sajeev_chacko Posted April 22, 2002 Share Posted April 22, 2002 Just curious to know it. <p> Its knows that a good (optically, ofcourse) lens give sharper image. But how much role does it plays in getting the correct exposure. Suppose, one shoots a subject, in <b>Av</b> mode at a <b>fixed apperture</b>, <b>same metering</b> and sufficient light, how much gain in shutter speed he gets with a optically good lens over a bad one. Ofcourse, it depends on the lenses. For instance, a 300mm f/2.8 Vs 300mm f/4.0 at f/8 probably won't make much difference in EV. A 75-300 set at 300mm will definitely make. But how much ? Link to comment Share on other sites More sharing options...
preston_merchant Posted April 22, 2002 Share Posted April 22, 2002 Not quite sure I follow your question, Sajeev. Are you asking about lens performance at different apertures? <p> The general rule is that "superior" lenses are sharper and offer better contrast at wide apertures. All lenses have apertures at which they perform the best. The most expensive lenses are designed to perform their best over a wider range of f stops. <p> Shooting with such a lens will mean that your pictures will be very sharp at f2.8. It MIGHT be the case that a cheaper lens delivers the same performance at f8. <p> As you know, wider apertures mean faster shutter speeds, which up to a certain point contribute to sharpness. So, shooting with a cheap lens at f8 might mean you have to use a shutter speed too slow to guarantee sharpness. <p> But optical quality (grade of glass, coatings, number of elements) does not affect exposure value. The meter in the camera determines EV without knowing or caring how good the lens is. Think also of a hand-held meter, which does not have a lens in the same way a camera does. <p> In sum, your camera's meter will return the same EV regardless of the lens--provided the conditions are identical (focal length, ambient light, etc.). The difference in sharpness between a super-expensive lens and a throwaway cheap one is minimal when it comes to the camera meter's ability to determine EV. Link to comment Share on other sites More sharing options...
s._fisher Posted April 22, 2002 Share Posted April 22, 2002 If it matters since SLR cameras like EOS measure light through the lens, if you had some strange horrid lens that was somehow darker than a "good" lens, you would still get the correct exposure since the camera is looking through the lens for how much light there is. Cameras automatically adjusting for filters is a good example of this... Link to comment Share on other sites More sharing options...
sajeev_chacko Posted April 22, 2002 Author Share Posted April 22, 2002 Preston, if you are right, then I got the answer. I didn't want toknow the performance of any lens at any apperture. I wanted to knowwhether a better lens can give higher shutter speed than a low qualitylens at the same apperture. I felt that there should be no difference,since many photographers use hand held meters, especially when theircamera is not capable of doing so. If it works with an external meterwithout the lens, it should probably work with any kind of lens,unless otherwise as pointed by Steven, that the lens isexceptionally bad. <p> Anyway, thanks both of you. Link to comment Share on other sites More sharing options...
isaac sibson Posted April 22, 2002 Share Posted April 22, 2002 By the very definition of aperture, there can't be any difference in EV between two lenses of the same focal length at the same aperture. <p> However, some cheaper lenses misreport apertures or have inaccurate apertures, which could cause variation. Link to comment Share on other sites More sharing options...
s._fisher Posted April 22, 2002 Share Posted April 22, 2002 Are you sure that the "definition of aperature" has something to do with the Ev level? <p> I thought the aperature was the ratio of focal length to aperature diameter (for example see http://members.tripod.com/~Prophotoman/fstop.html). <p> Of course a lens that screws with the Ev levels probably wouldn't be a great seller, but I don't see how (for example) painting over the front element would affect a lens aperature. Link to comment Share on other sites More sharing options...
isaac sibson Posted April 23, 2002 Share Posted April 23, 2002 The definition of aperture is indeed the ratio of focal length to aperture diameter. Thus, for two lenses of the same focal length, set to the same aperture, the amount of light being collected must be the same, barring of course any foolish stripes painted across the front of the lens. I was taking "under the same lighting conditions" as assumed. Link to comment Share on other sites More sharing options...
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