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.

-blabpictures-

Movies and IS - the examples

Well on Saturday, I promised you some example videos showing what IS does when you're using long telephoto lenses with the camera mounted on a tripod. Something I didn't say on Saturday was that this doesn't only appear in movies - if you're using Live View to compose or focus, you'll also see the image moving slightly. However, the benefits of IS if you're shooting stills are great enough to mean we'd suggest leaving IS on - The ability to manual focus with a more steady viewfinder being one of the main reasons.

So, on with the movies. What you'll notice is that in the first 10seconds, the image seems to move around and you can hear the IS motor spinning. In the second clip (after the cut) is that the image is stable and there is no IS sound.


EOS 5D Mark II Movie with IS on long telephoto lenses from Blab pictures on Vimeo.

-blabpictures-

Filming, tripods, long lenses and IS

We're all aware that IS is a wonderful invention whether you're shooting stills or video. However, there is one caveat to that - when using a tripod.
Most of the recent IS lenses don't require you to turn off the IS when you mount the camera on the tripod, however in our experience, if you're pretty certain the setup is stable, then turn it off otherwise you'll end up with slightly blurry images as the shutter speed drops.
For filming, you don't end up with blurry images. From our experience, we've seen that with longer lenses when filming from a tripod, you don't so much get blurry images, as motion in the images - in other words the image seems to drift back and forth as the IS spins but there is nothing to correct. Essentially the IS elements that move to counteract are creating a movement that isn't there. To avoid this, simply turn the IS off when you put a big lens on a tripod and plan to do some filming.
Look out for a post early next week where I'll put up two video clips, one with IS on and one with IS off to show the effect...

-blabpictures-