Mark Dubovoy's essay

[digitaldog]

As long as the histogram on the camera is a fair representation of what is going on ...

Its not, by a lot (unless you are only concerned with the JPEG and not the raw data itself).

[Alan Goldhammer]

Its not, by a lot (unless you are only concerned with the JPEG and not the raw data itself).

Actually on my Nikon D300 it's pretty darn good and useful in determining whether the correct exposure has been obtained.  I realize that it's from the JPEG but it is the one tool we have to gauge the exposure without bracketing every shot.  When I've done bracketing, it's usually the first exposure that's correct, but maybe I have the one camera in a 1000 that gives this result.

[digitaldog]

Actually on my Nikon D300 it's pretty darn good and useful in determining whether the correct exposure has been obtained.  I realize that it's from the JPEG but it is the one tool we have to gauge the exposure without bracketing every shot.  When I've done bracketing, it's usually the first exposure that's correct, but maybe I have the one camera in a 1000 that gives this result.

How are you determining its correct?

[BJL]

What I want from the exposure index (so-called ISO settings) on a camera is that when I choose f-stop, shutter speed on the basis of the ISO speed and my light measurements, I get correct exposure levels. What Canon and Nikon seem to be doing is ensuring exactly that, correcting for a well-known problem of most sensors with micro-lenses: their sensitivity is distinctly less to light that comes in at a highly off-perpendicular angle, which happens with the light at the outer edges of the very broad light cone from a very low f-stop. It was discusses years ago that microlenses limit the speed advantage of f-stop reductions once one goes much below f/2, and certainly below f/1.4.

To put it another way, the camera is simply adjusting the amplification needed to adjust for the fact that the sensitivity of the sensor actually declines as the f-stop gets very low, and so maintaing constant true sensitivity/exposure index. DxO has a controversial way of measuring "true ISO exposure index", as has been discussed in other threads.

So on the one hand, Canon and Nikon are doing exactly what should be wanted by anyone who wants to make precise manual exposure decisions but knows only the f-stop, not the effective T-stop of a particular lens+sensor combination.

On the other hand, this does show that somewhere between f/2 and f/1.4, further speed gains from reducing f-stop are mostly lost to this "microlens vignetting".

And to respond to a recent post: the DOF reduction is partly or mostly lost for exactly the same reason: it is the extra light coming in near the edges of the larger light cone of a lower f-stop that increases OOF effects, so if the sensor is not detecting much of that extra light when you open up beyond about f/1.8 or f/1.4, the DOF reduction is also mostly lost. By the way, OVF image brightness stops improving even earlier, about f/2 or above, for other reasons, to do with the ground glass of an OVF.

[Sekoya]

without actually knowing how the measurements were measured, the article is difficult to place in context. E.g., on the face of it, the Nikon data seems to contradict the whole CMOS versus CCD argument; the D50/D70 series (CCD sensor, if I recall correctly) is worse than the D300 series (CMOS sensor).

Well, how do you think they would measure it? You provide a target of known illumination, take pictures at various f-stops and compare the raw data, if no effects are there, opening the f-stop by one stop should give you twice the value in the raw data.

And of-course this CCD vs. CMOS argument is very dubious, as the data contradict it. But any argument LL makes that makes MF appear in a more positive light has a high potential to be dubious. Not to say also irrelevant in this case as there is no MF lens faster than f/2 anyway.

[Sekoya]

Can someone explain why the circle of confusion of a large aperture lens is impacted by the apparent higher iso of the sensor.

The first item (loss of 0.5 EV due to T stop) is probably true, nice to know but not significant for most shooting. However the DOF story in my mind is unfounded and FUD.

The long tube effect of the individual sensel has the same effect as a physical aperture (not in a perfect manner as it is in wrong position), thus in effect what Marc describes here is actually a smaller f-stop (except that is in the wrong position and thus behaves not really the same as a normal aperture but it behaves to some extent similarly).

[Sekoya]

What I want from the exposure index (so-called ISO settings) on a camera is that when I choose f-stop, shutter speed on the basis of the ISO speed and my light measurements, I get correct exposure levels. What Canon and Nikon seem to be doing is ensuring exactly that, correcting for a well-known problem of most sensors with micro-lenses: their sensitivity is distinctly less to light that comes in at a highly off-perpendicular angle, which happens with the light at the outer edges of the very broad light cone from a very low f-stop. It was discusses years ago that microlenses limit the speed advantage of f-stop reductions once one goes much below f/2, and certainly below f/1.4.

To put it another way, the camera is simply adjusting the amplification needed to adjust for the fact that the sensitivity of the sensor actually declines as the f-stop gets very low, and so maintaing constant true sensitivity/exposure index. DxO has a controversial way of measuring "true ISO exposure index", as has been discussed in other threads.

So on the one hand, Canon and Nikon are doing exactly what should be wanted by anyone who wants to make precise manual exposure decisions but knows only the f-stop, not the effective T-stop of a particular lens+sensor combination.

On the other hand, this does show that somewhere between f/2 and f/1.4, further speed gains from reducing f-stop are mostly lost to this "microlens vignetting".

I don't think that reducing the effect of one full stop (when going from f/2 to f/1.4) by a third to two-thirds of a stop (for the FF cameras) could be cold as 'mostly', 'mostly' would mean by more than half. But for some of the older and crop-cameras cameras it is can be called mostly as the effect is more than half of a stop.

And the proper thing to do would be to adjust the displayed f-stop, ie, you set your lens to f/1.4 and it reads f/1.5 or f/1.8 (but that would confuse a lot of people, as the same lens would show different values on different cameras).

And the interesting question is what the cameras do with non-CPU lenses.

[Alan Goldhammer]

How are you determining its correct?

Andrew,
Two ways.  First was to use a standard print designed for printer testing coupled with the X-Rite Passport (I used the print that Jack Flesher created that posted on the Outback Photo site.  Exposures were done under daylight conditions (no direct sun on the target as it was a slightly overcast day).  I used a 60mm macro lens and kept the aperture at 5.6 and bracketed by changing the exposure time (ISO kept at 200).  I had a series of images that could be evaluated both visually and via the histogram rendered in Lightroom and compare it to what was rendered in the camera.  In this case, it was the standard (unbracketed) that was closest.  Second test was to take bracketed pictures of a rose in bloom.  Same thing here, the initial camera reading was the best image both visually and via the histogram. 

Alan

[digitaldog]

I had a series of images that could be evaluated both visually and via the histogram rendered in Lightroom and compare it to what was rendered in the camera.  In this case, it was the standard (unbracketed) that was closest.  Second test was to take bracketed pictures of a rose in bloom.  Same thing here, the initial camera reading was the best image both visually and via the histogram. 

So you matched (as closely as possible) the histogram on the camera to the histogram in LR based on various settings (I would assume mostly Exposure) on the various brackets until the two histograms matched?
In terms of the “over exposed” but “normalized* images in LR, were they able to be matched?

*Normalized based on the article here on LL: http://www.luminous-landscape.com/tutorials/expose-right.shtml

[Alan Goldhammer]

And the interesting question is what the cameras do with non-CPU lenses.

I have three 'old' Nikkor lenses that I had adapted to fit my D300.  The f stop registers fine and is properly shown, but of course cannot be altered by the camera during exposure.  Whether anything else is going on is anyone's guess.

[Alan Goldhammer]

So you matched (as closely as possible) the histogram on the camera to the histogram in LR based on various settings (I would assume mostly Exposure) on the various brackets until the two histograms matched?
In terms of the “over exposed” but “normalized* images in LR, were they able to be matched?

*Normalized based on the article here on LL: http://www.luminous-landscape.com/tutorials/expose-right.shtml

The match is between the normal exposure and 1/3 over exposure which is why I find that only under strange light conditions that I need to expose to the right more than 1/3 of a stop.  I would estimate that 80% of the time I do not need to change from the original reading.  Again, this is with my camera and others may have different experiences.

[JohnTodd]

Assuming the cameras are making these amplification decisions based on the reported aperture of the lens, a simple way to test it would to be to take an adapted non-OEM lens with a programmable EXIF chip and compare images taken under identical circumstances with the chip set to report F1.0, F1,2, F1.4, etc. - the images should get progressively darker down to a plateau, right?

[dubomac]

Hello everybody.

Thank you for reading the open letter and thank you so much for all your comments.

Let me start with an OOOOPS, I apologize!  When the issue that prompted this open letter came about, it was because of a discussion of the structure of CMOS sensors. The whole discussion focused on these structures, and once I received the final data it escaped me that some of the cameras on the chart had CCD sensors. I apologize for any confusion this oversight might have caused. 

The whole point of the letter though, remains intact, which is that the light loss depends on the structure of the sensor, and that light loss at the sensor is a very real phenomena. 

On a related topic, and to respond directly to one of the comments on the forum: The mention of Medium Format has nothing to do with trying to put MF in a better light or to try to imply that somehow CCD sensors are better than CMOS.  Quite the contrary, the mention of MF was meant to point out that the MF back manufacturers face an even bigger problem, which has forced them to produce structures that are quite different. My understanding from talking to the CTO's of the major MF companies is that at this point in time they cannot produce a CMOS sensor that will work for MF backs. While the current CCD structures work well with camera movements, there is no free lunch and they require compromises in other areas, for example, MF CCD backs have a huge negative, which is that Live View is not possible. Personally, I wish I could have a MF back with Live view today!  There are other drawbacks to CCD's, but let us not digress.

Let me also respond directly to the comment that the data cannot be found on the DxO site.  There is a new release of the DxO site coming in a few weeks.  It will contain a lot of new data, including the data that support this open letter.

While there has been some discussion on the depth of field issue, l would like to repeat what I said in the open letter, which is that DxO is in the process of performing thorough focus measurements. Once the final focus measurement data is available, we will be able to put this issue to rest.

Finally, I hope that all of you understand that the reason this was written as an open letter as opposed to an essay is that there are a number of open questions.  I asked some of them, and the participants in the forum are asking many other valid questions. We at The Luminous Landscape would like to have an open and constructive dialog with the camera manufacturers to better understand what is going on "under the hood", and to better understand what we can expect from our tools as we use them.

It is my hope that we will receive some interesting feedback from some of these companies.  If we do, we will definitely share it with you.

Mark Dubovoy

 

[sandymc]

Well, how do you think they would measure it? You provide a target of known illumination, take pictures at various f-stops and compare the raw data, if no effects are there, opening the f-stop by one stop should give you twice the value in the raw data.

Well, no. That doesn't tell you (a) what impact the lens only has independent of the sensor and (b) what impact data processing the camera has. Be aware that the "raw" data in images files, especially for CMOS sensors, is nowhere near the actual sensor data. It's already been been processed to remove noise, dark current, etc, etc

Hopefully the additional info that Mark says in his post above will soon be available will clarify this.

Sandy

[pegelli]

The long tube effect of the individual sensel has the same effect as a physical aperture (not in a perfect manner as it is in wrong position), thus in effect what Marc describes here is actually a smaller f-stop (except that is in the wrong position and thus behaves not really the same as a normal aperture but it behaves to some extent similarly).

Since the optical circle of confusion is spread over multiple sensels I still do not understand how the long tube effect of one individual sensel inpacts this. So I can understand how the effect results in less light being recorded (hence larget difference between T and F stop) but not how it impacts the DOF.

[Christoph C. Feldhaim]

Since the optical circle of confusion is spread over multiple sensels I still do not understand how the long tube effect of one individual sensel inpacts this. So I can understand how the effect results in less light being recorded (hence larget difference between T and F stop) but not how it impacts the DOF.

I understood it so, that light beams coming from the outer part of the lens get more diminished at the sensor than the light beams coming from the central part, because of the angle towards the sensel tube. Therefore adding an F-Stop speed from a fast lens, lets say from f2.0 to f1.4 doesn't add proportionally more light. So - to get a correct +1EV exposure you'd need either longer exposure or ISO gain, since the additional F-Stop doesn't deliver the full amount of light. Shortly: the light from the center of the lens contributes dysproportionally more to the exposure than the light from the "outer rim" ... This appears similar to the leaf shutter effect at short exposure times. This changes the behavior of the transition from sharp to unsharp zones of the image and is one of the explanations, why digital images have a much harder transition from sharp to unsharp zones, as opposed to film. Modern lens design is another factor.
Cheers
~Chris

ADDENDUM: I made a mistake up there, since actually the described effect should make the sharpness transition smoother, not harder. I just don't want to edit above for accuracy of the record. Sorry for the confusion ... Now I myself have some more questions....

[MelissaF]

Hello,
There are so many glaring flaws in that logic. The data clearly shows that ultra large aperture lenses still collect more additional light than is lost by the sensor.Going from f/2.0 to f/1.4 may not yield the full stop of light we might expect but it still yields a net benefit of 2/3 stops of light. 
The data is explicitly showing that.An f/2.0 lens does not yield "virtually the same" results a f/1.4.
Regards,
Ali.

[pedro.silva]

[...] MF CCD backs have a huge negative [...]

you mean, they shoot film too?!  minted!
sorry, couldn't resist...

I'd like to know [...] whether the attenuation is measured in the center, averaged over the whole frame or via a center-weighted average?

i wondered about this too, details from mr dubovoy would be appreciated.

[Bart_van_der_Wolf]

Hello everybody.

Thank you for reading the open letter and thank you so much for all your comments.

Let me start with an OOOOPS, I apologize!  When the issue that prompted this open letter came about, it was because of a discussion of the structure of CMOS sensors. The whole discussion focused on these structures, and once I received the final data it escaped me that some of the cameras on the chart had CCD sensors. I apologize for any confusion this oversight might have caused.

Hi Mark,

Honestly though, that was not the only oversight. The most glaring omissions are the fact that microlenses reduce the sensitivity to angular rays being blocked by the exposure gates. In fact the microlenses are so effective that the light fall-off is less than with film!, What's more, CMOS devices currently are often used with lenses of a retrofocus design (for shorter focal lengths), and the mirrorbox dimensions/depth limits the maximum angle of incidence.

BTW, you also didn't mention in your introduction that the difference between F-stop and T-stop is caught by the internal exposure meter, and that wider aperture lenses exhibit more vignetting when used wide open (which will be compensated for by the internal exposure meter). I admit that it something that needs to be compensated for when using manual exposure settings, just like one needs to compensate for shorter focus distances (= larger magnification factors, bellows factor).

There are special considerations when the physical sensor array gets larger, and especially when lens shift is introduced. That makes fixed microlens designs (where the lens flange or rather the exit pupil is assumed to be in a fixed position and microlenses can be offset towards the corners) for these scenarios suboptimal, and as such introduces the angular masking issue (including color cast).

The only difference between CCDs and CMOS devices in the context of masking, is that a CMOS device has relatively more surface area that's opaque to incident light, but that doesn't say much about the sensitivity to angular effects. In fact as the presented data shows, CCDs seem to be more sensitive to angular effects. There does seem to be a trend that smaller sensel pitches suffer more from exposure gate masking effects than the larger pitches do, which seems logical from a geometric standpoint (smaller surface area means larger percentage shadow at a given depth).

The whole point of the letter though, remains intact, which is that the light loss depends on the structure of the sensor, and that light loss at the sensor is a very real phenomena.

Well, other than reducing the quantum efficiency of the detector which is a fact of life, it can only be improved by using back-illuminated sensors and improved exposure gate designs. An interesting observation is that apparently, according to DxO, there is a gain compensation based on aperture value setting. That's something that will need to be verified.

Cheers,
Bart

[scott kirkpatrick]

Sandymc (one of the correspondents above, and the developer of Cornerfix, which corrects both luminance and color-shift vignetting) surely understands what is missing from this article.  But he is tactfully suggesting that we wait for more information from DXO.  Fast lenses vignette, so it is important to know how the intensity that is received at the sensor varies as a function of distance from the center of the image.  Manufacturers who have announced that they correct for vignetting (only Leica, AFAIK) apply a correction which increases as you move from the center of the image to the outside.  In Leica's case, they appear to provide this correction for an aperture value about two stops below wide open, and let the corners darken at wider apertures.  (My observation from a few experiments -- Leica doesn't comment on this.)  This is a sensible approach, since the color shift is annoying but aperture independent, while luminance vignetting often improves a picture by solidifying the edges and is strongest at wide apertures.  Anyway, summarizing the effects as a T-stop to F-stop difference leaves a lot of questions unanswered.  It is worth digging into what corrections manufacturers slip in but the full answer requires a curve for each lens and maybe for each of several apertures, and can't be reduced to a single data point.

scott

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