Ramblings on Focal Length

I’ve been looking at the Sony A7s as a potential replacement for an ageing canon APS-C DSLR. However the range of FF1)full frame  – sensor is the same size as a 35mm film frame lenses available for the e mount is not great. I enjoy taking portraits, especially with the great value canon 50mm f1.8 mk ii. This gives an FF equivalent focal length of 80mm on an APS-C body. I found this to be a useful focal length for taking close portraits.

However there isn’t an e-mount prime over 55mm yet. I had wondered if I could get close and use the 55, but further research shows that getting closer with a wider lens does not give the same results as a longer focal length.

The following striking gif from Micaël Reynaud shows this effect. The block is a consistent size within the frame, but the resulting images vary massively. Mostly because the changing focal length affects the field of view as much as the size of the main subject.

Click to show Micaël Reynaud's focal length comparison gif


This gif also shows the relationship between working distance2)distance from camera to subject and compression. As the working distance and focal length increase, the distance between the block and the houses appears to decrease. Another interesting effect is that the relative sizes of foreground and background objects change, so the wider the focal length, the larger foreground subjects appear relative to background subjects. This idea is important for portraits as faces are not a flat plane but have various protrusions which may be amplified when using too wide a lens.

The following example from Anna Wu demonstrates the dangers of facial distortion when using a wide angle lense. Click the image to view her post which contains comparisons at intermediate focal lengths.

Focal length comparison by Anna Wu

Focal length comparison by Anna Wu


So please Sony, Zeiss or Sigma3)art e-mount?? produce an e-mount 85mm prime.

Further information


After some further research, I chose the Fuji X system. Still APS-C but with a good, ever increasing line of lenses, and great balance of size/weight to functionality. Yes, APS-C cameras have larger DOF for the same equivalent focal length, but the XF56/1.2 is shallow enough for me – too shallow for most shots.

Notes   [ + ]

1. full frame  – sensor is the same size as a 35mm film frame
2. distance from camera to subject
3. art e-mount??