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View Poll Results: f/2.8 + 1.4 TC =
3.5 1512.50%
4.0 9075.00%
4.5 97.50%
What are you talking about? 65.00%
Voters: 120. You may not vote on this poll

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11-22-2007, 02:39 PM   #16
Syb
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After all these maths, I consulted my K10d with a f/2.8A lens
It said: 4.5

So. I repeat thoughtlessly: 4.5

I am a minority, though.

11-22-2007, 03:44 PM   #17
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It shouldnt, it should say F4.0...
The easiest way to remember is to right down all the full stops on a piece of paper and stick it somewhere you will see it everyday, eventually you'll know them like the back of you hand!

Thanks
11-23-2007, 06:45 AM   #18
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Well when my Tamron 28-75mm f/2.8 and the 1.4x TC get here I will let you all know. I wasn't expecting the camera to tell me, if I knew that would happen I would never of asked the question.
11-23-2007, 07:10 AM   #19
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It's f4.0. A 39-105 f4 doesn't sound like that useful a lens to me.

11-23-2007, 01:29 PM   #20
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QuoteOriginally posted by Buddha Jones Quote
Well when my Tamron 28-75mm f/2.8 and the 1.4x TC get here I will let you all know. I wasn't expecting the camera to tell me, if I knew that would happen I would never of asked the question.
There are a couple of things you will need to figure out here,

the pentax electronics does not have any correction for teleconverter, therefore an F2.8 lens will read f2.8 with the TC attached.

the actual apature will be F4 but the impact to the camera is that the light coming through is less, and the metering will get the exposure right.

I have the sigma 1.4x TC for my 70-200 and htat is how it works.

You will also note there is no correction of lens focal length, with the TC attached. That means your shake reduction will be set to something other than optimum, and yoou can't change it.

Pentax is aware of these issues
11-23-2007, 02:41 PM   #21
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QuoteOriginally posted by SupremeMoFo Quote
It's f4.0. A 39-105 f4 doesn't sound like that useful a lens to me.
You're right, I did not buy it for that lens, but I bought it at the same time because it was a good deal. I intend to use it on the Tamron 70-300mm when I decide to get it and ultimately on the Tamron 80-200mm f/2.8 when it comes out .
05-30-2008, 10:03 PM   #22
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I thought this was 1st grade photography stuff. Ya'll did take photography in the 1st grade, right?

And why am I replying to a poll that's 6 months old!?

Anyway, maths isn't decided democratically.
05-31-2008, 02:05 AM   #23
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QuoteOriginally posted by RBellavance Quote
I'm pretty sure that it effectively "eats" 1.4 stops of light. However, it is usually rounded to 1 stop for ease of calculation. The correct answer would be 4.5, IMHO.
I'm pretty sure that this is not correct

One stop more or less means half or double the light. The max aperture is a function of the focal lenght f, thats why it's called f/2.8. So, a 28mm f/2.8 lens has an effective max. inner diameter of 28/2.8=10mm or 28mm/10mm = 2.8. When stopping down, say, 2 stops to f/5.6, this results in 28/5.6=5mm. The area that 'lets in' the light grows with the square of the diameter, so it's half the diameter, quarter of light (two stops).

Therefore one stop more is 1.4142 (square root of 2) times the diameter, resulting in 1.4141 ^ 2 = 2 times as much light.

If you increase the focal lenght f by 1.4(142) using a TC, while of course the inner lens diameter remains the same, you lose one stop = aperture value times 1.4.

With the 28mm lens from above:

28mm x 1.4 (from TC) / 10mm = f/4

05-31-2008, 06:52 AM   #24
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The teleconverter obviously eats light, but does it also increase depth of field? So, in the exampled discussed, do you get f/4 in terms of light but the depth of field of an f/2.8 lens? Or does both the amount of light and the depth of field go to f/4?
05-31-2008, 09:24 AM   #25
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QuoteOriginally posted by jms698 Quote
The teleconverter obviously eats light, but does it also increase depth of field? So, in the exampled discussed, do you get f/4 in terms of light but the depth of field of an f/2.8 lens? Or does both the amount of light and the depth of field go to f/4?
A 1.4x teleconverter on a 100mm f/2.8 lens will give you depth of field and field of view of a 140mm f/4 lens. So, you both lose DOF (by the longer focal length) and win DOF (by the smaller effective aperture). I assume DOF stays the same as with the master lens.
05-31-2008, 09:28 AM   #26
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Geeeze people, live in the now!! This is like a year old thread.
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