Big lenses

I've done it!

It took some doing mind: a lot of calculations, poring over spreadsheets, and projecting future credits, but at length I convinced myself. I've been nurturing  a pot of capital set aside to pay the interest on a legacy my mother left to her great-grandchildren when they reach adulthood. I am now confident that there is a surplus in the pot which I could justifiably spend! However,( I rebuke myself), it must only be spent on capital goods - things that retain their value; things like big lenses.

For wildlife photography, unless your concept of wildlife is limited to insects and small slow moving creatures, you need a lens which can magnify your subject at least as much as a pair of binoculars will. My present long lens is the new version of Canon's 100-400 f4-f5.6 lens, the mark 2. It's a wonderfully versatile lens and I've taken lots of excellent pictures with it, but it is limited in its range. Objects -  usually birds - in the middle distance are hard to get clear, so for some time I have been thinking about an upgrade. There are many classes of consumer goods where moving up a notch is relatively easy. You look at the specifications, decide what you can afford and buy. Lenses are horribly complicated. It's taken me a long time to learn all the complications and I would like to try to pass on that knowledge to anyone interested. Get yourself well caffeinated and concentrate!

These are the factors governing the choice:

• Price - of course. New Canon telephoto lenses  go from £1000 up to £10,000.
• Focal length in millimetres, from 200 up to 800.
• Widest aperture expressed as an f number
• Weight in kilos.
• Autofocus speed
• Shutter speed expressed as fractions of a second.
• Image stabilisation capability.
• For used lenses, which version of the lens, expressed as a mark number.
• Performance with tele-converter lenses.

Price is self-explanatory, so let's look first at focal length. With a bit of simplification we can say that the base line of the scale is a 50mm lens. That will deliver what you see - no magnification. A 100mm lens will magnify by 2, a 200 by 4 and so on. The binoculars which most bird-watchers use are x8 so the equivalent lens will be a 400mm, which is, with one important exception, the shortest length which will do the job.

 The "f stop" scale measures the size of the gap through which light passes into the lens - the aperture. The f-stop measurement is harder to grasp, because it also affects focus, and the terminology is confusing. "Depth of field" represents the amount of the picture, foreground to background,  which is in clear focus. A lens with a wide aperture - known as a "fast" lens - will have a very shallow depth of field. For example if you focus on the eyes of your subject, the nose and the ears will be out of focus - i.e. blurred. The f-stop scale starts at 1.4, and a lens with this figure would be considered the "fastest" available: one which lets in the maximum amount of light and delivers the shallowest depth of field. The next "stop down" is actually up: 2.8 at which aperture half as much light is being delivered to the sensor. Each time this figure is doubled: 5.6, 11.2, 22.4 the amount of light is halved, but a much deeper field of focus is available.

Next we have to look at what happens when you change the focal length of the lens. Let's take a small but very expensive lens with a focal length of 50 and a maximum aperture of f1.4. Now change it for a 100mm lens. Because the front of the lens is further away from the sensor, the amount of light will have been cut in half so the effective "speed" of the lens has reduced (ie gone up!) to 2.8. To keep the same amount of light getting to the sensor, the lens will need to have a lot more glass in it: it will be bigger and heavier. And this is where we get to the nub of the problem. If my pot of capital was big enough to by a decent family car I could buy the biggest lens which Canon offer: one with a focal length of 800mm. If Canon had simply lengthened the 50mm to 800mm in a tube of the same diameter, the "speed" of it - the f.stop - would have been way off the scale  - in the hundreds. Instead, what the lens manufacturers do is increase the amount of glass to compensate, which means that the 800 lens is very big and very heavy 4.5 kilos. It will also have quite a high f-stop: 5.6, and a very high price.

Are you keeping up at the back? The upshot of all this is that to get the best pictures of birds you need a lens which magnifies the image at least 8 times, delivers a lot of light, and is not too heavy to carry around. Most professional bird photographers use a 500mm f4 lens, which weighs over 3 kilos and costs new around £8000. A 400mm f4 will also do the job and will be cheaper. There is a 400mm f2.8 but it is heavier than the 500 and more expensive still.

I can stretch my pot of money to 2000, for which I can buy a used 300 f2.8 or, at a stretch, a 400 f4, but there are still two more factor to consider: autofocus, and image stabilisation. The slower the shutter speed, the more any movement of the camera will translate into blurred images. The ability of a camera and lens combination to  quickly focus on a moving subject depends on the sophistication of the camera's autofocus system and on the speed and accuracy of the motor drive in the lens.

Now let's bring shutter speed into the equation. The longer and heavier the lens, the faster the shutter speed will have to be to stop all movement. The general rule is that, without image stabilisation, each 100mm of length adds 100th of a second to the shutter speed, thus a 400 lens will need one/400th of a second to stop movement. Early IS systems offered a "2 stop advantage" meaning  you could have the equivalent of 4 times the amount of light reaching the lens and still have a clear image. With Canon lenses the ability of the lens/camera combination to compensate for physical movement resides in the lens, and is again measured in f-stops. The best IS systems (also called Vibration Control in other cameras) offer a 4 stop advantage.  The older variations of the big Canon telephoto lenses have no IS and slower AF (autofocus).

Last factor: tele-converters. These are small lenses which fit between the main lens and the camera, and magnify the image by either 1.4, 1.6,  or 2 times. The newest versions of the Canon ones cost around £300-400 each, a lot less than the extra cost of buying the bigger lens. They have electrical connections which enable IS and AF to operate, though at a reduced level. So, instead of buying an 800 lens, I can fit a 2x converter to a 400 lens and capture images from twice as far. There is however a big downside to these gadgets: loss of light. Here we go again: with a 2x converter you double the f number of the lens, so a 400 f4 becomes an 800 f8. Autofocus suffers in speed and range, IS is reduced, shutter speeds go up and apertures must be wider to get the image.

Now, at last, we are back where we started - my new lens. It's a second series 300 f2.8 with image stabilisation, and it's bigger and heavier than my 100-400 f4-5.6. With a lens this "fast" I can use a 1.4x tele-converter and have a 420 f4 lens, significantly better than my current lens at 400 f5.6. When I don't need the extra length I can use the full capabilities of a very fast lens at f2.8 - the gold standard for most lenses. The big attraction of this lens though was the thought that I could use it with a 2x converter and have a 600 f5.6 lens.

My first tests were disappointing. The light was poor, the subjects further away than I would have attempted with my zoom, and the images were all too soft. Without the converter the lens is wonderful, but 300 does not have the reach I need for wildlife. I've done more reading on the subject and now need a good sunny day to be sure it will be worth the extra capital. Watch this space.