HoppyUK
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Crop Factor vs Full frame (a different request)
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arad85
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Mine is busy so I'll be brief.Groan....Still, it's a slow day
The "problem" is the reading of this sentence:
This reads to me as "once you have chosen your camera, aperture and subject distance, the DoF you see will be the same for all print sizes and viewing distances. I read the sentence as "for any print size and viewing distance, the DoF is set (and equal)."
We actually all agree with each other - just looking at it from different ends of the stick
arad85
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<sigh>
Pookeyhead
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It's called a conversation. If it bores you, don't take part. Since when has anyone in here JUST typed things into threads only to answer questions, and who amongst us ignores threads just because the OP has received an answer?
Pointless comment.
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HoppyUK
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Unnecessary comment.
StewartR
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Unnecessary comment.
Phil V
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Actually I'd like to add....Unnecessary comment.
On 2nd thoughts, it's been covered.
sk66
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anyway, not sure if the past images were conclusive enough to demonstrate the effect of changing focus length on depth of fill with the same aperture setting...so i did a few shows on a focus chart.
Lens 1 - 17-50mm at 50mm @ f2.8 - DoF is 18mm front & back of the centre focus (with slight focus shift toward the front)
Lens 2 - 105mm @ f2.8 & f5.6 - DoF for f2.8 is 8/9mm front and back and Dof @ f5.6 is 16-17mm front and back. the reason for the slight discrepencies between 50mm and 105mm is the extra 5mm. as both lenses are on cropped body so the extra 5mm is almost 10mm.
so double the focal length half the DoF if the same aperture is to be used.
so the same principle applies to the lens on cropped bodies
50mm f2.8
100% crop
105mm f2.8
105mm f5.6
Lens 1 - 17-50mm at 50mm @ f2.8 - DoF is 18mm front & back of the centre focus (with slight focus shift toward the front)
Lens 2 - 105mm @ f2.8 & f5.6 - DoF for f2.8 is 8/9mm front and back and Dof @ f5.6 is 16-17mm front and back. the reason for the slight discrepencies between 50mm and 105mm is the extra 5mm. as both lenses are on cropped body so the extra 5mm is almost 10mm.
so double the focal length half the DoF if the same aperture is to be used.
so the same principle applies to the lens on cropped bodies
50mm f2.8
100% crop
105mm f2.8
105mm f5.6
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arad85
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which also nicely shows the effect of image size. Take:
To me, the DoF on the zoomed out is around 25-30mm front, 22-25mm to the back. The larger "zoomed in" image I'd agree with ~18mm each way.Lens 1 - 17-50mm at 50mm @ f2.8 - DoF is 18mm front & back of the centre focus (with slight focus shift toward the front)
50mm f2.8
100% crop
Same image captured, same processing, different viewing conditions aaaannnnddddd.... different DoF
which also nicely shows the effect of image size. Take:
To me, the DoF on the zoomed out is around 25-30mm front, 22-25mm to the back. The larger "zoomed in" image I'd agree with ~18mm each way.
Same image captured, same processing, different viewing conditions aaaannnnddddd.... different DoF
not right at all buddy. I have made a couple of slides to hopefully demonstrate the fundamental of capturing 3D information onto a 2D image the z-axis (depth) information is recorded and cannot be altered.
the image below shows a 3 layered scene that has been captured by a camera with a limited DoF that has focused on the middle layer. thus the triangle and circle will be blurred. the actual z-axis information is recorded by the senor and thus the image
the next slide shows when someone is looking at the picture flat on. they see the triangle that is blurred which should be existing on a plane in front of the image. so in the real scene as the previous slide the camera or person can simply move further away to get the DoF to be on the triangle. but when looking at the image, move further away will not give you focus on the triangle because the triangle only exists ont he plane of the paper and not on the actual visual plane (noted as dotted). therefore it will always be blurred.
now the only reason you think everything is sharp by moving further away is that the resolution of your vision cannot detect the small differences. this principle is the same as shrinking the picture by compressing the pixels together thus making the blur appear as if it is not blurry.
the basic point is an image has X and Y axis...DoF is all about z-axis. so a 2D image cannot have varying z-axis information because it is 2D, the z-axis is infinitely small it might as well be zero or in this case the thickness of a paper.
however a 2D image laid flat and recorded while focusing on different parts of the image will produce DoF information. this is how the focus chart i have used works. but the key is that every part of that image has to be sharp. otherwise you will just be blurring blurs or trying to focus on a part of an image that is already blurred
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StewartR
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@ricky1980 - Your views here seem to be at odds with others (and mine), so I'd be interested to know how you explain this observation which I made earlier:
I invited your thoughts at the time but I don't think you replied.
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sk66
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These examples must have been made at near minimum focus distances... They are almost examples of DOF in Macro work. In Macro work DOF isn't affected by FL, it's affected by magnification power (reproduction ratio). A 60mm 1:1 lens will have the same DOF as a 100mm 1:1 lens, and the DOF will be split evenly in front/behind.
In low magnification photography (greater focus distances/more typical photography) the DOF will be 1/3 in front and 2/3 behind. And the DOF will be 1/4 if you double the FL.
These examples fall somewhere in between...
What you are showing/explaining is that "focus" is locked into the image at the time of capture. And this is true. But only one point/area is exactly in focus and that is determined by the size of the projected dot (airy disk) on the sensor in relation to the size of a pixel/receptor. (this gets very complicated/involved)
DOF varies based upon perception... it is what we perceive to be "acceptably sharp." DOF isn't "real," it doesn't "exist" as a fixed entity. If you change the image size you change the DOF, and if you change the viewing distance you change the DOF. And, in fact, if you change your visual acuity (like my using reading glasses) it changes the DOF. That's why the DOF standard (COC) *requires* a print/display of a given size be viewed at a given distance. For example. As I sit here viewing the 105mm f/2.8 example on my laptop I judge the DOF to be ~ 4mm in front and 5mm behind.
You can take an image and enlarge it on your computer and the DOF (acceptable sharpness) will decrease. You can then move further away and the DOF will then return. The DOF will be different when using the DOF preview button and viewing thru the viewfinder, when viewing the image on the camera LCD, and when viewing on your computer.
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if the lens is for a cropped sensor , it wont make any difference , the full frame will either auto adjust , or you will notice some black edging on the image, i hope this helps
a full frame lens on a cropped cramera , will just act like a cropped lens
a full frame lens on a cropped cramera , will just act like a cropped lens
I am not debating that human DoF. I am debating that you cannot make an image to show something that isn't there, I.e. the out of focus details. I am sorry if u somehow can do that why on earth nations spend billions on military spy satellites? By your interpretation simply move away from the image and tada the secrete iraqi WMD site gonna pop out at ya.
Also all the images are take close to mind distance in order to achieve the shallow DoF to give a representation of what is going on. The maginification has zero to do with DoF that I am trying to show which is clearly indicated by the measuring shown in the images. Also the 105 is a macro lens and the 17-50mm isn't.
If you still don't appreciate how it all works. Simple print the image of 105 f2.8 then hang it up. And train any of you lens to it. By your theory, if you shift the focus marginally towards after obtaining focus on the paper then u will be able to focus sharply on the foreground blur and vice versa for the background blur. But what u gonna get is a big patch of blur.
Also all the images are take close to mind distance in order to achieve the shallow DoF to give a representation of what is going on. The maginification has zero to do with DoF that I am trying to show which is clearly indicated by the measuring shown in the images. Also the 105 is a macro lens and the 17-50mm isn't.
If you still don't appreciate how it all works. Simple print the image of 105 f2.8 then hang it up. And train any of you lens to it. By your theory, if you shift the focus marginally towards after obtaining focus on the paper then u will be able to focus sharply on the foreground blur and vice versa for the background blur. But what u gonna get is a big patch of blur.
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Another analogy is if you take landscape photo, you try to obtain hyperfocal so anything from 3 feet in front of the sensor to infinity are in focus. Now imagine there is a mountain in a long distance. The mountain in the image will be in focus and should be tact sharp anf have the same about of details as the object that is 3 feet away from u. but it is not and it is often look soft. This because the limitation of the sensor's resolution as well as optical resolution of the lens. OK this is the exact opposite phenomenon over human vision I.e the topic of our tangent discussion. But the same principle. It is not what u called perceptive DoF, it is simply things are beyond our visual capabilities.
sk66
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In fact, magnification (and individual acuity) determines the DOF. It does not determine what is actually "in focus" (pixel level).
arad85
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Yes it is
No one is saying it is. What people are telling you is that because the human eye cannot resolve infinite amounts of detail, the perceived depth of field changes depending on viewing conditions.
The whole concept of depth of field in a photo is not whether something is captured in or out of focus (because everything off the focal plane is out of focus by a greater or lesser extent), but whether you can see it as in or out of focus when you view it. That's what depth of field is. Full stop. End of story.
Can't you see atomic structures ? No, but can an electron microscope see it yes. Why because it has the resolution to detect the atoms. Simples. It's not DoF issue.
sk66
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You're confusing/combining the resolution/acuity of the system with DOF. They are separate things but interrelated...
A recorded image doesn't "have" a DOF, it does have a resolution. DOF doesn't "exist" until an image is displayed at a given size and viewed at a given distance. But "focus/resolution" does exist in a recorded image and DOF doesn't change that.
Of course it does. That's why the DOF is split evenly in front/behind, and it's also why you only see a reduction of DOF by 1/2 instead of 1/4 when you more than doubled the FL.
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Lol I am not combining resolution and DoF at all. You guys seems to be hung on the idea that an image gets bigger (effectively loosing resolution) and smaller (increasing resolution) has an effect on the recorded DoF information on an image.
Also the DoF is split 50+50 becuaee it is shot at 45 degrees not becuae of magificant. If I shot horizontally the DoF increase by single multiple to front and 2 times towards the back. Only shooting an image at 45 degrees can I represent the DoF to the OP otherwise by stepping down from f2.8 to f5.6 will cause disproportional more DoF and the images won't be comparable.
Also the DoF is split 50+50 becuaee it is shot at 45 degrees not becuae of magificant. If I shot horizontally the DoF increase by single multiple to front and 2 times towards the back. Only shooting an image at 45 degrees can I represent the DoF to the OP otherwise by stepping down from f2.8 to f5.6 will cause disproportional more DoF and the images won't be comparable.
arad85
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OK. I will try one more time.
- What gets recorded on the image as the out of focus area is technically called the Circle of Confusion. See: http://en.wikipedia.org/wiki/Circle_of_confusion It is not depth of field.
- Whether you see that blur circle or not as out of focus after the photo has been taken depends solely on the print size and viewing distance.
- Depth of field calculations take this into account in something called the Circle of Confusion diameter limit which is the size of blur that is predicted you can see based on standard human vision and magnification (i.e. how much bigger than the sensor the print is). See: http://en.wikipedia.org/wiki/Circle_of_confusion#Circle_of_confusion_diameter_limit_in_photography
You are using terminology that has a specific meaning incorrectly. Please.... go and read up on this and then come back and have the discussion.
martsharm
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What a bizarre discussion. Even though ricky1980 isn't giving the best explanations, he's basically correct. The viewing distance is only relevant *if you have moved too far away from the print to distinguish blurred detail from not focussed detail*. I.e. you are too far away, and your visual acuity has dropped in relation to the target. This is simple visual optics.
To take the circle of confusion example, of course any blur circle will look like a point source if you move far enough back. It's so obvious I'm surprised it's being discussed with such fervour.
But - imagine moving back towards the image. It will get clearer, and bigger, but once you are close enough to perceive all detail present in the target object - let's say a normal VDU viewing distance of about 50cm - *no further detail will be visible no matter how much closer one gets*. You can't see detail that isn't there in the original no matter how close you get. This is ricky's point about spy satellites.
All talk of print sizes is indeed relevant in the world of prints, but we are talking about viewing DOF scales on a computer monitor, in which case one's visual system (assuming normal visual acuity) is fully capable of distinguishing individual pixels (let's not get into retina-grade screens at this point). Getting any closer will not change the apparent DOF. Since we're all viewing this thread on computers, then talk of print viewing distances is irrelevant, unless one has particularly poor visual acuity.
For this DOF discussion we should assume the viewing distance gives full perception of all available object detail and leave it at that. Because that's the reality of viewing on VDUs.
To take the circle of confusion example, of course any blur circle will look like a point source if you move far enough back. It's so obvious I'm surprised it's being discussed with such fervour.
But - imagine moving back towards the image. It will get clearer, and bigger, but once you are close enough to perceive all detail present in the target object - let's say a normal VDU viewing distance of about 50cm - *no further detail will be visible no matter how much closer one gets*. You can't see detail that isn't there in the original no matter how close you get. This is ricky's point about spy satellites.
All talk of print sizes is indeed relevant in the world of prints, but we are talking about viewing DOF scales on a computer monitor, in which case one's visual system (assuming normal visual acuity) is fully capable of distinguishing individual pixels (let's not get into retina-grade screens at this point). Getting any closer will not change the apparent DOF. Since we're all viewing this thread on computers, then talk of print viewing distances is irrelevant, unless one has particularly poor visual acuity.
For this DOF discussion we should assume the viewing distance gives full perception of all available object detail and leave it at that. Because that's the reality of viewing on VDUs.
arad85
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Your argument of "full perception of all the detail because you can see the pixels" only works when viewing the pixels at 100%. We typically don't view the pixels at 100% unless we are zooming the image for editing otherwise you'd be viewing a 1.5m x 1m image from 50 cms. If you do do this, your perception of DoF changes to make the DoF shallower than predicted by a tool such as DoFmaster. Zoom back out so the diagonal is more like the viewing distance (which is the assumption made in most DoF calculations) and the perceived DoF changes - whether that is on a computer screen or not. That's actually shown by the images of the ruler above if you look....
That's so obvious, I'm surprised you thought it worth bringing up....
StewartR
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No. Your analogy is faulty.
If you focus at the hyperfocal distance, ONLY objects at that EXACT distance are in focus. The foreground object and the mountain are NOT in focus. They are "acceptably sharp" because (at standard print resolution, at standard viewing distance etc.) the average human eye cannot see the details well enough to see that they are not exactly in focus.
This really is the crux of the whole thing about DOF. Only objects precisely in the plane of focus are actually in focus. The perception that there is a zone in front and behind that plane where things are also in focus (the "depth of filed") is purely driven by human visual acuity. How big that zone is depends on the size of the image etc.
Please, please, tell us you understand this. Because as long as you're saying things like "the mountain will be in focus" it just sounds like (a) you don't understand, and (b) you aren't listening. I'd much rather believe you've just been misunderstood, and we can carry on with an interesting discussion.
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It has magnification rather than resolution to do this. An electron microscope cannot photograph a wide landscape view with single atom resolution.
Steve.
martsharm
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Your argument of "full perception of all the detail because you can see the pixels" only works when viewing the pixels at 100%. We typically don't view the pixels at 100% unless we are zooming the image for editing otherwise you'd be viewing a 1.5m x 1m image from 50 cms. If you do do this, your perception of DoF changes to make the DoF shallower than predicted by a tool such as DoFmaster. Zoom back out so the diagonal is more like the viewing distance (which is the assumption made in most DoF calculations) and the perceived DoF changes - whether that is on a computer screen or not. That's actually shown by the images of the ruler above if you look....
That's so obvious, I'm surprised you thought it worth bringing up....
Let's not confuse image pixels with VDU pixels. Forget software levels of magnification just for a second, that's a different issue, which I'll come to in a minute. My first point is if you're at a normal distance from your monitor your visual perception is outresolving the monitor's pixel pitch (retina screens excepted). So physical viewing distance, when considering VDU viewing, is not a variable that effects DOF. I hope we can agree on that point first of all.
At which magnification the image is viewed will indeed change the perceived DOF - again that's totally obvious
. The smaller it gets, the more perceived sharpness there is. But that's only because we're binning pixels in software, which is hardly a good way of gathering the best information about what's going on. I don't think binning the pixels that give the perception of blur and then saying the DOF has actually increased is a particularly useful strategy.You say looking at a 100% crop means a disconnect between the final image size and the VDU viewing distance, which is true to a certain extent. Although I do like to examine my prints at 50cm to check sharpness. Perhaps a casual viewer wouldn't get in that close. What's clear about viewing distance is that it's totally unpredictable. At the time the photo is taken, nobody knows what the viewing distance, or indeed the crop, will be with any certainty - it's just a vague assumption open to many sources of error.
What's needed to settle this is a standard for quantitatively comparing DOF between different formats and different lenses and I would suggest 100% viewing is the best solution. Of course I've looked at the ruler pictures, which make the case - the 100% view gives a clear, accurate picture of what's going on, regardless of the final image size. For these sort of academic discussions, 100% is the only way to compare like-with-like.
arad85
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No. This has nothing to do with pixels on the screen, all about how big you view the image.
GoodAt which magnification the image is viewed will indeed change the perceived DOF - again that's totally obvious
That should be the end of the discussion then.Well, that's exactly what you do when you make a print that is smaller than pixel by pixel - you are binning the pixels in software just before printing. Or you view the image from further away than its diagonal - you are binning the pixels because your eye can't resolve them.
You forgot to add the visual acuity of the viewer. That ALSO comes into DoF - DoF is perceived and can be different per person even at exactly the same viewing distances.... but you can guess that a 1.5m x 1m print is more likely to be viewed from 2m away than 50cm....
What's 100% viewing? I have 2 x 24" monitors and a 27" monitor on my desk. The pixels on the 27" monitor are smaller than the pixels than on the 24" monitor. Put the same image at 100% on the 24" monitor and it looks bigger than exactly the same image on my 27" monitor.
There is a standard, It's a 10 x 8 print viewed from around 12". When you use dofmaster, those are the assumptions made. It gives you DoF answers based on that assumption.
No you haven't because when you look at 100%, all you are doing is looking at the resolving power of the system. That will change as the lens changes, and the number of pixels on the sensor. Looking at this microstructure of the image just tells you how close you can get before you run out of resolving power and has very little to do with DoF as when you print and view from normal distances, the smallest objects you can resolve generally cover a number of pixels.
The mistake you appear to be making is confusing resolving power of the system with depth of field and looking for the limit you can get by printing big (a screen is, after all, just another medium for displaying an image).
I'm not suire what point you are trying to make here. That camera/lensd combinations have a resolving limit? Yes, I agree with that but it has very little to do with depth of field.. Something else?
QFT
Anyone who can't understand why viewing distance affects DOF should read this post carefully...
martsharm
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I just want to pick up the point above. If you read my previous post I made the caveat "assuming normal visual acuity". So your assertion that I've forgotten about it is demonstrably wrong, and implies that you haven't properly read what I've written. I am a professional expert in visual acuity - there's nothing you can teach me about it. I will say this - in the age population of, say, up to 80 years old, the vast majority of people have functionally the same level of corrected binocular VA. It's just not an issue here.
I'm sick of this discussion. As you freely admit, you don't understand what I'm saying so I'm going to leave it.
Phil V
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Ricky
Your writing suggests just 2 states though, in focus and not in focus, but forgive the pun, that line is very blurred.
Only the plane of focus is 'in focus'. A mm behind or in front of that whether at f/1 or f/16 is out of focus (though some will appear to be closer to in focus than another). The point about viewing distances and image size comes in to play here. on a small image from a fair distance, something might look sharp where on a larger image closer it'll obviously not be sharp.
You're right we can't physically print something sharper than it is. But no-one said we can. What we can do is perceive something to be sharper than it is. That's why it's called a circle of confusion, because it's what can be used to confuse us.
Or what Graeme said.
arad85
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I fully understand what you are saying, I just don't understand what point you are trying to make....
sk66
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I think what makes DOF so confusing is that we tend to talk about it as being "fixed" as a characteristic of an image... "I think the DOF is too shallow"... That's really technically "wrong."
The whole concept of DOF is in defining how an image is going to be perceived by a viewer. To do that it "defines/restricts" several variables. In the days of prints being the most common form of display we had more control of the perceived DOF. We determined the print size for the desired use. We knew that if the image was going to be in a book at full page size it had to be sharper overall or it would have less DOF. And if it was going to be printed at 1/4 page size it would have greater DOF (people tend to look at books from a fixed distance).
We also knew that if we printed an image at x size for display it would tend to be viewed from a typical distance assuming the situation allowed for that. But if we were printing larger or the typical viewing distance for that size couldn't be achieved (i.e. a hallway) the DOF would be less.
To have a known/fixed DOF you have to know the display size/viewing distance *before* taking the picture and then you have to accommodate/control those variables in actual application. (we never have/had control over visual acuity). If you do know the display size/distance and they are not "standard" then you can't use the "standard" COC to determine what the DOF will be (i.e. in a calculator).
These days, images are viewed digitally more often than not. It may be on a cellphone, a laptop, or a large high-res display and we have no control over that. As such, the DOF in our images is infinitely variable because the display size is infinitely variable (how often do you move closer/further from your monitor?).
What we really should say is "I think the DOF is too shallow for how I'm viewing this image"...
None of this has anything to do with resolution and what is actually in focus. What is "in focus" will always be in focus/sharp regardless of how it's viewed only limited by the resolution of the display medium (and visual acuity)
The whole concept of DOF is in defining how an image is going to be perceived by a viewer. To do that it "defines/restricts" several variables. In the days of prints being the most common form of display we had more control of the perceived DOF. We determined the print size for the desired use. We knew that if the image was going to be in a book at full page size it had to be sharper overall or it would have less DOF. And if it was going to be printed at 1/4 page size it would have greater DOF (people tend to look at books from a fixed distance).
We also knew that if we printed an image at x size for display it would tend to be viewed from a typical distance assuming the situation allowed for that. But if we were printing larger or the typical viewing distance for that size couldn't be achieved (i.e. a hallway) the DOF would be less.
To have a known/fixed DOF you have to know the display size/viewing distance *before* taking the picture and then you have to accommodate/control those variables in actual application. (we never have/had control over visual acuity). If you do know the display size/distance and they are not "standard" then you can't use the "standard" COC to determine what the DOF will be (i.e. in a calculator).
These days, images are viewed digitally more often than not. It may be on a cellphone, a laptop, or a large high-res display and we have no control over that. As such, the DOF in our images is infinitely variable because the display size is infinitely variable (how often do you move closer/further from your monitor?).
What we really should say is "I think the DOF is too shallow for how I'm viewing this image"...
None of this has anything to do with resolution and what is actually in focus. What is "in focus" will always be in focus/sharp regardless of how it's viewed only limited by the resolution of the display medium (and visual acuity)
sk66
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The orientation of the paper has noting to do with "how" the DOF functions/appears/is divided. It only changes what is within the DOF...
If the paper was perpendicular (vertical) the DOF would still be 50/50, there just wouldn't be anything there to see.
With focus distances @/beyond hyperfocal distance DOF is divided 1/3 in front, 2/3 behind the point of focus. If you change the aperture by a factor of 2 (2x or 1/2) you change the DOF by a factor of 2. If you change the FL by a factor of 2 you change the DOF by a factor of 4. And if you change the distance by a factor of 2 you change the DOF by a factor of ~4 (the more DOF you have the less FL and distance are a 1:1 swap).
With focus distances short of hyperfocal distance these behaviors change until ultimately DOF is only controlled by magnification, what's w/in the DOF is controlled by distance, and the DOF is divided 50/50.
And I should probably reiterate...this still does not make the DOF a fixed entity... it's only "assumed to be."
StewartR
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That's an excellent idea!
StewartR
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Not quite. You're right about the 50/50 split at close distances, but (like many many other people) you're wrong about the 1/3 - 2/3 bit.
I wrote about this back in 2008:
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sk66
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- Steven
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Yes, you are correct... the displacement is actually continually variable.
For instance, no matter how far into a scene you focus, the "near focus" will never be further than the hyperfocal distance. I.e. if you focus at infinity the displacement is 100% in front...
The 1/3-2/3 is another oversimplification that we tend to use too much.... Guilty as charged!