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12-31-2014, 11:07 AM   #16
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QuoteOriginally posted by jatrax Quote
Yep, looks quite like the spreadsheet chart I came up with. I suppose there might be some mathematical formula to describe the curves but that is beyond me. Consistent and repeatable but no linearity.
If I were to guess (and mind, I have no idea of the physics behind this) it would be like when you were a kid playing with a magnifying glass. Dead center, you get a tight, focused (and hot, but thats not the issue here) point of light. Cocked even slightly to the left or the right, or moved closer or further, and the spot of light will fade and be darker.

A lens is essentially the same thing, so if the lens isn't right where it needs to be, it will fail to transmit as much light as it would at its sweet spot (where ever that happens to be on that particular lens).

The result representing the refracted light would be a graph much like that posted above.


Last edited by Sagitta; 12-31-2014 at 11:12 AM.
12-31-2014, 11:16 AM   #17
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QuoteOriginally posted by jatrax Quote
I wonder if DA lenses have info in either the lens chip or the camera that allows more precise metering by describing the characteristics of the lens?

That agrees with what I found in my tests. I expected a linear error based on static parameters but all I could determine with accuracy was that there were errors in metering that varied with the aperture. It might be possible to chart those errors and determine something but I never carried it that far, it certainly was not consistent. My tests were all on Takumars so the aperture linkage was not a factor.
I believe you are right, modern lenses contain info re the kens characteristics, which is why MTF mode works in P mode. Presumably the curves shown by other posters re part of the info encoded to ensure that exposure is consistent. My Sigma 50-150 oth reads as "either Sigma 50-150xxxxx or Sigma 70-200xxx" or simply "Unknown lens" in exif data, implying that the info in its chip is not being read properly
12-31-2014, 12:35 PM   #18
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QuoteOriginally posted by jatrax Quote
Yep, looks quite like the spreadsheet chart I came up with. I suppose there might be some mathematical formula to describe the curves but that is beyond me. Consistent and repeatable but no linearity.
The correction is in body, adjusting to be neutral at F5.6
12-31-2014, 01:39 PM   #19
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This brings up a lot of memories. I always found it challenging to use the M lenses I had with my K10d. I remember all the related discussions including the chart Lowell posted. I created my own as well and found the same results. It was a headache with the K10d because I remember the error being as much as 1 stop depending on the aperture (under on one extreme and over exposured on the other extreme). Given the poor noise and lower dynamic range, PP couldn't help much if you didn't compensate correctly.

I think things got a lot better with the K5 and now the K3 seems to expose almost perfectly for me with those old lenses (or the curve has just flattened and I have more control in PP). The discussions have all but gone away, too.

07-30-2015, 09:35 PM   #20
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A couple of weeks ago I attached the Pentax-A 50mm f/1.7 lens (that was kitted with my Super Program in 1984) to my K-30 and took some pictures. I set the lens aperture to "A", the camera to Av or P mode, and took pictures as normal. The pictures were sharper than I remember that lens being (of course, with crop-sensor I'm not using all of the lens) and I was satisfied with the exposure. This is a very practical test, I wasn't comparing this lens to any other lens, but it left me comfortable that I can use the lens as normal and get results as normal. I'm not sure what else I could ask of this elderly lens.
07-31-2015, 04:31 AM   #21
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QuoteOriginally posted by reh321 Quote
A couple of weeks ago I attached the Pentax-A 50mm f/1.7 lens (that was kitted with my Super Program in 1984) to my K-30 and took some pictures. I set the lens aperture to "A", the camera to Av or P mode, and took pictures as normal. The pictures were sharper than I remember that lens being (of course, with crop-sensor I'm not using all of the lens) and I was satisfied with the exposure. This is a very practical test, I wasn't comparing this lens to any other lens, but it left me comfortable that I can use the lens as normal and get results as normal. I'm not sure what else I could ask of this elderly lens.
I use my collection of SMC-M lenses and also my M42 Super Takumar lenses from the 1960s on full-frame digital (Sony A7), and agree they generally look better than on film. I also use Leica lenses from as far back as the 1930s on my Leica M9, and they are amazingly good. Lens designers were very capable by that time. However, the camera industry needs to sell new lenses, so they generally ignore the usefulness of the old lenses.
08-01-2015, 01:08 PM   #22
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QuoteOriginally posted by TomB_tx Quote
However, the camera industry needs to sell new lenses, so they generally ignore the usefulness of the old lenses.
I also think they are changing to optical glass not containing lead (due to environmental issues) which in turn probably will necessitate changes in lens constructions (their optical formulas). Many (most?) of the best older Pentax (and other brands) lenses have fairly high content of lead.
08-02-2015, 01:04 PM   #23
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QuoteOriginally posted by TomB_tx Quote
Lens designers were very capable by that time. However, the camera industry needs to sell new lenses, so they generally ignore the usefulness of the old lenses.
This is so very true. The tendency is to think that high optical quality is a product of computerized design and manufacturing limited to the last decade. In fact, the fundamentals of optical design were worked out prior to WWII and there has been little innovation in single focal length design or optical theory since then. What has changed since then is the availability of appropriate plastics for cast aspheric elements, coating technology, and availability of computer-aided design*. Without that last factor, the advanced designs of modern zooms would be prohibitively expensive in terms of both time and money to develop.

BTW...I found the Wikipedia article on Optical Lens Design interesting, particularly in regards to the dates.


Steve

* The use of computers for optical design dates back to the 1960s, but it was not until the late 1970s that it became more common. The mid-90s saw the advent of powerful CAD workstations, design software, and the means to do design-time functional modeling.


Last edited by stevebrot; 08-02-2015 at 01:30 PM.
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