Exposure & Focus
While focal length determines how much of the bird fills the frame, the aperture (f-stop) determines the technical quality of the image, specifically the depth of field and the overall exposure.
Depth of Field (DoF)
Depth of Field is the distance between the nearest and farthest objects that appear acceptably sharp in an image. In wildlife photography, especially with long lenses, the DoF can be incredibly thin.
Depth of Field at 100m Distance
Calculated for a subject at 100m. Values represent the total focus zone in centimeters.
| Lens | f-stop | M4/3 (OM-S) | APS-C (R7/R10) | Full Frame (A1 II/R5 II) |
|---|---|---|---|---|
| 400mm | f/4 | \(18.8\text{ cm}\) | \(25.0\text{ cm}\) | \(37.5\text{ cm}\) |
| f/5.6 | \(26.3\text{ cm}\) | \(35.0\text{ cm}\) | \(52.5\text{ cm}\) | |
| f/8 | \(37.5\text{ cm}\) | \(50.0\text{ cm}\) | \(75.0\text{ cm}\) | |
| 600mm | f/4 | \(8.4\text{ cm}\) | \(11.1\text{ cm}\) | \(16.7\text{ cm}\) |
| f/5.6 | \(11.7\text{ cm}\) | \(15.6\text{ cm}\) | \(23.3\text{ cm}\) | |
| f/8 | \(16.7\text{ cm}\) | \(22.2\text{ cm}\) | \(33.3\text{ cm}\) | |
| 800mm | f/4 | \(4.7\text{ cm}\) | \(6.3\text{ cm}\) | \(9.4\text{ cm}\) |
| f/5.6 | \(6.6\text{ cm}\) | \(8.7\text{ cm}\) | \(13.1\text{ cm}\) | |
| f/8 | \(9.4\text{ cm}\) | \(12.5\text{ cm}\) | \(18.8\text{ cm}\) |
The Takeaway: At 800mm and f/4, your focus zone is less than 10cm deep. If the bird's chest is in focus, the head may already be blurred.
The Exposure Triangle
To freeze a Bald Eagle in flight, a shutter speed of at least \(1/2000\text{s}\) is required. Because the shutter speed is fixed, any change in aperture (to increase DoF) must be compensated for by the ISO.
ISO Compensation for Depth of Field
Assuming a fixed shutter speed of \(1/2000\text{s}\) and constant ambient light.
| Aperture Change | Light Loss | ISO Adjustment | Example |
|---|---|---|---|
| f/4 \(\to\) f/5.6 | 1 Stop | \(2\times\) Increase | ISO 400 \(\to\) ISO 800 |
| f/5.6 \(\to\) f/8 | 1 Stop | \(2\times\) Increase | ISO 800 \(\to\) ISO 1600 |
| f/4 \(\to\) f/8 | 2 Stops | \(4\times\) Increase | ISO 400 \(\to\) ISO 1600 |
| f/4 \(\to\) f/11 | 3 Stops | \(8\times\) Increase | ISO 400 \(\to\) ISO 3200 |
The Diffraction Limit
Closing the aperture too far (increasing the f-number) eventually leads to diffraction, where light waves bend around the edges of the aperture, creating a blur that degrades image sharpness regardless of the lens quality.
Diffraction Thresholds by Sensor
The f-stop at which diffraction begins to noticeably soften the image based on pixel size.
| Sensor Model | Pixel Size (approx) | Diffraction Limit | Note |
|---|---|---|---|
| Sony A1 II FF | \(3.76\mu\text{m}\) | \(\approx\) f/8 - f/11 | High density FF |
| Canon R5 II FF | \(4.4\mu\text{m}\) | \(\approx\) f/11 | More forgiving |
| Canon R7 APS-C | \(3.7\mu\text{m}\) | \(\approx\) f/5.6 - f/8 | High density APS-C |
| OM-S M4/3 | \(3.3\mu\text{m}\) | \(\approx\) f/5.6 | Most sensitive |
Recommendation: For maximum sharpness, try to stay within 1-2 stops of your lens's maximum aperture. Avoid f/16 or f/22 unless absolutely necessary for depth of field.