Doug Kerr
Well-known member
A recent article on Luminous Landscape:
http://www.luminous-landscape.com/essays/an_open_letter_to_the_major_camera_manufacturers.shtml
discusses a recent report from DXO labs that reveals how certain digital cameras apparently ameliorate an undesirable optical-photometric phenomenon. The author suggests that camera manufacturers either should not do that, or at least should "tell us" about it.
Here is my take on this situation.
**************
The basic technical story
a. As we increase the relative aperture (that's what the f-number tells us), the photometric exposure on the film or overall sensor surface changes in a predictable way.
A change in f-number from f/4 to f/1.4 increases the photometric exposure by almost exactly 8 times. This will be true for digital sensors as well as film.
b. In a digital camera, the "photodetectors at the bottom of chimneys" effect means that the light from a point on the object that passes through the outer regions of the aperture and lands on the sensor surface (at an oblique angle) is relatively less effective on the sensor than the same amount of light per unit surface area arriving via the central part of the aperture (at a less oblique angle).
c. Thus the impact on the sensor is no longer consistent with photometric exposure, a discrepancy that escalates when more of the light arrives at more oblique angles, as occurs as the aperture increases.
Thus, perhaps, a change in the f-number from f/4 to f/1.4 would increase the impact on the photodetectors by only about 6 times.
d. As a result, if we use shutter speed priority and
• set a shutter speed of 1/200 sec, and the metering system set an aperture of f/4, and we got an exposure result that we thought was "proper", and then
• set the shutter speed to 1/25 sec, and the metering system would then set an aperture of f/1.4, we might find that we had a result that was underexposed by the equivalent of 1/3 stop.
e. Evidently, to avert this effect, the camera manufacturers shift the sensor gain up a bit when a larger aperture is about to be used.
f. The charts in the article suggest that, for a Canon 550D, when the aperture to be used is f/1.2, the gain is bumped up by about 1/2 stop (that is, by a factor of about 1.4).
We are tempted to say that this is an increase in the ISO SOS (an actual objective measure of the sensor sensitivity) of 1/2 stop (1.4x). But, as we will see shortly, that is not the proper outlook.
For a Nikon D200, when an aperture of f/1.4 is about to be be used, the sensor gain is evidently bumped by about 1/3 stop (a factor of about 1.25).
g. The objective measures of sensor sensitivity stated as various "ISO" quantities (ISO speed, ISO SOS) are based on the response of the sensor to various photometric exposures. The phenomenon at issue here reduces the sensitivity of the sensor as a greater fraction of the arriving light arrives at increasingly oblique angles.
Thus, the increase in gain said to be applied in such cases does not increase the ISO sensitivity but rather partially restores it to the "set" value. (The compensation is apparently not complete.)
Complain! Sue!
Great outrage over this apparent camera feature is suggested. But over what? That the manufacturer causes the camera to unilaterally increase the ISO sensitivity of the sensor to compensate for the expected phenomenon when a large aperture is about to be used? Well, they don't increase the ISO sensitivity. They do increase the sensor gain to prevent the decline of the ISO sensitivity.
In any event, if we for some reason don't like that, what do we suggest the manufacturers do instead?
The phenomenon is present (although lenses designed for digital cameras often have their entrance pupils intentionally moved forward to reduce the magnitude of the phenomenon). We cannot complain it out of existence.
Possibilities:
1. Do nothing. People might want to employ exposure compensation (EC) to compensate for the metering inconsistency with large apertures. Or complain. Horn players have to move their fists inside the bells of their instruments to keep them properly on pitch for different notes. Singers move a hand microphone farther from their mouths when singing louder. It's part of the craft.
2. Automate the "application of EC". In aperture priority mode, set a shutter speed that differed from the basic result of the metering equations to compensate for the phenomenon expected at the aperture about to be used. In shutter speed priority mode, set an aperture that would take the phenomenon into account. In programmed exposure mode, set a combination that would take the phenomenon into effect. In manual exposure mode, make an adjustment to the "little meter".
A disadvantage is that the exposure set might be inconsistent with the indications of an external exposure meter (something that camera manufacturers strive mightily to avoid, since it leads to complaints about the camera's metering system being inaccurate).
3. Adjust the sensor sensitivity to compensate (apparently what is being done).
If the complaint is that the manufacturers don't tell us this, I could fill pages with things they don't tell us. Canon, for example, doesn't tell us that with a flash active, and the camera in programmed exposure mode, an aperture larger than f/4.0 will never be selected. Or how E-TTL metering works.
The argument is made that by doing this, the manufacturers are misrepresenting the "ISO something" of their cameras under this situation. Of course, as I pointed out, that is not really what happens.
In any case, note that Canon, for example, does not say that the "ISO" setting we make on the camera produces a corresponding value either of the two ISO objective measures of actual sensitivity, the ISO speed or the ISO SOS. They state instead the ISO REI, a value they are free to choose as they see fit.
Loss of light? Or just squandering it.
The author speaks of the underlying phenomenon as being a loss of light. It isn't so far as the overall sensor is concerned, although it is from the standpoint of the business end of individual detectors themselves.
What does the concept of the T-stop have to do with this?
The author also characterizes this phenomenon as a degradation in the T-stop of the lens. As he well discusses at the outset, the T-stop of a lens (for any given f-number setting) is sort of an "effective f-number" which takes into account that the lens does not have 100% transmission - that is, that part of the light it collects is not delivered to the focal plane.
But it is misleading to invoke this as a metaphor in this situation. The phenomenon at issue does not cause the lens to deliver less light to the focal plane. It does cause the individual photodetectors to catch and use less of that light when the light arrives at increasingly oblique angles.
Another aspect of the matter
The phenomenon of the sensor having less sensitivity in the case of "oblique rays" has another bad effect, not so correctable by a change in exposure or in sensor gain.
For a given scene point luminance, the photometric exposure falls off as we move from the center of the scene (the source of "natural vignetting"). One classical model of this phenomenon suggests that the falloff is proportional to the fourth power of the cosine of the angle the point of interest lies off the axis. Of course, this is rarely found to be the actual relationship.
But, whatever the relationship is for a particular lens at a particular aperture, at larger apertures, the phenomenon we discuss here (the "photodetectors at the bottom of chimneys" effect) accelerates the decline not in photometric exposure but rather in the impact on the sensor (which is of course what creates our image).
One can of course compensate for the overall effect during raw development (or later, if need be). Many processing software packages do this, based on tables for the lens noted to be in use), and some cameras are now beginning to have this internally.
Well, it's time for breakfast.
Best regards,
Doug
http://www.luminous-landscape.com/essays/an_open_letter_to_the_major_camera_manufacturers.shtml
discusses a recent report from DXO labs that reveals how certain digital cameras apparently ameliorate an undesirable optical-photometric phenomenon. The author suggests that camera manufacturers either should not do that, or at least should "tell us" about it.
Here is my take on this situation.
**************
The basic technical story
a. As we increase the relative aperture (that's what the f-number tells us), the photometric exposure on the film or overall sensor surface changes in a predictable way.
Photometric exposure is the physical phenomenon to which film responds (but not necessarily a digital sensor). It is the product of the illuminance on any point in the film times the duration for which it persists - the exposure time.
A change in f-number from f/4 to f/1.4 increases the photometric exposure by almost exactly 8 times. This will be true for digital sensors as well as film.
b. In a digital camera, the "photodetectors at the bottom of chimneys" effect means that the light from a point on the object that passes through the outer regions of the aperture and lands on the sensor surface (at an oblique angle) is relatively less effective on the sensor than the same amount of light per unit surface area arriving via the central part of the aperture (at a less oblique angle).
c. Thus the impact on the sensor is no longer consistent with photometric exposure, a discrepancy that escalates when more of the light arrives at more oblique angles, as occurs as the aperture increases.
Thus, perhaps, a change in the f-number from f/4 to f/1.4 would increase the impact on the photodetectors by only about 6 times.
d. As a result, if we use shutter speed priority and
• set a shutter speed of 1/200 sec, and the metering system set an aperture of f/4, and we got an exposure result that we thought was "proper", and then
• set the shutter speed to 1/25 sec, and the metering system would then set an aperture of f/1.4, we might find that we had a result that was underexposed by the equivalent of 1/3 stop.
e. Evidently, to avert this effect, the camera manufacturers shift the sensor gain up a bit when a larger aperture is about to be used.
f. The charts in the article suggest that, for a Canon 550D, when the aperture to be used is f/1.2, the gain is bumped up by about 1/2 stop (that is, by a factor of about 1.4).
We are tempted to say that this is an increase in the ISO SOS (an actual objective measure of the sensor sensitivity) of 1/2 stop (1.4x). But, as we will see shortly, that is not the proper outlook.
For a Nikon D200, when an aperture of f/1.4 is about to be be used, the sensor gain is evidently bumped by about 1/3 stop (a factor of about 1.25).
g. The objective measures of sensor sensitivity stated as various "ISO" quantities (ISO speed, ISO SOS) are based on the response of the sensor to various photometric exposures. The phenomenon at issue here reduces the sensitivity of the sensor as a greater fraction of the arriving light arrives at increasingly oblique angles.
Thus, the increase in gain said to be applied in such cases does not increase the ISO sensitivity but rather partially restores it to the "set" value. (The compensation is apparently not complete.)
Complain! Sue!
Great outrage over this apparent camera feature is suggested. But over what? That the manufacturer causes the camera to unilaterally increase the ISO sensitivity of the sensor to compensate for the expected phenomenon when a large aperture is about to be used? Well, they don't increase the ISO sensitivity. They do increase the sensor gain to prevent the decline of the ISO sensitivity.
In any event, if we for some reason don't like that, what do we suggest the manufacturers do instead?
The phenomenon is present (although lenses designed for digital cameras often have their entrance pupils intentionally moved forward to reduce the magnitude of the phenomenon). We cannot complain it out of existence.
Possibilities:
1. Do nothing. People might want to employ exposure compensation (EC) to compensate for the metering inconsistency with large apertures. Or complain. Horn players have to move their fists inside the bells of their instruments to keep them properly on pitch for different notes. Singers move a hand microphone farther from their mouths when singing louder. It's part of the craft.
2. Automate the "application of EC". In aperture priority mode, set a shutter speed that differed from the basic result of the metering equations to compensate for the phenomenon expected at the aperture about to be used. In shutter speed priority mode, set an aperture that would take the phenomenon into account. In programmed exposure mode, set a combination that would take the phenomenon into effect. In manual exposure mode, make an adjustment to the "little meter".
A disadvantage is that the exposure set might be inconsistent with the indications of an external exposure meter (something that camera manufacturers strive mightily to avoid, since it leads to complaints about the camera's metering system being inaccurate).
3. Adjust the sensor sensitivity to compensate (apparently what is being done).
If the complaint is that the manufacturers don't tell us this, I could fill pages with things they don't tell us. Canon, for example, doesn't tell us that with a flash active, and the camera in programmed exposure mode, an aperture larger than f/4.0 will never be selected. Or how E-TTL metering works.
The argument is made that by doing this, the manufacturers are misrepresenting the "ISO something" of their cameras under this situation. Of course, as I pointed out, that is not really what happens.
In any case, note that Canon, for example, does not say that the "ISO" setting we make on the camera produces a corresponding value either of the two ISO objective measures of actual sensitivity, the ISO speed or the ISO SOS. They state instead the ISO REI, a value they are free to choose as they see fit.
Loss of light? Or just squandering it.
The author speaks of the underlying phenomenon as being a loss of light. It isn't so far as the overall sensor is concerned, although it is from the standpoint of the business end of individual detectors themselves.
What does the concept of the T-stop have to do with this?
The author also characterizes this phenomenon as a degradation in the T-stop of the lens. As he well discusses at the outset, the T-stop of a lens (for any given f-number setting) is sort of an "effective f-number" which takes into account that the lens does not have 100% transmission - that is, that part of the light it collects is not delivered to the focal plane.
But it is misleading to invoke this as a metaphor in this situation. The phenomenon at issue does not cause the lens to deliver less light to the focal plane. It does cause the individual photodetectors to catch and use less of that light when the light arrives at increasingly oblique angles.
Another aspect of the matter
The phenomenon of the sensor having less sensitivity in the case of "oblique rays" has another bad effect, not so correctable by a change in exposure or in sensor gain.
For a given scene point luminance, the photometric exposure falls off as we move from the center of the scene (the source of "natural vignetting"). One classical model of this phenomenon suggests that the falloff is proportional to the fourth power of the cosine of the angle the point of interest lies off the axis. Of course, this is rarely found to be the actual relationship.
But, whatever the relationship is for a particular lens at a particular aperture, at larger apertures, the phenomenon we discuss here (the "photodetectors at the bottom of chimneys" effect) accelerates the decline not in photometric exposure but rather in the impact on the sensor (which is of course what creates our image).
One can of course compensate for the overall effect during raw development (or later, if need be). Many processing software packages do this, based on tables for the lens noted to be in use), and some cameras are now beginning to have this internally.
Well, it's time for breakfast.
Best regards,
Doug