How (and why) does sensor size change DOF?

[ecka]

Prepared to shrink yourself 100 times and tell me that I am mad, LOL! I'm laughing and crying at the same time! ;D :'( ;D :'(

Even you have confirm that shrinking picture increase DoF do have a real world implication, BUT

Shrink a picture 100 times? Yes! Shrink a human 100 times? OMGWTFBBQChickenWings!

In the end, isn't photography all about perception? :

I'm glad that you are enjoying the discussion. It wasn't my intent to offend anyone.
I didn't confirm the nonsense. Shrinking the picture simply makes it's details imperceptible to you. If you can't see a bacteria, it doesn't mean that there are none. In case with a magic shrinking machine, the details are made smaller so you can still see them by shrinking yourself or maybe using a microscope. However, when you shrink the image on your screen or print a thumbnail, you are just losing the information. Just like for a half-blind person all your images can look same "sharp" or same "blurry". In fact, for him, sharp and blur looks the same. CoC is about perception. DoF is not, it is about information, same as photography. Once the light of an optical image hits the sensor, it is gone, all that's left is the information gathered by the electronics. If you shoot a picture that has nothing in focus, it doesn't matter to what resolution you downsize it, no new information will occur (except the false one). You can manipulate the image in any way you want, but in relativity to reality DoF won't change a bit. If photography is just a form of art for you and perception is the only thing that matters, then perhaps you are not even trying to understand what I'm talking about.

 

[BozillaNZ]

I'm glad that you are enjoying the discussion. It wasn't my intent to offend anyone.
I didn't confirm the nonsense. Shrinking the picture simply makes it's details imperceptible to you. If you can't see a bacteria, it doesn't mean that there are none. In case with a magic shrinking machine, the details are made smaller so you can still see them by shrinking yourself or maybe using a microscope. However, when you shrink the image on your screen or print a thumbnail, you are just losing the information. Just like for a half-blind person all your images can look same "sharp" or same "blurry". In fact, for him, sharp and blur looks the same. CoC is about perception. DoF is not, it is about information, same as photography. Once the light of an optical image hits the sensor, it is gone, all that's left is the information gathered by the electronics. If you shoot a picture that has nothing in focus, it doesn't matter to what resolution you downsize it, no new information will occur (except the false one). You can manipulate the image in any way you want, but in relativity to reality DoF won't change a bit. If photography is just a form of art for you and perception is the only thing that matters, then perhaps you are not even trying to understand what I'm talking about.

Well by your standard, in reality there is nothing really 'in focus'. The focus 'plane' is a hypothetical thing that has zero thickness. Also on the 'true' focus plane every light point has diameter of 0. Anything in front, or behind this zero thickness hypothetical plane is deemed out of focus because they have a CoC > absolute 0.

The sensor sees something in focus not because they are in focus, but simply because the CoC is smaller than sensor's pixel could distinguish. So what you say? That the image the sensor captures is the real world? It is not.

If above assumption is correct, then take an example, if I shoot a photo with a 320x240 pixel FF sensor, what is my DoF? Even my lens gives a blurry mess I would still get a 320x240 photo that is sharp at pixel level. Does this represent the 'reality'?

The thing is, reality is far weirder than you can ever imaging. We are in a photographic forum, so yes, photography is just a form of art for me and perception is the only thing that matters. I learn from my output photos and prints so I can control my equipment to get the result I want.

Then we leave the underlying physical, electricial or philosophical discussions for some one else or somewhere else.

 

[ecka]

I'm glad that you are enjoying the discussion. It wasn't my intent to offend anyone.
I didn't confirm the nonsense. Shrinking the picture simply makes it's details imperceptible to you. If you can't see a bacteria, it doesn't mean that there are none. In case with a magic shrinking machine, the details are made smaller so you can still see them by shrinking yourself or maybe using a microscope. However, when you shrink the image on your screen or print a thumbnail, you are just losing the information. Just like for a half-blind person all your images can look same "sharp" or same "blurry". In fact, for him, sharp and blur looks the same. CoC is about perception. DoF is not, it is about information, same as photography. Once the light of an optical image hits the sensor, it is gone, all that's left is the information gathered by the electronics. If you shoot a picture that has nothing in focus, it doesn't matter to what resolution you downsize it, no new information will occur (except the false one). You can manipulate the image in any way you want, but in relativity to reality DoF won't change a bit. If photography is just a form of art for you and perception is the only thing that matters, then perhaps you are not even trying to understand what I'm talking about.

Well by your standard, in reality there is nothing really 'in focus'. The focus 'plane' is a hypothetical thing that has zero thickness. Also on the 'true' focus plane every light point has diameter of 0. Anything in front, or behind this zero thickness hypothetical plane is deemed out of focus because they have a CoC > absolute 0.

The sensor sees something in focus not because they are in focus, but simply because the CoC is smaller than sensor's pixel could distinguish. So what you say? That the image the sensor captures is the real world? It is not.

If above assumption is correct, then take an example, if I shoot a photo with a 320x240 pixel FF sensor, what is my DoF? Even my lens gives a blurry mess I would still get a 320x240 photo that is sharp at pixel level. Does this represent the 'reality'?

The thing is, reality is far weirder than you can ever imaging. We are in a photographic forum, so yes, photography is just a form of art for me and perception is the only thing that matters. I learn from my output photos and prints so I can control my equipment to get the result I want.

Then we leave the underlying physical, electricial or philosophical discussions for some one else or somewhere else.

No. In reality there is nothing that is a 0. Zero is not a thing, zero is just a tool in mathematics. Every point has dimensions and it can be represented as an image of at least 1 pixel. When you are viewing ~18mp image on a ~2mp screen, then 1 dot (color) on the screen represents a group of 9 pixels of the image. Sensor does not capture the real world. The projection of an optical image on the sensor is limited by all kinds of information manipulation by the lens (diffraction, aberrations, vignetting, coma, color tint, distortion, flares and CoC). If the 9 combined pixels carry enough information to represent 1 real world dot, then it will be sharp. If not, then it will be blur (or noise). At 1:1 (100%) it is similar, but with much more false color and noise. If you shrank the blur into oblivion and got some kind of real world information, then it only means that you've destroyed all the rest and the whole blurriness carried only this little.

The sensor and electronics "sees" nothing in focus, just color and contrast of the neighboring pixels.320x240 pixel FF sensor cannot mimic human vision. There are artificial eye implants that allow blind people to see the world in just a few hundred pixels and trust me, it's nothing like the real thing. It's a blurry mess and they can only see a letter or a digit in close-up.

 

[Pi]

No. In reality there is nothing that is a 0. Zero is not a thing, zero is just a tool in mathematics.

You are wrong about DOF but I will let others argue about it. But as a mathematician, I strongly object the statement that zero is not a thing, and that there is noting at zero. How many 200-400 lenses do you own?

Every point has dimensions and it can be represented as an image of at least 1 pixel,

What if you are shooting film?

 

[ecka]

No. In reality there is nothing that is a 0. Zero is not a thing, zero is just a tool in mathematics.

You are wrong about DOF but I will let others argue about it. But as a mathematician, I strongly object the statement that zero is not a thing, and that there is noting at zero. How many 200-400 lenses do you own?

Every point has dimensions and it can be represented as an image of at least 1 pixel,

What if you are shooting film?

I don't use superteles.
Film has it's minimum dot size that can be captured.

 

[dougkerr]

First recall that DoF is not a distance that comes solely from optical theory (like the distance to the image for a certain object location); it is a description of the range of object distances over which a certain arbitrarily-adopted criterion of "blurred-ness" is not exceeded, with the camera focused at a certain distance.

Often, for consistency, that criterion is a blur circle whose diameter is a certain fraction of the overall image size (maybe its diagonal size). This relates to a consistent diameter blur circle on an image produced at a consistent size (such as a consistent size print).

That having been said, when we compare the DoF attained for two values of some attribute of the camera (such as sensor size), "all other factors being equal", we must adopt and announce what we mean by "all other factors being equal".

Here is one set of such that we might adopt:

a. Focal length such that the field of view is the same in both setups. (Aha!)

b. Camera focused at the same distance. (Of course.)

c. Same f-number. (Of course.)

d. Criterion for "acceptable" blurring the same in terms of blur circle diameter as a fraction of the image size (diagonal will do).

Now, if we are comparing two cameras, with "B" having an image size 2x that of "A":

1. Under rule a, we must use a lens of twice the focal length in B as in A.

2. Under rule c, our acceptable diameter of blur circle is twice for B as it is for A.

Now, with regard to point 2, that means that we are more tolerant of blur in B than in A. Thus this consideration alone would lead to a greater depth of field for B than A.

This works essentially proportionally to the blur circle diameter criterion, and thus to image size (sensor size).

Remember, depth of field is not an creature of optical theory alone. It is a creature of what amount of blurring we consider "acceptable". If we increase the amount we consider "acceptable", then our focus distance can be more "off" and we still consider the result acceptable.

Now, to point 1. Because of the optical situation involved (and I will not attempt to describe this in detail here), for a greater focal length the "incorrectness" in focus distance to cause a certain diameter blur circle is less. Thus this consideration alone would lead to a lesser depth of field for B than A.

This works essentially proportionally to the square of the focal length, and thus (because of rule a) to the square of the image size (sensor size).

So as we move from A to B, we find that issue 2 gives an increase in the depth of field, and issue 1 gives a larger decrease in the depth of field (because it varies inversely as the square of the image size).

Thus, the overall effect is that (under the rules stated above), for an increase in sensor size we have a net decrease in depth of field.

Quod erat demonstrandum.

Best regards,

Doug

 

[bdunbar79]

Speaking about sensor size changing the DOF is a bit misleading. Do we keep everything else the same? This is actually impossible. You cannot keep the AOV and the FL the same. If we just swap the bodies, without changing (physically) anything else, smaller sensors have less DOF.

For the same AOV and same f/stop, larger sensors with the corresponding lenses have less DOF because the "physical aperture" is larger. For example, 80/2>50/2. There is no rule saying you must shoot at the same f-stops with both sensors, and if you shoot with equivalents ones (like 80/3.2 vs. 50/2) DOF is the same. That means "faster" lenses for the smaller sensor. They do not always exists, but sometimes, they do. For example, the m43 system has f/2 zooms, eq. to f/4 ones. They also have a f/0.95 prime, eq. to an f/2 one. But they do not have a match for f/1.4 on FF.

Bottom line: it is not the sensor size, it is the lens and the sensor but lenses with less DOF for larger sensors are easier to make, more common, cost less, and resolve more.

Yes you are correct obviously. But what I was getting at, is if you shoot with a 1Dx with a 50mm lens at f/4, and then you change to a 7D, and don't move: Obviously the DOF is not the same. Move back to the same AoV and the DOF is STILL not the same. Now you have to change aperture. So those that are saying sensor size has nothing to do with it, well...

Look at the things I had to change, simply because of a different sensor size. I guess I don't get why this has gone so far. That is all I am trying to say.

 

[Meh]

Shrinking the picture simply makes it's details imperceptible to you.

If I may, can I suggest that this one sentence sums up some of the disagreement in this thread. DoF is, in fact, a concept that is rooted in human visual perception. DoF is defined as the distance in front of and behind the plane of focus that appears in focus to a human being. The calculation requires assumptions regarding human visual acuity, print size, and viewing distance.

I believe, others can correct me if I'm wrong, it is also implicitly assumed that the print size and resolution is such that the individual pixels in the print are too small for the viewer to see them at the assumed print size and viewing distance. If the pixels are visible then the entire image would not appear sharp. That is why sensor resolution does not appear in the calculation.

So yes, print size matters and yes, if you print small enough the entire image would "magically" appear sharp. "Appear" is the operative word in that statement but it is relevant because "appears sharp" is fundamental to the concept of DoF. If you also shrunk yourself down, your visual acuity would likely also change so in fact DoF would be the same.

And it is a concept. It is a defined value based on some reasonable assumptions. DoF is not something that exists independent of human vision and is not a strictly defined measurement like mass, distance, size, etc.

If you're looking for a physically defined parameter, it exists. That is focus distance. The distance from the image plane that is precisely in focus (in practical terms it would be maximally in focus because there is no perfect focus). And there is only one distance that is maximally in focus... every plane in front of and behind the focus plane is less focused. If human visual acuity was infinite and the resolution of a print was infinite you would be able to see the tiniest difference in sharpness. But that's not the case, more than just the exact plane of focus appears sharp and we can define the depth in the image that appears sharp... i.e. Depth of Field.

 

[Marsu42]

DoF does not rely solely on optics, so badgerpiper's statement is false. DoF relies on apparent aperture (optics) and subject magnification (optics, reproduction size and viewing distance).

Thank you very much that you are so persistent explaining it, simple /me really learned a lot in this thread from you and the links you posted, esp. http://www.josephjamesphotography.com/equivalence/light.htm#dof

I was mislead by the dof calculators that output a precise dof range, I never read the fine print :-> ... but on the other hand I knew that the apparent dof changes drastically with perceived image size (like from camera display -> large lcd and your distance from it) which actually requires you to take multiple shots of the same scene with different apertures to have the dof *you* want for every output situation you have in mind. The CoC explains this and having read about it I don't know how I could have ever thought dof would only depend on optics

 

[privatebydesign]

DoF does not rely solely on optics, so badgerpiper's statement is false. DoF relies on apparent aperture (optics) and subject magnification (optics, reproduction size and viewing distance).

Thank you very much that you are so persistent explaining it, simple /me really learned a lot in this thread from you and the links you posted, esp. http://www.josephjamesphotography.com/equivalence/light.htm#dof

I was mislead by the dof calculators that output a precise dof range, I never read the fine print :-> ... but on the other hand I knew that the apparent dof changes drastically with perceived image size (like from camera display -> large lcd and your distance from it) which actually requires you to take multiple shots of the same scene with different apertures to have the dof *you* want for every output situation you have in mind. The CoC explains this and having read about it I don't know how I could have ever thought dof would only depend on optics

You are so so welcome.

For another mind numbing factoid related to dof, we all think DSLR's are WYSIWYG when shooting wide open, however look through a fast lens (>f2) mounted on your camera, then take a picture and compare the dof of the image to the scene through your camera. The image will have much narrower dof, focusing screens have an aperture! It is around f2.8 so when you shoot wider than that you get much shallower dof than you expect. As a follow on to this it is impossible to accurately manually focus an ultra fast lens with our standard focusing screens.

 

[Loswr]

DoF does not rely solely on optics, so badgerpiper's statement is false. DoF relies on apparent aperture (optics) and subject magnification (optics, reproduction size and viewing distance).

Thank you very much that you are so persistent explaining it, simple /me really learned a lot in this thread from you and the links you posted, esp. http://www.josephjamesphotography.com/equivalence/light.htm#dof

I was mislead by the dof calculators that output a precise dof range, I never read the fine print :-> ... but on the other hand I knew that the apparent dof changes drastically with perceived image size (like from camera display -> large lcd and your distance from it) which actually requires you to take multiple shots of the same scene with different apertures to have the dof *you* want for every output situation you have in mind. The CoC explains this and having read about it I don't know how I could have ever thought dof would only depend on optics

You are so so welcome.

For another mind numbing factoid related to dof, we all think DSLR's are WYSIWYG when shooting wide open, however look through a fast lens (>f2) mounted on your camera, then take a picture and compare the dof of the image to the scene through your camera. The image will have much narrower dof, focusing screens have an aperture! It is around f2.8 so when you shoot wider than that you get much shallower dof than you expect. As a follow on to this it is impossible to accurately manually focus an ultra fast lens with our standard focusing screens.

Yep - the stock screens are laser microetched to make them brighter with slow lenses, and that's the consequence. Not only are you getting the DoF of ~f/2.8 through the VF, you're getting that brightness even with a faster lens. You can see the effect of this with bodies with a transmissive LCD (7D, 5DIII, 1D X) - with a fast lens (f/1.2-1.4), the AF point display is a sort of washed out gray, not black. As you stop down with the DoF preview button pressed, the VF doesn't get dimmer, but the transmissive LCD gets progressively darker until ~f/2.5, then the whole VF starts getting darker, too.

 

[ecka]

Shrinking the picture simply makes it's details imperceptible to you.

If I may, can I suggest that this one sentence sums up some of the disagreement in this thread. DoF is, in fact, a concept that is rooted in human visual perception. DoF is defined as the distance in front of and behind the plane of focus that appears in focus to a human being. The calculation requires assumptions regarding human visual acuity, print size, and viewing distance.

I believe, others can correct me if I'm wrong, it is also implicitly assumed that the print size and resolution is such that the individual pixels in the print are too small for the viewer to see them at the assumed print size and viewing distance. If the pixels are visible then the entire image would not appear sharp. That is why sensor resolution does not appear in the calculation.

So yes, print size matters and yes, if you print small enough the entire image would "magically" appear sharp. "Appear" is the operative word in that statement but it is relevant because "appears sharp" is fundamental to the concept of DoF. If you also shrunk yourself down, your visual acuity would likely also change so in fact DoF would be the same.

And it is a concept. It is a defined value based on some reasonable assumptions. DoF is not something that exists independent of human vision and is not a strictly defined measurement like mass, distance, size, etc.

If you're looking for a physically defined parameter, it exists. That is focus distance. The distance from the image plane that is precisely in focus (in practical terms it would be maximally in focus because there is no perfect focus). And there is only one distance that is maximally in focus... every plane in front of and behind the focus plane is less focused. If human visual acuity was infinite and the resolution of a print was infinite you would be able to see the tiniest difference in sharpness. But that's not the case, more than just the exact plane of focus appears sharp and we can define the depth in the image that appears sharp... i.e. Depth of Field.

I don't know. I disagree, that when I photograph a ruler which clearly shows that the DoF is, let's say, ~15mm, I must let the shrink size decide that the DoF is actually half a meter or that I was shooting at hyperfocal and I'm crazy .
I thought that DoF is the thickness of the sharp focus plane your camera can capture (how is it called then?). Turns out I was wrong, it is what anyone wants it to be and if it can't be, then just get a better printer .
I say, if you have to shrink your images to make everything look sharp, then you are using the wrong camera format.
DoF area is sharp, but sharpness ≠ DoF

 

[BozillaNZ]

It seems there's nothing constructive I can add to this post now. Everyone except one agrees on the common knowledge about relationship between output magnification and DoF. The post starter can either agree we us, or him or come up with his own conclusion.

 

[Matthew19]

The simplest answer is from Gale Tattersall, DP of the TV show HOUSE : a larger sensor requires a longer lens to achieve the same field of view. The longer the lens the less DOF. (aperture staying the same of course)

 

[Marsu42]

It is around f2.8 so when you shoot wider than that you get much shallower dof than you expect. As a follow on to this it is impossible to accurately manually focus an ultra fast lens with our standard focusing screens.

For once, I knew this, but I've got a question since you or neuro probably have tried these screens: Amazon customer reviews tell me that the effect of the more visible dof is not that large, while the effect of a darker vf for slower lenses is significant - bottom line to only get this screen if you shoot f1.2-f1.4 wide open very often, correct?

As you stop down with the DoF preview button pressed, the VF doesn't get dimmer, but the transmissive LCD gets progressively darker until ~f/2.5, then the whole VF starts getting darker, too.

... meaning the exchangeable screen for lesser cameras also has zero effect for f2.8 lenses, but only for f2.5 or faster? I was never able to research how large the effect of the screen is for what aperture, and unfortunately you cannot try these in a shop.

 

[Sporgon]

... meaning the exchangeable screen for lesser cameras also has zero effect for f2.8 lenses, but only for f2.5 or faster? I was never able to research how large the effect of the screen is for what aperture, and unfortunately you cannot try these in a shop.

At 2.8 there is a slight benefit in real DoF, maybe just due to more definition between in and out of focus. However the viewfinder is slightly darker even at 2.8. Although they are called 'manual focus' screens I've found them to be beneficial only with very fast primes: f1.2 - 2, so they are more of a 'fast prime' screen really.

 

[Sporgon]

Yes you are correct obviously. But what I was getting at, is if you shoot with a 1Dx with a 50mm lens at f/4, and then you change to a 7D, and don't move: Obviously the DOF is not the same. Move back to the same AoV and the DOF is STILL not the same. Now you have to change aperture. So those that are saying sensor size has nothing to do with it, well...

There's a practical photographer speaking.

In practical use of course sensor size effects DoF. OK, it's influence is indirect because of the lens focal length and distance you're going to use, but as you cannot produce a picture without both lens and sensor, the sensor size does, indirectly, effect DoF.

 

[Loswr]

... meaning the exchangeable screen for lesser cameras also has zero effect for f2.8 lenses, but only for f2.5 or faster? I was never able to research how large the effect of the screen is for what aperture, and unfortunately you cannot try these in a shop.

At 2.8 there is a slight benefit in real DoF, maybe just due to more definition between in and out of focus. However the viewfinder is slightly darker even at 2.8. Although they are called 'manual focus' screens I've found them to be beneficial only with very fast primes: f1.2 - 2, so they are more of a 'fast prime' screen really.

Chuck Westfall has stated, 'faster than f/4'. Personally, I don't see a lot of difference at f/2.8, but there is some. It's not really enough to change how I'd compose a shot, but at f/1.2 or f/1.8, the difference is very noticeable.

 

[Pi]

The simplest answer is from Gale Tattersall, DP of the TV show HOUSE : a larger sensor requires a longer lens to achieve the same field of view. The longer the lens the less DOF. (aperture staying the same of course)

Which aperture: the f-stop or the physical one?

If it is the former, start with the 200/2 on crop. How do you get less (EDIT) DOF with 320mm on FF?

 

[privatebydesign]

The simplest answer is from Gale Tattersall, DP of the TV show HOUSE : a larger sensor requires a longer lens to achieve the same field of view. The longer the lens the less DOF. (aperture staying the same of course)

Which aperture: the f-stop or the physical one?

If it is the former, start with the 200/2 on crop. How do you get more DOF with 320mm on FF?

Mr Tattersall was over simplifying.

How do you get more DOF with a FF than a 200/f2 on a crop camera. Easy, use a lens with a smaller physical aperture than 100mm, a 100-400 f4-5.6 would do the job.

If you wanted equivalency, i.e close to identical images from both cameras from the same spot you'd need to do this
[list type=decimal]
[*]Crop camera: 200mm, f2, 100iso
[*]FF camera: 320mm, f3.2, 200iso
[/list]

Obviously a 300 f2.8 on a ff camera outperforms a 200 f2 on a crop camera with regards thin dof, it also makes smoother and much more blurred backgrounds. People who espouse the 200 f2 as having a "unique look", are missing the point, a 300 f2.8 demonstrates all the "unique" qualities of the 200 f2, isolation, big background blur, narrow dof, only it does them all "better".