masjah Posted September 29, 2006 Share #1 Posted September 29, 2006 Advertisement (gone after registration) Apologies to all if this is by now common knowledge. The latest LFI has an extended piece on the M8. They explain what the blue dot is for. It's all about correction of vignetting. As we all know, vignetting has a component related to the focal length of the lens, and a component related to the selected aperture. The "bar code" will tell the camera the focal length of the mounted lens. But how does the camera know what aperture has been selected, when M lenses have no aperture coupling, and TTL M metering is therefore "stopped down"? Well, the blue dot is a separate metering cell independent of lens aperture. The camera compares the reading from the blue dot meter with the stopped down TTL reading, and estimates the set aperture from the difference. LFI report Leica as saying it gives the camera the set aperture to within +/- one stop, which they say is sufficiently accurate for the correction of the aperture dependent vignetting. Quote Link to post Share on other sites More sharing options...
Advertisement Posted September 29, 2006 Posted September 29, 2006 Hi masjah, Take a look here What the M8 blue dot is for. I'm sure you'll find what you were looking for!
adan Posted October 1, 2006 Share #2 Posted October 1, 2006 Here's an interesting tidbit supporting this. My Leica rep let me look at a few M8 shots today on his laptop computer (ones I'd just shot with his sample M8 and his card). The EXIF data for JPEGS (but not RAW shots) listed both the shutter speed AND shooting aperture used. EVEN WITH UNCODED LENSES! E.G. - a shot with my 15mm C/V had: Exposure: 1/60 @ f/3.4 (which is a bit off, since the CV is f/4.5, but what the hey!). Same for a 40-year-old 135 - Exposure, 1/1500 @ f/6.5. As I said, no aperture recorded in the EXIF for .DNG files - but this was with beta firmware v. 0.xx, so meaningless in terms of final product. Quote Link to post Share on other sites More sharing options...
rvaubel Posted October 1, 2006 Share #3 Posted October 1, 2006 If the blue dot, in fact, interpolates the taking aperture of the lens, that would explain a lot. Somehow it never made much sense that the function of the blue dot was simply to change the illumination of the finder LEDs. Does anyone else have any confirmation? If true, one could build custum vignetting actions for non coded lenses. Maybe,,,I have to think about that one. Rex Quote Link to post Share on other sites More sharing options...
marknorton Posted October 1, 2006 Share #4 Posted October 1, 2006 I must admit I was wrong about this. I didn't think it would be possible to obtain an accurate estimate of the shooting aperture by comparing the two light levels. Even so, I still wonder about using a long lens to focus on a bright object in an otherwise dark scene, for example a spot-lit singer on stage. It will also be interesting to see how well it works at small apertures. At f16, the level of the light reflected off the shutter into the sensor could be as low as 1/1000 of the incident light. Presumably the knowledge of the actual lens mounted could allow them to make the estimate more accruately, not least to avoid coming up with apertures that don't exist, like f0.8 for a Nocti or f45 for the Macro-Elmar. I'm also wondering whether the blue dot plays a role in flash metering... Quote Link to post Share on other sites More sharing options...
rvaubel Posted October 1, 2006 Share #5 Posted October 1, 2006 I must admit I was wrong about this. I didn't think it would be possible to obtain an accurate estimate of the shooting aperture by comparing the two light levels. Even so, I still wonder about using a long lens to focus on a bright object in an otherwise dark scene, for example a spot-lit singer on stage. It will also be interesting to see how well it works at small apertures. At f16, the level of the light reflected off the shutter into the sensor could be as low as 1/1000 of the incident light. Presumably the knowledge of the actual lens mounted could allow them to make the estimate more accruately, not least to avoid coming up with apertures that don't exist, like f0.8 for a Nocti or f45 for the Macro-Elmar. I'm also wondering whether the blue dot plays a role in flash metering... Mark The blue dot could not function like an incident meter to work. Think of the blue dot as a little 90mm telephoto lens that is spot reading the center of the frame and comparing it's light value to the prime len's. This comparision has to be a little rough but thats OK for the purpose it serves. Vignetting correction is a matter of judgement anyway. Rex Quote Link to post Share on other sites More sharing options...
marknorton Posted October 1, 2006 Share #6 Posted October 1, 2006 True, we don't know what the viewing angle of the blue dot sensor is but a spot meter would make sense. Quote Link to post Share on other sites More sharing options...
rvaubel Posted October 1, 2006 Share #7 Posted October 1, 2006 Advertisement (gone after registration) True, we don't know what the viewing angle of the blue dot sensor is but a spot meter would make sense. Mark The quality of the blue dot "lens" could be of very minimal quality as it wouldn't have to form a very good image, in fact a blurry image would probably work better. Rex P.S. I feel like I'm playing tag with you on RFF Quote Link to post Share on other sites More sharing options...
rvaubel Posted October 1, 2006 Share #8 Posted October 1, 2006 Mark The static on the other forums is reporting the same thing. I hope that this isn't a mass hallucanation. Rex Quote Link to post Share on other sites More sharing options...
chrism Posted October 1, 2006 Share #9 Posted October 1, 2006 About 4'30" into Michael Reichmann's videoblog about the M8, the Leica rep says the blue dot is to 'assist the camera by metering for the right flash mode, it's for the M-TTL metering.' Chris Quote Link to post Share on other sites More sharing options...
cbretteville Posted October 1, 2006 Share #10 Posted October 1, 2006 If it is a secondary meter that reads the ambient light levels , there is no reason why the data couldn't be used for all three explanations we've heard: - Dim the lens in the view finder - Assist in the semi-ttl flash mode - Measure it agains the light level hitting the meter through the lens. - Carl Quote Link to post Share on other sites More sharing options...
rvaubel Posted October 1, 2006 Share #11 Posted October 1, 2006 If it is a secondary meter that reads the ambient light levels , there is no reason why the data couldn't be used for all three explanations we've heard: - Dim the lens in the view finder - Assist in the semi-ttl flash mode - Measure it agains the light level hitting the meter through the lens. - Carl Your exactly right, an external secondary meter could perform all three functions. That would be totally cool, an elegant solution to three apparently unrelated problems with one little "blue dot"! Kudos, Leica Rex Quote Link to post Share on other sites More sharing options...
ho_co Posted October 1, 2006 Share #12 Posted October 1, 2006 Your exactly right, an external secondary meter could perform all three functions. That would be totally cool, an elegant solution to three apparently unrelated problems with one little "blue dot"!--Kudos, Leica! Rex--Kudos is right! I say kudos to Leica if even one of the three is realized via the blue dot. And didn't we hear somewhere that it also serves as indicator for self-timer? Remember the US Sylvania flashbulb advertising slogan "Blue Dot for Sure Shot"? Leica has done it in spades! -HC Quote Link to post Share on other sites More sharing options...
marknorton Posted October 2, 2006 Share #13 Posted October 2, 2006 The self-timer LED is in the main viewfinder window. Quote Link to post Share on other sites More sharing options...
ho_co Posted October 2, 2006 Share #14 Posted October 2, 2006 The self-timer LED is in the main viewfinder window. Mark--Thanks for the correction! I guess if I'm using the self-timer, I don't need to worry about a light source within the finder! I would never have come up with the idea that one could put any kind of light in the finder, so again I say: KUDOS to those guys! --HC Quote Link to post Share on other sites More sharing options...
adan Posted October 2, 2006 Share #15 Posted October 2, 2006 Yep - the self-timer indicator is just visible as a little dark speck centered behind the upper mirror-coated band on the front finder window. The aperture recording is definitely "approximate", as the LFI magazine quote says. Anywhere within a 2-stop window. Quote Link to post Share on other sites More sharing options...
rvaubel Posted October 2, 2006 Share #16 Posted October 2, 2006 Yep - the self-timer indicator is just visible as a little dark speck centered behind the upper mirror-coated band on the front finder window. The aperture recording is definitely "approximate", as the LFI magazine quote says. Anywhere within a 2-stop window. Andy Does that mean +/- 1 stop? That's not so bad for it's intended purpose. It would be pretty rough for exposure control but for indicating the amount of vignetting or gemeral EXIF information, its OK. I am always annoyed that my RD1 doesn't give me a clue as to the aperture used. I can generally figure it out by the shutterspeed, ambient light and DOF but the M8 system will definetly be a step up. The possibility of creating my own custom vignetting filters in Photoshop also exists. However the aperture data would need tobe read to the RAW file for this to work. Or you could take a simultaneous RAW+JPEG and write some sort of complex action that would bring up the appropriate vignetting filter. Fun and games for a rainy Sunday afternoon:) Rex Quote Link to post Share on other sites More sharing options...
masjah Posted October 2, 2006 Author Share #17 Posted October 2, 2006 Rex Yes, according to the LFI piece, a tolerance of +/- 1 stop is quoted, which is not good enough for serious exposure assessment, but which they say is OK for estimation of vignetting. Quote Link to post Share on other sites More sharing options...
wlaidlaw Posted February 17, 2007 Share #18 Posted February 17, 2007 The Contax G2 had exactly the same system for reporting the interpolated lens aperture for the purposes of the data-back. I believe that originally, it had been intended that the lens would report aperture, which is borne out by the fact that there are spare unused contacts on the lens to body connection but they decided that an interpolated figure was good enough. However, maybe only in the later M8 like mine, when fitted with a Zeiss Biogon 35, with no hand-done coding, aperture is not being reported in the EXIF. Wilson Quote Link to post Share on other sites More sharing options...
carstenw Posted February 17, 2007 Share #19 Posted February 17, 2007 Heh, I rest my case. I have reported that this was the case from an earlier LFI article, but was pooh-poohed at the time. Trivia of the day: bluedot in German is Blaupunkt. Quote Link to post Share on other sites More sharing options...
stumac Posted February 17, 2007 Share #20 Posted February 17, 2007 Would be an interesting experiment to put a black tape over the blue dot and see the effect. Has anyone done this? Regards, Stuart Quote Link to post Share on other sites More sharing options...
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