Macro Exposure Compensation Calculator

Frequently Asked Questions

Does my camera compensate for this automatically?

If you use TTL (through-the-lens) metering or TTL flash, yes - the camera sees the dimmed image and compensates automatically. External flash on manual power, handheld meters, and guide-number setups do not compensate and require this correction.

What is 1:1 magnification?

1:1 means the subject is reproduced on the sensor at its actual physical size. A 24 mm wide subject fills a 24 mm-wide sensor. Most dedicated macro lenses reach 1:1 at their closest focus distance.

How do I find magnification from extension tubes?

Magnification = extension / focal_length. A 50 mm extension tube on a 50 mm lens gives M = 50/50 = 1.0 (1:1 magnification). The calculator accepts both methods.

Why does the effective aperture matter for diffraction?

Diffraction depends on the physical f-number of light reaching the sensor, which is the effective aperture. At 1:1 with f/8 set, light behaves as f/16 for diffraction purposes - so choose your aperture knowing the effective value, not the marked one.

Provided by AllCalculators.io
Free online calculators for everyday. No registration required.

Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

JavaScript is required to use the interactive calculator above. The questions and answers below remain readable without JavaScript.

How It Works

At high magnification, the lens is extended far from the sensor to focus closely. This extra distance spreads the same amount of light over a larger projected image, dimming the exposure - the bellows factor or extension factor. The effective aperture is: N_eff = N_set x (1 + M), where M is the magnification ratio (1:1 = M 1.0, 2:1 = M 2.0).

The light lost in stops is: stops = log2((1 + M)^2). At 1:1 (life-size), the factor is (1 + 1)^2 = 4, which is exactly 2 stops. At 2:1 the factor is (1 + 2)^2 = 9, about 3.2 stops. You can enter magnification directly as a ratio (e.g., 1.0 for 1:1) or derive it from the extension tube length and focal length.

The tool then computes the corrected shutter speed: if a meter or TTL exposure reads 1/125 s at M = 1, the adjusted shutter needed for correct exposure is 1/125 / 4 = 1/31 s.

Use Cases

Macro exposure compensation is essential in any high-magnification close-up work:

Extension tubes: TTL metering works through the lens but guide-number flash and handheld meters do not account for the bellows factor

Bellows systems: long bellows extensions at 2:1-5:1 magnification can require 3-5 stops of compensation

Reversed lenses: when a wide-angle lens is mounted backwards for high magnification, the camera cannot meter through it

Copy stand work: reproducing flat art at 1:1 where consistent, repeatable exposure is critical

Scientific macro photography: ensuring accurate density measurements require knowing the effective exposure

Tips

If you use TTL metering through the lens, the camera already accounts for the bellows factor - do not apply this correction a second time.

Diffraction softening tracks the effective aperture, not the marked aperture. At 1:1 with f/8 marked, diffraction behaves as f/16, which matters when choosing your working aperture.

At 1:1 and beyond, depth of field shrinks to millimetres. Focus stacking is often the only way to keep an entire subject sharp.

Manual studio flash and guide-number calculations must add the full stop loss because they cannot meter through the lens.

A ring flash or twin-flash system sits close to the subject and largely eliminates the inverse-square falloff issue at macro distances.

FAQ