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Author Topic: How (and why) does sensor size change DOF?  (Read 28243 times)

Don Haines

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Re: How (and why) does sensor size change DOF?
« Reply #15 on: July 15, 2013, 08:08:46 AM »
This is a confusing topic because the term "1.6 x crop" is a bit confusing to some. You hear that when you mount a 100mm lens on an APS-C camera it behaves like a 160MM lens on a FF camera. Really, what it means is that the field of view is the same as if you had a 160m lens on a crop camera, but it does not mean you have a 160mm lens.

The lens remains a 100mm lens. It's optical qualities do not change, it's depth of field does not change, it's aperture does not change. When you mount that lens on an APS-C camera you are using a sensor that will only sample the center 40% of the image ( 1/(1.6^2)).

If you were to use a FF sensor with 46Mpixels that was built with the same technology as an 18Mpixel APS-C sensor, the center part of the FF image capture would be ABSOLUTELY identical to that of the APS-C image.

So, sensor size has no effect on DOF or aperture.
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Re: How (and why) does sensor size change DOF?
« Reply #15 on: July 15, 2013, 08:08:46 AM »

neuroanatomist

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Re: How (and why) does sensor size change DOF?
« Reply #16 on: July 15, 2013, 08:42:29 AM »
There's no "story". DOF is a characteristic of a lens and has nothing to do with sensor size. End of story.

You couldn't be more wrong. Perhaps you're stuck in the circle of confusion...   ::)

If you take two pictures at the same position with the same lens on both a crop and a full frame, the distance between the nearest in focus object and the furthest away in focus object will be the same, so the depth of field does not change. Obviously the image captured will be different on each.

However, if you're talking about the depth of field for an equivalent picture, you'd have to move the full frame camera closer which would result in a reduced depth of field.

In the former case, my head shot just became an eyes-and-mouth shot that I had to delete because most people like to see their whole face in a portrait. The latter case is far more relevant to most types of photography. The exception might be macro photography, where at 1:1 magnification, you are at the MFD of the lens, regardless of sensor size.  But at macro distances, DoF is incredibly thin anyway (and the assumptions made by most DoF calculators don't hold).

As for DoF being solely a property of the lens, what, subject distance no longer matters??

The real story is that DoF is determined by subject magnification and lens aperture.
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rs

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Re: How (and why) does sensor size change DOF?
« Reply #17 on: July 15, 2013, 08:47:28 AM »
This is a confusing topic because the term "1.6 x crop" is a bit confusing to some. You hear that when you mount a 100mm lens on an APS-C camera it behaves like a 160MM lens on a FF camera. Really, what it means is that the field of view is the same as if you had a 160m lens on a crop camera, but it does not mean you have a 160mm lens.

The lens remains a 100mm lens. It's optical qualities do not change, it's depth of field does not change, it's aperture does not change. When you mount that lens on an APS-C camera you are using a sensor that will only sample the center 40% of the image ( 1/(1.6^2)).

If you were to use a FF sensor with 46Mpixels that was built with the same technology as an 18Mpixel APS-C sensor, the center part of the FF image capture would be ABSOLUTELY identical to that of the APS-C image.

So, sensor size has no effect on DOF or aperture.
That is all absolutely correct, except for one thing - in reality, if you moved from a crop system to a full frame system, you wouldn't take photos of the same scene from the same location with the same lens to end up with what is a wider framed photo.

As you say, if you do that and you crop it down, its identical to taking it with a crop camera.

But if you frame up the shot as you envisioned, either approximating it by altering your perspective by getting close, or ideally by keeping your perspective the same with a longer lens, presuming the aperture remains the same, the DoF is narrower with FF.
« Last Edit: July 15, 2013, 08:51:11 AM by rs »
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Re: How (and why) does sensor size change DOF?
« Reply #18 on: July 15, 2013, 09:09:43 AM »
This is a confusing topic because the term "1.6 x crop" is a bit confusing to some. You hear that when you mount a 100mm lens on an APS-C camera it behaves like a 160MM lens on a FF camera. Really, what it means is that the field of view is the same as if you had a 160m lens on a crop FF camera, but it does not mean you have a 160mm lens.

The lens remains a 100mm lens. It's optical qualities do not change, it's depth of field does not change, it's aperture does not change. When you mount that lens on an APS-C camera you are using a sensor that will only sample the center 40% of the image ( 1/(1.6^2)).

If you were to use a FF sensor with 46Mpixels that was built with the same technology as an 18Mpixel APS-C sensor, the center part of the FF image capture would be ABSOLUTELY identical to that of the APS-C image.

So, sensor size has no effect on DOF or aperture.

Good explanation, with wrong conclusion. Using similar logic we can say that lens-sensor combo has no effect on DoF as well, only subject (scene) size and distance matters. DoF is not a constant describing the lens or the sensor, but it is described by the combination + distance.
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Re: How (and why) does sensor size change DOF?
« Reply #19 on: July 15, 2013, 09:26:13 AM »
DoF ~ focal length * aperture * subject distance
If you want to shoot the same picture using the same lens with both FF and crop sensor cameras, you need to be closer to the subject with FF camera to get the same framing and that's the only difference.


this is really it... good simple explanation...
 and badgepiper added  a bit more....

so
to get the same framing ...in the result.....you move closer on FF...
and closer....shrinks the DOF...
in basic terms...

this is all you need to keep in mind .....IMO....


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Re: How (and why) does sensor size change DOF?
« Reply #20 on: July 15, 2013, 09:43:27 AM »
Here is a comparison I did a while ago for another thread much like this one. It is a comparison based on the above criteria. The three images are identical in dof, noise, perspective, angle of view, etc etc.

Admit it, since you knew they'd be the same you were lazy and they're all from the same camera :-p ... ok, just kidding.

Why does the sensor size affect the DOF and why do FF cameras have a smaller DOF?

In addition to the good explanations from privatebydesign above, here's what I personally tell myself: To get the same shot (i.e. field of view) on a ff camera in comparison to a crop, you have to walk towards the object with a ff, resulting in a thinner dof @same aperture.

This also means that in real life, camera-object range is equaly important for the resulting dof than your sensor size, you can get very thin dof shots out of a longer lens even @medium aperture like the 70-300L@f4 which is able to focus very near objects. Play around here: http://www.dofmaster.com/dofjs.html

Last not least, note that if people rave about thin dof they often mean "strong background blur" except if you like the "only the nose in focus" type shots, and same thing here: bokeh also strongly depends on focal length and object/background distance relationship.

PS: The phrase "ff has thinner dof" indeed confusing at first, /me also German :-p and I also had to refer to this site to get a proper explanation :-)

privatebydesign

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Re: How (and why) does sensor size change DOF?
« Reply #21 on: July 15, 2013, 09:49:14 AM »
Or you can mostly keep discussing it whilst completely ignoring the linked articles that explain it far better then any of us.  :)

What is dof? It is the distance at which a point is no longer a point but a circle (where something sharp is no longer sharp). The size of the circle or point is determined by the physical aperture (not the f stop value) and the subject magnification (not distance). Alone.

Depending on how you draw your comparison the sensor size can make a difference, or not.

Typical photo situations.
  • Where you have different sensors and want to take exactly the same image from the same place. To do that you use a shorter focal length lens on a crop camera, this has a smaller physical aperture for the same framing and f stop value, if you use a 160mm @ f2.8 on your FF camera you are shooting through a 57mm aperture . Whereas for your same framed shot with a crop camera a 100mm @ f2.8 gives you an aperture of 35mm (to get the same dof you need the same 57mm aperture, with your crop sensor and 100mm lens that would be f1.8 ). The smaller the aperture, in absolute terms, the greater the dof. Think pinhole cameras where everything is sharp.
  • Where you are focal length limited. If you are shooting :) taking a picture of a bird with your two cameras, but you don't have a lens long enough to fill the frame even with the crop camera. If you crop both images to the same size, the dof is the same in both images. The physical aperture you shot through was the same, and the subject magnification is the same, so you get the same depth of field.

If you change either the physical aperture, not necessarily the f stop value (50mm f2 is the same as 100mm f4, they both equal 25mm), or the subject magnification, you change the dof characteristics.

Subject magnification is not limited to the size it is reproduced on the sensor, it continues through to final output size. Take a slightly blurry image and down size it to a thumbnail, it will look sharp, if you make something smaller the dof increases, conversely enlarging something more makes those points and circles easier to differentiate. Take a razor sharp image and enlarge it 400%, it will look blurry from the same place, step back and it will regain its apparent sharpness. Look at a billboard from across the street, it looks sharp, now walk up to it and the "pixels" can each be two inches large.

You have to determine your specific image and comparison criteria to determine dof.
« Last Edit: July 15, 2013, 11:06:56 AM by privatebydesign »
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Re: How (and why) does sensor size change DOF?
« Reply #21 on: July 15, 2013, 09:49:14 AM »

neuroanatomist

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Re: How (and why) does sensor size change DOF?
« Reply #22 on: July 15, 2013, 09:52:44 AM »
Last not least, note that if people rave about thin dof they often mean "strong background blur" except if you like the "only the nose in focus" type shots, and same thing here: bokeh also strongly depends on focal length and object/background distance relationship.

Except that bokeh really refers to the quality of the OOF blur, independent of quantity.  But maybe that's a story for another day.  ;)
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privatebydesign

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Re: How (and why) does sensor size change DOF?
« Reply #23 on: July 15, 2013, 10:04:10 AM »
Here is a comparison I did a while ago for another thread much like this one. It is a comparison based on the above criteria. The three images are identical in dof, noise, perspective, angle of view, etc etc.

Admit it, since you knew they'd be the same you were lazy and they're all from the same camera :-p ... ok, just kidding.


 :) No I promise they were from three different cameras. Having said that you could mimic the test with one camera and a zoom lens, just crop the zoomed in images.

I actually did the original for a talk I did on sensors, dof and other related camera stuff. Somebody said they were the same image until I pointed out some of the greenery moves between them!
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Re: How (and why) does sensor size change DOF?
« Reply #24 on: July 15, 2013, 10:42:13 AM »
This has been confusing for me for a long time thank you everyone for clarifying it.

Here's my new understanding.
• Physically the DOF does not change because your not changing the lens (ie. set up a shot,  switch a crop to a FF, and you'll just get a wider field of view but same DOF).
• But in practice it essentially does change (ie set up a shot, switch bodies, now move the came to have the same field of view, now the DOF of the crop camera photo will be larger. Same goes for not moving the camera and changing focal length).

privatebydesign

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Re: How (and why) does sensor size change DOF?
« Reply #25 on: July 15, 2013, 11:04:53 AM »
This has been confusing for me for a long time thank you everyone for clarifying it.

Here's my new understanding.
• Physically the DOF does not change because your not changing the lens (ie. set up a shot,  switch a crop to a FF, and you'll just get a wider field of view but same DOF).
• But in practice it essentially does change (ie set up a shot, switch bodies, now move the came to have the same field of view, now the DOF of the crop camera photo will be larger. Same goes for not moving the camera and changing focal length).

If you change your physical aperture you change dof. If you change your subject magnification you change your dof. If you don't change either you don't change dof.

So both your points are valid.
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Re: How (and why) does sensor size change DOF?
« Reply #26 on: July 15, 2013, 11:47:37 AM »
This has been confusing for me for a long time thank you everyone for clarifying it.

Here's my new understanding.
• Physically the DOF does not change because your not changing the lens (ie. set up a shot,  switch a crop to a FF, and you'll just get a wider field of view but same DOF).
• But in practice it essentially does change (ie set up a shot, switch bodies, now move the came to have the same field of view, now the DOF of the crop camera photo will be larger. Same goes for not moving the camera and changing focal length).

Bingo! :)
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Re: How (and why) does sensor size change DOF?
« Reply #27 on: July 15, 2013, 01:30:52 PM »
Well, thank you guys for the great explanation! I got confused because so many people stated that the DoF for a lens will be different when you use it on FF. The thing that changes the DoF is the distance to the subject to get the same framing. Basic answer to a basic question. :)
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Re: How (and why) does sensor size change DOF?
« Reply #27 on: July 15, 2013, 01:30:52 PM »

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Re: How (and why) does sensor size change DOF?
« Reply #28 on: July 15, 2013, 01:43:41 PM »
picture is worth a thousand words

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Re: How (and why) does sensor size change DOF?
« Reply #29 on: July 15, 2013, 02:08:09 PM »

Excellent comparison!

Oh dear,

Anybody that wants to argue the point really needs to read my two links, and get a background understanding on the CoC, circle of confusion.

The first thing you have to do when trying to understand this dof stuff is set a standard for your comparison. If we are going to assume a same sized reproduction, ie 8"x10" print or "fill screen" on the same monitor, and frame the image the same while shooting from the same place, you have to take everything into consideration.

Here is a comparison I did a while ago for another thread much like this one. It is a comparison based on the above criteria. The three images are identical in dof, noise, perspective, angle of view, etc etc.

If you read and understand the quote from Wikipedia I posted you will understand if you change your comparison criteria you will change the dof characteristics.
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Re: How (and why) does sensor size change DOF?
« Reply #29 on: July 15, 2013, 02:08:09 PM »