Try this 10-second test. Take one of your favorite portrait prompts and swap “85mm” for “18mm”. Hit generate. If the two images look almost the same, your model is ignoring the lens and just nodding along. Welcome to the club, Captain.
A Redditor (u/ItsMia06) on r/PromptEngineering ran a proper experiment on this, and the fix is simpler than you’d think. It takes about ten minutes, a single prompt, and a little patience while you wait for six renders.
🎯 The Challenge: Run the Lens Ladder
The idea is to lock everything down except the lens. Same subject, same outfit, same location, same camera (a Sony A7 IV), same aperture (f/5.6). Then change only two things: the focal length and the camera distance. Write your base prompt once, copy it six times, and edit only the lens line and the distance line in each copy. That’s the whole trick.
Here’s the ladder, step by step:
- 18mm (ultra wide): close camera, exaggerated foreground, strong perspective expansion, distant background looks smaller. Think a hand or shoulder looking oversized while the skyline shrinks behind it.
- 24mm (wide): fairly close, dynamic environmental portrait, surroundings clearly visible. Great for a street scene where the place is half the story.
- 35mm (environmental): moderate distance, subject and background in balance, natural perspective. This is the “documentary photographer” look.
- 50mm (standard): natural framing, balanced proportions, moderate distance. It’s the closest to how your eye sees things, so it makes a handy baseline.
- 85mm (portrait telephoto): longer distance, flatter facial perspective, background elements look larger next to the subject. Faces get more flattering here because noses and ears stop poking out.
- 135mm (telephoto): camera far away, strong background compression, flattened perspective, big distant architecture. Buildings and mountains stack up right behind the head like a painted backdrop.
See the pattern? Each line describes what the lens does to the picture, not just its name. When you write your own, borrow that formula: lens name, then camera distance, then what happens to the face, then what happens to the background.
🔍 The Secret Sauce: Camera Position
Here’s the part that makes this work. A lens doesn’t create perspective. Camera position does. If you want the subject to stay about the same size in frame, you move closer with wide lenses and step back with long ones. Picture a photographer physically walking backward as the lens gets longer. That walk is what changes the look, and your prompt needs to describe it.
That’s why “shot on 85mm” alone gives mushy results. The model gets a number but no sense of where the camera is standing. Words like “camera two meters from the subject” or “photographer standing far back across the street” give it something concrete to draw.
The OP also adds this guard instruction to every prompt:
Keep the same subject, outfit, pose, location, lighting, camera model, and aperture. Maintain approximately the same subject size by adjusting camera-to-subject distance. Do not simulate focal length changes by simply cropping or changing background blur.
That last sentence stops the model from cheating with a crop or a blur. Paste it at the end of every rung so each image gets the same ground rules.
📊 What Your Results Mean
- All six images look the same: the model is reading the lens name as decoration. Add the distance and perspective words.
- Backgrounds shift but faces stay identical: the model is faking it with blur or cropping. Add the “do not simulate” line.
- Wide shots look warped and tele shots look flat: that’s the win. The model is finally respecting perspective.
- Subject size jumps around: you forgot the distance instruction. Tell it to hold the subject size steady.
- Only the extremes (18mm and 135mm) look different: that’s common. Lean harder on the middle rungs by spelling out the distance in meters.
One honest note from the OP: this doesn’t guarantee physically accurate lens simulation. It just gives the model far more to chew on than a bare number. And one commenter (u/trite_advert) nailed it: most people slap “85mm” on a prompt and wonder why everything looks alike.
💡 Extra Tips
- Change one variable at a time. Two changes at once and you learn nothing.
- Keep a seed or reference image if your tool supports it, so the face stays put.
- Save your best ladder as a template and reuse it for every new subject.
- Use 18mm to 24mm for scenes where the place matters, and 85mm to 135mm when you want the person to own the frame.
- Line the six images up in a grid and compare them side by side. Differences you miss one at a time jump out when they sit together.
- If one rung keeps failing, rewrite only that line and leave the rest alone, so you know exactly what fixed it.
🏴☠️ Your Mission, Captain
Run the full six-lens ladder on your own subject this week, then tell us which focal length surprised you most! Drop your results in the comments and let’s see whose model actually listens.
Frequently Asked Questions
Q: Why does adding ‘shot on an 85mm lens’ often change nothing?
Models don’t measure optics, so a lens number is only a loose hint. They respond better to the picture the number suggests: how far back the camera sits, how large the background looks, and how much of the scene is visible. Describe those things in plain words, like ‘camera stands back, background elements look large and close,’ and the results become more predictable.
Q: Do different AI models respond to distance cues the same way?
The test in this post used one setup, so it doesn’t answer that question. Commenters asked the same thing, and the answer will likely vary by model. Run your prompt through each model you use and compare subject size and background scale side by side. If a model ignores distance language, add a sentence about where the camera sits relative to the subject.
Q: Why does a long lens up close just look like a crop?
Telephoto compression comes from distance, not from the lens number alone. Moving in close with a long lens gives you a tight crop with close-range perspective, not the flattened look you’d expect. To get that telephoto feel, pull the camera back and let the focal length do the framing.
Testing 6 focal lengths in AI image prompts: 18mm vs 135mm
by u/ItsMia06 in PromptEngineering