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05-13-2010, 07:02 PM   #1
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Regarding optimal F Stop and Teleconvertors

Say I have a 70-200 2.8 lens that is sharpest at f 4 if I add a 1.4x TC to it will it be theoretically sharper at F4 (2.8+1 for the TC) or F5.6?
(This is excluding the effect the extra glass of the TC has on IQ)

05-13-2010, 07:35 PM   #2
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Generally, to get the best out of a teleconverter, you also need to stop the lens down a couple of stops.
However, there are some "matched multipliers" (I believe this is a Tamron term) which allow for decent results at wide apertures.
The whole teleconverter thing is a crap shoot however, so it's best to check it out with your own equipment.
05-14-2010, 05:58 AM   #3
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It's always worth carrying out tests with a TC to see if it actually works with your lens in the 1st place and also to find the lowest aperture that you can get acceptable results at. Also look at exposure it can also be different. In the case you quoted check that you are losing 1 stop. The 1.4x or 2x quoted by the manufacturer is a nominal figure probably referring to a 50mm lens at their unspecified test distance.

The Pentax AF 1.7x for instance on a 210mm lens is a 1.3x at 2m and a 2.7x at infinity!!! so a big variation in the light loss and for people who prefer it over a 2x (me included ) it comes as a big surprise to find that the Teleplus 2x MC7 has exactly the same magnification and light loss...... just different manufacturers, with different ways of specifying products

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05-14-2010, 12:47 PM   #4
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Think about it. TC is but a magnifier, taking the center of the image produced by the lens and magnifying it to fill the sensor.

In the OP's case, F/4 of the combo (lens at F/2.8 X 1.4) will not be optimal. If the lens itself is sharpest at F/4, there is a good chance that the combo will be sharpest at F/5.6 (lens at F/4 X 1.4).

05-15-2010, 04:57 AM   #5
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QuoteOriginally posted by SOldBear Quote
Think about it. TC is but a magnifier, taking the center of the image produced by the lens and magnifying it to fill the sensor.

In the OP's case, F/4 of the combo (lens at F/2.8 X 1.4) will not be optimal. If the lens itself is sharpest at F/4, there is a good chance that the combo will be sharpest at F/5.6 (lens at F/4 X 1.4).
It's not quite as simple as just a magnifier The TC loses definition and contrast more at the edges than at the center.... stopping down might improve this. It can also increase purple fringing on some lenses and stopping down will usually reduce this.

One disadvantage with TC's that never gets mentioned is the reduced DOF due to the focal length being increased. The change in aperture is not enough to compensate and you may have to stop down even further if DOF is important.

shim
05-15-2010, 06:07 AM   #6
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If a perfect TC is a simple magnifier, it must be the case that the best actual aperture diameter for the lens does not depend on the presence of a TC (in the OP's case, the lens should be set so its indicator says f/4.)
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