You can think of this as using the camera to photograph through a window that has wavy glass. And because the increased magnification is between the subject and the aperture, it also equally magnifies/increases the size of the entrance pupil (effective size of the aperture opening as seen by the subject). It increases the magnification of the scene prior to entering the lens. it does not convert or change the lens' optical formula in any way. If your added magnification is at the objective end it is a diopter or telescope type addition. I'm not sure I understand the description/question. Or tell your camera your converter's magnification factor in its menus, but that is rarely available. Now here is another rub with cheap teleconverters: they tend to lose more resolution via their optical quality than they gain by enlarging the image.Īnother problem with those converters is not really much of their fault: image stabilisation will undercompensate since it doesn't expect the image to move as much as it does with a tele converter in front. So you are more likely in the 120mm/2.8 area (assuming that the glass and coatings are good enough not to cause significant light loss). Once fantasy numbers come into play, they may try to justify themselves as indicating the area size factor rather than linear size. Typical factors for realistic converters with reasonable quality are 1.4, 1.5, 1.7. Now here is the rub: almost everything marketed as a "2x teleconverter" for the front of the lens is quite far from actually being a 2x teleconverter. A 2× teleconverter would thus turn an 80mm/2.8 lens into a 160mm/2.8 lens. Having said all that, I bet that you won't be able to see the converter front lens edge, and that the exposure-time experiment will result in a factor of 4.Ī teleconverter attaching to the front of your lens and not vignetting will retain the aperture number (that is quite different from a tele extender put between lens and camera body). If there's a factor of 4 between the two times, the classical calculation applies, if it's a factor of about 2, your 40mm-based calculation is correct. If you don't, that means that the 30mm opening of the base lens still is the limiting factor, and the 30mm calculation will probably give better results.Īnd, if you want to do some experiments, compare the exposure times for full-open shots with and without the converter (of course, in a constant-lighting situation). If you can fully see the circular edge of the converter's front lens, then the 40mm-based calculation is indeed valid (all the light collected on that 40mm circle reaches the sensor). Or, to put it another way: Does all the light collected in your converter's front lens reach the sensor, or is some part of it blocked by the front opening of your base lens? If part is blocked, then the large converter front lens doesn't help and is just a waste of material.Īs a quick check, you can detach the combo from the camera body and look into it from the rear side, a few centimeters behind the lens, roughly where you'd expect the sensor. The relevant question is where the light path is effectively limited. With a rear-mount converter, it's always the factor of 2 (unless there's a gross mismatch between lens and converter), but with your front-mount one, it's not that simple.
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