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10-17-2020, 08:39 PM   #1
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Weight of the K series 105mm F2.8

Regarding the SMC Pentax 105mm F2.8 lens (https://www.pentaxforums.com/lensreviews/SMC-Pentax-K-105mm-F2.8-Lens.html):


It has been brought to my attention that the weight we originally specified in the database, 331g, was incorrect. Upon checking the original K-series Pentax manual, it looks like the correct weight should be 294g. However, other sites on the web indicate a different weight.

So, if you have this lens, could you please weigh it (without hood/caps) and post the result here? I will update the DB if/as appropriate. Thanks!


Adam
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10-17-2020, 09:47 PM   #2
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SMC Pentax K 105mm f2.8 Serial 5211055 = 304gm

EDIT For calibration purposes same scales - SMC Pentax K 50mm f1.4 serial number 1091695 = 271gms (reviews say 265gm)
10-18-2020, 03:01 AM   #3
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SMC Pentax 1:2.8/105 S/N 5038087: 306g
10-19-2020, 01:52 PM   #4
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Hello Adam, yes that was me, and on a fairly accurate balance the 105mm K 2.8 measured 304 grams.
I discovered another possible adjustment: The SMC PENTAX-A 50mm f/2.8 macro is possibly 50mm long, the 40mm as the description on its page says cannot be true.

10-19-2020, 03:43 PM   #5
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I like this sort of attention to detail!
10-19-2020, 06:02 PM   #6
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QuoteOriginally posted by habrune Quote
I discovered another possible adjustment: The SMC PENTAX-A 50mm f/2.8 macro is possibly 50mm long, the 40mm as the description on its page says cannot be true.
Yep 50mm minimum from mounting flange to front edge at infinity.
10-19-2020, 07:49 PM - 1 Like   #7
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Well that opened an interesting can of worms. I have 8 x M50 1.7. Thought I would weigh them and see how consistent the weight was. 4 @ 183 - 1 @ 186 - 3 @189/90. Serials for the 4 were all 6million the one was 4 million and the 3 was 3million. Guessing it was a trend from metal to plastic.
Moral of the story = +-5g is near enough.
10-19-2020, 10:25 PM   #8
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QuoteOriginally posted by GUB Quote
Well that opened an interesting can of worms. I have 8 x M50 1.7. Thought I would weigh them and see how consistent the weight was. 4 @ 183 - 1 @ 186 - 3 @189/90. Serials for the 4 were all 6million the one was 4 million and the 3 was 3million. Guessing it was a trend from metal to plastic.
I only have one, purchased fall of 1982...

180gm ser.# 6596445

I prefer to think the lower weight was due to different and better grades of helicoid and aperture mechanism lubricants. Just kidding of course, though I have carried that lens over many miles of rugged trail and off-trail routes in conditions ranging from well below freezing to desert heat. Despite having never been coddled, focus action is still smooth and well-damped and the aperture action is still snappy and without slop.

Six grams is not very much, the mass of 0.2 fluid ounces of water.


Steve

10-19-2020, 11:18 PM   #9
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QuoteOriginally posted by stevebrot Quote
I only have one, purchased fall of 1982...

180gm ser.# 6596445

I prefer to think the lower weight was due to different and better grades of helicoid and aperture mechanism lubricants. Just kidding of course, though I have carried that lens over many miles of rugged trail and off-trail routes in conditions ranging from well below freezing to desert heat. Despite having never been coddled, focus action is still smooth and well-damped and the aperture action is still snappy and without slop.

Six grams is not very much, the mass of 0.2 fluid ounces of water.


Steve
"can of worms" was perhaps a mite enthusiastic
Given that brass has a specific gravity of around 8 it represents less than 1cubic mm . A tiny sliver of brass somewhere.
I was most surprised that the variation corresponded with the serial number.
10-19-2020, 11:27 PM   #10
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QuoteOriginally posted by GUB Quote
Well that opened an interesting can of worms. I have 8 x M50 1.7. Thought I would weigh them and see how consistent the weight was. 4 @ 183 - 1 @ 186 - 3 @189/90. Serials for the 4 were all 6million the one was 4 million and the 3 was 3million. Guessing it was a trend from metal to plastic.
Moral of the story = +-5g is near enough.
I assume you weighed them without the front or rear caps?
10-19-2020, 11:34 PM   #11
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QuoteOriginally posted by leekil Quote
I assume you weighed them without the front or rear caps?
Yes.
Just weighed my M50 f2s (3 of them) The serial 4.8million loses 6g to the 4.1 million and earlier ones. (166g and 160g.)
EDIT All metal lens barrels
10-19-2020, 11:41 PM   #12
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QuoteOriginally posted by GUB Quote
Given that brass has a specific gravity of around 8 it represents less than 1cubic mm .
750mm^3 = 6000 mm^3 8

A somewhat larger sliver of brass.


Steve
10-19-2020, 11:57 PM - 1 Like   #13
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QuoteOriginally posted by stevebrot Quote
750mm^3 = 6000 mm^3 8

A somewhat larger sliver of brass.


Steve
Ouch yes - only about three decimal points out. !
Given that the one with the 4mill serial had its own intermediate weight it would suggest there was two changes.
But it could be as basic as a brass element separator going plastic.
Despite these variations I was wondering if the M50 1.7 was a good calibrator for our various scales.
10-20-2020, 03:02 PM - 1 Like   #14
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QuoteOriginally posted by GUB Quote
Well that opened an interesting can of worms. I have 8 x M50 1.7. Thought I would weigh them and see how consistent the weight was. 4 @ 183 - 1 @ 186 - 3 @189/90. Serials for the 4 were all 6million the one was 4 million and the 3 was 3million. Guessing it was a trend from metal to plastic.
Moral of the story = +-5g is near enough.
I identified three slightly different versions of the M 1:1.7 50mm lens long (20 years?) ago.
Somewhere I must have a picture explaining those, but where...
It was mainly in the finish, the oldest version had the best finish/engravings.
10-25-2020, 02:53 PM   #15
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Clearly, the weight differences are due to photonics impaction...
All of the photons that have gone through the lens have left their marks...

The light ones just haven’t taken as many pictures...

OK, as none of that is true, I expect it’s sample variation for an item that had a lot of hand work in it....

-Eric
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