When I attach a 4x magnifier to my 75-300 lens it won't focus on anything.

When I attach a 4x magnifier to my 75-300 lens, it won't focus on anything, even a mile away.

Answer:

The reason your lens will not focus correctly is most likely because it does not have enough light to do so. A 75-300 lens is normally a variable aperture lens meaning that as the lens is zoomed the largest aperture the lens can open to changes. These lenses are generally considered to be "slower" lenses meaning they require more light than a "fast" fixed aperture professional-grade lens does. This is why they are not suited to shooting in lower light conditions or with lens extenders attached to them.

All cameras require a certain amount of light to focus properly. The amount of light needed for your camera to auto-focus depends on the brand and type of camera but all cameras even the high-end professional models will not auto-focus without adequate light reaching the focus sensor.

Adding a teleconverter or magnifier to a lens reduces the amount of light the lens passes through to the sensor. Typically a high quality 2x extender will reduce the available light by one half. Therefore the effective aperture of your lens is decreased. For example, a 3.5F aperture with a 2x teleconverter essentially becomes a 7F lens. With a 4x extender your effective aperture would be reduced to F14 which is too small to allow your camera to auto-focus.

Another drawback of using teleconverters or lens magnifiers is that another side effect is they often reduce the sharpness of the lens. The combination of not enough light and reduced sharpness of the image would be enough to cause the issues you are having with focusing when using a 4x magnifier on your lens.

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