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07-25-2010, 03:53 PM   #16
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QuoteOriginally posted by Clinton Quote
So, I'm a little slow in sharing the response from support, but here it is:



So it would sound like either answer is plausible. These lenses may be slightly wider than 1.8 and hence display at 1.7, or it may be possible that our modern bodies just read these lenses at 1.7.

I'd kinda hoped Pentax would have a clearer answer. <shrug>
the simple answer is that the lens coding is in half stops and 1.7 is the half stop. Any error will be taken care of with the light hitting the meter and shutter speed. The error in any case is 1/6 of a stop and not noticable

07-25-2010, 04:14 PM   #17
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personally, a register of 1.7 or 1.8 doesn't really matter or complain especially if a lens advertised as 1.8 turns out a 1.7. you might just give them an idea on pricing the lens even more. but for the benefit of giving an explanation, I could only say that such lenses aren't really exactly 1.8 but approx. near that value. the camera simply rounded it off since it doesn't really work as a super-accurate calculator unless camera makers would start building lens camera aperture reading cameras complicated and accurately exposing the real aperture value of lenses.
07-25-2010, 07:04 PM   #18
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QuoteOriginally posted by Pentaxor Quote
I could only say that such lenses aren't really exactly 1.8 but approx. near that value. the camera simply rounded it off since it doesn't really work as a super-accurate calculator unless camera makers would start building lens camera aperture reading cameras complicated and accurately exposing the real aperture value of lenses.

that what T stops are for mate, the transmittance for the Pentax 55mm f/1.8* according to my test data is T=1.84.So that is the net amount of light transmitted by this lens, including the light absorbed by the glass in it. incidentally the F number according to my calculations; and corroborated my my test data, the actual f stop of the lens is f/1.78 which means our cameras aren't that far off the mark. my tests give lens transmittance somewhere in the vicinity of 93% so it is an efficient design, more efficient than the olympus 50mm f/1.8 which transmits only 84%

*Serial No 1141378 -SMC version tested so results don't apply to earlier versions- light transmittance tested from the centre of the lens to prevent vignetting from contaminating results.

Last edited by Digitalis; 07-25-2010 at 07:20 PM.
07-26-2010, 02:38 AM   #19
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QuoteOriginally posted by Digitalis Quote
that what T stops are for mate, the transmittance for the Pentax 55mm f/1.8* according to my test data is T=1.84.So that is the net amount of light transmitted by this lens, including the light absorbed by the glass in it. incidentally the F number according to my calculations; and corroborated my my test data, the actual f stop of the lens is f/1.78 which means our cameras aren't that far off the mark. my tests give lens transmittance somewhere in the vicinity of 93% so it is an efficient design, more efficient than the olympus 50mm f/1.8 which transmits only 84%

*Serial No 1141378 -SMC version tested so results don't apply to earlier versions- light transmittance tested from the centre of the lens to prevent vignetting from contaminating results.
Don't confuse T stops with designs that push the limit of tolorances on international standards.

Many lenses are not necessairly exactly on the money in terms of design, in meeting their stated values.

It used to be standard practice when magazines tested lenses to report the actual focal lengths, and apertures of the lenses that they tested. This stopped a while back and now all they report is the resolution, and a bunch of very subjective criteria, but never report on the real focal lengths, or the accuracy of each aperture stop.

07-26-2010, 04:44 AM   #20
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QuoteOriginally posted by Lowell Goudge Quote
Don't confuse T stops with designs that push the limit of tolorances on international standards.
how does this invalidate my testing? photographic lenses are limited by the physics of light which, like acoustics, is a phenomenon that is pretty well understood these days; and is easily quantifiable and measurable. I know exactly what T stops are, I'm pretty thorough with my optical tests. But if you have any suggestions on how to improve...i'm all ears.

QuoteOriginally posted by Lowell Goudge Quote
Many lenses are not necessairly exactly on the money in terms of design, in meeting their stated values.
I agree, do Leica lenses transmit light better than any other lens? Of course not. I have seen lenses that are from the same manufacturer, optically very similar deliver markedly different results. SMCP-A 135mm f/2.8 and SMCP-A*135mm f/1.8 spring to mind

Last edited by Digitalis; 07-26-2010 at 04:57 AM.
08-30-2010, 08:34 PM   #21
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QuoteOriginally posted by feverbeaver Quote
My A*135 and K-7 is stopping down a little bit if I select f=1.8.
At f=1.7 it is wide open.
Interesting, I'll need to do some tests with mine.
08-31-2010, 08:18 PM   #22
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My guess is all of our 135/1.8's will say 1.7. Now if we only had the gear to measure the lens wide open and see what it's aperture really is...
09-01-2010, 12:09 PM   #23
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QuoteOriginally posted by Clinton Quote
My guess is all of our 135/1.8's will say 1.7. Now if we only had the gear to measure the lens wide open and see what it's aperture really is...
Clinton what are you like, when this thread first started, I thought to myself why worry about this.

Seeing your reply today and the fact the thread is still going on, I actually took the time to look at mine and yes is does report f1.7 on all my K20D's and also the couple of K10D's I have left.

I can honestly say I have never noticed this before.

Strangely enough wide open it reports f1.7 up a click f2.0, but going down the way from f2.0, a click f1.8 down again f1.7. i.e. an extra click on the way down.

Anyway it's just a great lens, which I use all the time and never noticed this 'till you pointed it out. You must have special powers.................................

09-01-2010, 01:14 PM   #24
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QuoteOriginally posted by Clinton Quote
My guess is all of our 135/1.8's will say 1.7. Now if we only had the gear to measure the lens wide open and see what it's aperture really is...
there is a simple way, just measure the grey scale value when you shoot a frame against a uniformly lit wall, and also do the same with an F2 lens, noting also the difference in aperture and shutter speed, this gives you a relitive comparison
QuoteOriginally posted by kerrowdown Quote
Clinton what are you like, when this thread first started, I thought to myself why worry about this.

Seeing your reply today and the fact the thread is still going on, I actually took the time to look at mine and yes is does report f1.7 on all my K20D's and also the couple of K10D's I have left.

I can honestly say I have never noticed this before.

Strangely enough wide open it reports f1.7 up a click f2.0, but going down the way from f2.0, a click f1.8 down again f1.7. i.e. an extra click on the way down.

Anyway it's just a great lens, which I use all the time and never noticed this 'till you pointed it out. You must have special powers.................................
the reason it does this is because you probably have the increments in 1/3 stops in the camera body, which is fine.

normally I use 1/2 stop increments, in which case it will show F2 and F1.7.

the question is, does the exposure change between the 2. as I suggested to clinton, take shots at all apertures available (F1.7 and F1.8 specifically and measure greyscale value and compare shutter speeds also,. you may see a change in the lens or you may not.

If you are not sure how to measure the data, let me know and i will do it for you
09-01-2010, 02:18 PM   #25
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QuoteOriginally posted by Lowell Goudge Quote
the reason it does this is because you probably have the increments in 1/3 stops in the camera body, which is fine.

normally I use 1/2 stop increments, in which case it will show F2 and F1.7.

the question is, does the exposure change between the 2. as I suggested to clinton, take shots at all apertures available (F1.7 and F1.8 specifically and measure greyscale value and compare shutter speeds also,. you may see a change in the lens or you may not.

If you are not sure how to measure the data, let me know and i will do it for you
Aye your right Lowell, mine's set to 3rds

The shutter speed does appear to show a change i.e. in my example here tonight a 60th @ f1.8 to a 80th @ f1.7, but hey Lowell don't go to any trouble over this for me, the lens takes great images, which for me is the critical bit.
09-02-2010, 09:11 PM   #26
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QuoteOriginally posted by Lowell Goudge Quote
normally I use 1/2 stop increments, in which case it will show F2 and F1.7.

the question is, does the exposure change between the 2. as I suggested to clinton, take shots at all apertures available (F1.7 and F1.8 specifically and measure greyscale value and compare shutter speeds also,. you may see a change in the lens or you may not.
It's definitely stopping down at 1.8, but just a little bit, and more substantial amount to F2. I'm using 1/3 stops as well. It's easy to put it in bulb mode and observe the aperture blades.
09-02-2010, 09:12 PM   #27
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QuoteOriginally posted by kerrowdown Quote
Strangely enough wide open it reports f1.7 up a click f2.0, but going down the way from f2.0, a click f1.8 down again f1.7. i.e. an extra click on the way down.
Yup. At least when you're using 1/3 stops you can only select 1.8 on the way down, not the way up.
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