Long and short on teleconverters

When you're a wildlife photographer working in a public park, the equipment you use undoubtedly draws attention. This summer, I've been documenting the development of a pair of young bald eagles. To get the images I need without disturbing the birds, I've been using a 600mm lens – a 13-pound monster of a lens that has a front element only slightly smaller than a dinner plate.

A small number of people come up and ask me questions about the birds. Many, many more grill me about my equipment. The vast majority say something like, "I bet you can see every nose hair with that." I cannot. In terms of magnification, the lens falls in between a pair of binoculars and a birder's spotting scope. The lens is physically big because it lets a lot of light in allowing me to capture action images at high resolution.

It's the second most common question that I'm going to address in this blog post. It comes from amateur photographers who want to know about my use of a teleconverter with this lens.

A teleconverter is essentially a small secondary lens. It's basically a magnifying glass that fits between a lens and the camera body, magnifying the power of the lens. Teleconverters typically come in two strengths: 1.4x and 2x. To make the math simple, if you added a 1.4x teleconverter to a 100mm lens, it would give you a focal length of 140mm. A 2x teleconverter, also known as a doubler, would double the magnification of the lens to 200mm.

As with anything in photography, however, there are tradeoffs. That extra magnification comes at the expense of the volume of light passing through the lens. While a 2x teleconverter doubles the magnification of the lens, it also cuts the amount of light reaching the camera in half. Unless it's a really bright sunny day, that sharp reduction in light can pose some challenges in capturing sharp, action images. When you use a doubler, you also have to double the shutter speed.

The photography questions I get are all related to the second tradeoff involved in using teleconverters: their impact on the sharpness of the image.

Using a teleconverter does reduce the quality of the image somewhat. The teleconverter magnifies the center of the lens. Any flaws in that original lens will be magnified. You may not notice those flaws when using the lens at its intended focal length, but they will appear when you try to capture twice as much detail through a small section of glass.

That is what I try to explain to photographers who want to buy a cheap 300mm or 400mm lens, add a teleconverter to it, and expect the results that I get from my equipment. A doubler cannot improve the quality of your lens; it just magnifies the output coming out of it. If you start with a marginal lens to begin with, you're going to be very disappointed with the quality the teleconverter produces. Doubling marginal quality doesn't produce good quality; it results in lower quality.

The other questions I get are from photographers who understand the quality impact and are puzzled that I would be satisfied with the quality loss. First, I am starting with a very, very sharp lens. The quality loss is minimal. Second, to avoid doing any harm to the young eagles, I observe them from a distance of 150 feet or more. The teleconverter is my only option for giving the birds their space while still being able to get the close-up images I need. Canon no longer makes a 1200mm lens – I couldn't afford the $100,000 lens anyway – and 600mm/teleconverter combination results in a clearer image than I would be able to get by cropping into and using software to enlarge a tiny section of an image from the 600mm lens by itself.

Unlike other forms of art where you can create whatever image you want, with photography you have to make do with what you have. To do that, it's critical you understand the limitations and tradeoffs.

If you're serious about wildlife photography, but don't have the budget for a new, powerful telephoto lens, you will likely produce better images – and be happier producing them – by saving for a used telephoto lens than you will by adding a teleconverter to a weak lens.

If you already have a sharp telephoto lens, understand that using a teleconverter is sometimes the best option in a difficult situation. I will always choose a slight loss in image quality over any loss of life.

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