Depth of field (DOF) does NOT change with sensor size

[markjwyatt]

...
Do not make the error of assigning perspective to lens focal length. Perspective is a function of where you put the camera, period. Lens focal length does not determine perspective. So perspective means both photos taken from the same place. This is a critical factor. In order to compare same photos they must both be taken from the same place -- same perspective.

Joe

Maybe "perspective" is not the correct term(s). By perspective I am talking about the effect I get when I stick a 300mm lens on a 35mm camera, or a 12mm lens for a different perspective (i.e., scrunching or widening of distance, as well as distortions, etc.). A "normal" lens is intended to give a normal (to the typical human eye) relation between far and near distances. Telephotos and wide angles tend to change those relations.

 

[markjwyatt]

...that means sensor size is a determinant factor for DOF...Joe

Generally when I talk to people in camera stores I basically say that and they always try and shoot me down. When I bought my Fuji XT-2, I tested it in place, and the guy kind of looked at me funny as though thinking, 'ok, if it makes you fell better, but it does not matter'. It does matter, but does not necessarily make one format better or worse than another, just different, and you need to understand those differences.

 

[Ysarex]

Joe- I think we are entirely consistent.

Yes. Sensor size is absolutely a DOF determinant factor. Both of the YouTube vidiots featured in this thread presented their nonsense recently. I've had to address both of them in different forums for the last two weeks and clear up the rubbish they've been dumping. Don Quixote here tilting at YouTube windmills -- good thing I'm retired.

Joe

Sorry for not considering your "how not to do it" aspect! We actually use that approach in selling software, we say How Not to Do "X", then list three easy ways that people use to try and handle "X" which we then demonstrate not to be correct. We then follow by demonstrating how our software [or more generally the theoretically preferable approach] is the best way to handle "X".

 

[Ysarex]

...
Do not make the error of assigning perspective to lens focal length. Perspective is a function of where you put the camera, period. Lens focal length does not determine perspective. So perspective means both photos taken from the same place. This is a critical factor. In order to compare same photos they must both be taken from the same place -- same perspective.

Joe

Maybe "perspective" is not the correct term(s). By perspective I am talking about the effect I get when I stick a 300mm lens on a 35mm camera, or a 12mm lens for a different perspective (i.e., scrunching or widening of distance, as well as distortions, etc.). A "normal" lens is intended to give a normal (to the typical human eye) relation between far and near distances. Telephotos and wide angles tend to change those relations.

Be careful with that thinking. This is a case of cause and effect and correlation being very closely related. It can matter at some point that you have the cause/effect relationship straight. Perspective is the rendering of the relationship of objects in 3D space. It is only determined by camera position. We get a chicken/egg thing happen here with very long and short lenses. A long lens is only good for photographing distant objects and so we render those distant objects with what appears to be compressed perspective. Is that a function of the lens or a function of the camera subject distance? We normally use long lenses to photograph far away objects but is the compressed perspective due to the lens or due to the fact that we're far away? It's not the lens it's our physical distance from the subject -- our 3d relationship with the other objects in 3d space.

Joe

 

[Ysarex]

...that means sensor size is a determinant factor for DOF...Joe

Generally when I talk to people in camera stores I basically say that and they always try and shoot me down. When I bought my Fuji XT-2, I tested it in place, and the guy kind of looked at me funny as though thinking, 'ok, if it makes you fell better, but it does not matter'. It does matter, but does not necessarily make one format better or worse than another, just different, and you need to understand those differences.

Different is good! I have a little compact I use to take a lot of photos of my wife's garden and the flowers there. It has a dinky little 1/1.7 sensor in it. I can set the f/stop to f/3.2 and get great photos of close-up flowers that a FF camera would struggle to give me at f/9.5. Understand the differences and take advantage of them. What a mess we'd be in if we were all restricted to using only one type of camera!

P.S. Your people in the camera stores need some education.

Joe

 

[Tim Tucker 2]

...
Do not make the error of assigning perspective to lens focal length. Perspective is a function of where you put the camera, period. Lens focal length does not determine perspective. So perspective means both photos taken from the same place. This is a critical factor. In order to compare same photos they must both be taken from the same place -- same perspective.

Joe

Maybe "perspective" is not the correct term(s). By perspective I am talking about the effect I get when I stick a 300mm lens on a 35mm camera, or a 12mm lens for a different perspective (i.e., scrunching or widening of distance, as well as distortions, etc.). A "normal" lens is intended to give a normal (to the typical human eye) relation between far and near distances. Telephotos and wide angles tend to change those relations.

Be careful with that thinking. This is a case of cause and effect and correlation being very closely related. It can matter at some point that you have the cause/effect relationship straight. Perspective is the rendering of the relationship of objects in 3D space. It is only determined by camera position. We get a chicken/egg thing happen here with very long and short lenses. A long lens is only good for photographing distant objects and so we render those distant objects with what appears to be compressed perspective. Is that a function of the lens or a function of the camera subject distance? We normally use long lenses to photograph far away objects but is the compressed perspective due to the lens or due to the fact that we're far away? It's not the lens it's our physical distance from the subject -- our 3d relationship with the other objects in 3d space.

Joe

To be a little more precise, perspective is entirely from the position of the observer.

It is a *chicken and egg* as what is actually happening is easy to mis-interpret if you only try to define it by how a lens works. Focal length has no effect on perspective, it is entirely dependant on the camera position. But...

Perspective is also dependant on the distance you view the print. Though because your eye presents you with a consistent view of the world, one where distances are constant and understood perspective doesn't really change as you change your position to a print. It's not a linear relationship and another demonstration that relationships in images are not defined entirely from the lens but also from our human perception when we view the images. It is why standard lenses are called *standard*, because they present a natural perspective, or the same perspective when we view the print. If you view a print from the same relative position as the camera we see the perspective from the camera position (standard lens for the format), if you view a print from a position that's not the same relative to the camera position you see a distortion of perspective.

Very basically FOV is not recorded in a print so when you view a print you make an assumption of FOV. Two prints of the same size next to each other then you assume similar FOV's which affects your assumptions of scale and distance when you view. It's easy to demonstrate with the same photo, perspective does't really change but you assumptions of the size and distance away the objects are does:

...that means sensor size is a determinant factor for DOF...Joe

Generally when I talk to people in camera stores I basically say that and they always try and shoot me down. When I bought my Fuji XT-2, I tested it in place, and the guy kind of looked at me funny as though thinking, 'ok, if it makes you fell better, but it does not matter'. It does matter, but does not necessarily make one format better or worse than another, just different, and you need to understand those differences.

You are correct, see my first, and the second, post on this thread. Because of the nature of the relationships of f-stop, focal length and sensor size it is possible of fix some as constant and therefore cancel others out of the equation. If you hold total light as constant then FOV and aperture diameter are constant. If this condition is set then dof must also be constant between equivalent photos. But to achieve this exposure must also be different between equivalent photos. If you hold exposure as constant then dof must vary between photos.

The trouble is that if you cancel focal length from the equation you must also ignore a fundamental relationship between real world sensor size, available f-stops and achievable dof. I've tried to express it in the diagram below, but don't know if it's understandable. What it tries to show is why the 35mm film format is unique. It's easier to understand if you assume *hand held* photography, or photography with a non-static subject, deep dof being easy to achieve to the left of the line:

 

[Tim Tucker 2]

I was always taught with portraits that the key is to have the catch light in the eyes sharp. Get that and the impression of sharpness is favorable. Lose that and you lose the image. In your image, the catch light is pretty sharp.

Here is a picture I took of my dog. I really did not get the catch lights perfectly sharp, and there is an impression of softness (I still like the image but would prefer the eyes to be sharper). If you look closely, the collar is pretty sharp.

Absolutely. Eyes have a natural moisture that acts as a reflector of light, eyes therefore have a higher acutance and we simply expect them to look sharper in a photograph because they always do in the real world, and we always notice the eyes. If you compare my image against yours you will see that they have similar dof, but one you perceive as sharp and the other as soft. It is especially true in portraits that the actual point of focus is more critical than dof and does affect you assumption of sharpness when viewing a print.

 

[zulu42]

I have to admit I love it when people post misinformation here. Just love it. The resulting education is priceless. Long live TAP!

 

[markjwyatt]

...

Be careful with that thinking. This is a case of cause and effect and correlation being very closely related. It can matter at some point that you have the cause/effect relationship straight. Perspective is the rendering of the relationship of objects in 3D space. It is only determined by camera position. We get a chicken/egg thing happen here with very long and short lenses. A long lens is only good for photographing distant objects and so we render those distant objects with what appears to be compressed perspective. Is that a function of the lens or a function of the camera subject distance? We normally use long lenses to photograph far away objects but is the compressed perspective due to the lens or due to the fact that we're far away? It's not the lens it's our physical distance from the subject -- our 3d relationship with the other objects in 3d space.

Joe

This does make sense given that for instance for an 8x10 format, a "normal lens" is 300-350mm which for 35mm format (again for instance) is a pretty long telephoto.

 

[Ysarex]

To be a little more precise, perspective is entirely from the position of the observer.

It is a *chicken and egg* as what is actually happening is easy to mis-interpret if you only try to define it by how a lens works. Focal length has no effect on perspective, it is entirely dependant on the camera position. But...

Perspective is also dependant on the distance you view the print. Though because your eye presents you with a consistent view of the world, one where distances are constant and understood perspective doesn't really change as you change your position to a print. It's not a linear relationship and another demonstration that relationships in images are not defined entirely from the lens but also from our human perception when we view the images. It is why standard lenses are called *standard*, because they present a natural perspective, or the same perspective when we view the print. If you view a print from the same relative position as the camera we see the perspective from the camera position (standard lens for the format), if you view a print from a position that's not the same relative to the camera position you see a distortion of perspective.

Tim, I like the But... this has long been a topic of interest for me. It begs the question, how do we present an image such that the viewer experiences as faithful a rendition as possible to what we experienced when taking the photo. And that begs the question of why a standard or normal lens is normal. It's taken on faith (possibly a mistake in this business) that a normal lens presents a natural perspective. You hear, a normal lens mimics human vision, or the normal lens sees the scene the way you do, etc. Apart from just empirically testing that, (in most cases with the scene photographed no longer available), what's the criteria for assigning "normal" to a lens focal length for a given format? The answer that a lens is normal if it's focal length matches the diagonal measure of the film/sensor never sounded to me like an adequate answer if achieving natural perspective was the goal.

So this should interest you: Leslie Stroebel was likewise interested in this question and added some additional information for us to consider. It's obviously limited but what he did was measure an average viewing distance that people naturally stood back to view displayed images. Given the freedom in a gallery to view an image from a distance of choice he was able to say that people on average preferred to view an image from a distance equal to twice the long side of the image. So viewing an 8 x 10 print = 20 inches, viewing a 16 x 20 print = 40 inches, etc. That gives us a number to plug into an equation, and if we run that math the answer is that a normal lens as determined by the film/sensor diagonal comes up a little short.

So by conventional determination a normal lens for a 35mm camera is 43mm (we round up to 50). Using what I'll call the Stroebel method a normal lens for a 35mm is 70mm. I've long had this fantasy that I'd mount a 75mm f/1.4 Summilux on an M4 and live happily ever after.

Joe

 

[markjwyatt]

...So this should interest you: Leslie Stroebel w...

Joe

I just got this book from Amazon and look forward to digging into it: Photographic Materials and Processes

As an aside, you stated, "how do we present an image such that the viewer experiences as faithful a rendition as possible to what we experienced when taking the photo", is this the "standard for photography" (I think it is one standard, but maybe not the only one). I proposed a group on Photrio, which was not well received: PHOTRIO.COM

I also picked up the domain name http://Standards.photography , currently empty, but...

 

[Ysarex]

...So this should interest you: Leslie Stroebel w...

Joe

I just got this book from Amazon and look forward to digging into it: Photographic Materials and Processes

Leslie Stroebel is the best.

As an aside, you stated, "how do we present an image such that the viewer experiences as faithful a rendition as possible to what we experienced when taking the photo", is this the "standard for photography"

Not at all, but it is a unique opportunity for photography and I think worth exploring. Just like it's really good that we have lots of different camera types to chose from there is richness in diversity and there are lots of different ways to approach and practice photography.

Joe

(I think it is one standard, but maybe not the only one). I proposed a group on Photrio, which was not well received: PHOTRIO.COM

I also picked up the domain name http://Standards.photography , currently empty, but...

 

[john.margetts]

It begs the question, how do we present an image such that the viewer experiences as faithful a rendition as possible to what we experienced when taking the photo.
Joe

I think that I can safely say that none of my photographs are faithful renditions of what I see. I am more interested in conveying smell (usually musty), temperature (usually cold), age (usually ancient) and other non visual aspects of my subject (mediaeval churches). Lenses pay only a small part in this.

(I do note that Joe says 'experienced' in the quote above rather than saw. I am really commenting on the desire for faithful which I (and many other photographers) do not share)



Sent from my 8070 using Tapatalk

 

[Tim Tucker 2]

So this should interest you: Leslie Stroebel was likewise interested in this question and added some additional information for us to consider.

Thanks, just ordered his book on visual principles to take a look, promises to be interesting.

Given the freedom in a gallery to view an image from a distance of choice he was able to say that people on average preferred to view an image from a distance equal to twice the long side of the image. So viewing an 8 x 10 print = 20 inches, viewing a 16 x 20 print = 40 inches, etc.

Though I don't doubt this....

That gives us a number to plug into an equation,

I do question this. You can't equate human vision to equations that easily.

The best illustration I can think of is the 3D pavement art. As you walk around it then you have a fairly consistent view of what you see, until you reach the exact point at which the perspective is designed to create the illusion. At that point your *understanding* or interpretation of the visual cues changes dramatically and you see the 3D effect. It is much the same with images, in that your view of them will be fairly consistent until you stand at a point that coincides with the camera position and a different reality is presented. This different interpretation then takes over as being more consistent with what you understand through experience and therefore the one you brain registers as *the truth*. It is not a gradual process but a more sudden one as your brain is also trying to maintain a *consistent* understanding of what you see. For instance you do not see a world in motion as you drive down the *freeway* but understand a static world and yourself moving through it. It's quite a computation of the absolute reality that your eyes *see*.

EDIT: For the diagonal I agree, I don't think that it's actually based on anything other than observation and a useful coincidence.

This is further compounded because we *learn* through experience. As photographers we learn the effects of wide angle lenses and so recognise them more readily than someone who has never seen a photograph. As photographers our interpretation of the image will have a grounding in out experience of learning to see through a camera whereas the non-photo aware eyes will base understanding on reality alone. It is a very real truth in photography that we teach ourselves to view images in terms of our understanding of the photographic process alone.

 

[Ysarex]

So this should interest you: Leslie Stroebel was likewise interested in this question and added some additional information for us to consider.

Thanks, just ordered his book on visual principles to take a look, promises to be interesting.

Given the freedom in a gallery to view an image from a distance of choice he was able to say that people on average preferred to view an image from a distance equal to twice the long side of the image. So viewing an 8 x 10 print = 20 inches, viewing a 16 x 20 print = 40 inches, etc.

Though I don't doubt this....

That gives us a number to plug into an equation,

I do question this. You can't equate human vision to equations that easily.

I made no such suggestion -- just that Leslie's observation gives us a hard number. Further below you agree the use of the format diagonal isn't especially meaningful. So otherwise we have nothing but repeated empirical observation. We know as you pointed out that perspective is also dependent on print viewing distance. What is or should that distance be -- a normal distance? If we have an average distance that applies to most viewers we can back calculate the lens focal length required to match the FOV to the print. It is then fair to say that if you view a 16 x 20 inch print from 40 inches you will be in the correct position to experience perspective as it appeared from the camera position using a 70mm lens on a 35mm camera. It's also fair to say that if you stay at that 40 inch distance and are presented a 16 x 20 image taken with a 20mm lens on a 35mm camera you're likely to experience the image as distorted, but not necessarily as it's actually pretty easy to fool your brain about what it's seeing.

The numbers along with understanding are useful tools.

Joe

The best illustration I can think of is the 3D pavement art. As you walk around it then you have a fairly consistent view of what you see, until you reach the exact point at which the perspective is designed to create the illusion. At that point your *understanding* or interpretation of the visual cues changes dramatically and you see the 3D effect. It is much the same with images, in that your view of them will be fairly consistent until you stand at a point that coincides with the camera position and a different reality is presented. This different interpretation then takes over as being more consistent with what you understand through experience and therefore the one you brain registers as *the truth*. It is not a gradual process but a more sudden one as your brain is also trying to maintain a *consistent* understanding of what you see. For instance you do not see a world in motion as you drive down the *freeway* but understand a static world and yourself moving through it. It's quite a computation of the absolute reality that your eyes *see*.

EDIT: For the diagonal I agree, I don't think that it's actually based on anything other than observation and a useful coincidence.

This is further compounded because we *learn* through experience. As photographers we learn the effects of wide angle lenses and so recognise them more readily than someone who has never seen a photograph. As photographers our interpretation of the image will have a grounding in out experience of learning to see through a camera whereas the non-photo aware eyes will base understanding on reality alone. It is a very real truth in photography that we teach ourselves to view images in terms of our understanding of the photographic process alone.