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I have been comparing my Pentax Takumar 28mm f3.5 lens (via an adapter) to my Canon 18-55mm 3.5-5.6 IS lens.

When I have them both set to the exact same f-stop, the exposure meter of the camera is a few ticks lower when the Takumar lens is being used than it is when the Canon lens is being used.

What could be contributing factors to this? Is it the lens coating? I have read that t-stop takes into account the actual transmitted light, as opposed to the theoretical based f-stop. So my question still sort of remains that same. Why does my one lens have a different t-stop to my other lens, when both are at the same f-stop?

What are the contributing factors to t-stop?

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    Possible duplicate of What is T-number / T-stop? – Robin Gertenbach Mar 31 '17 at 20:29
  • I think the underlying questions are different. One is asking what is t-stop, specifically what is it measuring. Where as my question is asking what factors or characteristics of a lens contribute to varying t-stop. – ScottF Mar 31 '17 at 20:35
  • Are you using both lenses on the same camera? Is the zoom lens set at 28mm? – Laurence Payne Mar 31 '17 at 21:21
  • Yes I am using the same camera. I have not perfectly controlled for the zoom. i will try another test. – ScottF Mar 31 '17 at 21:31
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The modern camera lens is an array of individual glass lenses. Some cemented together, others air-spaced. Each lens element has two polished surfaces that act to reflect away some of the exposing light. The recipes used to make the optical glass are diverse. Surface reflection is the cause of considerable light loss. Worst, much of this reflected light will become stray rays that bath film or digital sensor, during the exposure, inducing flare which distresses the image.

To mitigate flare and improve light transmission, the surfaces of the lens elements are coated with a thin film of minerals. Modern lenses will be multi-coated. It is the coat thickness that does the trick. The optimized coat thickness is ¼ of wavelength of the color of the light to be diminished as to its reflectivity. A modern lens might have 6 to 8 coats or more, each with a different thickness. An uncoated lens will reflect away about 8% of the light. A coated lens will reflect away less than 2%.

How to set the camera for proper exposure? This is more of a problem than you might think. Our complex lenses are not perfect transmitters of light. The amount of light passes is a function of the working diameter of the lens intertwined with the working focal length. Each doubling of the focal length, say from 100mm to 200mm, reduces image brightness 4X (four time). We must double the working diameter to regain this light loss. We are taking a significant problem.

The f-number to the rescue: The f-number or focal ratio, takes into account the working diameter of the lens and the working focal length. We divide the focal length by the diameter to obtain the focal ratio. The fact that this number is a ratio is the key to solving the problem. A ratio is dimensionless. An f/8 lens 1000mm focal length with a diameter of 125mm, passes the same amount of light as a 50mm focal length with a diameter of 6.25mm. Both function at focal ratio f/8. So can we say, any lens set to the same f-number passes the same amount of exposing light? Yes we can say this within reason!

The f-number system is pretty good but – it does not take into account surface reflection loss or loss due to the fact that the glass is not faultlessly transparent. For the most part, still photographers need not worry about minor differences in exposure calculations. That’s because film has latitude and digital sensors are adjusted pre and post exposure by software. For the cinema photographer, it’s a different story. Often, in the movies, the scene is changed. Whenever the camera is changed, the differenced in exposure will be notice. Cinema camera use T-stops. These are f-numbers computed not just by ratio, instruments are used calibrate the light transmitted. This method improves the uniformity of different lenses set to the same aperture setting. Some still camera lenses feature T-stops, but this is not common.

No need for special calibration as the modem camera features thru-the-lens exposure metering. This method minimizes exposure setting glitches. Will the color be different, one lens compared to another. Yes!. Each lens is individual. Even lenses of the same make and model come of the line with slightly different transmission profiles. With film, this was a bigger problem. With digital, the problem is mitigated with camera and editing software. This is all part of our skill set.

  • A good example of f-stop and t-stop to divergence is the mirror in a catadioptric lens physically blocking the passage of light. – user50888 Apr 2 '17 at 3:34
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Sad but true - lenses are different. I've got two copies of the same lens (Nikon 85mm 1.8G), one is slightly darker than the other, both are 'QA-certified'.

Would not think to much into it, especially when comparing different lenses. (different materials, coatings, diameter, internal setup...)

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Different absorption capacity of light specifically. For example, my 18-55 always ticks lower than my 35mm prime. Lens coating, material and sometimes lens construction contribute to this.

Simply speaking, more the number of elements in a lens, more its absorption. Companies tweak the coating and material to lower this. Also, in some cases, chromatic aberrations take place due to this.

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