Teleconverter Exposure Loss Calculator

Enter your lens focal length, base aperture, and teleconverter factor to calculate stops of light lost, effective aperture, light transmission, and more.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Base Focal Length (mm)
Input the focal length of your lens without the teleconverter, e.g., 200mm for a 70-200mm lens.
2
Enter Base Aperture (f-number)
Input the maximum aperture of your lens, e.g., 2.8 for an f/2.8 lens.
3
Enter Teleconverter Factor (x)
Input the magnification factor of your teleconverter, commonly 1.4x or 2x.
4
Review Exposure and Focal Length Changes
The calculator instantly displays the light loss in stops, the new effective focal length, and the effective aperture.

Example Calculation

A wildlife photographer attaches a 1.4x teleconverter to their 200mm f/2.8 lens and wants to know the resulting light loss and effective focal length/aperture.

Base Focal Length (mm)

200

Base Aperture (f-number)

2.8

Teleconverter Factor (x)

1.4

Results

0.97 stops

Tips

Compensate with ISO

To maintain a usable shutter speed after adding a teleconverter, you'll likely need to increase your ISO. A 1.4x teleconverter costs about 1 stop of light, so doubling your ISO (e.g., from 100 to 200) can compensate for the exposure loss.

Mind Your Autofocus

Many cameras struggle with autofocus at apertures slower than f/5.6 or f/8. Be aware that a teleconverter will make your effective aperture slower, potentially impacting autofocus speed and accuracy, especially in low light.

Consider Tripod Use

The increased effective focal length from a teleconverter also magnifies camera shake. For critical sharpness, especially with a 2x teleconverter (which can create effective focal lengths up to 800mm or more), using a sturdy tripod and remote shutter release is highly recommended.

Calculating Teleconverter Exposure Loss and Effective Focal Length

The Teleconverter Exposure Loss Calculator helps photographers understand the optical changes that occur when adding a teleconverter to a lens. This accessory extends a lens's focal length but comes with trade-offs, primarily a reduction in light transmission and a slower effective aperture. For instance, attaching a 1.4x teleconverter to a 200mm f/2.8 lens will result in a light loss of approximately 0.97 stops, while extending the focal length to 280mm and slowing the aperture to f/3.9.

The Physics of Light Transmission in Optics

Light transmission in optical systems, such as lenses and teleconverters, is governed by fundamental principles of physics. As light passes through any glass element, a small percentage is absorbed or reflected, leading to a cumulative loss of light. In a teleconverter, additional glass elements are introduced between the primary lens and the camera sensor, further attenuating the light. This reduction in light intensity is directly related to the square of the teleconverter's magnification factor. For example, a 2x teleconverter effectively doubles the light path, leading to a four-fold reduction in light (or a 2-stop loss), consistent with optical principles that dictate light intensity decreases with the square of the distance or the square of the magnification factor. Modern lens coatings help minimize these losses, but they cannot be entirely eliminated.

The Mathematics of Teleconverter Impact

The calculation for teleconverter exposure loss and its effect on focal length and aperture is straightforward, based on the teleconverter's multiplication factor.

Here are the key formulas:

New Focal Length:

New Focal Length = Base Focal Length × Teleconverter Factor

Effective Aperture (f-number):

Effective Aperture = Base Aperture × Teleconverter Factor

Light Loss in Stops:
```
Stops Lost = log2(Teleconverter Factor ^ 2)
```
This formula quantifies the reduction in light, where a factor of 1.4x results in approximately 1 stop of light loss, and a 2x factor results in 2 stops. The calculator also derives light transmission percentage, magnification gain, and the equivalent ISO needed to compensate for the light loss.

💡 If you're comparing the effective reach of different camera and lens setups, our Millimeters to Inches Converter can help you understand lens dimensions, while our Telephoto Reach Comparison Calculator can show you the overall magnification.

Analyzing a 200mm f/2.8 Lens with a 1.4x Teleconverter

Consider a photographer using a professional 200mm f/2.8 prime lens, a common choice for sports or wildlife. They decide to add a 1.4x teleconverter for extra reach.

Input Base Focal Length: 200 mm
Input Base Aperture: f/2.8
Input Teleconverter Factor: 1.4x
Calculate New Focal Length: 200 mm × 1.4 = 280 mm. The lens now behaves like a 280mm lens.
Calculate Effective Aperture: f/2.8 × 1.4 = f/3.92. This is typically rounded to f/4.0 in camera displays.
Calculate Light Loss in Stops: log2(1.4^2) = log2(1.96) ≈ 0.97 stops. This means nearly one stop of light is lost, requiring an exposure compensation.

The photographer now has a 280mm f/3.9 lens, gaining reach but losing almost a full stop of light. This requires adjusting shutter speed or ISO to maintain proper exposure.

💡 For quick conversions of time-based photography settings, like shutter speeds, our Milliliters to Fluid Ounces Converter is not directly related, but for general unit conversions you might encounter, it can be useful.

ISO Standards for Optical Transmission Loss

In the world of optics and photography, various ISO standards exist to ensure consistent measurement and reporting of optical properties, including light transmission and loss. While there isn't one single ISO standard exclusively for "teleconverter exposure loss," broader standards like ISO 9022 (Environmental test methods for optics and optical instruments) or ISO 10110 (Optics and optical instruments — Preparation of drawings for optical elements and systems) touch upon how optical performance, including transmission, should be specified and verified. These standards help manufacturers maintain quality control and provide reliable specifications for their products. For instance, the light transmission percentage, a direct output of this calculator, is a critical metric for assessing optical efficiency, and its measurement often adheres to specific industry protocols to ensure accuracy and comparability across different brands of lenses and teleconverters. This standardization ensures that photographers can trust published specifications and make informed decisions about their equipment.

Frequently Asked Questions

How do teleconverters affect a lens's focal length and aperture?

Teleconverters increase a lens's focal length by their magnification factor (e.g., a 2x teleconverter turns a 200mm lens into a 400mm lens). Simultaneously, they reduce the maximum effective aperture by the same factor, resulting in a loss of light (e.g., a 2x teleconverter changes an f/2.8 lens to f/5.6, losing two stops of light).

What is 'light loss in stops' when using a teleconverter?

Light loss in stops refers to the reduction in the amount of light reaching the camera sensor due to the teleconverter. Each 'stop' represents a halving or doubling of light. A 1.4x teleconverter typically causes a 1-stop loss, while a 2x teleconverter results in a 2-stop loss, requiring adjustments to exposure settings.

Does a teleconverter reduce image quality?

Yes, teleconverters can inherently reduce image quality to some extent due to the addition of more glass elements into the optical path. This can manifest as reduced sharpness, increased chromatic aberration, or vignetting. The degree of impact depends heavily on the quality of both the lens and the teleconverter.

When is a teleconverter a better choice than a longer lens?

A teleconverter is often a more cost-effective and portable solution for extending focal length, especially when a dedicated longer lens is prohibitively expensive or bulky. It's ideal for situations where occasional extra reach is needed, or when carrying less equipment is a priority, despite the trade-off in light and potential image quality.