Keep the eyes sharp, not just the background soft.
A fast portrait lens gives you options. Its widest aperture is not automatically the most useful setting for every pose.
A repeatable two-metre portrait
Imagine a person two metres from an 85 mm lens, with a wall eight metres from the camera. Use a 36 × 24 mm sensor and a 0.030 mm acceptable circle of confusion. The wall is six metres behind the subject, not eight. Keep these distances unchanged while adjusting only aperture.
| Aperture | Near limit | Far limit | Total DOF |
|---|---|---|---|
| f/1.8 | 1.972 m | 2.029 m | 5.73 cm |
| f/4 | 1.938 m | 2.066 m | 12.74 cm |
| f/5.6 | 1.915 m | 2.093 m | 17.85 cm |
What these numbers tell you
At f/1.8 there are only about 28 mm of acceptable depth in front of the focused plane and 29 mm behind it. A turned face can put the eyes at different distances. Subject movement or a small focus error can consume that allowance. Eye autofocus helps place focus; it does not increase depth of field.
Stopping to f/4 more than doubles the geometric sharp range in this example. That does not guarantee both eyes in every pose, but it gives you more tolerance. Pin the wide-open shot as setup A, choose f/4 for B, and compare the numerical limits rather than relying only on the preview.
Keep separation without opening all the way
Move the person away from the wall while keeping the camera-to-person distance fixed. The background blur diameter grows toward a finite limit as the wall recedes. This does not change subject magnification or the DOF limits around the face. It changes where the background sits relative to the focused plane.
For a group, bringing faces onto a similar distance plane often helps more than asking the lens to cover a deep staggered arrangement. Increase the f-number, then check shutter speed and light separately: this simulator does not calculate motion blur, flash exposure, or camera shake.
A field test, not a promise
- Focus on the nearer eye and make a reference frame.
- Repeat at f/4, compensating exposure without introducing subject motion.
- Inspect both eyes at your intended display or print size.
- If you still need more tolerance, change pose, distance, or aperture and repeat.
The cutout is a flat teaching subject. It cannot prove that the nose, ears and eyes all fall within DOF. Use the calculated distance interval and a real photograph together.
Next: compare the same portrait across sensor formats, or inspect the equations and assumptions.
Verify the numbers: reproducible reference examples · equations and limitations · publisher and corrections.