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Old Feb 26, 2009, 2:27 PM   #1
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I took a few shots with and without the Tamron 1.4 teleconverter on my Sigma 150-500mm APO HSM and I set everything to full auto, hand held. I noticed that the teleconverter shots are overexposed. I thought the TC would cause the shots to be underexposed...

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Old Feb 26, 2009, 9:43 PM   #2
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Hi Fl_Gulfer,

There are a lot of factors in play here.

With multisection metering, any bright area anywhere in the frame will cause the metering system to bias towards less exposure. In the case of the antennae, you had one shot with clear blue sky, and one shot with 90% cloud cover. With the flowers, you have that bright spot on the wall in the upper left corner which would cause the overall scene to expose less. You really want as close to the same lighting with and without the TC, and it changes moment to moment, so the closer the two shots are together in time, the better the comparison.

Another factor is that you really need to match the FOV from the two shots to get equivalent metering. If you do this in the viewfinder, it's more accurate than if you go by the zoom indicator on the lens. Many zooms will show yield less Field Of View (longer FL) at greater distances than they do at close distances at the same zoom setting, thus you might have to set the FL differently on the lens to get an equivalent FOV for close shots than you did for the longer shot. Your best bet is to frame the shots the same in the viewfinder, regardless of what the zoom setting is.

The reason why FOV is important is that you want the camera metering as close to the same scene as possible. This will matter most in Multisegment Mode, less with Center Weighted, and probably the least on Spot as long as the center spot is aimed exactly in the same place.

I'd repeat the test, picking subjects that would be easy to set the right and left borders of the frame. With the flower box, set up so the end of the box touches the each side of the frame, with the antenna, set up so the it just fits into the frame, regardless of the zoom setting. Set the cam up on a tripod, then take consecutive shots, with and without the TC, taking care to use the zoom to frame the shots as closely as possible, then repeat at a different subject and distance. If you used Av or Tv priority, it would be easier to calculate any exposure differences than if both were variable.

You'll see if the TC has any real effect on the exposures so you can possibly Ev compensate whenever you use it. Somehow I doubt that this will be the case.

. . . Sorry for the long post. . . as usual. . .:roll:

Scott
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Old Feb 26, 2009, 11:38 PM   #3
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Ok since the FOV is smaller with the TC I need to move back so the shot looks the same?

I'll try it tomorrow if thats correct.

Thanks for the explanation..
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Old Feb 27, 2009, 12:51 AM   #4
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Fl_Gulfer wrote:
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Ok since the FOV is smaller with the TC I need to move back so the shot looks the same?

I'll try it tomorrow if thats correct.

Thanks for the explanation..
I'd pick an intermediate zoom setting -- let's say 250-300mm, shoot without the TC, then, staying at the same position, add the TC and zoom back until the framing matches the first shot.

I think I'd also try to shoot the first shot at least one f-stop smaller than max so any max aperture change because of the shorter FL on the lens (when you zoom out) doesn't come into play. Keep the aperture value on the lens the same for both shots)

You should see a one stop difference because of the magnification of the TC, but that should be it (with the aperture at the same value, the shutter speed should be about twice as long).

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