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Re: [OM] Tamron 90mm

Subject: Re: [OM] Tamron 90mm
From: "John A. Lind" <jlind@xxxxxxxxxxx>
Date: Mon, 06 Aug 2001 23:11:28 +0000
At 02:24 8/7/01, Jeff Bellin wrote:
OK, so now I went and got the 49mm version of the Tamron 90mm 2.5 and it
really does seem to be extremely well made. At least to me the "fit and
finish" are great. It may be a little on the large and heavy size but for
me the feel more than makes up for it. It seems to have a lot of metal in
it.
Anyway I also got the extension tube with it. So now my question is;  is
there a "light loss" when using the tube and if so how much do you figure
it would be? Are we talking an f stop or so?

Thanks again to everyone for their help,

Jeff Bellin
Newton, MA

If you are using the TTL metering, you don't need to worry about compensation for the extension tube, because it's through the lens.

Technically you always lose light when focusing at anything closer than infinity by extending a lens, whether it's the lens focusing helical, extension tube, or bellows. From a practical standpoint, the loss doesn't become significant enough to worry about until you are focusing closer than about 7 or 8 times the lens focal length (for a 90mm lens this is 60 to 70 centimeters or about 2 to 2.5 feet). How much light is lost depends on the total extension of the lens from infinity focus. If you still want to compute how much light you will lose (correction factors), either by total lens extension or by magnification, see my tutorial and the examples given on how to calculate it here:
  http://johnlind.tripod.com/science/scienceexposure.html#Macrophotography

I don't know the length of the Tamron tube, nor the total extension the lens alone has from infinity to minimum focus distance so I can't calculate it for you from lens extension. I measure the total lens helical extension from infinity to minimum focus distance by placing camera with lens on tripod and using a small flexible tape measure marked in millimeters held along the side of the lens. Make note of the measurement from body to a good reference point, such as the filter ring, then turn the focus ring from infinity to closest focus distance, and measure it again. The difference is the total possible lens extension at minimum focus distance. An extension tube is straightforward; simply its length.

The correction factor at 1:1 magnification is easy, if total extension of your lens will achieve it (tube plus focusing): it's 4x. This means the shutter must be open 4x longer compared to the same subject brightness at infinity focus. For aperture compensation (or shutter speed by using stops), it's 2 stops more exposure.

-- John


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