I have a telephoto lens made to screw on to another lens. Which one should I attach it to?

Question

I have a prime lens, a zoom lens and a telephoto lens. My Telephoto lens is made to screw onto another lens. Should I attach it to the prime lens or does it matter?

Answer 1

The lens you have that goes on the front of another lens is not, technically, a a telephoto lens. That's a specific kind of lens design made to give a narrow field of view. Usually, when we use the term "telephoto lens", we mean any lens with a narrow field of view (a focal length of 100mm or higher, give or take.) What you have is a "secondary" lens, sometimes called a "frontside teleconverter".

As a general rule, these are of very low quality and might be worse than just taking your photograph with the basic lens and cropping. See Is it worth buying cheap lens attachments for my camera? for more.

All that said, you ask:

Should I attach it to the prime lens or does it matter?

Well... it's probably meant to attach to the filter threads on your "real" (non-secondary) lenses. If it fits on both, give it a try — there's nothing to lose (except the image quality of your shots). But, lenses have different filter thread diameters, and you'll need to match that of your secondary lens to that of your other lenses. See How do I find the right size of filters for a lens? for how to figure that out.

Answer 2

Should I attach it to the prime lens or does it matter?

That all depends on some things you haven't told us:

• What is the thread size of the "teleconverter" lens adapter?
• What are the thread sizes on the front of your two lenses?
• What is the focal length and maximum aperture of your prime lens?
• What is the focal length range and maximum aperture range of your zoom lens?
• How far away is the subject you want to photograph?
• How big is your intended subject?

What you describe is not an actual telephoto lens. It is a spin-on lens that makes another lens have a narrower angle of view. This is the same thing as saying it gives your camera more magnification or "zoom".

In general, assuming the spin-on lens adapter has threads that fit both of your lenses:

• If your intended subject is far away and relatively small so that you want maximum magnification, try the adapter with the lens that has the longest focal length. If you have a 50mm prime lens and an 18-55mm zoom lens it won't make much difference. But if you have a 50mm prime lens and a 10-22mm zoom lens, you'll get a lot more "zoom" with the 50mm + magnifying lens than you would putting it on your 10-22mm. On the other hand, if your zoom is an 18-85mm zoom, it has a longer focal length at maximum zoom than your 50mm prime.
• If low light is a concern, try it on the lens with the largest maximum aperture which will most likely be your prime lens.
• If your subject is very small and you want to shoot as close as possible to make it as large as possible in the photo, use the lens that has the closest minimum focus distance (MFD). This could be either your zoom or your prime, depending on exactly which zoom and exactly which prime lens your have.

In the end, spin-on magnifying lenses usually aren't every good in terms of image quality. But they can be fun to play with at times. So why not try it on both of your lenses and see what you can get? It might help you find what you need for the kind of pictures you want to take when the time comes to buy an actual telephoto lens.