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Thread: F-stop measurement across different brands

  1. #21
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    Quote Originally Posted by kegler
    300D kit lens 18mm(1.6x) which is 28.8mm with a 3.5f?
    who taught you that, ought to be shot. apparent focal length has nothing to do with the aperture, it's the sensor crop factor that's important by itself.

    Quote Originally Posted by kegler
    compare as in f828 at 28mm f2.0 and with 300D kit lens 18mm(1.6x) which is 28.8mm with a 3.5f? i think do u think they r on par? or how different?
    maybe on par in terms of amount of light goes into the cam?
    the apparent angle of view would be pretty similar, but the DOF is affected by the obviously smaller aperture on the 828. you have mixed and compared oranges and pineapples this time.

    the 828 has the focal length at the widest end, at 7.1mm. the EF-S 18-55 has the shortest focal length at 18mm.

    the EF-S 18-55 has (at wider end) an aperture of 18/3.5 = 5.14mm diameter wide.
    the 828 has (at wider end) an aperture of 7.1/2.0 = 3.55mm diameter wide.

    the angle of view at both cannot be compared directly, because the EF-S lens has a reduced imaging circle size, that gives the apparent 1.6x sensor crop factor, hence the apparent 28.8mm FOV which gives 74.27 degrees of view, while the 828 at widest gives 75.02 degrees of view. but IF the EF-S was a full image circle lens, it would have been giving 100.11 degrees FOV.

    at ANY given aperture, ANY lenses compared will have the same amount of light going though. the only difference would be the DOF affected by the actual absolute size of the aperture diaphragm.

  2. #22

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    Quote Originally Posted by sehsuan
    who taught you that, ought to be shot. apparent focal length has nothing to do with the aperture, it's the sensor crop factor that's important by itself.



    the apparent angle of view would be pretty similar, but the DOF is affected by the obviously smaller aperture on the 828. you have mixed and compared oranges and pineapples this time.

    the 828 has the focal length at the widest end, at 7.1mm. the EF-S 18-55 has the shortest focal length at 18mm.

    the EF-S 18-55 has (at wider end) an aperture of 18/3.5 = 5.14mm diameter wide.
    the 828 has (at wider end) an aperture of 7.1/2.0 = 3.55mm diameter wide.

    the angle of view at both cannot be compared directly, because the EF-S lens has a reduced imaging circle size, that gives the apparent 1.6x sensor crop factor, hence the apparent 28.8mm FOV which gives 74.27 degrees of view, while the 828 at widest gives 75.02 degrees of view. but IF the EF-S was a full image circle lens, it would have been giving 100.11 degrees FOV.

    at ANY given aperture, ANY lenses compared will have the same amount of light going though. the only difference would be the DOF affected by the actual absolute size of the aperture diaphragm.
    somehow its abit too much for me to absorb....but i got a rough idea of wats going on liao....the first paragraph...i wanna compare 2 lens at the same distance n same F no..... wonder if the difference would be great or not
    but from wat u mention.... i think its hard to compare this way...heehee....
    thx

  3. #23
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    Firstly do people the basic of the F-stop? What make a F-stop. Let get a few basic right! When on the lens or the camera they show F2.8 it is actually stand for 1/2.8 and F22 is stand of 1/22. So by mathmatic rules the aperture open for F2.8 or 1/2.8 is bigger than F22 or 1/22.

    Then poeple will be wondering how do these F-stop number come from. F-stop is a ratio that come from the calculation of diameter of the aperture opening divided the lens focus length. Thus the smaller the aperture opening diameter divided by the focus length of the lens will result in a smaller the f-stop like F16, F22 and F32, which actually is 1/16, 1/22 and 1/32. A lot people confuse by the number thinking F32 is biggger than F16.

    So this calculation SHOULD be same throughout any lens design or formula. But sadly lens manufacter dun alway give the right maxium aperture opening because it could be F2.68 or F2.9 they will state it is F2.8. To conclude all F-stop calculation is base on this formula thus it is no matter what the lens brands or the type of lens the F-stop should be standardize.

  4. #24

  5. #25

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    I have a simple mind.

    Frankly I do not care whether one uses a digital camera, a film camera, a medium format camera, a larger format camera.

    When a lens says that its maximun aperture is f2.8, it means just that. Barring production quality control, all lenses with f2.8 should let in the same amount of light provided the shutter speed is the same. This is the principle of the light meter. If the meter have to take into account whether the camera is a digital one or otherwise, it will be a nightmare! I am not sure if there is such a thing as a light meter made for digital camera.

    However the quality of the light that falls on the sensor, whether it be digital or film, can be different because of things like the number of lens elements that can degrade image, and coatings which can reduce flare.

    Another thing. Looking through the lens, a f4.5 with a shorter focal length can actually look darker than a f8 in one with a longer lens. This is due to the fact that in a longer focal lens, the light is concentrated whereas in a wide angle lens, the light spreads out.

    So when I meter a scene, and it says F2.8 at 1/125, I can use any lens on any camera, and get a correct EXPOSURE. The quality of the image is another thing!

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