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09-26-2011, 03:17 PM   #16
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Wow, sooo cool pictures But Rupert, this thread would have been worthless without a squirell

09-26-2011, 04:12 PM   #17
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QuoteOriginally posted by SlickYamaha Quote
Just as a really dumb-amateur-sure go ahead and laugh at me question...

Would shooting the 70-200 at 2.8 be LESS light than the 17-50 at 2.8? or is 2.8 still 2.8 at any zoom range?
My mind says, when zoomed at 200mm, you would have less light to fill, at 2.8 or not, where as 17mm, you have a much broader angle to capture more light... correct? So in the future, should i put the 70-200 on the K5 and the 17-50 on the K7 to compensate?
A concept called T-stop should help you understand what's happening. Two lenses marked f/2.8 may not necessarily transmit the same amount of light; hence, they may have different T-stop values. The T-stop is similar to the f-number, but is adjusted for actual light transmission. An f/2.8 lens may actually be T3.1, for example. One of your f/2.8 lenses may be T3.0 and the other may be T3.2. While this is not normally important when shooting still images, it is important in cinematography. Photo lenses are generally not marked for T-stops, but cine lenses usually are.

09-26-2011, 05:15 PM   #18
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QuoteOriginally posted by DragonLord Quote
A concept called T-stop
DxO now includes this in their lens measurements.

Also sometimes the manufacturer is not always 100% accurate in their designation. A Pentax 70-200/F4 (non-SMC version) has a noticeable decrease in effective aperture to about f/4.5 (based on the increase in shutter speed in Av mode) at the telephoto end.


Last edited by dosdan; 09-26-2011 at 09:57 PM.
09-26-2011, 06:48 PM   #19
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Thanks for all the replies. I guess my main question was. Is the light difference NOTICEABLE? When it comes down to it, should i be throwing the 70-200 on the K5 since it can do better ISO, or is it such an insignificant difference that it really doesnt matter?

09-26-2011, 08:00 PM   #20
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QuoteOriginally posted by kcobain1992 Quote
Wow, sooo cool pictures But Rupert, this thread would have been worthless without a squirell
Otis certainly appreciates that comment, although he understands that not everyone feels that way..... It is a good thread with or without a Squirrel, some good information here, as is often the case throughout PF. Little bits and pieces I pick up here and there do help. We have some knowledgeable and accomplished shooters around here.

Back to the subject of my earlier post...using the high ISO in decent light for is you another Squirrel, in fact it is Otis, shot for speed in medium light.

1/ 160 for speed F10 for resolution ISO 6400 I think the K5 shines when used in this manner, anyhow, Otis sure likes it!
Best Regards!

I have posted this shot before...and it caused me some problems with Otis, so now I must post a clarification when posting it again. "Otis had just been swimming in the cold Bird Bath, and was suffering from "shrinkage". This shot in no way represents the true manhood of Otis Squirrel." I can too?
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