Of the same poster, yes.
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A couple of questions for Great Bustard…
- Thread starter Ziggie
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Of course the ISL applies. The only difference is, with optics in the path, the point of origin (r = 0) is not necessarily at the position of the last optical element.
Which position are you agreeing with?
That light from a reflected source or a focused system doesn't obey the inverse square law, or that it does?
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Great Bustard
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But it doesn't.
If the sensor is twice as far away, it still records the same amount of light. Hence, the ISL does not apply.
But it does. The inverse square law is a direct consequence of geometry and the conservation of energy/photons/whatever.
Unfortunately, you are wrong. The ISL applies to intensity, not to total light. The sensor needs to be twice as big (in linear dimension) to capture the same amount of light if it is placed at twice the distance from the origin.
I am really surprised that you don't understand this, given all your involvement in the arguments about "Equivalence."
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Great Bustard
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But this was presumed from the outset -- note the text I highlighted in bold above.
I am more surprised that you are arguing against what I wrote given that it is being presumed that the sensor is large enough to record the projected scene with respect to the OP's question.
The text highlighted above exactly demonstrates the ISL. The screen needs to be larger to capture all the light because of radial spreading, i.e., the ISL.
See above.
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Formulate it.
Great Bustard
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OK, let's say it like this:
The light from the exit pupil spreads according to the ISL. However, the size of the sensor is such that all the light from the scene falls on it at the focal plane. So, if the focal plane were twice as far from the exit pupil, then the sensor would have to be twice as large (4x the area).
Even that is untrue. For the ISL, you need to have a point source.
The light from the exist pupil is not coming from a point source (I am not talking about the object side), case closed. You cannot even say that is coming from a set of point sources (even if it did, the ISL would not hold). Those rays have directions, it is not like they propagate from each point of the exit pupil in all directions.
Imagine you shoot a bright point, like the sun or so. At any distance from the exist pupil, the total light is constant. The intensity varies on the plane - from a defocused to a focused then to defocused again, depending on the distance to the sensor. The ISL does not even make sense to be formulated.
Are you serious? Do you want me to write the equation for the area of a sphere?
J
Joe Pineapples II
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J
Joe Pineapples II
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You took your time getting here! Is the Bat Phone busted? Or would that be the Bustard Phone busted?
Joe
Joe
This has no relevance to the camera though. What you say makes total sense and I agree fully. Except it doesn’t apply to the camera. It only applies to light sources. The camera is not a light source. The Inverse Square Law applies to light sources, not camaras.
The only instance where this would apply to the camera is when your scene is exclusively lit by on-camera flash in which case the camera would indeed become a light source. That’s seldom the case though.
J
Joe Pineapples II
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The Inverse Square Law applies to all light, not only point sources. My studio softbox and my focal spot strobe both adhere to the law in the excact same manner despite being of different sizes. The softbox is quite large but the focal spot is essentially a point source. The light from both becomes less intense as I move them further away form whatever I am photographing and more intense as I move them closer. The change in intensity is according to the Inverse Square Law. The smaller the light source (i.e. point sources) the more accurately it adheres to the law though.
The light intensity inside the camera decreases as the distance between lens and image plane increases. A 25mm lens will basically project a brighter image than a 50mm lens unless compensated for by using a larger aperture.
This is why large format view cameras shooters have always had to figure in the bellows extension factor when shooting close-ups and using a hand-held light meter. As the lens gets racked forward for close focusing it moves further away from the image plane and the image gets progressively darker requiring the photographer to adjust the exposure accordingly. While this also holds true for the smaller format SLR or mirrorless cameras most people don’t notice because the built-in light meter automatically compensates for this while the view cameras of course don’t have built-in meters. I have encountered this numerous times in real life scenarios when photographing with a view camera.
In fact anyone can easily verify this if they have access to a view camera and an adapter that allow you to stick a smaller format camera with a built-in light meter on the back. It will fit securely into the Graflok clips on the rear standard. Aim your camera at a target that is both evenly lit and uniform. A uniform wall or solid overcast sky will do fine. As you rack the lens back and forth you can observe the camera light meter alternate between overexposure and underexposure. Brighter when the lens is close to the camera and dimmer when the lens is further away from the camera. I’ve tried it. It works excactly according to the Inverse Square Law. The lens projection inside the camera is indeed subject to that law. Theory and real world experience dovetails nicely.
In the case of general outdoor photography however you are right that the Inverse Square Law does not apply. According to the law the light intensity diminishes to ¼ with every doubling of distance from the light source. The sun is so enormously far away that by the time the light finally reaches us way down here on Earth the relative distances will be so great as to nullify any effects of the Inverse Square Law. It’s only when we use light sources at short distances that we need take the light fall-off described by the Inverse Square Law into account. Like when working in the studio or using on-camera flash.
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Ziggie
It applies in the case of the image the lens projects onto the camera’s image plane but not in the case of scene to camera distance.
The light intensity inside the camera decreases as the distance between lens and image plane increases. A 25mm lens will basically project a brighter image than a 50mm lens unless compensated for by using a larger aperture.
This is why large format view cameras shooters have always had to figure in the bellows extension factor when shooting close-ups and using a hand-held light meter. As the lens gets racked forward for close focusing it moves further away from the image plane and the image gets progressively darker requiring the photographer to adjust the exposure accordingly. While this also holds true for the smaller format SLR or mirrorless cameras most people don’t notice because the built-in light meter automatically compensates for this while the view cameras of course don’t have built-in meters. I have encountered this numerous times in real life scenarios when photographing with a view camera.
In fact anyone can easily verify this if they have access to a view camera and an adapter that allow you to stick a smaller format camera with a built-in light meter on the back. It will fit securely into the Graflok clips on the rear standard. Aim your camera at a target that is both evenly lit and uniform. A uniform wall or solid overcast sky will do fine. As you rack the lens back and forth you can observe the camera light meter alternate between overexposure and underexposure. Brighter when the lens is close to the camera and dimmer when the lens is further away from the camera. I’ve tried it. It works excactly according to the Inverse Square Law. The lens projection inside the camera is indeed subject to that law. Theory and real world experience dovetails nicely.
In the case of general outdoor photography however you are right that the Inverse Square Law does not apply. According to the law the light intensity diminishes to ¼ with every doubling of distance from the light source. The sun is so enormously far away that by the time the light finally reaches us way down here on Earth the relative distances will be so great as to nullify any effects of the Inverse Square Law. It’s only when we use light sources at short distances that we need take the light fall-off described by the Inverse Square Law into account. Like when working in the studio or using on-camera flash.
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Ziggie
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Andre Affleck
Senior Member
Here's my take. Let's say you are photographing a point source. The light radiates out from the point source and enters the lens. The amount of light entering the lens is proportional to the flux density and the area of the entrance pupils. Once inside the lens, the elements focus that light to a point again at the sensor. If you now move twice as far, the ISL says the flux density will be reduced by the square of the distance. However, the entrance pupil remains the same (IOW the steradians change). Therefore, even though the lens will again focus the light to a point source, the total amount of light has reduced by the square of the distance. So the ISL holds even for a lens.
However, once the source grows from a point to an area, things change. First of all, you can consider an area of emission as the integration of many point sources over the area. At close proximity, the integral starts to become closer to first order relationship (and reduces further down to unity at very close distances). At longer distances however, the error rapidly drops and quickly approaches second order again. But in general, the illuminance falls with distance (except at very close proximity.
Now, let's examinine what happens to an area of emission through a lens as one moves further away. The ISL still holds for the integral of all those point sources that make up the area of emission (just like the first scenario), and therefore, the light drops by the square of the distance (at sufficient distances), but since the projected area is also dropping by the square, it cancels, and density of light remains the same independent of distance.
So it depends on how you look at it. One may break it down by components and say the component of ISL holds, but is canceled by the area change component, or one may simply say the net effect is that the captured luminance remains the same independent of distance regardless.
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In fact it is indeed applicable to the lens projection. Like I said in another reply:
Anyone can easily verify this if they have access to a view camera and an adapter that allow you to stick a smaller format camera with a built-in light meter on the back. It will fit securely into the Graflok clips on the rear standard. Aim your camera at a target that is both evenly lit and uniform. A uniform wall or solid overcast sky will do fine. As you rack the lens back and forth you can observe the camera light meter alternate between overexposure and underexposure. Brighter when the lens is close to the camera and dimmer when the lens is further away from the camera. I’ve tried it. It works excactly according to the Inverse Square Law. The lens projection inside the camera is indeed subject to that law. Theory and real world experience dovetails nicely.
This is observable, repeatable, verifiable real world empirical evidence I have a hard time ignoring.
Finally, a quote from Langford's Basic Photography textbook:
“For distant subject (lens focused on infinity) the image is formed at one focal length from the lens. The inverse square law of light shows that doubling the distance of a surface from a light source quarters the light it receives. Therefore a lens of (say) 100mm focal length basically forms an image only one-quarter as bright as a lens of 50mm”
Andre Affleck
Senior Member
Which it does by design in order to keep the exposure the same at a particular f stop regardless of FL. The physical aperture changes with FL in order to keep relative aperture (f-stop) the same.
Not quite. As mentioned, the physical aperture increases with FL to maintain the same F-stop. So considering f2 on a 25mm lens, the physical aperture is smaller than f2 on a 50mm lens.
Sure, but It's a different scenario for view cameras. They were more primative, and physical aperture was not linked to bellows travel.
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Andre Affleck
Senior Member
Ok, but your enlargement factor is changing. Of course the exposure is going to change. You are taking the same amount of light and spreading it over a larger area so the density is going to change.
This is not the same thing as moving the camera to subject matter distance. Here, the light is changing, but it is being offset by the change area of the prejected image, so they cancel, leaving you with the same brightness
Correct for a point source. It does not apply to areas since as mentioned, the area drops by the square of the distance, cancelling the drop in light by the square. So the object is the same brightness, but occupies a smaller area on the sensor.
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