Regarding optimal F Stop and Teleconvertors

Tonto · 05-13-2010, 07:02 PM

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)

Wheatfield · 05-13-2010, 07:35 PM

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.

shim · 05-14-2010, 05:58 AM

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

shim

SOldBear · 05-14-2010, 12:47 PM

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).

shim · 05-15-2010, 04:57 AM

Originally posted by SOldBear

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

newarts · 05-15-2010, 06:07 AM

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.)

05-13-2010, 07:02 PM	#1
Tonto Inactive Account Join Date: Apr 2010 Location: Wellington Posts: 969	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)

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05-13-2010, 07:35 PM	#2
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 14,820	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
shim Junior Member Join Date: Mar 2010 Location: Merseyside UK Posts: 34	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 shim

05-14-2010, 12:47 PM	#4
SOldBear Veteran Member Join Date: Feb 2009 Location: Alameda, CA Posts: 3,206	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
shim Junior Member Join Date: Mar 2010 Location: Merseyside UK Posts: 34	Originally posted by SOldBear 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
newarts Inactive Account Join Date: Dec 2008 Location: Ames, Iowa, USA Photos: Albums Posts: 2,965	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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