Do lens focal length and sensor size influence depth of field

[daved31415]

I've read some conflicting information on this topic so I'm hoping to get it clarified here.

Does lens focal length and sensor size influence depth of field at all?

 

[Ysarex]

Yes.

Do you need specifics?

 

[fjrabon]

yes and no.

sensor size doesn't directly influence depth of field. But to get an equivalent frame, a larger sensor will require more magnification, and more magnification equates to shallower depth of field.

Focal length will typically mean you're more zoomed in on a subject and will thus equate to a shallower depth of field, again because of the increased magnification.

But the only things that change depth of field in a direct way are aperture and magnification.

 

[Derrel]

Q: Does lens focal length and sensor size influence depth of field at all?
A: Yes. However, there is a lot of conflicting information available, which is incorrect, wrong, internet-era nonsense that many people and web sites propagate. The B.S. is rampant.

Here is the basic idea: the larger the sensor or film, the LESS depth of field there is at each, equivalent picture angle, using format-appropriate lens lengths.

Each film or sensor size uses a normal lens that is roughly/approximately as long as the diagonal measurement of the format. So, for a square-format rollfilm TLR or SLR, the normal lens has been the 75mm f/2.8 or 80mm f/2.8 lens. For the 35mm film's 24x36mm size, the diagonal is 43mm, but the 50mm normal lens has been the standard for decades. For the APS-C sensor, the normal lens length is around 27mm, so the common 28mm lens can be considered a normal lens for an APS-C sensor camera. On the micro 4/3 sensor cameras, like those from Olympus, the diagonal measurement of the format is 22.4mm.

Very simply: LARGE format cameras, like those using 8x10 inch sheet film, will have very minimal depth of field. A smaller 4x5 inch sheet camera will have more depth of field than an 8x10 camera, but it's still fairly minimal. 120 rollfilm has a number of film formats, with 6x6 "square" and 6x7 fairly popular; this format does not have an overabundance of depth of field with its normal lens. 35mm film cameras shooting in the "135 format" (24x36mm image size) or so-called FF digital (also a 24x36mm size) will have a reasonable amount of depth of field with its normal lens.

An APS-C camera will give us more DOF than a 35mm film camera or a FF sensor digital. A camera with an m4/3 sensor has even more depth of field than an APS-C camera. The tiny sensors, like the itty-bitty ones used in smart phones allow users to create photos that have tremendously deep depth of field, even when their lenses are at wide apertures like f/2.8.

 

[ruifo]

Yes. Watch this video to see how:

 

[Ysarex]

yes and no.

sensor size doesn't directly influence depth of field. But to get an equivalent frame, a larger sensor will require more magnification, and more magnification equates to shallower depth of field.

Focal length will typically mean you're more zoomed in on a subject and will thus equate to a shallower depth of field, again because of the increased magnification.

But the only things that change depth of field in a direct way are aperture and magnification.

Have to argue with you on that one. If by magnification you mean as a function of focal length and distance. Then aperture and magnification are not enough to calculate DOF. You also need a value for circle of confusion and that value is derived from the sensor size.

Therefore: Same lens focal length at same f/stop from same distance, one shot FF and another APS, the APS version will have slightly less DOF because of the sensor size.

Joe

 

[fjrabon]

yes and no.

sensor size doesn't directly influence depth of field. But to get an equivalent frame, a larger sensor will require more magnification, and more magnification equates to shallower depth of field.

Focal length will typically mean you're more zoomed in on a subject and will thus equate to a shallower depth of field, again because of the increased magnification.

But the only things that change depth of field in a direct way are aperture and magnification.

Have to argue with you on that one. If by magnification you mean as a function of focal length and distance. Then aperture and magnification are not enough to calculate DOF. You also need a value for circle of confusion and that value is derived from the sensor size.

Therefore: Same lens focal length at same f/stop from same distance, one shot FF and another APS, the APS version will have slightly less DOF because of the sensor size.

Joe

right, but that's due to the magnification of the image, not the sensor size directly. If you kept the image sizes as constant multiples of sensor sizes, they would have equivalent depths of field.

 

[Ysarex]

yes and no.

sensor size doesn't directly influence depth of field. But to get an equivalent frame, a larger sensor will require more magnification, and more magnification equates to shallower depth of field.

Focal length will typically mean you're more zoomed in on a subject and will thus equate to a shallower depth of field, again because of the increased magnification.

But the only things that change depth of field in a direct way are aperture and magnification.

Have to argue with you on that one. If by magnification you mean as a function of focal length and distance. Then aperture and magnification are not enough to calculate DOF. You also need a value for circle of confusion and that value is derived from the sensor size.

Therefore: Same lens focal length at same f/stop from same distance, one shot FF and another APS, the APS version will have slightly less DOF because of the sensor size.

Joe

right, but that's due to the magnification of the image, not the sensor size directly. If you kept the image sizes as constant multiples of sensor sizes, they would have equivalent depths of field.

Yes, but that basically negates the sensor size difference. What you're saying is if you pretend there is no sensor size difference then there's no difference. Or if you crop the FF image so you're only using the APS equivalent then there's no difference -- same as saying APS is the same as APS. The whole point of having different size sensors is to use them.

DOF is always calculated to a standard print size. That is a requirement of any DOF calculation. You can't run the DOF math and compare DOF between an 8x10 print from one camera and a 5x7 print from another. All DOF calculations require a value for the CoC and that CoC value changes with sensor size. You can't calculate DOF without the CoC value.

Joe

 

[Derrel]

Yes, but that basically negates the sensor size difference. What you're saying is if you pretend there is no sensor size difference then there's no difference. Or if you crop the FF image so you're only using the APS equivalent then there's no difference -- same as saying APS is the same as APS. The whole point of having different size sensors is to use them.

Joe

Yes, a thousand-thousand times, yes!

 

[fjrabon]

yes and no.

sensor size doesn't directly influence depth of field. But to get an equivalent frame, a larger sensor will require more magnification, and more magnification equates to shallower depth of field.

Focal length will typically mean you're more zoomed in on a subject and will thus equate to a shallower depth of field, again because of the increased magnification.

But the only things that change depth of field in a direct way are aperture and magnification.

Have to argue with you on that one. If by magnification you mean as a function of focal length and distance. Then aperture and magnification are not enough to calculate DOF. You also need a value for circle of confusion and that value is derived from the sensor size.

Therefore: Same lens focal length at same f/stop from same distance, one shot FF and another APS, the APS version will have slightly less DOF because of the sensor size.

Joe

right, but that's due to the magnification of the image, not the sensor size directly. If you kept the image sizes as constant multiples of sensor sizes, they would have equivalent depths of field.

Yes, but that basically negates the sensor size difference. What you're saying is if you pretend there is no sensor size difference then there's no difference. Or if you crop the FF image so you're only using the APS equivalent then there's no difference -- same as saying APS is the same as APS. The whole point of having different size sensors is to use them.

DOF is always calculated to a standard print size. That is a requirement of any DOF calculation. You can't run the DOF math and compare DOF between an 8x10 print from one camera and a 5x7 print from another. All DOF calculations require a value for the CoC and that CoC value changes with sensor size. You can't calculate DOF without the CoC value.

Joe

No, I agree with you, but my point is that sensor size, directly, doesn't do anything. It's the magnification that sensor size leads us to that causes changes in depth of field.

 

[Ysarex]

yes and no.

sensor size doesn't directly influence depth of field. But to get an equivalent frame, a larger sensor will require more magnification, and more magnification equates to shallower depth of field.

Focal length will typically mean you're more zoomed in on a subject and will thus equate to a shallower depth of field, again because of the increased magnification.

But the only things that change depth of field in a direct way are aperture and magnification.

Have to argue with you on that one. If by magnification you mean as a function of focal length and distance. Then aperture and magnification are not enough to calculate DOF. You also need a value for circle of confusion and that value is derived from the sensor size.

Therefore: Same lens focal length at same f/stop from same distance, one shot FF and another APS, the APS version will have slightly less DOF because of the sensor size.

Joe

Saying
right, but that's due to the magnification of the image, not the sensor size directly. If you kept the image sizes as constant multiples of sensor sizes, they would have equivalent depths of field.

Yes, but that basically negates the sensor size difference. What you're saying is if you pretend there is no sensor size difference then there's no difference. Or if you crop the FF image so you're only using the APS equivalent then there's no difference -- same as saying APS is the same as APS. The whole point of having different size sensors is to use them.

DOF is always calculated to a standard print size. That is a requirement of any DOF calculation. You can't run the DOF math and compare DOF between an 8x10 print from one camera and a 5x7 print from another. All DOF calculations require a value for the CoC and that CoC value changes with sensor size. You can't calculate DOF without the CoC value.

Joe

No, I agree with you, but my point is that sensor size, directly, doesn't do anything. It's the magnification that sensor size leads us to that causes changes in depth of field.

Your original statement that DOF is a function of aperture + magnification is correct but very much simplified. Magnification has to be expanded to not only mean the magnification of the image on the recording media, film/sensor, but also final magnification of the entire image to the standard print.

Saying sensor size doesn't do anything directly is no different than saying focal length doesn't do anything directly. What if I said this: Focal length does not directly determine DOF since DOF is a function of f/stop and magnification. Since I can get the same magnification with different focal lengths (just alter distance), then focal length is not directly a factor in DOF. Is that going to work?

Sensor size is a required variable in the DOF equation. DOF can't be calculated without it. I think that's direct enough. DOF by any standard definition does not exist without sensor size as a variable in the math.

What you're trying to do is make the DOF determination out of place at the film/sensor recording stage -- laying one image over the other and if they're the same size (and same f/stop) then the DOF is the same. No such calculation exists for DOF. You can find DOF calculations (typically used in macro work) that take out the lens focal length (not a direct determinant) and use magnification and f/stop and CoC -- can't get away from CoC -- that's sensor size.

Joe

 

[Derrel]

I've read some conflicting information on this topic so I'm hoping to get it clarified here.

Does lens focal length and sensor size influence depth of field at all?

This web page Depth of Field Digital Photography and Crop Sensor Cameras - Bob Atkins Photography
will probably help you answer questions you have. Please pay careful note to the caveat about relative depth of field between different-sized capture formats! Atkins writes: "Again, this simple analysis only applies at "intermediate" distances, but we have to have that limitation if we want a "simple" formula. It only really breaks down when the lens is focused further than about halfway to the hyperfocal distance or when we get to magnifications near 1:1". And...keep in mind the TRUTH you seek, which is, "So the bottom line - and all you really need to know - is that DOF is inversely proportional to format size."

 

[fjrabon]

Since I can get the same magnification with different focal lengths (just alter distance), then focal length is not directly a factor in DOF. Is that going to work?

Yes, sure that works, that's exactly what direct versus indirect causation means. A direct cause causes something... directly. An indirect cause causes something through its influence on another factor.

The direct factors are aperture and magnification.

Magnification has two components, image magnification and viewer magnification.

Image magnification further has three subcomponents: subject distance, focal length and format size.

Viewer magnification is determined by how close the viewer is to the image, and how large the physical image is (and also how well they can see, but let's not go down that rabbit hole, unless not talking about viewer resolution is also too simplified).

None of the subcomponents themselves effect DoF except to the degree they impact magnification. Which is what I've been saying all along. Sensor size impacts DoF because it impacts magnification.

It's like saying temperature changes energy costs. It is true that when the temperature moves away from room temp, energy costs go up. But temperature itself doesn't charge us money, it causes us to adjust the heat/air, which then causes us to pay more for energy. Saying that the determinants of DoF are aperture and magnification is no more simplified than saying the determinants of energy costs are energy used and cost per unit of energy. Sure, those rates have different factors in them, but it isn't oversimplified to state that the two fundamental factors are magnification and aperture. It's just a different meta level.

 

[fjrabon]

And as far as the fact that DoF calculators use sensor seize to calculate DoF. Yes, obviously. But that's because DoF calculators use sensor size, focal length and subject distance to... calculate magnification.

 

[Ysarex]

Since I can get the same magnification with different focal lengths (just alter distance), then focal length is not directly a factor in DOF. Is that going to work?

Yes, sure that works, that's exactly what direct versus indirect causation means. A direct cause causes something... directly. An indirect cause causes something through its influence on another factor.

The direct factors are aperture and magnification.

It would be nice if it was that simple but DOF is too messy. The lens focal length as just a component of magnification only works to a degree. It falls apart when infinity winds up in the equation and it also doesn't work when you start to look at the DOF distribution around the focus plane. You can't really pull lens focal length out as just a sub factor canceled by distance -- it's more than that. In the macro equations you can do it but that only covers that one special use.

I really understand the seduction of simplification. I have to teach this mess and when it comes up in class I say the exact same thing to start off -- I even write it on the blackboard if the room still has one: DOF = f/stop + magnification. I agree it's the right place to start.

But if it could be that simple then you should be able to run variants on the DOF Master calculator using different focal lengths and as long the magnification is the same you should get the same DOF -- you'll get the same total DOF but you'll get different front/back distributions so focal length isn't really canceled out.

DOF = f/stop + magnification. (I even have that as the right answer on an exam!) But the caveat is that it's an oversimplification. When you break down magnification you can't say that the specific factor values cancel as long as they add up to the same magnification value -- it's not that clean; be nice if it was. So we at least have to write this: DOF = f/stop + magnification*

And when you flesh out that asterisk the sub factors carry weight.

Joe