Comparing Background Separation of 85mm Lens on Crop Sensor with 135mm Lens on Full-Frame

The concept of background separation or depth of field is a crucial consideration for photographers, as it significantly affects the aesthetics of an image. This article breaks down how an 85mm lens on a crop sensor compares to a 135mm lens on a full-frame camera, examining key factors such as focal length, sensor size, and distance to the subject.

Focal Length and Sensor Size

The effective focal lengths of lenses on different camera sensors are crucial for determining background separation. A 1.5x crop factor for an 85mm lens on a crop sensor translates to an effective focal length of approximately 127.5mm (85mm x 1.5). In contrast, a 135mm lens on a full-frame camera provides a true focal length of 135mm. While the 85mm on a crop sensor has a slightly shorter focal length, the difference is not substantial when comparing them directly.

Background Separation and Depth of Field

Background separation and depth of field are strongly influenced by the focal length, aperture, and the distance to the subject. Longer focal lengths generally produce shallower depth of field, which can enhance background separation. It is essential to use the same aperture setting across both setups to accurately compare their effects.

Comparative Analysis

The effective focal length of the 85mm lens on a crop sensor (127.5mm) is quite close to the 135mm lens on a full-frame camera. If both lenses are used at the same aperture and the subject distance is the same, the background separation should be quite similar due to the comparable effective focal lengths. However, the actual framing and depth of field can differ slightly due to the crop factor.

Theoretical vs. Practical Considerations

Theoretically, if you use a 33.75mm aperture for the 85mm on a crop sensor (since 85mm/33.75 f/2.5), you should get a similar background separation to a 135mm f/2.8 lens on a full-frame camera. However, practical limitations often prevent achieving this perfectly. For instance, crop sensors often do not have fast enough lenses to match the same aperture as a full-frame equivalent.

To achieve a similar aperture, you often need to adjust the lens to an f/2.8 setting on the full-frame camera. When you do this, while the results are quite close, small differences in framing and depth of field can still be noticeable.

Hyperfocal Distance and Depth of Field

Using a hyperfocal distance and depth of field calculator can provide concrete numerical values. For example, a Nikon crop sensor camera with an 85mm f/1.8 lens at a subject distance of 10 feet has a depth of field of 0.29 feet. A Nikon full-frame camera with a 135mm f/2.8 lens at the same subject distance of 10 feet has a depth of field of 0.27 feet. These values indicate that the depth of field is approximately equal but not exactly the same.

Furthermore, if you want to achieve the same framing of a subject (e.g., from head to waist), you need to move farther away with the crop sensor camera to capture the same amount. At 15 feet, the depth of field for a crop sensor camera becomes 0.66 feet, which would be noticeable in a photograph.

Conclusion

While the background separation with an 85mm lens on a crop sensor and a 135mm lens on a full-frame camera will be similar under ideal conditions, other factors such as lens design, aperture, and shooting conditions can influence the final look. Always test both setups in your shooting environment to achieve the desired aesthetic. Understanding these nuances will help you make informed decisions and improve your photography.

Keywords: background separation, depth of field, crop factor