Photographing A Chestnut Sided Warbler and Why Maximum Aperture Matters

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Photographing A Chestnut Sided Warbler

This fetching male Chestnut Sided Warbler arrived one late morning to the Service Berry tree in our yard. Lots of bird activity draws even more birds, I believe, and there was partying going on at the feeders.

The light was just right. The histogram revealed a nice balance of dark and light variation. The 1/400 shutter speed captured the moment sharply thanks in part to the relatively low ISO.

What a beauty! This spring specimen flaunts a bright yellow cap, black elongated eye stripes atop a white face, glossy chestnut colored streaks on its flanks and a complementing touch of yellow on the wing bars.

I photographed the female Chestnut Sided Warbler last Fall. I was not so lucky in proximity for that shoot and had to use a 2.0 extender on my 500mm lens to get the magnification I needed.

Photo of Chestnut Sided Warbler
Chestnut Sided Warbler
ISO800; f/6.3; 1/400 Second

Why Aperture Size Impacts Autofocus

Whenever I read about tele extenders, dire warnings about potential focusing problems surface. I find the literature on this subject to be somewhat baffling, or perhaps just incomplete, so I did some more research.

The basic question:  Why might autofocus be compromised when I attach a 1.4 or 2.0 extender to a telephoto lens?

It really depends on the lens – or more specifically, the maximum aperture of the lens. (See this post to learn more about the basics of aperture.)

Attaching an extender to a lens effectively narrows the the size of the len’s max aperture. A 1.4 extender shifts the len’s max aperture one stop smaller; a 2.0 extender shifts the len’s max aperture two stops smaller.

Photo of Chestnut Sided Warbler
Chestnut Sided Warbler
ISO1250; f/6.3; 1/500 Second

The Downside – Less Light

If the len’s maximum aperture becomes smaller when you attach an extender, light transmission through the lens is reduced. Example: My 300mm lens 2.8 L II lens with a 2x extender would lose 2 stops. As I look through the viewfinder I would see the equivalent of a f/5.6 aperture. That’s significantly less light. So What?

In order for you to see optimally through the viewfinder, you need light. In order for the camera and lens to accurately auto focus, they need light. When preparing to take a photo, the len is always set to its widest aperture for auto focus purposes. It immediately stops down to the aperture needed for correct exposure when the shutter is released.

Modern AutoFocus Point Technology

Modern multiple focus point technology (selectable and non-selectable assist points) is engineered to deal with a wide variety of action, accurately and fast. But autofocus points need light to work. With an active bird, you increase your chances of accurate focus if the maximum aperture of your lens lets in a lot of light. As the max aperture is reduced (as is the case when you attach an extender) the lens slows down and does a lot more hunting, especially on a low light cloudy day, and most especially when multiple focus points are activated.

(NOTE: See this post to learn more about using multiple focus points.)

Should I Attach A Tele Extender to My Lens?

If you have a lens letting in a lot of light (2.0 or 2.8 maximum aperture), the camera’s access points have sufficient light with which to autofocus. Attaching a tele extender to this lens would be no problem. However, a lens with an extender narrowing the max aperture to f/5.6 or f/8 may have insufficient light and/or contrast, causing some autofocus points to be slow or not operational. NOTE: The center auto focus point will most likely work because it will get the most light.

If you have a modern DSLR camera and are wondering how many auto focus points are actually working with a lens and extender attached, you can check. Activate all auto focus points, look through the viewfinder, press the shutter half way, and see how many light up.

Manual Focus to the Rescue

If autofocus does not work, you are left with manual focus. Looking through a relatively dark viewfinder may hinder your ability to get an accurate fix in manual focus.  NOTE: I find it hard to manually focus without lots of light so I generally switch the camera to Live View and focus from the LCD screen instead of thru the viewfinder. (More info on Live View is available at this post.)

Variable Aperture Zoom Lenses

Just to complicate this issue, some zoom lenses have variable apertures; meaning the max aperture available changes depending on the focal length. Example: Canon EF 100-400mm f/4.5-5.6L IS II USM Lens. The widest aperture of this lens is f/4.5 at 100mm and f/5.6 at 400mm.


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