Correct recalculation of EGF, when it is done and for which lenses. Why does the conversion need to be performed even for lenses originally designed for crop cameras? This is the most frequent question that is asked here in the comments on Radozhiv or to me in private messages. I answer this question so often that I have come up with my own set of simple and uncompromising rules on this matter.
Rule 1: EGF (Equivalent Focal Length) is linked to the frame size 36 X 24 mm.
It is the EGF that is recalculated, but not the focal length (FD). Everything is recalculated / reduced exactly under the frame 36 X 24 mm (full frame, narrow format, small format photography).
EGF is used for convenience. The 36 X 24 mm frame was chosen because it is one of the most common frame sizes. The EGF calculation provides a quick understanding of the viewing angles of a particular lens. Photographers are not used to operating with viewing degrees or angular field; it is much easier to express themselves in terms of FR, EGF and physical frame size.
Rule 2: the lens always indicates the focal length (FD), not the EGF. Focal length (FR) is a physical parameter and does not depend on the size of the used frame.
Various markers in the lens name such as Nikon DX, Canon EFS / EF-S, Pentax DA, Sony DT, Konica Minotla DT, Sony E, Sigma DC, Tamron DI II, Tamron DI III, Tokina DX, 7Artisans APS-C ( they are different for each manufacturer) indicate that the lens can only project an image onto a circle / rectangle no more than a certain radius / diagonal. Crop factor (Kf) of this or that lens or sensor just shows how much this diagonal is larger or smaller than the diagonal of a 36 X 24 mm frame.
An important example: 'DX' marker on lens Nikon DX AF-S Nikkor 35mm 1: 1.8G SWM Aspherical for DSLR cameras with APS-C sensor, which use crop factor Kf = 1.5X, does not meanthat this lens gives a 35mm EGF when used on a cropped camera with an APS-C sensor with Kf = 1.5X (Nikon DX format).
35 mm is FR, not EGF. The Nikon DX mark only means that this is a lens that cannot project images onto a 36 X 24 mm full-frame sensor (Nikon FX), but can only work with cropped sensors no larger than Nikon DX / APS-C Kf = 1.5X.
In fact, this is only an indication / recommendation to use this lens exclusively on Nikon DX series cameras / cameras (which crop factor Kf = 1.5X). Therefore, the EGF of such a lens on a Nikon DX camera will be 52.5 mm (35 * 1.5). The same remark applies to all other systems and other markers described above.
Rule 3: EGF recalculation is always performed for all lenses. To find the EGF, you need to multiply the focal length of the lens (FR) by crop factor camera (Kf, it is also called “focal length multiplier”).
Simplification for the rule: as crop factor of full-frame cameras with a frame size of 36 X 24 mm is equal to one, then it can be argued that in this case the numerical value of the FR will be equal to the numerical value of the EGF. Large and medium formats have Kf <1. A full frame has Kf = 1. Cropped cameras have Kf> 1.
Example 1... A lens designed for use on cameras with a cropped (reduced) sensor. Nikon DX VR AF-P Nikkor 18-55 mm 1: 3.5-5.6G... DX marker.
- EGF on cropped cameras with an APS-C sensor and a crop factor of Kf = 1.5X (Nikon DX) will be 27-82.5 mm.
- The EGF on cameras with a full-frame sensor and a crop ratio of Kf = 1X (Nikon FX) will be 18-55 mm (but the lens will not be able to fully work).
- EGF on medium format cameras and crop coefficient Kf = 0.82X will be equal to 14.76-45.1 mm (but the lens will not be able to fully work).
Example 2... A lens designed for use on full frame cameras. Sigma 50 mm 1:1.4 DG ART... DG marker.
- EGF on cropped cameras with an APS-C sensor and a crop factor Kf = 1.6X will be 80 mm (50 * 1.6).
- EGF on cameras with full-frame sensors with a crop factor of Kf = 1X will be 50 mm (50 * 1).
- EGF on medium format cameras with a crop coefficient Kf = 0.55X will be equal to 27.5 mm (50 * 0.55, but the lens will not be able to fully work).
Example 3... A lens designed for use on medium format cameras. FUJINON GF LENS 50 mm 1: 3.5 R LM WR... GF marker.
- EGF on cropped cameras with Micro 4/3 sensor and crop factor Kf = 2X will be equal to 100 mm (50 * 2).
- EGF on cameras with full-frame sensors and crop coefficient Kf = 1X will be 50 mm (50 * 1).
- EGF on medium format cameras and crop coefficient Kf = 0.79X will be 39.5 mm (50 * 0.79).
The focal length is the number or numbers that are written on the lens itself. For some reason, manufacturers of interchangeable optics are not accustomed to writing EGF, which is why a lot of misconceptions and disputes arise among amateur photographers. Only rare compact cameras write several numbers for their lenses - with FR and side by side with EGF, which is very convenient. Smartphone manufacturers often write only EGF.
Materials on the topic
- Matrix size matters
- Crop identification
- Resurrection of the crop
- Relationship between focal length, viewing angle, and focus distance
- Spherical mirrorless crop in vacuum
- What is the focal length of lenses
- The dependence of the viewing angle of the lens on the focusing distance (Focus Breathing)
- About choosing a fixer for a crop - 35 or 50 mm