Ad
Ad
Ad
Pages: « 1 2 [3] 4 5 ... 8 »   Bottom of Page
Print
Author Topic: Mark Dubovoy's essay  (Read 29264 times)
dreed
Sr. Member
****
Offline Offline

Posts: 1169


« Reply #40 on: October 28, 2010, 05:42:32 AM »
ReplyReply

Until I read this, I thought that I was crazy for noticing that the shutter speed for a given photo was faster when using a prime than a zoom (at the same focal length.) i.e. using a 50mm f/1.4 at f/4 resulted in a faster shutter than a zoom at 50mm that was stopped down to f/4.

Now I know that I'm not quite as crazy as I thought I was Smiley
Logged
FranciscoDisilvestro
Sr. Member
****
Offline Offline

Posts: 323


WWW
« Reply #41 on: October 28, 2010, 06:54:49 AM »
ReplyReply

Quote
Until I read this, I thought that I was crazy for noticing that the shutter speed for a given photo was faster when using a prime than a zoom (at the same focal length.) i.e. using a 50mm f/1.4 at f/4 resulted in a faster shutter than a zoom at 50mm that was stopped down to f/4.

I would say that the effect you observed is due to the transmission losses of the zoom lens, since it has more elements than a prime. Both lenses at F/4 will have different T stops. I don´t think the sensor issue explained here is doing anything in your case
Logged

PierreVandevenne
Sr. Member
****
Offline Offline

Posts: 508


WWW
« Reply #42 on: October 28, 2010, 07:26:24 AM »
ReplyReply

Leaving aside the technical details and the logic lapses in the main article, isn't this a storm in a teacup? There are dozens of things that are in a way or another normalized for the comfort of photographers and to support legacy hardware. ISO really isn't anything more than a convenient abstraction and we already know different manufacturers ISOs are not really equivalent. Incidence issues are well understood: microlens geometry and lens design have already evolved.

Manufacturers automagically play with the whole process in order to extract the best out of their sensors and stay as close as possible to the legacy photographer's assumptions. So what? At another level, this isn't different from "discovering" they are "cheating" on real pixels counts by using bayer matrices.

AFAIC, I wouldn't mind having a slider for aperture and a slider for gain and some presets.
Logged
Gary Gray
Newbie
*
Offline Offline

Posts: 25


« Reply #43 on: October 28, 2010, 08:54:39 AM »
ReplyReply

Well, I was going to spout off in detail here but it appears that a large number of readers have already nailed this down for what it is.  A tempest in a tea-cup.  The article is a bit long winded, uninformative and poorly explains the fact that a digital camera sensor is not a piece of film and as such doesn't behave the same way film does, and intimates that the camera manufacturers are all in a conspiracy to keep important information from the consumer.  Knowing the gain curve for a particular camera isn't high on my list of priorities in photography.
Logged
bjanes
Sr. Member
****
Online Online

Posts: 2714



« Reply #44 on: October 28, 2010, 08:56:37 AM »
ReplyReply

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

Mark,

Thank you for posting a very informative article. Your use of T-stops actually impaired my comprehension of the article, since T-stops deal with the transmission of light by the lens and do not vary with the aperture, whereas the tunneling effect deals with loss of light at the level of the sensor, and this loss is unlike vignetting, since it also occurs in the central part of the field of view. I was not aware of this effect.

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.

Yes, right now your essay raises more questions than it answers, but it is a good starting point for additional study and discussion. I would be interested in getting more information on the geometry of CCD and CMOS sensors. As Bart pointed out, CCDs also suffer from this problem, which was particularly acute with the Leica M9 in which the flange to sensor distance is small since there is no mirror box requiring the use of retrofocus lens designs necessary for SLR cameras.

Back illuminated CCDs are available in specialized sensors used for science and space applications (as from Fairchild) and these would greatly reduce the tunneling effect, but I am not aware of their use in general purpose dSLRs or MFDBs. Sony has introduced a back illuminated CMOS for compact cameras, and the graphics in the link demonstrate the geometry of both front and back illuminated CMOS sensors.
Logged
jenbenn
Jr. Member
**
Offline Offline

Posts: 68


WWW
« Reply #45 on: October 28, 2010, 09:19:17 AM »
ReplyReply

On a side note: Not being an optical technician or a physisist I cannnot comment on the viability and reliability  of the measurment techniques used by DXO. I do have serious doubts about their reliability, however. There are many instances (far more than with other test sites) where their measurements have absoltly nothing to do with practical results. In fact, they are so far off, that its almost ridiculous. Just take the DXo resolution measurements of the Canon 16-35 mm lens, for example. At 16mm the DXO graph shows that the lens' edge and corner reslution decreases when stopping it down. DUDE!  Just shoot the lens in real life and report back. Its certainly not a perfect lens and the edges/corner are far from optimal even stopped down. But the corner resolution at f/8 is certainly better than at f/2.8. No question. What a joke.

Same goes for their camera sensor maesurments. Lookin at real life pics and prints from differnt camera models  I often find that their rankings are just upside down.

I am therfore very critical about  the findings in the current article.
« Last Edit: October 28, 2010, 09:33:00 AM by jenbenn » Logged
Robert Roaldi
Sr. Member
****
Offline Offline

Posts: 446


WWW
« Reply #46 on: October 28, 2010, 11:03:49 AM »
ReplyReply

Read this interesting comment on another forum http://www.dvxuser.com/V6/showthread.php?227234-Small-bomb-from-LL-and-DxO&p=2142565&viewfull=1#post2142565

I've wondered for years (even before digital) why we still use the f/stops and shutter speed combos that we do, figuring that it was just historically convenient.
Logged

--
Robert
robertroaldi.zenfolio.com
FranciscoDisilvestro
Sr. Member
****
Offline Offline

Posts: 323


WWW
« Reply #47 on: October 28, 2010, 02:04:19 PM »
ReplyReply

Reading the other forum comments,

The author seems to ignore the autoISO function in some cameras that allow you to set a fixed aperture and a shutter speed and the camera automatically adjust the ISO.
Somewhere down that thread, another poster makes a wrong claim about how the White Balance works, suggesting that there is a gain adjustment in each channel (RGB). White Balance does not work that way, it scales the numerical values of each channel in post processing.

I think that an important issue with varying ISO is the effect on Noise and Dynamic Range. Increasing ISO on any sensor will increase noise and reduce DR.
Logged

BartvanderWolf
Sr. Member
****
Online Online

Posts: 3012


« Reply #48 on: October 28, 2010, 02:26:20 PM »
ReplyReply

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.

An quick evaluation (EOS 1Ds3 + EF 85mm f/1.2 L II), using IRIS software to study the Raw data before demosaicing in linear gamma space, indeed suggests a minute increase in read-noise at apertures wider than f/2. Whether this is also true for other lenses remains to be seen.

The increase in read-noise is a function of gain, and appears to be in the order of a fraction of 1/3rd stop boost. More rigorous testing is needed, but I wanted to share the initial/preliminary finding to allow the discussion to progress on a more fact based second opinion, rather than on speculation or hearsay alone.

Cheers,
Bart
« Last Edit: October 28, 2010, 03:35:26 PM by BartvanderWolf » Logged
Sekoya
Newbie
*
Offline Offline

Posts: 16


« Reply #49 on: October 28, 2010, 03:27:32 PM »
ReplyReply

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
The fact that the Canon data are shown for f/1.2 and the Nikon data for f/1.4 clearly shows that actual lenses were used. And of course, the results will vary from lens to lens, there is nothing you can do about it. Unless there is an agreed ideal model how a f/1.4 lens should look like (and somebody builds such a lens) you always have to deal with real lenses. As long as the difference between several real f/1.4 lenses is noticeably smaller than the difference between f/2 and f/1.4, you can conclude that the average f/1.4 data are somehow representative of the 'f/1.4 effect'.
Logged
wolfnowl
Sr. Member
****
Offline Offline

Posts: 5560



WWW
« Reply #50 on: October 28, 2010, 04:06:43 PM »
ReplyReply

Agreed but this was also a key sentence: This graph clearly shows that camera manufacturers “game the system” by increasing the ISO without the photographer's knowledge.
For those of us that like to know what is happening behind the scene, I found the article highly enlightening! And I agree with Mark’s idea of an open letter to resolve this if possible. But like world peace, unlimited free and clean energy and cameras that produce an open, non proprietary raw file, I think it unlikely we users will be heard. Still, I’m thankful for the article, I learned something.

Me too.  Thanks for the info, Mark!

Mike.
Logged

If your mind is attuned to beauty, you find beauty in everything.
~ Jean Cooke ~


My Flickr site / Random Thoughts and Other Meanderings at M&M's Musings
dubomac
Contributor
Sr. Member
*
Offline Offline

Posts: 555


« Reply #51 on: October 28, 2010, 04:39:43 PM »
ReplyReply

Folks:

Since there has been some discussion on vignetting, and some postings about microlenses near the edges of the frame, I would like to point out that the DxO measurements were performed at the center of the frame.

There have also been some questions about how DxO performed the measurements, so I think that the following direct quote from the technical experts at DxO should clarify things for everybody:

First, we measure the ISO sensitivity of a sensor without optics, then since all sensors we currently measure are linear (or can be linearized with a simple “Dark value” subtraction) we can use the sensor as a photometer with an optics between the sensor and very stable luminous surface which luminance is precisely measured with a calibrated luminance-meter. It is then easy to compute the actual T-Stop of the lens and sensor combination: one part of the loss comes from then lens itself, another part is due to the sensor inability to capture light from some angles. That measurement is performed at image center, what happens in the field is a different measurement, available on www.dxomark.com as “vignetting”.


Mark Dubovoy
Logged
sandymc
Full Member
***
Offline Offline

Posts: 235


« Reply #52 on: October 28, 2010, 11:33:14 PM »
ReplyReply

Folks:

Since there has been some discussion on vignetting, and some postings about microlenses near the edges of the frame, I would like to point out that the DxO measurements were performed at the center of the frame.

There have also been some questions about how DxO performed the measurements, so I think that the following direct quote from the technical experts at DxO should clarify things for everybody:

First, we measure the ISO sensitivity of a sensor without optics, then since all sensors we currently measure are linear (or can be linearized with a simple “Dark value” subtraction) we can use the sensor as a photometer with an optics between the sensor and very stable luminous surface which luminance is precisely measured with a calibrated luminance-meter. It is then easy to compute the actual T-Stop of the lens and sensor combination: one part of the loss comes from then lens itself, another part is due to the sensor inability to capture light from some angles. That measurement is performed at image center, what happens in the field is a different measurement, available on www.dxomark.com as “vignetting”.


Mark Dubovoy

Mark,

I'd have at least two concerns with that:
  • Firstly, using that procedure, DxO would have to have tested a large number of samples of each individual camera, else all you're looking at is sample to sample variation.
  • Secondly, I think there's a logical inconsistency between the measurement process and the conclusions made. If you're using the camera as a photometer, to believe you're getting valid data you have to assume that the in-camera processing isn't changing your measurement from exposure to exposure. But the conclusion of the article is that the in-camera processing does change from exposure to exposure, specifically with aperture! Now perhaps you can argue that processing only changes with aperture, and design the test round that assumption, if you were confident enough that aperture is all that's driving the changes. But if there is in-camera processing occurring as claimed in the conclusions, the initial assumption required for the data to be valid starts to look really shaky.

So going from an interesting factoid (the first graph in the article) to the conclusion as to CMOS sensor design starts to feel to me like a huge leap....

Sandy
Logged
dubomac
Contributor
Sr. Member
*
Offline Offline

Posts: 555


« Reply #53 on: October 29, 2010, 02:38:10 AM »
ReplyReply

I would like to address the last set of comments on this thread:

I am not sure where the concept of using "the camera with in-camera processing" came from. Please read the DxO statement carefully, they measure THE SENSOR, and they use THE SENSOR as a photometer.

Similarly, regarding the comment about sample to sample variation: Professional testing labs always work using enough samples and scientifically proven statistical methods before they publish any measurements or conclusions.

I believe that we have reached the point where we have beaten this horse to death, so this will be my last post on this thread.

I sincerely hope that all of you have learned something useful from this discussion. I know I have.

May the photons smile on all of you.

 Smiley

Mark Dubovoy
Logged
Chris_J
Newbie
*
Offline Offline

Posts: 1


« Reply #54 on: October 29, 2010, 02:39:11 AM »
ReplyReply

Hi Folks,

having read both Mark's essay and the responses with interest, it strikes me that there are two areas of contention: the "attittude" side, (i.e. does it matter?), and the technical side (i.e. is the data analysed and interpreted correctly?).

The first area, the "attitude," will always be personal, and people have to judge for themselves whether it matters to them. Hence I prefer not to comment on that.

However, regarding the technical issues, I would like to point out that Mark is not the first to describe the phenomenon that is subject of his open letter. With that I do not wish to take any credit away from Mark; he is the first (as far as I am aware) to describe the light loss backed by current and more extensive data.

I in no way endorse their system, or make any intimations regarding the overall quality of the results from their system, but the four-thirds consortium were making a lot of noise about light/ sensor issues. In fact, it was put forward as one of the main rationales behind the four-thirds design. They have gone a bit more quiet about it since advances in micro-lens design, but here is the still relevant version of the issue from them (half-way down the page):  http://www.four-thirds.org/en/fourthirds/index.html

Hope this helps,

Chris
Logged
Christoph C. Feldhaim
Sr. Member
****
Offline Offline

Posts: 2508


There is no rule! No - wait ...


« Reply #55 on: October 29, 2010, 05:04:05 AM »
ReplyReply

For me the light loss and possible hidden ISO change is not that much a possible issue,
but more the effect on the sharp-unsharp transitions of images, means the bokeh.

What I'd really like to see is an experiment of
- a test image of a motive with enough depth,
- taken with an open aperture, like F1.4
- with 2 sets of lenses: One digitars/modern lens, one old lens/designed for film
- and with film and sensor as media.
and this in a FF/35 mm format and in MF/MFDB format.

2/3 of an F-Stop more or less is a thing, one might care about or not,
but the rendering of the image / bokeh characteristics is something which will be seen, I believe.
A side by side test would be great for that.

Just my $ 0.02
Logged

sandymc
Full Member
***
Offline Offline

Posts: 235


« Reply #56 on: October 29, 2010, 07:13:48 AM »
ReplyReply

I am not sure where the concept of using "the camera with in-camera processing" came from. Please read the DxO statement carefully, they measure THE SENSOR, and they use THE SENSOR as a photometer.

Mark,

Sorry, but I think you misinterpret what DxO are saying; unless both Canon and Nikon gave DxO cameras modified to allow direct access to sensor, DxO are looking at processed data. The best you can get with an unmodified camera is the so called "raw" image data, but that's already been processed extensively. And if by some chance Canon and Nikon did both provide all those models of cameras with special firmware or test connections, they sure didn't do so in sufficient numbers for a statistically valid sample; 20 or 30 samples per model and you'd be dealing with thousands of specially modified cameras! Canon and Nikon may well be super nice guys, but I have trouble believing they're that nice.

Sandy
Logged
pegelli
Sr. Member
****
Offline Offline

Posts: 580



WWW
« Reply #57 on: October 29, 2010, 07:34:56 AM »
ReplyReply

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

Thanks for taking the time to explain the effect. I now understand it better, but also understand it has more to do with the shape of the bokeh, rather than the "exact dof" (I know "exact dof" is an oximoron).

However if the outer parts of the lens contribute less light vs. the center it reminds me of what the heritage Minolta (now Sony) 135 STF does, they built in a filter close to the aperture which is clear in the center and slowly gets denser as you move outward. This has the effect to make the bokeh really buttery smooth, so if a digital sensor used with a large aperture lens does the same thing (less contribution from the outer rims of the lens) why would the bokeh become harder rather than softer as happens with this STF lens.

Maybe I'm missing a trivial point, so pls. bear with me, I'm just trying to understand this whole story better.
Logged

pieter, aka pegelli
Christoph C. Feldhaim
Sr. Member
****
Offline Offline

Posts: 2508


There is no rule! No - wait ...


« Reply #58 on: October 29, 2010, 07:46:48 AM »
ReplyReply

Light coming from the lens and forming a point (ideally) has the shape of a cone with its base at the lens.
That is why wide angles are so sensible against defocusing errors.
The greater the angle at the tip of that cone is, the faster and intenser defocusing leads to blurring.
Light coming from the "outer rim", which is only revealed at open apertures, forms a cone with a greater angler than light coming from the central parts, which are always used, even at high F-Stops=small apertures.
The light forming a point comes from the central beam, beams from the outer rim and all between.
But: Due to the different angles, the light from the "outer rim", which is only there if the aperture is wide open,
is more sensible to the blurring effects of defocusing.
The light from the center is not.
Therefore - the larger the amount of light from the outer rim is, the faster defocusing effects become visible, and we see that as low DOF at open apertures.
Theoretically you could put a center filter with varying density (transparent to grey or grey to transparent) at the plane of the aperture and use this to manipulate the behavior of the bokeh. You could make the transition from sharp to blurred areas softer or harsher. I guess, the more light from the outer rim is there, the harsher the transition would be, the more from the center the softer.

To Pegelli:
ADDENDUM: Erm ... I just realized my error above ... Yup - the bokeh should be softer. But generally digital files seem to have a harder transition - Maybe its the thickness of the image plane film vs. sensor which comes into play ? Or is it the construction of the modern digital lenses ... ? Or is it "just" the generally smaller formats/sensor sizes?
Sorry for the mistake ... I'd really like to see a test and have some enlightenment from the tech nerds here ...

What remains for me is, that from experience there is a clear difference between the sharpness behavior of digital vs. film imaging and I thought (which I had to revise) the described problem could be an explanation. Now it appears the opposite is true and the light loss at sensor level is probably working against the otherwise harsh sharpness of digital systems. So - if I look at it from this side, this could actually be something good ...
« Last Edit: October 29, 2010, 08:38:00 AM by Christoph C. Feldhaim » Logged

PierreVandevenne
Sr. Member
****
Offline Offline

Posts: 508


WWW
« Reply #59 on: October 29, 2010, 08:22:23 AM »
ReplyReply

They are testing the sensors in the camera. If they wanted to test the sensor outside the body, they would have to provide custom electronics anyway, and their results wouldn't be relevant to the camera anymore. What manufacturer extract from their camera depends a lot on the electronics behind the sensor (see for example the different behavior of the KAF-3200ME in the QSI and SBIG astro cameras).

Here an interesting link on light transmission.

http://www.cctv-information.co.uk/i/Light_Transmission_Through_Lenses

Not the final word, for sure, but I think some of us will be surprised by the numbers.
Logged
Pages: « 1 2 [3] 4 5 ... 8 »   Top of Page
Print
Jump to:  

Ad
Ad
Ad