Theoretically, all lenses, when set to the same f/stop, should deliver the same amount of light to the film/sensor. In practice, there are differences in effective light transmission, due to different optical designs, and different materials and coatings.
Below find a table of observations of differences between lenses. Since most of us don't really have the equipment to do these tests scientifically, we'll do them in terms of comparisons between lenses.
- Shoot under the same conditions
- At the same subject
- At the same or similar focal length
- With the same settings. (Meaning, bracket shots with each lens covering the same range of settings, adjusting ISO or shutter consistently.)
|Lens 1||Lens 2||Results||Observer||Sample Set/Thread|
|35mm f/2 O||35mm f/1.4 AIS||35mm f/1.4 AIS 2/3 stop faster at f/2||Kukkurovaca||Link|
|50mm f/1.4 S||35mm f/1.4 AIS||35mm f/1.4 AIS slightly faster at f/1.4||Kukkurovaca||Link|
|105mm f/2.5 K||55-200 VR||55-200 VR roughly 1/3 faster at f/5.6||Kukkurovaca||Link|
|50mm f/1.8 AF-D||18-55||18-55 faster at f/5.6||Eddie Barksdale||Link|
|180mm f/2.8 P||55-200 VR||At f/5.6, minimal difference. At f/11, 55-200 somewhat (1/2-stop?) faster.||Kukkurovaca||Link|
|35mm f/2.8 S||50mm f/1.8D||50mm f/1.8D roughly 1/2 stop faster at f/2.8||David Chu||No Pictures|
|50mm f/1.4 S||50mm f/1.8D||f/1.8D very slightly (less than 1/3 stop) faster at f/2.||Kukkurovaca||Images pending|
|35mm f/1.4 AIS||50mm f/1.8D||35mm f/1.4 as fast at f/2 as the 50mm f/1.8 AF-D at f/1.8.||Kukkurovaca||Images pending|
|300mm f/4.5 Late non-AI||400mm f/5.6 ED AIS||Very little, if any, difference in transmission||Kukkurovaca||Images pending|