 feet (rounded) from sea level to the top of Mt. Whitney may seem significant to you if you try and climb it but relative to 93 million miles it ain't spit.

So, the relative significance of your subject to background distance changes with the distance between the subject and camera. Work an example: Subject is a couple seated 6 feet in front of a flowering shrub. You're 12 feet away with the camera (75mm lens on a 35mm full frame) and your DOF at f/8 is a total of 4 feet. How significant is the shrub 6 feet behind the subject? Does it appear out-of-focus? Is it possible the magnification reduction from the extra 6 foot distance will make the shrub appear fairly sharp? Oh no! Now back up to 24 feet and switch to a 150mm lens. The framing is the same. At f/8 the DOF is the same total 4 feet. BUT! You've reduce the significance of the distance to the background shrub and so reduced the significance of the magnification reduction. As a result the shrub appears blurrier in the second photo.

Again you can see how photographers over time have made spurious connections for causality here. In the second photo taken with the longer lens it's not really the focal length that's responsible for the blurrier shrub, it's the alteration in significance distances -- it's because you backed up and made the subject to background distance less significant.

Joe

 

[Dao]

I read this article awhile back and it is quit useful.

DOF2

 

[bp4life71]

Focal length DOES affect DOF. A 20mm lens on an APS camera, focused at 10 feet, at f/4 will have a DOF from 5'-8" to 41'. A 200mm lens with the same settings will have a DOF from 9'-11" to 10'-1"

Where is the math on this? Id like to be able to calculate this! Good stuff!

 

[table1349]

Focal length DOES affect DOF. A 20mm lens on an APS camera, focused at 10 feet, at f/4 will have a DOF from 5'-8" to 41'. A 200mm lens with the same settings will have a DOF from 9'-11" to 10'-1"

Where is the math on this? Id like to be able to calculate this! Good stuff!

How to Calculate Depth of Field | Camera Dojo - Take better pictures with our photography tutorials and podcast

 

[zcar21]

I thought: Aperture controls depth of field, then why does focal length control depth of field too?

I used to think that the longer the focal length, the lesser the depth of field, but not anymore.
Since longer focal length compresses the distance between objects, it increases the size of everything further away. Thus, the circle of confusion (bokeh/blur) gets larger too, which makes the background LOOKS more blurry. In reality, the depth of field is just the same, only that the defocused area has larger circle of confusion. The defocused area in shorter focal lengths is not able to be seen because of diffraction/anti aliasing etc.

I might be wrong so, point out any mistakes of my point of view. I don't really understand what I'm talking... lol

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.

 

[JClishe]

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

 

[Ysarex]

I thought: Aperture controls depth of field, then why does focal length control depth of field too?

I used to think that the longer the focal length, the lesser the depth of field, but not anymore.
Since longer focal length compresses the distance between objects, it increases the size of everything further away. Thus, the circle of confusion (bokeh/blur) gets larger too, which makes the background LOOKS more blurry. In reality, the depth of field is just the same, only that the defocused area has larger circle of confusion. The defocused area in shorter focal lengths is not able to be seen because of diffraction/anti aliasing etc.

I might be wrong so, point out any mistakes of my point of view. I don't really understand what I'm talking... lol

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 blurryLen.

Lenses focus on a 2D flat plane. Objects in front of or behind that focus plane appearing in focus IS DOF.

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.

If you used f/stops 2-4 or 8-16 then distance wouldn't be the main aspect affecting DOF?

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

 

[Rephargotohp]

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

 

[EchoingWhisper]

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.

 

[jake337]

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

The lens you choose defines your bokeh. Bokeh cannot be changed.

 

[zcar21]

I thought: Aperture controls depth of field, then why does focal length control depth of field too?

I used to think that the longer the focal length, the lesser the depth of field, but not anymore.
Since longer focal length compresses the distance between objects, it increases the size of everything further away. Thus, the circle of confusion (bokeh/blur) gets larger too, which makes the background LOOKS more blurry. In reality, the depth of field is just the same, only that the defocused area has larger circle of confusion. The defocused area in shorter focal lengths is not able to be seen because of diffraction/anti aliasing etc.

I might be wrong so, point out any mistakes of my point of view. I don't really understand what I'm talking... lol

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 blurryLen.

Lenses focus on a 2D flat plane. Objects in front of or behind that focus plane appearing in focus IS DOF.

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.

If you used f/stops 2-4 or 8-16 then distance wouldn't be the main aspect affecting DOF?

Joe

Objects appear focused, but enlarge it to full size you'll notice it Is out of focus. About aperture I either use f4, f5.6, or f8 only three different DOF, but if I add all the different distances to subject the DOF is ilimited.

 

[zcar21]

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.