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Lens Sharpness with Teleconvertors
My Impressions Based On Field Experience Using D2X Camera

I use both a 1.4X and a 1.7X teleconverter on my 70-200mm 2.8 and my 300mm 2.8 lenses.

The pro is obvious, a longer lens e.g. my 300mm with a 1.7X teleconverter gives me a 510mm lens.

The cons are as follows.

1. An increase in the minimum f stop e.g. with the 1.7X , the minimum stop on a 2.8 lens is 4.8. This increases the minimum depth of  field I can get which is normally not a big deal.  BUT it also decreases the ability of the lens to focus in low light situations since all lenses focus at wide open ... for me is is a big deal. So much so that I normally only use a teleconverter when there is lots of available light.

2. A decrease in lens sharpness performance. Here a lot of variables come into play, but in general, sharpness with a teleconverter is not as sharp as without one, particularly with zooms wide open. So to compensate, I usually shoot at 1 stop over the minimum when using a teleconverter.
 

Nikon 70-200 2.8 VR Lens

 

F Stops in 1/3 Increments

  2.8 3.2 3.5 4.0 4.5 5.0 5.6 6.3 7.1 8.0
Bare Lens

Sharp

1.4x Tele

F4.0 Is Minimum

Caution

Sharp

1.7x Tele

F4.8 is Minimum

Caution

Sharp

 
Interpretation - The Nikon 70-200 2.8 VR Lens produces sharp images from wide open (F2.8 ) and up to at least F8. With a 1.4x teleconverter images are not as sharp as without a teleconverter until F5.6 and with the 1.7x teleconverter until F7.1.

My bottom line, I think teleconverters are great for f2.8 lenses and maybe even f4.0 lenses with a lot of light.  Beyond f4.0 I think the ability to focus in all but the brightest of days would be seriously compromised.

  

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