In times of trouble when your lens just won’t reach the subject or fill the frame, people often turn to tele-converters to extend their optical reach. Here are a couple of examples from my 25 years in photography. The first is a Vivitar 2X converter for my old Minolta film cameras. The second is the Canon 1.4 converter that I carry today.
The standard line in converter today is that the 1.4x variety, at least as Canon is concerned, cut the light entering the camera by 1 F-stop. The 2x cuts the light by 2 F-stops. Most people realize this and compensate for it by opening their lens an extra stop and shooting away. There is far more to think about.
Let’s start with the Sunny 16 Rule
The Sunny 16 Rule that most of us were taught when we picked up a non-instant camera says that our shutter speed should equal the inverse of the ISO Speed of the Film/Sensor at F16 under a sunny sky.
Shutter = 1/ISO @ F16
Then there is the Inverse Shutter Handhold Guideline
This says that the shutter speed needs to be the inverse of the focal length of the lens in order to get a reasonably sharp picture while hand-holding the camera.
Shutter = 1/Apparent Focal Length
Without getting too ‘mathy’ with you, we have two equations that equal each other because they both equal shutter speed.
Another thing to think about is the sweet-spot of your lens. Most lenses are sharpest at 2 F-stops below the wide-open position.
With those three things in mind let’s look at an example lens. For the test shots included today, I used a Canon 300MM F4L IS Lens. Sweet spot of this lens is F8 or two stops brighter than the Sunny 16 rule of F16.
Let’s say we want to shoot a picture-handheld with this lens on a Canon 20D. The Inverse Shutter Handhold Guideline tells us we need 1/300th of a second to secure a reasonably sharp image. Ah-but the 20D has a 1.6 sensor crop which makes the lens behave like a 480MM lens. This kicks our minimum hand-held exposure to 1/500th of a second. Sounds reasonable. Let’s apply the Sunny 16 rule.
1/500th = 1/ISO so at F16 our ISO needed is 500. We want to shoot in the sharper range of our lens so we shoot at F8. This is two stops brighter than F16. This allows us to raise our shutter speed or lower our ISO. Let’s lower our ISO to get a more noise-free photo. Halving the ISO twice means we can shoot 1/500th of a second at ISO 125.
Let’s add a converter to the mix and see what happens.
300MM * 1.6 Sensor Crop * 1.4 Converter = 672 MM so the hand-held shutter speed needs to be 1/800th of a second as 1/640th is marginal. Apply the Sunny 16 Rule and you get an ISO of 800 at F16. But you didn’t want to shoot at F16 before you wanted to shoot at the Sweet Spot of your lens which, for review, is F8. But hold on! F8 is now one stop darker because of the converter so your Sweet Spot is at F11 now. This means you only get to open up one stop on Sunny 16 which means your ISO becomes 400.
Do the math: Add 1.4 to your focal length and increase your ISO by 3.4, or by 1 2/3 stops, to get the equivalent handheld shutter speed.
Now, if you go to the Canon 2x converter, you lose two stops and gain focal length. That shutter speed for handholding goes to 1/1000th of a second. That means an ISO 1000 at F16 (remember your sweet spot is F8 + 2 stops for the converter). Double your focal length and your ISO must increase by a factor of 8, or 3 full stops, to compensate.
Now stack those converter like some bold photographers do: ISO becomes 3200 for you to shoot at the Sweet Spot. Focal Length increased by 2.8 and ISO is increased by 25, or 4 2/3 stops.
This all assumes you have bright sun-which is seldom the case. Consider the examples above the best case scenario as your light will almost always be at or less than the Sunny 16 rule.
You can see by the sample shots (Tripod, Mirror Lockup and Remote Released) that the image quality doesn’t suffer greatly from the addition of the 1.4 converter provided you take steps to ensure a sharp image.
Remember- a converter means sacrifices in some area whether it be ISO, lens sharpness, or tethering yourself to a monopod or tripod to reduce your shutter speed. When you see a converter cuts the light by a stop, consider the focal length change costs you another stop in hand-held shooting.
Rikk Flohr © 2008