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01-02-2020, 07:45 AM   #1
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Off-the-film metering after the Pentax LX

One of the vaunted features of the Pentax LX is "off-the-film" metering, giving amazingly accurate automatic exposures in exceedingly low light. As it has been described, once the shutter opens, metering continues off the surface of the film, allowing an accurate reading of the accumulated exposure, even in changing lighting conditions. The shutter will eventually close once sufficient light has been captured by the film. I can attest to it working very well -- I have taken some nice nightime landscapes and cityscapes that would have been very tricky to accurately meter -- and the LX gave perfect exposures automatically.

"Off-the-film" metering was clearly a new approach to metering when the LX was introduced. It was promoted and discussed widely at the time.

But what about Pentax (or other) cameras since the LX? Did newer cameras, after the LX, incorporate this technique? None of the descriptions of metering on newer cameras (A-series, Z-series, or MZ-series) mention "off-the-film" metering. Did the approach die with the LX? If so, why?

01-02-2020, 08:29 AM   #2
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All the cameras with true TTL flash (not the P-TTL of today's digital cameras) have "off-the-film" metering to monitor the flash exposure and cut off the flash at the instant the scene reached the desired light level. For example, the MZ series has a silicon photodiode in the floor of the mirror box that watches the film plane.

However, whether these models had the sensitive exposure integration circuitry of the LX that's needed to monitor the accumulated light of a very dim scene is another matter entirely!
01-02-2020, 08:57 AM   #3
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I think the Olympus OM-2 series was the first camera to offer off the film metering. The Minolta CLE also did.

One issue that off-the-film, or TTL flash fans fail to mention is that exposure varied, depending on the reflectivity of the film you were using - at least in the early days.

For instance, my old Minolta X-700 made nice TTL flash exposures when using colour film. But black and white film was frequently a stop underexposed - only with flash, not regular ambient metering.

The difference was that at that time (mid-1980s) colour film emulsion was a darker brown colour. Ilford HP5 was a light, pale green. Consequently, the TTL flash system saw more light reflected off the film and reduced the flash output. I had to remember to set my compensation dial to +1 when shooting flash on black and white film. I expect off-the-film systems like the LX and OM-2 would have had similar issues.

I believe that later on, an industry standard was set for film emulsion reflectivity, as more and more cameras were using TTL flash (very few used off-the-film ambient measuring).
01-02-2020, 09:27 AM - 1 Like   #4
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I got some wonderful long exposure shots with my LX using this feature. You could set the camera on a tripod, stop down to f/16 or so, select auto and fire away with the self timer or cable release. The camera would sit there for a few (or many) seconds and then you would hear the mirror drop. Magic!


I've been told that TTL auto flash couldn't quite work with digital cameras. The reflections off the sensor was not quite the same as with film. The *istD generation had support for TTL but after that it went away. I was not impressed about that when I got my K10D.

01-02-2020, 10:12 AM   #5
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I have searched far and wide regarding this and AFAIK the LX is the only camera ever - by any brand - film or digital, that can aperture priority for as long as it takes.

Regarding metering accuracy of OTF, here is a snip from 1976 Modern Photography about this. Canon and Minolta also conducted testing. MP's conclusion is 1/6 of an f stop variation max.


The OM2 did introduce OTF but incorporated two sets of metering cels in the VF and in the body. It can only aperture priority up to 19 minutes and with a lot of caveats -> Tech Pages - OM-2/2N.

I've only used modern Canon and Nikon film cams and none of them can aperture priority anywhere near the lenght of time the LX can. All Canons - even digitals, can only aperture priority for 30 seconds max.

Of course all DSLRs use OTF except that sensor replaces film. AFAIK there are no digicams that can meter in the minutes let alone hours that the LX can.
01-02-2020, 11:54 AM   #6
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QuoteOriginally posted by filmamigo Quote
One of the vaunted features of the Pentax LX is "off-the-film" metering, giving amazingly accurate automatic exposures in exceedingly low light. As it has been described, once the shutter opens, metering continues off the surface of the film, allowing an accurate reading of the accumulated exposure, even in changing lighting conditions. The shutter will eventually close once sufficient light has been captured by the film. I can attest to it working very well -- I have taken some nice nightime landscapes and cityscapes that would have been very tricky to accurately meter -- and the LX gave perfect exposures automatically.



"Off-the-film" metering was clearly a new approach to metering when the LX was introduced. It was promoted and discussed widely at the time.



But what about Pentax (or other) cameras since the LX? Did newer cameras, after the LX, incorporate this technique? None of the descriptions of metering on newer cameras (A-series, Z-series, or MZ-series) mention "off-the-film" metering. Did the approach die with the LX? If so, why?
The SuperProgram (SuperA) had TTL flash off the film surface...not sure about non-flash but I always assumed it did. I'll have to test one of mine.

01-02-2020, 01:23 PM   #7
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The LX was the only Pentax to have that feature. It is tough to know why. Was Pentax paying Olympus to use the patent? Every feature has a cost. Did eliminating that feature make Super Program to P series more affordable even if they weren't paying to use a patent? I like the feature but rarely use it and do use shutter priority and sometimes even program mode.
01-02-2020, 03:29 PM   #8
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I don't know if Pentax had to license the technology. Keep in mind that the LX was available at the same time as the Super Program (or Super A). If you wanted off the film metering the LX was it. Interestingly enough the LX outlived the Super Program and remained available well into the AF era. It only disappeared in 2001 when the MZ-S was introduced.

01-02-2020, 03:35 PM   #9
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The most likely reason the LX was the only one is that it was the only body with a removable viewfinder. Irrespective of the finder, the meter still works.
01-02-2020, 03:57 PM - 2 Likes   #10
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QuoteOriginally posted by pentaxus Quote
The LX was the only Pentax to have that feature. It is tough to know why.
There is a very good discussion of Pentax IDM from the LX that includes comparison with other similar technologies at the page linked below:

Pentax LX - How does its metering works

As can be seen from the diagrams, the metering system used a pellicle (85% reflectance) main mirror to allow for preview exposure information in the viewfinder display. According to the Pentax specifications, actual metering for automated exposure was done either off a pattern on the front curtain or a combination front curtain and the film. An interesting sidelight is that the LX used stop-down metering for the actual exposure. Manual metering was done through the pellicle mirror. This is all very cool, IMHO.

As for why only the LX, the complexity speaks for itself as might the price points of competing products.


Steve
01-02-2020, 04:26 PM   #11
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QuoteOriginally posted by stevebrot Quote
There is a very good discussion of Pentax IDM from the LX that includes comparison with other similar technologies at the page linked below:

Pentax LX - How does its metering works

As can be seen from the diagrams, the metering system used a pellicle (85% reflectance) main mirror to allow for preview exposure information in the viewfinder display. According to the Pentax specifications, actual metering for automated exposure was done either off a pattern on the front curtain or a combination front curtain and the film. An interesting sidelight is that the LX used stop-down metering for the actual exposure. Manual metering was done through the pellicle mirror. This is all very cool, IMHO.

As for why only the LX, the complexity speaks for itself as might the price points of competing products.


Steve
Excellent link!


Thanks.
01-03-2020, 10:19 AM   #12
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QuoteOriginally posted by stevebrot Quote
Manual metering was done through the pellicle mirror.
Yep and that's why you need a CPL for manual metering, but can use a LPL for auto metering off the film plain. (That little mirror flipped up when auto metering, so did not interfere with the polarizer.)

Phil.
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