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Author Topic: Mark Dubovoy's essay  (Read 34038 times)
jenbenn
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« on: October 27, 2010, 09:47:59 AM »
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Hm, sounds a bit unfinished to me. Ok, manfacturers increase the "real" iso without the photographers knowledge, which possibly results in more noise. But hey at f/1.2 its about half a stop only. So the gain of a f/1.2 lens  over an f/2 lens is still there. As canon doesnt offer an 85mm f/1.4, which according to Mr Dubovoys argument would be the same noise wise as an f/1.2, I see no alternative to that very lens (unless you want to buy a missfocussing Sigma).

Regarding the depth of filed /maximum blur/bokeh thing: It would have been nice if Mr Dubovoy had facts. This way he is just creating a big stir on the basis of a few assumptions. Better do the tests first before publlishing some wild guessing.

In the end I fail to see the issue. The results of my 85/1.8 are different from the 85/1.2. I can see that in real life, without doing measurements.  This "revalation" will certainly not make the f/1.2 less usefull or replacable by an f/1.8 in practice. So yeah, for the sake of a technical dsicussion, why not ask the manufacturers to let us in on the topic. But  if they dont, its no big deal, because there is no alternative right  now than the technology which is available.
« Last Edit: October 27, 2010, 09:51:11 AM by jenbenn » Logged
billrickman
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« Reply #1 on: October 27, 2010, 10:02:43 AM »
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I would like to know if this tactic is used on cpu lens only or on any optic. I use an Olympus E-p1, primarily with non-cpu Zukios. During live view the camera adjusts the aperture on the system lenses as the brightness in the scene changes but cannot when a non-cpu lens is attached Grin If one where to repeat these test with non-cpu lenses, would the camera still adjust iso? Does entering the lens data as on a Nikon change this?

This oculd be a work around for critical work Wink but would drive up the prices on old lenses Sad

Bill
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Bill

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John.Murray
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« Reply #2 on: October 27, 2010, 10:37:03 AM »
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But  if they dont, its no big deal, because there is no alternative right  now than the technology which is available.

Which is *precisely* the point of his article.
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digitaldog
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« Reply #3 on: October 27, 2010, 11:11:40 AM »
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Which is *precisely* the point of his article.

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.
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Andrew Rodney
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cosmo.6792
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« Reply #4 on: October 27, 2010, 11:48:53 AM »
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It doesn't look like the author knows how to interpret the very data he is quoting in his own article.


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Bottom line: Due to the complexity of design and manufacture (let alone the high cost and weight) of large aperture lenses, one may actually end up with better results at virtually the same ISO and depth of field using lenses with more modest maximum apertures.

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.



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When you look at the structure of CMOS sensors, each pixel as basically a tube with the sensing element at the bottom.  If a light ray that is not parallel to the tube hits the photo site, chances are the light ray will not get to the bottom of the tube and will not hit the sensing element. Therefore, the light coming from that light ray will be lost. It appears from this graph that when using large aperture lenses on Canon cameras, there is a substantial amount of light loss at the sensor due to this effect. In other words, the "marginal" light rays coming in at a large angle from near the edges of the large aperture are completely lost.

The article fails to mention the important fact that there are microlenses at the top of these "tubes" to funnel light into the sensing elements at the bottom.  And no, "marginal" light rays coming at a large angle are not completely lost.  As microlenses become more efficient at collecting light, so will the sensor.

The article fails to point out that the amount of light lost at the sensor is decreasing with each camera generation.  It's right there in the charts, if the author had bothered to look.  The 1Ds3 sensor has the same pixel pitch as a an EOS 20D sensor, but the 1Ds3 loses only 1/3 stop of light @ f/1.4 compared to 2/3 stops by the 20D.  The pixel pitch of a 1D4 sensor is nearly identical to that of a 450D sensor, but the 1D4 loses only 0.4 stops of light @ f1.4 instead of 0.7 stops.  There is progress is being made in minimizing the light loss at the sensor level, and so there is no reason for companies to stop producing ultra large aperture lenses.



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I wish to re-emphasize that these issues apply only to the current crop of cameras with CMOS sensors

Odd that the article would be constantly laying the blame on the structure of CMOS sensors, when the very data quoted clearly indicates that the problem is far more severe with CCD sensors.  The graphs in the article clearly shows that CCD sensors were losing significantly more light than CMOS sensors from the same generation.  The author fails to mention that important fact, and instead misinterprets the data as a reason for the medium format backs and Leica to opt for CCD sensors.

I do not think the author even realized that a lot of the sensors he plotted on those charts are, in fact, CCD sensors.  He keeps emphasizing that his article refers to CMOS sensors, when the charts he is using is showing a good mix of CMOS and CCD sensors.  Is he not aware that the sensor for the A350, D200, D80, D70, D60, D50, and D40x are all CCD?  Is he not aware that the bottom feeders on his charts are populated by these very same CCD sensors?  And that the charts are clearly showing a greater efficiency among CMOS sensors?


« Last Edit: October 27, 2010, 12:23:23 PM by cosmo.6792 » Logged
Mark D Segal
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« Reply #5 on: October 27, 2010, 12:30:16 PM »
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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 Mark.

Andrew, the camera companies may not respond to this item - for some of them the culture is in the bones not to respond in public to very much at all. But behind the scenes they may well have a look at how they can address the basic issue with improved lens and sensor designs - as indeed some have been doing; then they will market these new designs with all the associated hype about the nature of the improvements, which is fine - gets us where we now know we want to go. I think the more important aspect of the contribution is that we - the customers and users - now know more about this than we did yesterday, and it helps us think about what to buy and how to set our cameras and lenses more intelligently than we knew yesterday.
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Mark D Segal (formerly MarkDS)
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AFairley
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« Reply #6 on: October 27, 2010, 12:34:34 PM »
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I wonder how much Olympus' telecentric design reduces (or eliminates) the "lost light" effect with its fast (f2) 4/3 format lenses?
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jenbenn
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« Reply #7 on: October 27, 2010, 12:47:47 PM »
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yup, to the point. Interesting as the article is,  it is impossible to understand what the problem/issue is. The author seems to have found a slight discrepancy between the "real" and the "apparent" ISo and completly blown the causes out of proportion. Why the hell is he asking the manufacturers to answer him? The whole article sounds like the camera makers are somehow cheating on us, by not providing us with the last fraction of a stop of "real" iso. Come on Mr Dubovoy, a good journalist is not the journalist who always finds something to critizise. An objektive  article simply describing the technical background would have been enough. No assumptions, no complaining about irrelvant performnce differences. Really, these days I dont want to be a camera manfacturer with all those experts on the web constantly complaining about issues which dont exist.
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Sekoya
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« Reply #8 on: October 27, 2010, 01:16:32 PM »
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I would like to know if this tactic is used on cpu lens only or on any optic. I use an Olympus E-p1, primarily with non-cpu Zukios. During live view the camera adjusts the aperture on the system lenses as the brightness in the scene changes but cannot when a non-cpu lens is attached Grin If one where to repeat these test with non-cpu lenses, would the camera still adjust iso? Does entering the lens data as on a Nikon change this?
I'd like to know this too and also whether the attenuation is measured in the center, averaged over the whole frame or via a center-weighted average? I would say it has to be in the center, otherwise the attenuation would also have to be lens/focal length dependent.
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EricV
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« Reply #9 on: October 27, 2010, 01:20:54 PM »
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The meaning of ISO or EV sensitivity of digital sensors is a bit murky.  Do you define proper exposure as filling the sensor charge capacity to a certain level?  Then a sensor with a lower full-well capacity would have a higher ISO rating, which seems wrong.  Do you base ISO rating on sensor sensitivity, ignoring full-well capacity?  Then you cannot properly equate exposures (shutter speed and f/stop and ISO) across different sensors.  Given this murkiness, comparing EV settings across cameras introduces a lot of questions, unrelated to lens light loss at large apertures. 

If the subject of interest is light loss on digital sensors with large aperture lenses, why not plot sensor response (image brightness corrected for any gain tricks) as a function of lens aperture?  Do this for a variety of lenses to see the effect of lens design, and do this for a variety of sensors to see the effect of sensor architecture.
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feppe
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« Reply #10 on: October 27, 2010, 01:23:27 PM »
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Both vertical and horizontal axis titles for the last graph were lost in transit. Anyone care to decipher those?

While it was an interesting article, it reminds me of the definition of kilo in computer terms: 1 kilo is defined as 1,024 by everyone in IT, except hard disk manufacturers who define it as 1,000. The marketing benefit of that is an exercise left to the reader.
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Sekoya
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« Reply #11 on: October 27, 2010, 01:27:58 PM »
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It doesn't look like the author knows how to interpret the very data he is quoting in his own article.
Hey, no reason to get worked up, it is well-known that we should not trust this site on technical details, all the points you point out are so obvious that it almost feels like Mark and Michael intentionally play this game, as if they wanted to say, we do not care about the details, that is below our attention level or as if they lived in alternative universe where intent and basic idea is everything and execution irrelevant.

What we have learned from this article is that on digital sensors the effects that cause them to vignette more than film (ie, acceptance angle) also cause some measurable 'vignetting' in the center of the lens (I am more and more convinced that these data have to come from the center or at the very least from a central area) and the that the camera manufacturers 'compensate' for this by increasing the ISO as a function of f-stop (and so few cameras are shown in this last graph that it has little practical value and merely generally proves the point).

This is good to know and we should simply ignore all the other obfuscation and declarifications in the article.
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Sekoya
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« Reply #12 on: October 27, 2010, 01:33:49 PM »
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The meaning of ISO or EV sensitivity of digital sensors is a bit murky.  Do you define proper exposure as filling the sensor charge capacity to a certain level?  Then a sensor with a lower full-well capacity would have a higher ISO rating, which seems wrong.  Do you base ISO rating on sensor sensitivity, ignoring full-well capacity? 
If you want to compare cameras (in a somewhat rigorous way as DxO does), you have to use some definition. Whether it is a certain percentage of the full well capacity or the median between a certain noise level and full well capacity does not matter, you just have to do it consistently.

And any ISO amplification factor defacto reduces the full well capacity, thus any ISO higher than base ISO is only higher because you have 'reduced' the full well capacity.
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Sekoya
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« Reply #13 on: October 27, 2010, 01:34:48 PM »
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Both vertical and horizontal axis titles for the last graph were lost in transit. Anyone care to decipher those?
Same as the other graphs, x-axis is f-stop, y-axis is delta EV stops.
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professorgb
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« Reply #14 on: October 27, 2010, 01:40:38 PM »
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If the subject of interest is light loss on digital sensors with large aperture lenses, why not plot sensor response (image brightness corrected for any gain tricks) as a function of lens aperture?  Do this for a variety of lenses to see the effect of lens design, and do this for a variety of sensors to see the effect of sensor architecture.

I also have this concern.  All of the graphs presented in the open letter are based on a single lens/aperture.  I would like to see comparisons among lenses with similar focal lengths/focal length ranges to determine if smaller-aperture lenses do, in fact, lose less light.  I'm not convinced that they do.  The only way in which a smaller-aperture lens, at the same focal length, could lose less light at the sensor is if the light is more collumnated in the smaller-aperture lens.  One could argue for this, by arguing that a smaller pupil is going to cut off light coming in from more extreme angles.  However, this loss of off-axis light is going to be balanced by light loss due to increased diffraction from a smaller aperture.  I don't know the balance between the two sources of light loss, but it's an easy empirical question to answer.
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feppe
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« Reply #15 on: October 27, 2010, 01:57:30 PM »
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Same as the other graphs, x-axis is f-stop, y-axis is delta EV stops.

Horizontal is pixel pitch on others, but thanks Smiley

Pretty modest increases, but ~0.4 EV increase in ISO from f/1.8 to f/1.2 might make a significant difference in IQ at very high ISOs. Probably a non-issue below 1600 ISO with most modern DSLRs.
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bobtowery
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« Reply #16 on: October 27, 2010, 02:11:31 PM »
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Maybe I'm missing the point of this article (it is a little thick to get through), but it seems to me he's missing the point entirely.

My reference points that seem in line with his theme are the 85mm f/1.2, and f/1.8, which I have had both. Saying that "the f/1.2 offers one more stop" is not doing it justice.

That is like saying "this Mercedes has only 40 more horsepower than this Ford, so it isn't worth it."

It's not just about f/stops. The 1.2 is a completely different lens aimed at a different market (i.e. one willing to spend $2k for a lens). It seems to me the 1.2 is the finest 85mm Canon knows how to make, regardless of price.  The 1.8 is a great lens at the price point.

I have never heard anyone say that Canon/Nikon/Leica rip off their customers.  Seems to me that this article is saying you'll achieve just as good results with the 1.8 as the 1.2, so why bother (i.e. you are being ripped off)?

Compare images with both lenses and you'll easily see why one would bother, assuming you have the $$ required.
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Alan Goldhammer
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« Reply #17 on: October 27, 2010, 03:09:00 PM »
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What was the old saying we had in the film days?  Something like f8 and be there.  While there may be something to all this, does it really have any practical implications over 99% of the images we capture?  As long as the histogram on the camera is a fair representation of what is going on and the lenses do not have any focus aberrations, I suspect all is well in the world.  We already know about the diffraction problems at small f stops (in contrast to what we used to do with film to get the great depth of field, f64 anyone?) and now there are some relatively minor issues at the other end.  I guess we should go back to the aphorism I already stated, set your camera to f8 and be happy (with some exceptions).

Alan
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sandymc
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« Reply #18 on: October 27, 2010, 03:17:43 PM »
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Well, "interesting" article, but very difficult (for me anyway) to evaluate as written. Is it possible to get more detail on how these measurements were performed? Notably:
  • Light loss at the sensor
  • The "sensor gain function"

A search on the DxO website gave me no hits, and 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). In fact, if I look at the Nikon data, I'd say the cameras with CMOS sensors show only a small difference (about .2 EV) versus pitch. Now maybe that discrepancy is due to other effects, but then whatever the other effects are would have to be taken into account in the Canon comparison.

Sandy
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pegelli
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« Reply #19 on: October 27, 2010, 03:19:06 PM »
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Quote from: Mark Dubovoy
In fact, is not even clear that large aperture lenses will deliver a shallower depth of field as intended.

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.
« Last Edit: October 27, 2010, 03:22:30 PM by pegelli » Logged

pieter, aka pegelli
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