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07-03-2009, 12:08 AM   #1
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Does internal focusing cause brightness issues?

If I have two different lenses, and I take a shot at the same aperture, shutter, iso and focal length, should brightness levels be equal?

Right now I have a Pentax 18-35mm and a Tamron 28-75mm. Shooting conditions was Raw with no image processing. 28mm, two 60w incandescent overhead lights on ceiling, wall is approx 7ft from lens.

As you will see below, the pentax is brighter all the way through until f4.5, to which both lenses are about even. Is this difference due to a faulty lens, or the fact that the Tamron is an IF lens, blocking some of the light? Or, is this just normal variance for different lens manufacturers?

Tamron on left, Pentax on right (sorry, the pentax was a bit misfocused). All shots at 28mm, ISO 800, 1/10 sec, and respective aperture:




Last edited by rmtschanz; 07-03-2009 at 12:23 AM.
07-03-2009, 08:06 AM   #2
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Probably not the internal focus per se. I would suspect some variance in the aperture actuation mechanism. Does the Tammy show a similar light fall-off when using the aperture ring?

Steve
07-03-2009, 07:30 PM   #3
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I think you're seeing a difference because you're comparing a variable-aperture zoom (the FAJ 18-35) to a constant-aperture zoom (the Tamron).

As I recall, the body can compensate for focal length with KAF-and-later variable-aperture zooms (so f/8 on the body is approximately equal to f/8 in reality, even though this corresponds to a nominal aperture setting of about f/6.3*) but the body doesn't know the exact focal length setting of the lens, so the adjustment isn't perfect.

No adjustment needed for the Tamron; f/8 really is f/8 regardless of focal length.

*That is, if FAJ lenses had aperture rings
07-03-2009, 09:31 PM   #4
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QuoteOriginally posted by rmtschanz Quote
Is this difference due to
en.wikipedia.org/wiki/T_stop#T-stops

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