CCD Advantage

[Bill Pierce]

Here is an interesting piece which, among other things, points out the advantage and necessity with wide angle lenses of the CCD sensor in digital M Leicas as compared to the CMOS sensor in many DSLR's.


http://www.luminous-landscape.com/essays/an_open_letter_to_the_major_camera_manufacturers.s html

[johnnygulliver]

a very interesting and thought provoking article Bill, many thanks for posting the link.

[antiquark]

http://www.luminous-landscape.com/essays/an_open_letter_to_the_major_camera_manufacturers.s html

I'm not quite sure what the writer is saying, but it sounds like he doesn't understand how ISO works. Is he saying that a D700 should produce brighter pictures than a D40 (at the same ISO) because the sensor is better?

[semilog]

That is a fascinating and thoughtful article.

Moreover, it is extremely funny if you think about all the baying for "fast, wide primes" for various digital systems (especially micro 4/3). Apparently, super-fast lenses not only aren't that much brighter, but in some cases they won't even buy you shallower DOF!

Now, that said, the "CCD advantage" in the M8/9 has almost nothing to do with CCD structure (as implied but NOT demonstrated with data in the linked article). Rather, the advantage of the M8/9 comes from the use of an eccentric microlens array. In fact, the requirement for that specialized array is specifically because the CCD sensor prefers to see light entering normal (perpendicular) to the sensor plane.

Note that an eccentric microlens array is also being used by Fuji on the APS-C, CMOS sensor for the X-100 — which is fitted with a fast (f/2) wide angle lens.

Fuji knows what they're doing.

For BOTH CCD and CMOS sensors, the best approach will eventually be to use backside-illuminated sensors (back-illuminated CCDs have been common in scientific imaging for well over a decade, and back-illuminated CMOS sensors are increasingly prevalent in consumer devices (http://www.tested.com/news/how-iphone-4s-new-camera-sensor-illuminates-low-light/396/) with small sensors). A good article that shows how beneficial backside illumination can be is here (http://www.microscopyu.com/articles/digitalimaging/digitalintro.html) (scroll down to Figure 3).

By putting the photosites closer to the sensor surface, the "tunnel effect" is minimised and quantum efficiency can be nearly perfect (approaching 100%).

Unfortunately, making large back-illuminated sensors is still very expensive. Too expensive, at present, for 4/3, APS-C, and larger sensors. And these sensors, because they are very very thin, are (mechanically) fragile — and the bigger they get, the more fragile they are. Not rugged enough to put in a Nikon D4, yet. But maybe by the time the D5 is out...

[semilog]

I'm not quite sure what the writer is saying, but it sounds like he doesn't understand how ISO works. Is he saying that a D700 should produce brighter pictures than a D40 (at the same ISO) because the sensor is better?

He clearly understands how ISO works, and that is not what he's saying.

[antiquark]

I re-read it... he's saying that the ISO is cranked up for wider apertures. That means that the camera would have to query the lens about the aperture, then adjust the amplifier gain accordingly.

A simple test of this theory would be to use a lens that doesn't communicate its aperture to the camera, and check if wide aperture images are darker than expected.

However, it would be a huge flaw if camera makers didn't normalize the ISO for wide apertures. Basically, every wide aperture photo would be underexposed. I can't see how people would perceive this as a "feature."

[semilog]

However, it would be a huge flaw if camera makers didn't normalize the ISO for wide apertures. Basically, every wide aperture photo would be underexposed. I can't see how people would perceive this as a "feature."

Of course that's right. People will complain, regardless. They want to have their f/1.2 cake and eat it, too. :rolleyes:

But this also provides a rather vivid demonstration of the fact that RAW files and EXIF data don't always tell the "raw truth."

[eddie1960]

I re-read it... he's saying that the ISO is cranked up for wider apertures. That means that the camera would have to query the lens about the aperture, then adjust the amplifier gain accordingly.

A simple test of this theory would be to use a lens that doesn't communicate its aperture to the camera, and check if wide aperture images are darker than expected.

However, it would be a huge flaw if camera makers didn't normalize the ISO for wide apertures. Basically, every wide aperture photo would be underexposed. I can't see how people would perceive this as a "feature."

On Pentax cameras there are some metering exposure issues with k and m series lenses ( i have 4 i frequently use) and typically they are out about a third to a half stop

[Soothsayerman]

Here's an interesting article on CCD tech vs CMOS tech...

http://www.dalsa.com/corp/markets/ccd_vs_cmos.aspx

[semilog]

Here's an interesting article on CCD tech vs CMOS tech...

http://www.dalsa.com/corp/markets/ccd_vs_cmos.aspx

^---- solid summary.

[antiquark]

In the article he says: "One might be better off purchasing smaller aperture lenses and increasing the ISO."

However, if my calculations are correct (big if), you're still benefiting from fast lenses. The worst EV adjustment for f1.4 is 0.5, which means that the ISO will have to go from, say, 100 to 140. If you choose an f2.0 lens, you'll have to increase the ISO to 200, which will be noisier than ISO 140. Thus, the 1.4 lens isn't exactly a waste of money.

[semilog]

In the article he says: "One might be better off purchasing smaller aperture lenses and increasing the ISO."

However, if my calculations are correct (big if), you're still benefiting from fast lenses. The worst EV adjustment for f1.4 is 0.5, which means that the ISO will have to go from, say, 100 to 140. If you choose an f2.0 lens, you'll have to increase the ISO to 200, which will be noisier than ISO 140. Thus, the 1.4 lens isn't exactly a waste of money.

Assuming, of course, that the T stops of both lenses (lens T-stops, not sensor T stops) are actually proportional to their marked f/ numbers. That's not always a valid assumption, especially when you're comparing a 5 or 6 element f/2 lens to a 10 or 12 element f/1.2 lens.

Note that when people working in filmmaking buy a lens for $10, 20, 30k they expect that the aperture ring will show T-stops!

[semilog]

A major issue with CCD sensors on a camera that isn't sealed, i.e. fixed lens, is dust.

Don't remind me. All our (scientific) cameras are Peltier-cooled, so to prevent condensation they are under vacuum. We just lost a vacuum seal on our best EMCCD camera... :bang:

[JPSuisse]

This is really a very technical issue here. Although I'm an engineer and can even follow part of these very interesting articles (luminous landscapes and dalsa.com), I'm disappointed that we are stooping so low here. Or, that we have to go to such lengths to understand what big corporations are doing to us....

"The morality of art consists of the perfect use of an imperfect medium." The problem with all these in camera modifications is that the character of your lenses is probably being hidden. In such a way, you'll never master your imperfect medium.

For me, it's another reason not to replace my failing D100 with a new DSLR. I hate deception.

Kind regards,

JP

[Brian Sweeney]

In the end, you pick a digital camera that gives you the image that you want. With a film camera, you can just load it with the film that gives the look you want. Digital camera- stuck with the sensor. Comparing images from my M8 with the EP2, shooting both at Low ISO, 160 and 200 respectively, the biggest difference that I see is more noise in the shadow areas on the EP2 images. The M8 images are smoother, less noisy. CCD vs CMOS? Olympus boosting gain because of sensor vignetting? More signal processing on the M8- I know that is not true! They are fairly minimalist on the in-camera processing. I don't care- the M8 images look better.

The EP2 is fun, makes a good image, uses lots of different lenses, and I can use it for making movies. But the M8 images are "cleaner".

[JPSuisse]

PKR,

don't call me a hypocrite now...

Good thing I don't have to pay my bills with the equipment I've got. I can view it as pure art. The bills get paid by making semiconductor components for you guys buying this stuff! :-)))

Now where's that NEOPAN 1600...

JP

[j j]

Can someone explain why any of this is a problem at all in any way? Even if true is it not just a camera correcting exposure?

[semilog]

Do you have some kind of vacuum pressure alarm.. ? What Torr is the sensor under?


No alarm -- it's a sealed assembly. We realised there was a problem when sensor performance plunged, it wouldn't hold temp at -80 (there's a thermocouple), and then we found condensation on the inside of the faceplate... :bang:

[Brian Sweeney]

No one knows who Jack Kilby or Fred Terman were. All they know is "Steve Jobs", a marketing guy..

I was using some 7400 series logic today.

[jrv]

If the author in the first article is upset to learn the camera applies a gain (ISO change) to normalize for different aperture settings, I wonder what they think of what the in-camera imagine processing is doing when a high-ISO setting is used? They might think "colorization" tame by comparison.

I would be very surprised if any camera CCD sensor has only a single readout channel. There are simply too many pixels, read too quickly, for ~ 14 bits of A/D. Uniformity is an issue for camera CCDs, but there are fewer places "drift" is introduced and calibration/correction should be easier than with CMOS.

[rxmd]

Here is an interesting piece which, among other things, points out the advantage and necessity with wide angle lenses of the CCD sensor in digital M Leicas as compared to the CMOS sensor in many DSLR's.

Firstly, I think a fair share of the light loss exhibited in the graphs can be attributed to manufacturers mislabeling their lenses - it's been known for ages that when you buy an f/1.2 lens, you may actually get a T/1.3 or even an f/1.3 lens. Lens tests have regularly shown this since the 1950s, and it's a bit laughable when people rediscover that a f/1.2 lens is a third of a stop slower, talk about "T-stop loss at the sensor" and never measure what T-stop the lens actually has. (DxO doesn't do that, all they do is look at RAW files.)

The article also fails to explain why T-stop losses at the sensor should be dependent on the aperture set in the lens at all. Why should there be more light loss at the sensor at f/1.4 than at f/5.6? And if light loss at the sensor is the same across apertures, his whole argument about fast wide lenses being unnecessary breaks down, because slow tele lenses have the same problem.

Also note that his argument about T-stops and depth of field is completely meaningless. Depth of field is determined by the ratio of focal length to the diameter of the projected aperture. You can have an f/1.2 lens with a T-stop of 8 and depth of field will still be that of a f/1.2 lens. The argument that depth of field is different because "marginal light rays don't hit the sensor" is comical at best, because that solely depends on sensor size, which he doesn't talk about at all.

Finally, the article says absolutely nothing about CCD vs. CMOS sensors. A fair share of the cameras in the comparison actually have CCD sensors in them. All Nikons before the D300 used CCD sensors, and the D300's CMOS sensor exhibits the same "light loss" as the D200's CCD sensor. In short, what the article describes is completely independent of sensor technology.

[semilog]

Firstly, I think a fair share of the light loss exhibited in the graphs can be attributed to manufacturers mislabeling their lenses - it's been known for ages that when you buy an f/1.2 lens, you may actually get a T/1.3 or even an f/1.3 lens. Lens tests have regularly shown this since the 1950s, and it's a bit laughable when people rediscover that a f/1.2 lens is a third of a stop slower, talk about "T-stop loss at the sensor" and never measure what T-stop the lens actually has. (DxO doesn't do that, all they do is look at RAW files.)

The article also fails to explain why T-stop losses at the sensor should be dependent on the aperture set in the lens at all. Why should there be more light loss at the sensor at f/1.4 than at f/5.6? And if light loss at the sensor is the same across apertures, his whole argument about fast wide lenses being unnecessary breaks down, because slow tele lenses have the same problem.

Finally, the article says absolutely nothing about CCD vs. CMOS sensors. A fair share of the cameras in the comparison actually have CCD sensors in them. All Nikons before the D300 used CCD sensors, and the D300's CMOS sensor exhibits the same "light loss" as the D200's CCD sensor. In short, what the article describes is completely independent of sensor technology.

It's pretty obvious that at least a major fraction of the T stop losses that DxO is seeing are indeed sensor-dependent and NOT (as you suppose) due to lens T stop values, because (1) presumably the same Canon f/1.2 lens is used across all the bodies tested, and they vary considerably in T stop at the sensor, and (2) there is a strong correlation with pixel size, with smaller pixels showing greater losses at wider apertures – precisely as we'd expect if angle-of-incidence-dependent shading is an issue.

Your point about CMOS vs. CCD is one I that made above, and is correct.

[antiquark]

The article also fails to explain why T-stop losses at the sensor should be dependent on the aperture set in the lens at all. Why should there be more light loss at the sensor at f/1.4 than at f/5.6?

The idea is, with a larger aperture, more light is arriving at the sensor from the edges of the aperture at a glancing angle. This would cause a reduction in sensitivity because sensors need the light to be coming in perpendicular to the sensor.

Other than that, I think the article is an attempt to make a mountain out of a molehill!

[rxmd]

The idea is, with a larger aperture, more light is arriving at the sensor from the edges of the aperture at a glancing angle. This would cause a reduction in sensitivity because sensors need the light to be coming in perpendicular to the sensor.

That phenomenon is well-known as well. It's called "vignetting". (If anything, it should lead to better results from smaller sensors.)

[semilog]

Other than that, I think the article is an attempt to make a mountain out of a molehill!

The difference in price (and size) between an f/1.4 lens and an f/1.2 lens can be large indeed. If, as seems possible, many of these sensors deliver (1) identical sensitivity at f/1.4 and f/1.2 [or worse] and the same DOF at f/1.4 and f/1.2, but the cameras disguise this fact by upping the ISO and then modifying the RAW data so that it looks as though you're really getting an extra half stop when you're not... I'd say that's a significant issue.

[semilog]

That phenomenon is well-known as well. It's called "vignetting". (If anything, it should lead to better results from smaller sensors.)

Wrong. What they are talking about here will be true at the center, as well as at the corners, though perhaps worse at the corners.

[antiquark]

That phenomenon is well-known as well. It's called "vignetting". (If anything, it should lead to better results from smaller sensors.)

Isn't vignetting a lens effect? I.e., even if you're using film, some lenses will still produce vignetting? I think the article was talking about an effect that would be seen even if the lens was free of vignetting.

[antiquark]

The difference in price (and size) between an f/1.4 lens and an f/1.2 lens can be large indeed. If, as seems possible, many of these sensors deliver (1) identical sensitivity at f/1.4 and f/1.2 [or worse] and the same DOF at f/1.4 and f/1.2, but the cameras disguise this fact by upping the ISO and then modifying the RAW data so that it looks as though you're really getting an extra half stop when you're not... I'd say that's a significant issue.

Based on some calculations, I think you're still ahead if you choose a faster lens. That is, the "secret" ISO increase is still less than if you chose a slower lens.

Also, the DOF argument seems bogus to me. If a f1.2 lens had the same DOF as an f1.4 lens, then the bokeh blobs would be the same size in both lenses. That would be a really simplistic test to run. (I can't because I don't have a 1.2 lens).

[semilog]

You have it.. big problem. Some designers tried pointing the pixel sites off axis, at the cost of resolution. This critical angle thing is the cause for the difference in DOF in FX sensors vs film. Those little silver rocks accept photons at a greater range , producing a greater illusion of depth..

I don't think that's right, PKR. Accepting photons from shallower angles of incidence should give you larger circles of confusion and less DOF. Excluding those photons should mimic a lens with smaller exit pupil.

The article says that DxO is now doing critical focus measurements to test this hypothesis.

[semilog]

Based on some calculations, I think you're still ahead if you choose a faster lens. That is, the "secret" ISO increase is still less than if you chose a slower lens.

How much would you pay for a quarter of a stop?

Also, the DOF argument seems bogus to me. If a f1.2 lens had the same DOF as an f1.4 lens, then the bokeh blobs would be the same size in both lenses. That would be a really simplistic test to run. (I can't because I don't have a 1.2 lens).

As mentioned above, the article says that DxO are doing these tests in a serious way.

[rxmd]

Your point about CMOS vs. CCD is one I that made above, and is correct.

Of course, you're right on that they should be affected all the same. I just thought it was worth pointing out that there is a fair number of CCD sensors in the test (I didn't see you mentioning that, maybe I overlooked it). The test actually shows that there is no correlation between CMOS vs CCD on the one hand and "T-Stop" (rather: readout) losses on the other hand at all.

It's pretty obvious that at least a major fraction of the T stop losses that DxO is seeing are indeed sensor-dependent and NOT (as you suppose) due to lens T stop values, because (1) presumably the same Canon f/1.2 lens is used across all the bodies tested, and they vary considerably in T stop at the sensor, and (2) there is a strong correlation with pixel size, with smaller pixels showing greater losses at wider apertures – precisely as we'd expect if angle-of-incidence-dependent shading is an issue.

Firstly, I don't think we're contradicting each other that much. I said "a fair share" can be attributed to the lens; in his f/1.2 test, there is a solid baseline of -0.4 EV, and that's what I'm talking about. Of course I don't deny the variation in the rest.

Regarding your points:

(1) you're right. However, the article forgets to tell us which lenses they are looking at. Canon sells f/1.2 lenses in 35, 50 and 85mm and we can only speculate. Basically, the f/1.2 test doesn't provide us with a strong argument that what we see depends on the angle of incidence or happens only in wideangles.

(2) I don't see angle-of-incidence-dependent shading should depend on pixel size. As far as I can see, it depends purely on the cosine of the angle of incidence. The relative amount of shading of a 1 cm² area is the same as a 1 µm² area. (EDIT: If you take the equation, you see that the area cancels itself out in the numerator and denominator.)

Again, if angle of incidence was an issue, I'd expect better results from cameras with smaller sensors. The oblique angles are (EDIT: predominantly) met with at the edge of the frame, so that smallers sensors are less affected, the results in the DxO tests being averaged across the frame. However, what we see is the opposite: smaller sensors are more affected, so angle of incidence can't be the main issue.

I presume what plays a large role is the way sensor readout is optimized. In cheap consumer cameras, the sensor readout electronics are less sophisticated and less effort is invested into postprocessing. That's about it.

[semilog]

In practice, a FX (not a smaller APS-C) sensor, exhibits about 1/3 less DOF than a piece of 35mm film.

Interesting. The only obvious (to me) explanation is that the light sensitive surface on a digital sensor is thinner than in a photosensitive silver emulsion.

But perhaps there's another explanation? I'm certainly open to ideas, here.

[jrv]

As mentioned above, the article says that DxO are doing these tests in a serious way.
DXO is using RAW files, right? How much do RAW files really say about the sensor?

RAW files are heavily processed by the camera at high ISO settings: is it really safe to draw conclusions about sensor performance from a RAW file even at low ISO settings?

I always think of a RAW file as being "normalized" to whatever the camera maker thinks a RAW should be like. A camera RAW isn't the unprocessed output of the sensor A/D.

[j j]

Did you read the citation in the first post?

"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."

From:

http://www.luminous-landscape.com/essays/an_open_letter_to_the_major_camera_manufacturers.s html

That's fairly simplified, but there are many other issues.

If you're truly interested, I suggest you do some reading.

p.

Oh dear. Yes, I read the article. It read like a load of hot air about nothing, and the quote you kindly included in place of an example of how this so-called issue is detrimental to actual photos shows that to be the case: the author does not say this is what happens, but what MAY happen.

[antiquark]

Here's a simple test comparing the bokeh of a 50/1.4 lens to a 50/1.8, on a Nikon D40x.

This shows that the depth of field is as it should be for the 1.4 lens, and isn't reduced due to the ISO increase at wide apertures.

http://www.rangefinderforum.com/forums/attachment.php?attachmentid=82260&d=1288306150

[Brian Sweeney]

It would be interesting to see the ray-tracing that goes on from entering the lens until being converted to charge by the detector. Somehow, I do not think they are going to provide that information.


Guess I could throw the 50/1.2 Canon onto the EP2 and then onto the M8, crop the M8 photo to overlay with the EP2 image. That could possibly be more boring than using Fourteen 50mm f2 lenses on my M3 setup on a tripod looking at a tree branch and testing at F2 and F4. The 1955 J-8 was really good, compared well with the Summicrons, Sonnars, and Nikkors. Come to think of it, I've added two Summars since that test and a Coated wartime 5cm f2 Sonnar T...

[Brian Sweeney]

TTL?? ground the unused input pins.. TTL is very robust.. CMOS is less likely to be damaged by RF.

I never keep my cell phone (world phone 4 bands) near my digital camera gear!!!!!

I had problems with damaged memory, traced to my assistant keeping my phone in the same bag with DSLRs..

p.

TTL yesterday, 4000 series CMOS today. I don't worry about my cell phone damaging anything. It's in the car and turned off.

[antiquark]

I don't know what a D40x is.. is it an APS-C sensor? The FX sensor is a completely different issue. .

Yes, it's APS-C.

[Brian Sweeney]

It would be interesting to see what goes on with fast lenses on the Forscher Polaroid backs that uses fiber optic bundles as optical relays. The acceptance angle of the fiber is small compared with film. Use the same camera, same lens, shoot negative film at the backplane and then compare with the Polaroid image using the Fiber bundle. I do not have one.

[semilog]

Thanks, Antiquark.

Here are Antiquark's COF images, as surface plots with intensity coded as height. 1.4 on the left, 1.8 on the right. What an interesting result: the COF's are asymmetric, and (apparently) more so at wider aperture!

http://semilog.smugmug.com/Other/35-Biogon-C-test-photos/falloff/1067955801_UrTfd-L.jpg

At f/1.4 we see very obvious horizontal bands of light falloff at the top and bottom, and a trace of this behavior is still visible at f/1.8.

This result is consistent with a photosite geometry in which light rays can be detected at shallower angles when coming from the left or right sides of the lens's exit pupil than when they are coming from the top or bottom of the exit pupil.

This behavior could be due to the design of the photosites, the microlenses, or both.

One question: how far from the center of the APS-C frame are we with these crops?

[Brian Sweeney]

Why would anyone design a detector array without two-axis symmetry? Aside from the D1x, which used odd binning.

Time to drag out the DCS200 which does not have a Mosaic filter.

[antiquark]

Thanks, Antiquark.

Here are Antiquark's COF images, as surface plots with intensity coded as height. 1.4 on the left, 1.8 on the right. What an interesting result: the COF's are asymmetric, and (apparently) more so at wider aperture!

http://semilog.smugmug.com/Other/35-Biogon-C-test-photos/falloff/1067955801_UrTfd-L.jpg

At f/1.4 we see very obvious horizontal bands of light falloff at the top and bottom, and a trace of this behavior is still visible at f/1.8. This result is consistent with a photosite geometry in which light rays can be detected at shallower angles when coming from the left or right sides of the lens's exit pupil than when they are coming from the top or bottom of the exit pupil.

This behavior could be due to the design of the photosites, the microlenses, or both.

I noticed that too, but wasn't sure if it was due to my setup. The light source was a flashlight with aluminum foil on the front punctured with a 2mm hole. I thought maybe the beam was directional, so I re-tested and added two layers of wax paper behind that to diffuse things.

Still the bokeh showed the darkening at the top and bottom. However, when I tried my D90 (much better sensor) the effect seems to have vanished. So yes, something's not optimal with the D40 sensor design.

An odd effect of that "anisotropic" bokeh is that the DOF would (theoretically) be different along the X vs the Y axis. However, it might not be measurable in a practical sense.

[semilog]

@ Brian,

It's probably not the array that lacks symmetry (although many arrays with rectangular pixel pitch have been made, predominantly for video applications) — it's the photosites themselves.

Even if the pixel *array* is a square grid, the photosites may have anisotropic light sensitivity due to the geometry of the wiring at each pixel. Asymmetric layouts may make possible lower shading factors (and thus higher overall quantum efficiency) at the cost of some angle-of-incidence anisotropy.

[semilog]

I noticed that too, but wasn't sure if it was due to my setup.

Easy to check: rotate the camera 90 degrees to the right or left.

[antiquark]

Easy to check: rotate the camera 90 degrees to the right or left.

Good idea. I tried it, and the dark edges stay aligned with the top and bottom of the CCD, so it's definitely the sensor.

[semilog]

especially in APS-C sensors. I think the pixel alignment within the sites is different in an FX sensor.. just guessing though!
p.

I suspect it's not so much APS-C vs. FF, but (1) sensor generation, and even more so (2) pixel size, with smaller pixels having more shading issues. This stands to reason: think of the acceptance angle of a cardboard tube like the one at the center of a toilet paper roll, versus the acceptance angle of a coffee cup. Same well height, different cross-sectional area.

Look at the charts in the Luminous Landscape article (http://www.luminous-landscape.com/essays/an_open_letter_to_the_major_camera_manufacturers.s html). The Canon 7d, with its teeny tiny pixels, is recording enormous losses (a full stop at f/1.2) for such a recent camera.

[antiquark]

Here's a better pic of the bokeh blob. I decreased the focus distance to the minimum, 0.45m, used a tripod, lower ISO to decrease noise.

You know, if you look closely, it looks like the left and right edges ALSO have some darkening. So it's like there's a rectangular mask that's slightly smaller than the circle of confusion.

Now I'm REALLY interested to see what an f1.2 lens would look like on an older sensor like a D40. If you expanded the disc below, the top and bottom edges might get quite dark with at f/1.2... in effect, confirming the article at luminous landscape!

http://www.rangefinderforum.com/forums/attachment.php?attachmentid=82262&d=1288312738