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07-01-2012, 05:02 PM   #16
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QuoteOriginally posted by joe.penn Quote
F-stops are purely geometrical...
Joe, that's a quote from the wikipedia page I already linked to in a post I made earlier in this very thread (06-29-12, 10:22 PM). Should I call out my links to things like wikipedia entries more clearly, rather than just linking a single phrase, like "t-stop"? I've noticed a lot of threads seem to have discussions that ignore embedded hyperlinks I make, and this is a really literal example of it... I must not be making myself clear if people are just following up to threads with copy-pastes of information I've already provided in a more subtle fashion.

07-01-2012, 05:27 PM   #17
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@nater - ya, I didn't notice the "t-stop" was actually a link in your post, kind of blended in with the text. - maybe on your other posts where it happened before could also be the same thing. Sorry...
07-01-2012, 05:58 PM   #18
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QuoteOriginally posted by joe.penn Quote
@nater - ya, I didn't notice the "t-stop" was actually a link in your post, kind of blended in with the text. - maybe on your other posts where it happened before could also be the same thing. Sorry...
No problem, good to know, and I'll make an effort to be more clear in the future (and maybe more complete quotes are the way to go).
07-01-2012, 07:43 PM   #19
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QuoteOriginally posted by nater Quote
No problem, good to know, and I'll make an effort to be more clear in the future (and maybe more complete quotes are the way to go).
Since the forum itself likes to create links for certain words, embedded links I totally ignore if I even notice them. If you want people to follow, make it clear you're posting a link, ala

Read this: F-number - Wikipedia, the free encyclopedia

07-01-2012, 08:05 PM   #20
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QuoteOriginally posted by vonBaloney Quote
Since the forum itself likes to create links for certain words, embedded links I totally ignore if I even notice them.
I had been wondering if that hyperlinking I'd seen in some posts was automatic... that's kind of lame. I guess it's some way to generate additional revenue for the forum, but it's like a robot sort-of editing what I write, some of the time, and without warning me. And the side-effect is that it massively degrades the value of all non-explict links anyone creates. (I bet Tim Berners-Lee would consider it an abuse of what he wanted HTML and the web to be.)
07-02-2012, 05:06 AM   #21
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in short, not very.

Basically all lenses apart from those exclusively designed for cinematography, and microscopy aren't all that well calibrated. For instance the Canon EF 70-200mm f/2.8 L IS lens has a T stop of about T=3.11 and Canon labels it f/2.8 lens. Although it has lens elements that are accurately sized for an f/2.8 lens, there is always going to be a big loss of light from internal reflections and the fact is the more glass you have in a lens the more problematic internal reflections are going to be. Also bear in mind that glass absorbs some of the energy from the photons travelling through it as well which further reduces the intensity of light at the focal plane. The Canon 70-200mm f/2.8 L IS, is constructed with 23 lenses in 18 groups - that is a considerable amount of glass* that light has to travel through. So how does canon get away with such a relatively large amount of light loss? firstly the lens has IS and secondly Canon DSLR cameras also give the ISO values a behind the scenes push or pull depending on the transmission of the lens attached to the camera - only a year ago this fact came to light. (pun intended)

Pentax are just as bad as everyone else is at stating the actual F/ number of their lenses, I wouldn't worry too much about it.



* The Canon 70-200mm f/4L only uses 20 elements in 15 groups- and it is widely considered to be sharper than the f/2.8 version. For comparison the Sigma 70- 200mm f/2.8 APO EX DG only uses 18 Elements in 15 Groups.

Last edited by Digitalis; 07-02-2012 at 05:21 AM.
07-02-2012, 10:00 AM   #22
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There's definitely some variability. The Pentax 31mm limited requires a much slower shutter speed at f1.8 than the Sigma 30mm at f1.8.
07-02-2012, 10:17 PM   #23
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QuoteOriginally posted by Dirtistasty2 Quote
The Pentax 31mm limited requires a much slower shutter speed at f1.8 than the Sigma 30mm at f1.8.
My FA31mm f/1.8 Measures at T= 1.84 I don't have a sigma 30mm f/1.4 nor would I want to, but I suspect that the aperture lever on pentax camera bodies especially when used with 3rd party lenses certainly adds an additional degree of variability to the accuracy of reported F values. One very good reason why I prefer lenses with aperture rings on them - you can cut out one more variable when testing the transmission of lenses, by using the lens to control the aperture - not the camera.

07-03-2012, 02:49 PM   #24
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QuoteOriginally posted by Digitalis Quote
One very good reason why I prefer lenses with aperture rings on them - you can cut out one more variable when testing the transmission of lenses, by using the lens to control the aperture - not the camera.
A pet peeve of mine. The weak link is the aperture actuator lever and the mechanism that supports it. I much prefer the original K-mount design where the stop-down state is a simple True/False.


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07-03-2012, 04:58 PM   #25
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QuoteOriginally posted by Digitalis Quote
My FA31mm f/1.8 Measures at T= 1.84 I don't have a sigma 30mm f/1.4 nor would I want to, but I suspect that the aperture lever on pentax camera bodies especially when used with 3rd party lenses certainly adds an additional degree of variability to the accuracy of reported F values. One very good reason why I prefer lenses with aperture rings on them - you can cut out one more variable when testing the transmission of lenses, by using the lens to control the aperture - not the camera.
Interesting, never thought about that...
07-03-2012, 07:34 PM   #26
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QuoteOriginally posted by stevebrot Quote
A pet peeve of mine. The weak link is the aperture actuator lever and the mechanism that supports it. I much prefer the original K-mount design where the stop-down state is a simple True/False.


Steve
I agree, I'm a huge fan of the KISS principle. And sticking to the design of the original K-Mount would have been a step in the right direction. They have done it with the 645D: you can use the aperture rings on 645 lenses without completely fouling up AE functionality, why can't they do it with the smaller K mount?
07-04-2012, 07:12 AM   #27
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I've owned a lot of lenses, and the only one I've ever noticed to have a noticeably inaccurate f-stop was the Tamron 17-50mm I bought new. Wide open at an alleged-f2.8, it underexposed by half a stop compared to my other f2.8 lenses, so it was more like a max aperture of f3.2 or 3.3. When stopped down to around f9 or f10, the problem was worse, with a full stop of underexposure compared to all my other lenses. I returned it, since I don't believe it was a problem unique to my copy. Here's my original thread:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/124013-tamron-...-k-x-pics.html
07-04-2012, 03:58 PM   #28
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It isn't whether they can, but whether they want to. Pentax is competeing against companies that don't care about compatability that much. Having an internal aperture feeler and the associated circuit boards etc costs money. They are just trying to be price competitive. As far as the aperture ring goes, however, the internal cam has to be shaped and adjusted properly to be accurate. Pentax K , 6x7 and 645 lenses are very well made in this regard but lesser brands are historically less well made. Anyway, just having a diaphragm ring doesn't necessarily mean it is more accurate.
07-04-2012, 10:26 PM   #29
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QuoteOriginally posted by pentaxus Quote
Anyway, just having a diaphragm ring doesn't necessarily mean it is more accurate.
At least it is going to produce more consistent results than relying on the camera's aperture actuator arm getting it right 100% of the time, that we can be certain of. As a studio photographer I have experienced instances where the lever didn't correctly strike the aperture lever causing severe overexposure - subsequent exposures were correct but it is a concern of mine. So you can imagine my annoyance at the fact that DA series lenses do not have aperture rings.

Last edited by Digitalis; 07-04-2012 at 10:33 PM.
07-05-2012, 10:20 AM   #30
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QuoteOriginally posted by Digitalis Quote
At least it is going to produce more consistent results than relying on the camera's aperture actuator arm getting it right 100% of the time, that we can be certain of. As a studio photographer I have experienced instances where the lever didn't correctly strike the aperture lever causing severe overexposure - subsequent exposures were correct but it is a concern of mine. So you can imagine my annoyance at the fact that DA series lenses do not have aperture rings.
what I have found, and this is limited to the 50 or so lenses I own is that the metering and errors have proven consistent over time. I am much less worried about the camera controlling the aperture than individual lenses especially third party makers meeting the requirements or inconsistancies from one lens model to another
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