F1

[maddog]

Whats the complexity in building F1 lenses. Isin't f-stop just the reciprocal of the aperture. Why can't the aperture be fully opened to give you F1? And why are fast lenses more expensive.

 

[roygoh]

f-stop is not just the reciprocal of the aperture. the f-stop number is given by :

focal length divided by aperture diameter

The bigger the aperture, the bigger the construction of the lense needs to be, thus higher cost.

For a 50mm f/1.4 lens, the aperture diameter when fully open is 35mm. To achieve f/1, then the diameter has to go up to 50mm, which means a more challenging mechanical design.

The price increase will most of the time be exponantial rather than linear.

Now consider a 200mm lens. To achieve a f/1 maximum aperture, the aperture diameter has to be 200mm. The outer diameter of the lens will easily exceed 250mm, if it is even possible to design. That will be a real monster.

Hope that answers your question.

Ian would be able to go into more detail on lens design, if he has the time to do that.

 

[maddog]

Originally posted by roygoh
f-stop is not just the reciprocal of the aperture. the f-stop number is given by :

focal length divided by aperture diameter

The bigger the aperture, the bigger the construction of the lense needs to be, thus higher cost.

For a 50mm f/1.4 lens, the aperture diameter when fully open is 35mm. To achieve f/1, then the diameter has to go up to 50mm, which means a more challenging mechanical design.

The price increase will most of the time be exponantial rather than linear.

Now consider a 200mm lens. To achieve a f/1 maximum aperture, the aperture diameter has to be 200mm. The outer diameter of the lens will easily exceed 250mm, if it is even possible to design. That will be a real monster.

Hope that answers your question.

Ian would be able to go into more detail on lens design, if he has the time to do that.

Whats the point in having a 200mm aperture diameter when the negative is only 35mm?

 

[Jed]

Actually, while the mechanics would be more problematic, that's really not a major source of the problem at all. It's more a case of cost of manufacturing a huge lump of optical glass that is free from blemishes that optically corrects an f1.0 lens. More importantly, the demand for f1.0 optics is very limited and will likely stay as such unless they can drastically reduce the prices of these lenses, which is unlikely to happen unless the prices of lenses fall across the board. Which means, less economies of scale and a higher development cost to be bourne on a per lens basis.

 

[ckiang]

Originally posted by maddog



Whats the point in having a 200mm aperture diameter when the negative is only 35mm?

Lens aperture has nothing to do with film size. The aperture and rear element diameter of the lens determines the amount of light going through the lens. For a 200 f/1, you will need 200mm diameter, regardless of film size.

Regards
CK