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Originally Posted by netwolf
I need a Camera 101 class. How to the F ratio's in a camera work, and how are they adjustable. What is 200mm? Is there a good intro somewhere?
Regards.
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Hi Netwolf
I too find this confusing, and difficult to move between telescopes and camera lenses. The use of the term aperture for photographic lenses and telescopes is potentially confusing.
For a
telescope, the aperture or diameter of the objective or mirror, is (usually) fixed. That is;
- A 10inch F5 dob has an aperture of 250mm and a focal length of 1250mm.
- An 8inch F10 SCT has a 200mm aperture, a focal length of 2000mm.
- A 6 inch F6 Mak-Newt has a 150mm aperture and a focal length of 900mm.
- A 4inch F9 refractor has a 102mm diameter aperture and a focal length of 918mm.
If you place an F6.3 Reducer/Corrector on the 200mm SCT, the aperture remains the same. That is, it is still an 8 inch SCT. However, the focal length is now effectively 2000mm x 0.63 = 1260mm and it is now an 8” F6.3 system.
Now let’s look at
telephoto lenses on cameras:
- My 200mm F4-F32 telephoto lens for my Pentax has a focal length of 200mm.
- My 135mm F2.8-F22 telephoto lens for my Pentax has a focal length of 135mm.
- My 50mm F1.8-F32 standard lens for my Pentax has a focal length of 50mm
- My 28mm F4-F22 wide angle lens for my Pentax has a focal length of 28mm.
The above 4 lenses all have a built in diaphragm or Iris. On the 200mm, the Iris is “wide open” at F4 meaning that the lens is operating at full aperture. When I twist the aperture ring to say, F32, this “stops” the lens down making it “slower”. The focal length does not change, it still is a 200mm telephoto lens, but now, only 1/64 of the light is getting through to the ccd chip. As we “stop down the lens” in steps of 1 stop on the aperture ring, we halve the amount of light reaching the ccd.
F4, F5.6, F8, F11, F16, F22, F32
On a telescope, there is no Iris, so it is always “wide open”, as the aperture cannot change, unless you place a sub-aperture mask over the objective.
Now here’s where it gets really confusing. In photography, a numerically large F-Ratio of say, F11 only lets in 1/16 of the amount of light compared to a numerically small F-Ratio of F2.8. How come, ‘cos the number 2.8 is smaller than the number 11? Well, in terms of light gathering, F2.8 is considered “larger” than F11 as it lets in light 16 times faster, so your camera shutter speed at F2.8 will be 16 times faster than at F11 to record the same amount of light.
So, an F2.8 lens is considered “faster” than an F4 lens and lets you shoot in lower ambient light, but you usually pay a lot more for a faster lens, as it has a larger aperture i.e. a bigger piece of expensive glass at the front end.
Hope that helps.
Cheers
Dennis