Let's Learn Light! Studio Lighting 101: Main Light vs Fill Light
Many studio portraits start with two lights doing two very different jobs. The main light shapes your subject, and the fill light decides how deep the shadows go. Get the balance between them right and you control the mood, contrast, and depth of every image you make.
In this guide we explain what each light does, then run a simple simulated test: we keep the main light at full power and raise the fill light one stop at a time, so you can see exactly how the look changes. We finish with main to fill ratios and when to use each one.
Shop Godox Studio FlashesTL;DR
The main light (key light) is your brightest light and creates the highlights and shadows that give your subject shape. The fill light is weaker and sits on the opposite side to soften those shadows. The balance between them is the main to fill ratio: 1:1 is even and flat, 2:1 is a flattering portrait starting point, and 4:1 to 8:1 is dramatic. Each flash power step (1/1, 1/2, 1/4) is one stop, and every stop doubles or halves the light. For accurate ratios, measure the light on your subject with a light meter.
Main Light vs Fill Light: What Each One Does
The main light and fill light are the foundation of most photography and video lighting setups. They serve different purposes, and they work together to create a balanced, appealing image.
The Main Light
The main light, also called the key light, is the primary light source in your setup. It provides most of the light on your subject and sets the overall look and mood of the scene. It is usually placed to one side of the subject, creating the highlights and shadows that add depth and dimension.
The Fill Light
The fill light complements the main light by reducing the contrast its shadows create. Its job is to soften those shadows and gently light the areas that would otherwise fall into darkness. The fill light is usually less powerful than the main light, so the main light still leads.
How we simulated this: we used set.a.light 3D, a lighting simulator for photographers and filmmakers, to recreate a studio space. Use the code CAMERASTUFF for 10% off this useful software.
The Test: Our Main and Fill Light Setup
Our setup uses two lights, each with a specific job. We keep the main light fixed and change only the fill light's power, so every difference you see comes from the fill.
- Main light: placed on the left of the subject, from the photographer's point of view, and set to full power (1/1). At 100% output it gives strong, defined light that shapes the subject and sets the mood.
- Fill light: placed on the right, from the photographer's point of view. Its job is to balance the shadows from the main light and reduce harsh contrast. It starts at 1/64 power, just 1.56% of its full output.
From 1/64, we raise the fill light one full stop at a time. A full stop means doubling or halving the light's output, so each step up doubles the fill.
Camera settings: 1/125 s, f/8, ISO 100, set on the virtual camera in our set.a.light 3D simulation. These settings are relative to many variables, including the equipment used, the camera settings, and the position of the lights, so treat them as a reference, not a recipe.
Studio Flash Power Settings Explained
Studio flashes adjust their output in fractions of full power. Each step halves the output, and each halving is one stop. Here is how each setting compares to full power:
| Flash Setting | % of Full Power | Stops Below Full |
|---|---|---|
| 1/1 | 100% | Full power |
| 1/2 | 50% | 1 stop |
| 1/4 | 25% | 2 stops |
| 1/8 | 12.5% | 3 stops |
| 1/16 | 6.25% | 4 stops |
| 1/32 | 3.13% | 5 stops |
| 1/64 | 1.56% | 6 stops |
At full power (1/1), the flash gives its brightest, most intense burst. At 1/2, it emits half its full strength, and at 1/4 a quarter. Lower down, the steps get finer: 1/8 is 12.5% of full power, 1/16 is 6.25%, and 1/32 is just 3.13%.
Some flashes go even lower than 1/64. Many strobes in the Godox PRO range go all the way down to 1/256, giving you the tiniest amount of light when you need a very subtle fill.
The Results: Fill Light From 1/64 to 1/1
Here is what happens as the fill light rises one stop at a time while the main light stays at full power.
The results show how much the balance between the main light and fill light shapes an image. At 1/64 the shadow side falls away into deep, moody contrast. As the fill rises, the shadows open up, until at 1/1 both sides are almost evenly lit and the face looks flatter. Neither end is wrong; it depends on the look you want.
Good to know: if both lights are the same model, with the same modifier, at the same distance from the subject, the fill power maps roughly to the ratio. Fill at 1/2 gives about 2:1, 1/4 about 4:1, and 1/8 about 8:1. Change any of those variables and the ratio changes, which is why a light meter matters.
Main Light and Fill Light Ratios
The main to fill ratio describes the relationship between the intensity of the main light and the fill light. It is one of the biggest factors in the overall look and mood of a photograph or video.
Ratios are written as numbers, such as 1:1, 2:1, or 4:1. The first number is the main light's intensity and the second is the fill light's. A 1:1 ratio means both lights are equal, while 2:1 means the main light is twice as bright as the fill.
Important: light intensity means the amount of light falling on your subject, not the setting on the flash. To measure the difference between your main and fill accurately, use a light meter and measure each light on its own.
How the Ratio Changes Your Image
- Contrast and mood: a higher ratio, like 2:1 or 3:1, creates stronger highlights and shadows for a more dramatic, moody image. A lower ratio, like 1:1, gives softer, even light with a lighter, more natural feel.
- Depth and dimension: stronger shadows give the subject a more three-dimensional form, helping it stand out from the background. A lower ratio reduces the shadows and can make the scene look flatter.
- Emphasis on the subject: a higher ratio draws attention to the subject with more pronounced highlights. A lower ratio lets other parts of the scene share the spotlight.
- Style: high ratios with strong contrast suit dramatic or artistic work, while lower ratios are common in soft, flattering portrait lighting.
- Creative control: adjusting the ratio gives you control over the whole setup, letting you create very different looks with the same two lights.
Main to Fill Ratio Cheat Sheet
| Ratio | Stop Difference | Description | Uses |
|---|---|---|---|
| 1:1 | No difference | Main and fill equal | Even, flat lighting |
| 2:1 | 1 stop | Main twice as bright as fill | Moderate contrast, adds depth and dimension |
| 3:1 | About 1.5 stops | Main three times as bright as fill | Bolder contrast, deeper shadows |
| 4:1 | 2 stops | Main four times as bright as fill | Strong contrast, ideal for artistic lighting |
| 8:1 | 3 stops | Main eight times as bright as fill | Intense contrast for special creative effects |
Some photographers calculate ratios differently, adding the main and fill together on the lit side. The approach above, comparing the main and fill light directly, is the simplest way to start.
When to Use Each Ratio
1:1: Equal Main and Fill
- Portraits: soft, flattering light that reduces harsh shadows on the face.
- Still life: balanced light with minimal contrast for capturing fine detail.
2:1: Main Twice as Bright
- Portraits: moderate contrast that adds depth and dimension to the face.
- Fashion: defines the subject's contours while keeping a pleasing balance of shadow.
3:1: Main Three Times as Bright
- Dramatic portraits: higher contrast and deeper shadows for a bold, intense mood.
- Artistic work: adds a strong sense of drama to the scene.
4:1: Main Four Times as Bright
- High-contrast portraits: a striking mix of highlight and shadow for an expressive, edgy look.
- Creative lighting: ideal for experimenting with bold effects.
8:1: Main Eight Times as Bright
- Special effects: highly stylised shots where the light itself becomes part of the image.
- Experimental work: unconventional setups with a distinct visual impact.
These ratios are starting points. Adjust them to suit the scene, the subject, and your creative vision. Once you understand how the main and fill work together, you can create almost any look with just two lights.
Studio Flashes for Your Main and Fill
A few flashes from our range that work well as a main light, a fill light, or both.
Studio staple
Godox SK400III-V
A 400Ws mains-powered studio monolight, a solid main or fill for a home studio.
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Compact PRO strobe
Godox AD300Pro II
A compact all-in-one battery strobe that works in studio or on location.
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Serious power
Godox AD600Pro II
A 600Ws TTL battery strobe with the output to drive large modifiers as your main light.
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Need Help Setting Up Your Studio?
CameraStuff has been lighting South Africa's creatives since 2006, with genuine stock and local support. Tell us what you shoot and the size of your space, and our team will help you put together the right main and fill setup. Pop into our Randburg showroom to see the lights in person, or get in touch online.
Visit Our StoreMain and Fill Light FAQs
What is the difference between a main light and a fill light?
The main light, or key light, is the brightest light and shapes the subject with highlights and shadows. The fill light is softer and weaker, placed on the opposite side to lift those shadows and control contrast.
What is a good main to fill ratio for portraits?
A 2:1 ratio, where the main light is one stop brighter than the fill, is a flattering starting point for most portraits. Go to 1:1 for very soft, even light, or 4:1 and beyond for more drama.
Do I need a light meter to set lighting ratios?
For accurate ratios, yes. A light meter measures the light actually falling on your subject from each light, which the power setting alone cannot tell you, because distance and modifiers change the result.
How many stops is each flash power setting?
Each step halves or doubles the output, which is one stop. Going from 1/1 to 1/2 is one stop, and 1/1 to 1/64 is six stops.
Conrad Knuist
Head of Marketing and Photography Specialist
Conrad Knuist is Head of Marketing and Photography Specialist at CameraStuff, South Africa's specialist camera and lighting retailer. With over 18 years of hands-on experience across portrait, event, and commercial photography, Conrad helps South African photographers build studio lighting setups that work.
Comments (2)
This was enlightening, thank you so much as a beginner photography I needed this lesson. It so helpful please continue to teach us this mysteries
This is a very well-presented lesson and have helped me a lot to get the setup right. Could you please advise the distances from the object i.e., camera to object, main light to object, and fill light to object.