Hoher Göll & Lens Transmission

I did the “Kleine Reib’m” hike in September 2010. This is a very popular loop hike in Bavaria’s Nationalpark Berchtesgaden. It’s two primary destinations are the peak of Schneibstein, probably one of the easiest mountains that is over 2000m* high, and Seeleinsee (which literally translates to “Little-Lake lake”, how silly is that?).

Variations exist as to the start- and endpoint of the hike, depending on the desired difficulty and length – which shall not be the topic of this post, though.

I was using a Tamron 24-135mm full frame vari-aperture zoom lens on the D700 back then as my “hiking lens” because it was relatively compact, had good sharpness, and a very versatile zoom range. Nikon’s own vari-aperture 24-120mm pretty much sucked (I mean the older f/3.5-5.6 model; the new constant aperture f/4 version is probably my favorite zoom lens), and back then, I thought that the Tamron’s primary flaw was its lack of stabilization (VR, OS, IS, you name it).

Hiking in broad daylight and thus without a tripod, I was using Auto-ISO to avoid blurry photos. It always felt a little bit odd how the ISO would literally sky-rocket: combined with a polarizer, I often ended up at ISO1600 or even 3200. I didn’t think about it too much because with plenty of light to gather, the D700’s high ISO image quality is just great in that ISO range.

One day though I tested some lenses against each other at 50mm focal length, and one thing about the Tamron really stuck out: the exposure times were always a full stop longer than with the other lenses! And that was the day I learned about the T-value.

This “T” stands for transmission. And despite the fact that it is actually a quite important value, it is often omitted from lens tests and specs. One of the few sites that lists the T-stop for lenses is DxOmark.com.

Ideally, the T-stop would be equivalent to the largest F-stop of the lens, but that is not always the case – which means that the lens absorbs a little more light. That’s particularly true for zoom lenses because they’re generally more complex and contain more lens elements. You may find a fast f/1.4 prime with a T-stop value of 1.7 – which means that the lens itself absorbs about half a stop of light (the next “full stop” from f/1.4 would be f/2.0). This is rather common. A zoom lens absorbing a full stop more than other zoom lenses though… is not so common. 🙂

The ending of my copy of the Tamron 24-135 is unknown. After replacing it with the 24-120/f4, I sold it on ebay and a buyer from Turkey made the highest bid (he had not read my shipping conditions, where I stated that I would only ship to the United States). He insisted that I send the lens nevertheless, as cheap as possible (ie. without insurance). About a month after shipping he contacted me and told me that he still hadn’t received the lens.

So the lens had taught me about T-stops, and him about insurance and tracking.

*) funny how this threshold makes little to no sense when converted to feet – which is why I kept it metric. 🙂

Thoughts? Let me hear them.

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