# What is a T-stop vs f-stop?

> An f-stop is a theoretical calculation based on the lens focal length and aperture diameter. A T-stop (Transmission stop) measures the actual amount of…

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## An f-stop is a **theoretical calculation** based on the lens focal length and aperture diameter. A T-stop (Transmission stop) measures the **actual amount of light transmitted** through the lens, accounting for light lost to glass absorption and reflections. T-stops are always slightly higher than f-stops.

## **Why the difference matters**

Light passes through multiple glass elements inside a lens. Each element absorbs and reflects a small percentage. An f/2.0 lens with 15 elements might only transmit as much light as f/2.2 — the missing light was lost in the glass.

For **photography**, this difference is handled by the camera meter — it measures actual light hitting the sensor, not theoretical transmission.

For **cinema and video**, consistent exposure between different lenses is critical. If you cut between two f/2.0 lenses with different transmission, brightness shifts visibly. T-stops solve this.

## **Where you see T-stops**

**Cinema lenses** are always rated in T-stops. A T2.1 lens transmits exactly the same light as any other T2.1 lens, regardless of brand or design.

**Photo lenses** are rated in f-stops. Two different f/1.4 lenses may transmit slightly different amounts.

High-quality lenses with advanced coatings have T-stops very close to their f-stops — a well-coated f/1.4 lens might be T1.5, meaning very little light is lost.

## **Cinema lenses at Camera Shop Egypt**

## **Should photographers care about T-stops?**

For most photography, **no**. Your camera meter compensates automatically.

For video where you switch between lenses, understanding T-stops helps maintain consistent exposure.

The practical takeaway: a lens with better coatings and fewer elements will transmit more light — one more reason quality optics matter.

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If you are building a cinema lens kit, matching T-stops across your set ensures exposure consistency between cuts. This is a key reason cinema lenses are sold in matched sets.

## Gear that does this

Available in Egypt and suited to what this guide covers.

- [Canon EF 50mm f/1.2L USM Lens](https://camerashop.com.eg/product/canon-ef-50mm-f-1-2l-usm-lens/) - 69300 EGP, in stock, SKU 2040
- [Sony FE 24-105mm f/4 G OSS Lens](https://camerashop.com.eg/product/sony-fe-24-105mm-f-4-g-oss-lens/) - 57500 EGP, in stock, SKU 2374
- [Canon EF 35mm f/1.4L II USM Lens](https://camerashop.com.eg/product/canon-ef-35mm-f-1-4l-ii-usm-lens/) - 75000 EGP, in stock, SKU 2355