As always, click the photo to view at full size
A white-eyed vireo whisper-sings as he works through a dense snarl of vegetation. I made this image yesterday morning in Hocking County, Ohio, before the sun had fully illuminated the vireo's shady haunts. Experienced photo critics will notice the flaws in this image, perhaps most notably the graininess. Even at a low shutter speed of 1/400 - given the lens I was using and the bird's movement - I was still at an ISO of 3200 - far higher than I'd want. Good noise reduction programs can't completely vanquish digital noise. Even with excellent full frame cameras, I really don't like the ISO rising above 800, and 500 or lower is far better yet.
Low light and high ISO settings are not the point of this post, though. The lens and its accouterments is. I shot this with Canon's remarkable 400mm f/2.8 II, which is great for low light shooting and insanely fast at attaining focus and holding it. But 400mm is at the low end of reach insofar as birds go. Birds are often tough to approach closely, and oftentimes size matters. The bigger the lens, the better.
I have both 1.4x and 2x teleconverters (Canon version III). Teleconverters are stubby little magnifiers that can be quickly mounted between the lens and camera, and increase magnification by the power indicated on the converter. In this case, my 400mm lens becomes a 560mm lens with the 1.4x teleconverter, and obviously an 800mm lens with the "doubler" mounted. Voila! Like magic, we're in truly big lens territory, and much more able to hunt those bashful birds.
But few things come without cost. For sure, the teleconverters are WAY less expensive than are huge lenses, but they exact a different price. One, you're shooting through more glass - the teleconverters have internal lenses - and the more glass one shoots through, the more the image quality suffers. This deterioration, however, is almost negligible with Canon's 1.4x. It's more noticeable with the 2x, but still the image quality can be quite reasonable even with this super-magnifier. Probably a bigger burden brought on by teleconverters is the loss of aperture. With the 1.4x, you lose one stop, and the 2x causes a loss of two stops. So, with the 1.4x mounted on my 400mm f/2.8, I'm at f/4. With the doubler, f/5.6. If conditions are sunny and light is abundant, this may not be much of a problem - you can still use a sufficiently fast shutter speed, yet maintain a low ISO. In the case of this vireo, that wasn't possible.
For this photograph, I used the 2x doubler, giving me a much-appreciated 800mm.That got me in good working distance to my bashful subject. But at 1/400 shutter speed - what I felt was about the lowest I could go - it gave me that dreaded 3200 ISO. If the doubler did not cause a loss of two stops and I was able to shoot at f/2.8, my ISO would have been 800. But as I said, there's no free lunches.
In great light, especially, teleconverters are a fantastic tool for extending reach. I use the 1.4x all the time, and see nearly no downsides to it. Autofocus is nearly unaffected, the loss of a stop is almost never an issue - ideally, I like to stop down to f/8 or so if light permits - and image quality remains superb. It's nearly magic without strings.
The 2x is a different story, and I usually don't recommend them, as I and many others have found that attaining sharp focus can be difficult with one. Micro-adjustments are usually necessary, and even with such tweaking it can still be hard to make crisp images. Autofocus is lost on some lenses, and if it's still enabled, it's slowed significantly. I've only had one lens where I could just slap on the 2x and take consistently sharp images and that's the Canon 300mm f/4 (600mm with the doubler). Until I got this miracle-working 400mm. With no micro-adjusting, it took perfectly to the 2x teleconverter. Not only that, but autofocus remains incredibly fast, and images look sharp and clear, especially under good lighting.
If you want more reach for birds, it might be worth renting a teleconverter or two. If they mate well to your lens and you're happy with the results, a teleconverter(s) is well worth the investment. The recent Canon version III lenses - like the 400mm and 600mm - are the best yet at mating effectively to teleconverters. They'll only get better, too.