Please explain why a Tamron lens is a stop faster than a Nikon lens

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MrFlash Contributing Member • Posts: 777
Please explain why a Tamron lens is a stop faster than a Nikon lens

I've been at this photo thing for a few decades now, and I thought I had a pretty good handle on why things work the way that they do. Until now.

I just bought a Tamron 28-300 f/3.5-6.3 PZD. I am testing it against my Nikon 24-120 f4 on my D800. The Tamron is surprisingly close in sharpness, but that is another post.

What I can't grasp is that the Tamron, apparently, is gathering twice as much light as the Nikon lens. (??)

I am shooting a static subject with constant lighting, RAW, AE, ISO 800. At every aperture and focal length the Nikon lens requires a shutter speed that is twice as long as the Tamron requires. If the Nikon indicates f/8 & 1/15 the Tamron will indicate f/8 & 1/30, etc., etc.

When I review the files in LR, the luminance of the images are identical, even though the files that were shot with the Tamron were shot at the same aperture and ISO, but at twice the shutter speed!

I checked this result with my other Nikon zooms and primes. All of them require the same exposure, more or less, as the 24-120. The Tamron delivers similarly exposed files as the Nikon lenses, but with a full stop less exposure.

Over the years, I've seen variances in exposure between bodies and lenses, but it is usually within a 1/3 of a stop or so. This is a FULL stop difference.

A first, I thought the Tamron was just opening the aperture more than indicated, but the results are the same when it's shot wide open. It can't open more than that!

Is this slow f/3.5-6.3 lens really a speedy f/2.5-4.5 lens?  If so, I'm keeping it!   But I know that is not possible.

It appears that either the Tamron is a stop faster than claimed or the Nikons are all a stop slower than claimed. Neither of those conclusions is believable.

Does anyone have an explanation for this dramatic discrepancy?

P.S. I owned this same lens a couple of years ago when I was shooting a Canon 5D II and I did not notice any apparent variance with the Canon lenses.

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