Originally posted by reh321 Are you saying that for a given sensor size, reducing the registration distance increases the size of the lens?
If that is true, then for a given registration distance, does reducing the sensor size decrease the size of the lens?
It is a bit more complex than that:
- Basically when you are near to the registration distance the lenses can be very small
- When a focal length is significantly shorter than the registration distance, you need a retrofocus design. The shorter the focal length and the more distance to the registration distance, the bigger it become.
- When a focal length is significantly longer than the registration distance, you need tele design. A naive design would make the lens the size of the focal length minus the registration distance. A tele allows to do better.
- Of course this should be all else being equal... Apperture, image circle.
Sony E registration distance is 18mm. K mount is 45.5...
Example of Sony A7 tele lenses vs K-mount equivalent.
- Sony FE 55mm f/1.8, 70mm long, 281g ... Same length as DA*55 f/1.4 that is 70mm long too but half a stop faster. Still longer than FA77 f/1.8 that has longer focal length and same apperture. 48mm long, 270g.
- Sony FE 90mm macro f/2.8 130mm long, 602g. Much longer/heavier than 100mm macro WR 80mm long, 340g even through the Pentax focal length is longer.
Here the shorter registration distance is not an asset, it is drawback: it make your lenses bigger.
Example of Sony A7 moderate WA:
- Sony 28mm f/2 60mm long 200g vs FA31 f/1.8 69mm long, 345g. The FA31 is a bit widder but the focal length start to show the difference. The longer registration distance add complexity for K-mount.
Then the issue is because Sony A7 is a FF, it need basically longer focal length for the same framing. So while it is good it can have sightly smaller 28mm and that this benefit grows for 24mm and less, you are likely to want something that goes up to 200mm at least and a prime for portraiture is more going to be in the 85mm-135mm range...
Anyway, let's look how Sony FE zooms fare:
Sony 24-70 f/4, 424g, 94.5mm long vs Cannon 24-70 f/4, 600g, 93mm long
Sony 70-200 f/4, 175mm long, 840g vs Canon 70-200 f/4, 172mm long, 705g.
Indeed on the zoom there less difference: they are the same size even through it is interresting to note that is the transtandard is lighter than the Cannon, the 70-200 while being the same size is heavier than the Canon. Strange when you think the A7 familly argument is the size/weight.
One thing is sure through, the short registration distance on Sony FE mount didn't provide the gain it was supported to bring. Only the camera body is smaller, but the lenses tend to be bigger. To me this is because they designed the mount to be for APSC first. For an FF, 18mm is too short, the increased need for short focal length negate all the gains you could have for a few UWA.