What is a telephoto lens?

If asked the question above, what would you say the answer is? Is it:
a) A lens with a focal length greater than 200mm?
b) A lens which can change it's focal length?
c) A lens whose physical length is less than the focal length?

I expect most of you will go for answer "A" but you'd be wrong. In real terms, anything with a focal length over 200mm is termed a telephoto lens in common parlance...but, there is a clue in the fact that you often find lenses shorter than 200mm refered to as telelphotos. So what are they talking about?

Well, the answer lies in techie talk.

According to the Canon Lens Work book, a telephoto lens is defined as follows:
With general photographic lenses, the overall length of a lens (the distance from the apex of the front lens element to the focal plane) is longer than its focal length. This is not usually the case with lenses of particularly long focal length, however, since using a normal lens construction would result in a very large unwieldy lens. To keep the size of such a lens manageable while still providing a long focal length, a concave (negative) lens assembly is placed behind the main convex (positive) lens which is shorter than its focal length. Lenses of this type are called telephoto lenses. In a telephoto lens, the second principal point is located in front of the frontmost lens element.

Translated into simple terms, a telephoto lens is one that has a physical length (from the front lens element to the the focal plane) which is less than its stated focal length.

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Focus distance limit switches on lenses

Some of the Canon lenses feature a little slide switch to let you restrict the range of focus distances the lens will operate over, but why? 

With the longer focal length lenses there's often quite narrow depth of field and also quite a large range of movement for the optics to cover from the closest focus distance out to infinity. If your subjects easily fall in to the range covered by a lens restricted limit switches then you get a much faster focus shift from minimum to maximum subject distance as the focus assembly only moves a small set of it's possible range. There are quite a lot of lenses in the range with focus limit switches including the EF100mm f/2.8 MACRO USM, EF135mm f/2L USM, EF70-200mm f/4L IS USM. In fact most L series lenses with focal lengths longer than 100mm have focus limit switches. In the case of a mega lens like the Ef800mm f/5.6L IS USM there's three positions for the limiter including the full 6 metre close focus to infinity range!

Coming back to something more normal the popular EF70-200mm f/4L IS USM has focus limit ranges 1.2m to infinity and 3m to infinity. So if you are shooting say motorsport and are over 3m from the track boundary choose the restricted range of focus, the lens will move from the new 3m minimum focus to infinity much faster than it would from 1.2m to infinity. This makes the AF seem more snappy when you pick up a camera and the initial focus is way off. There's just less time spent hunting for the subject while the lens racks in and out from minimum to maximum focus distances.

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IS Modes

I wrote about Image Stabiliser lenses with movies just the other week as well as shutter speed choices to avoid camera shake, so I thought I'd follow it up with some advice on the Image Stabiliser modes found on Canon lenses.

Most of the IS lenses in the Canon range feature two IS modes, cunningly named Mode 1 and Mode 2. These two modes cause the IS system to work differently so you can choose the mode depending on what sort of shot you're taking. The question is, when do you use each setting?

Mode 1 - General shooting. This is the mode you will have the camera set to most of the time. The IS mechanism will try and remove camera shake in both the horizontal and vertical direction, to give a sharp image.

Mode 2 - Panning. If you are shooting moving subjects, then one technique you might choose to use is a slow shutter speed and panning with the subject. If you get it right, this creates and image with a sharp subject and blurred background. It's a very effective technique and is used widely in motorsport photography especially. If you decide to have a go at panning, then you should set the IS Mode to 2. This locks the IS mechanism in one direction so say you are panning from left to right, the IS mechanism will not try to compensate for the horizontal motion, instead concentrating only on any vertical fluctuations.

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A star in the range - EF100mm f/2.8 Macro USM

The second part of our 'star in the range' series sees us looking at the EF 100mm f/2.8 Macro USM lens. Those of you with this lens will know how good it is, those without can only wonder what all the fuss is about. The truth is, this lens really is quite remarkable. It offers full 1:1 (lifesize) reproduction without having to change the length of the lens when focusing, and the results are stunningly sharp. So sharp in fact, that a common question asked is "Why is it not an L-Series lens?" The answer is that it doesn't use some of the technology required to make it an L-Series model, however the optical performance is more than enough to justify it.

If there are any complaints leveled at this lens, it's that it is in fact too sharp in some situations. Portraits usually. This lens seems to resolve detail that you just don't see, meaning that in a portrait shoot (for which the 100mm focal length is actually very useful) you end up with lots of images that need retouching to remove skin blemishes you didn't know where there!

So, should you buy it? Well, if you want to get into macro photography, it is the obvious choice in the range. The MP-E 65mm f/2.8 1-5x Macro photo is too specialised and lacks the ability to focus at infinity, the EF50mm f/2.5 Macro only does shalf life size unles you add the life size adaptor and the EF180mm f/3.5L Macro USM is more designed for subjects that need a longer working distance - butterflies and dragonflies for example. The 100mm is the all-rounder. You can shoot insects with it as long as you're careful about what you're doing, you can get in close to flowers and fungi and you can use it for portraits and general photography.

Is there anything missing from this lens? Well, I wish they'd box it with a lens hood included and I'd like to have this lens with an image stabiliser fitted like the N!kon VR Macro lens. Other than that though, it's a stunning lens that anyone interested in macro photography should seriously consider.

Have you got one? If so, what are your thoughts on it? Would you buy it again if you had to?




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A star in the range - EF 70-200mm f/4L IS USM

What, no Tuesday 'tog? Yes, it it is Tuesday, no, you're calendar is not wrong and no, there is no Tuesday 'tog today. Don't worry, there'll be one next week, we just thought we'd give it a break for a week while we take stock of where the blog is going and what it's going to do.

So this week, this is the start of a new mini-series picking out some of our favourite lenses in the Canon range. Some may be familiar to you, some maybe not, but all of them are lenses we've used and found to be cracking. First up this week is the EF 70-200mm f/4L IS USM lens. Now this lens is a great favourite of ours and is in fact well like by many photographers. Sadly though, it is often overlooked in favour of the EF 70-200mm f/2.8L IS USM in the belief that that f/2.8 lens is better.
Well, for all those that bought f/2.8 lenses - sorry! The f/4 lens is actually as good or better than the f/2.8 lens at any aperture. It's a much newer optical design and hence improvements have been made.

When looking at lenses, there are four things you're likely to consider:
Optics
Image Stabiliser
Weight
Price

If we take the optics first, as we said above the EF 70-200mm f/4L IS USM more than holds its own.
Image Stabiliser: f/2.8 = 3 stops, f/4 = 4 stops
Weight: f/2.8 = 1470g, f/4 = 760g
Price: f/2.8 = c.£1550, f/4 = c.£980

To make it simpler, you get better hand-holding ability, a weight saving of 710g (that's nearly half the weight) and a cost saving of about £570. Pretty clear cut isn't it?!

So, with the optic quality being equal and everything else being better on the EF 70-200mm f/4L IS USM lens, you may wonder why people still buy the f/2.8 lens. Well, the answer is simple - they shoot in low light or they use extenders. With extenders of in low light conditions, the f/2.8 lens is still king becasue it focuses better and keeps a larger maximum aperture. If you fit a 1.4x Extender the max perture drops by 1 stop taking the f/2.8 lens to f/4 and the f/4 lens to f/5.6 In bright light, this doesn't make too much difference, but once the light levels drop, having an aperture a stop wider can help with focusing.... not exposure though. Let's be clear on this = most people still have the film or early digital mindset. In other words "I can't take the ISO over 400 or I'll see noise". If you're shooting on one of the latest cameras, this doesn't matter. You can up the ISO further, still avoid undue image noise, and effectively counter the exposure loss of one stop.

For all those that do have the EF 70-200mm f/2.8L IS USM lens and are thinking you'd be better off with the f/4L IS version - don't worry. If your lens is in good condition, you should be able to sell it second-hand for more than the cost of a new f/4L IS lens. Just don't send any prospective purchaser to this page first!



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