On Lenses

I took a long, critical look the other day at some of the photos I'd recently taken with my Canon 17-85mm lens [left]. It's built for digital and as such, can't be used on any 35mm or full-frame censor CCD cameras (such as Canon's 5D - not that I have one!).

I've had it a couple of years, bought as a replacement for the original 18-55mm kit lens which came with my EOS 300D in 2004. That lens was pretty ropey - very plastic and the image quality was far from great.

Now I'm using my Canon 30D, I've begun to notice that the 17-85 lens isn't particularly sharp around the edges, especially at the wide 17mm end. It also suffers from chromatic abberation:

[Buttsbury Church wide angle shot looks OK at web resolution]

[Detail of the trees on the left]

As you can see, this 100% view from a section of the trees on the left shows a distinct magenta halo on one side of the birch trunk and a slight green halo on the other side. When trying to make prints at A3, these lens "features" are beginning to be irritating...

So I have been thinking about a better lens for my short/standard zoom. I initially looked at Canon's 24-70mm EF f/2.8 L USM but it costs nearly £900, so that was out of the question. I finally settled on the Canon 24-105mm EF f/4 L IS USM [right]. Yes it's a stop slower than the expensive lens, but has the bonus of Image Stabilisation, and f/4 is still pretty fast, especially as it's throughout the zoom's range.

And the L stands for Low Dispersion glass - Canon's best quality lens. On paper my old Sigma 135-400mm lens had a similar spec to the Canon zoom I now use for rugby, 100-400mm EF f4-5.6 L IS USM, but the pictures the Canon produces with that "L" quality glass are so much better. So I thought another L-series lens would be a good investment. Particularly if I ever get myself a Canon 5D full-frame D-SLR as my existing 17-85mm EF-S won't fit.

I had the pleasure of trying out the new toy this morning, taking a few quick shots on the way home from Chelmsford. So far, I'm impressed; the image quality seems first rate. I haven't yet missed the extra 7mm range at the wide end, but am glad of the extra 20mm at the tele end of things.

[Can Impression #2 shows buildings and blue sky reflected in the River Can as it flows through a concrete bit of Chelmsford]

[Branches shows a 100% enlargement of another image taken at the wide angle. Not sure I can see any chroma artifacts in that!]

Of course, I've yet to give the lens a proper run out, but I'm certainly happy with its first impression. And I'm rather pleased with today's Picture of the Day, Covert Surveillance, taken with the new beastie on the way home from the shop!

Lenses & Perspective

Background
You might have heard some lenses described as "wide angle" or "telephoto". This article should help you get to grips with different types of lenses and what they do.

Theory
With a 35mm film camera, a 50mm lens is called a "standard" lens because it shows the same angle of view and perspective as the human eye. It's focal length is said to be 50mm. Using one of these lenses, you will see objects in the correct size and relationship to each other. Other lens types are:

• Wide angle - below 50mm, eg 24mm
  Shows a greater angle of view in the viewfinder than 50mm, but things "seem" to be further away.
• Telephoto - lenses above 50mm, eg 300mm
  Shows a smaller fraction of the scene than 50mm, so things "seem" to be nearer.

Some lenses have just one focal length - eg 100mm macro or portrait lenses. Others are able to zoom between two focal lengths:

• Wide angle zoom - eg. 18-55mm lens often comes as part of a "kit" with a camera
• Telephoto zoom - eg. 70-210mm is a zoom which might be included for taking larger images at a distance.

Conversion Factor
So far, we have been talking about lenses in relation to film cameras (35mm). With many digital SLRs, the CCD sensor it smaller in area than a 35mm negative, and this means there is a conversion factor which needs to be considered - this gives us the 35mm equivalent focal length for the lens.

For my Canon EOS 300D, the conversion factor is x1.6. Here are some common lenses that I use with it, and their 35mm equivalent focal lengths:

• 17-85mm EF-S wide zoom -> 27-136mm equivalent with 35mm
• 70-300mm EF telephoto zoom -> 112-480mm equivalent with 35mm
• 50mm EF standard lens -> 80mm equivalent with 35mm
• 100-400mm EF telephoto zoom -> 160-640mm equivalent with 35mm

Similarly, my compact camera has a lens which is 5-17mm. But the lens is much closer to the CCD sensor than it is with an SLR, so again there is a conversion factor. This time, it's x6.4, giving the zoom a 29-109mm equivalent focal length.

Examples
What effect does the focal length of the lens have on the pictures, and what are the advantages and disadvantages of using them?

Wide Angle

• Good for use in confined spaces - where you can't "stand back" as far as you might like
• Perspective is widened - allows you to "get more in" the picture
• Things in the background will seem a lot further away than they do to the naked eye
• Size of objects in the foreground are exaggerated
• Often leads to distortion of verticals - they become curved instead of straight, and/or appear to converge

[Wide angle - 29mm equivalent focal length - taken right next to the building. It appears to lean backwards, and the verticals are converging in a triangular formation]

[29mm equivalent shows things in thebackground to be very small, and foreground is larger. The post on the left is also showing a curved distortion]

Telephoto

• Good for magnifying the view - where you can't get as close as you might like
• Perspective is compressed - things will seem to be squashed together with not as much separation
• Things in the background will seem a lot closer than they do to the naked eye

[58mm equivalent - the building looks much more natural and there is much less distortion of vertical lines]

[Telephoto - 109mm equivalent, compresses perspective so the lamps apear closer together. Distant objects appear nearer than they are.]

You have probably already used the different zoom lengths on offer from your camera and lenses. But why not try taking the same subject from close to, using a wide angle, and then move further away and use the telephoto setting. This should give you some different views of the same subject to compare.

All About Apertures & F-numbers

Background
What is meant when referring to the "aperture" (or f-numbers) of a lens?

Photographic lenses contain an iris, made up of solid vanes, built in a similar pattern to those on the left. The vanes can move in and out, thus making the central hole larger or smaller - and so letting more or less light through the lens. This is what's called the aperture, and is a guide to the speed of the lens.

The f-stop, (f-number) or aperture is not an absolute number, it's also related to the physical diameter of the lens. So we can compare the relative apertures of a small compact camera's lens with that of a large telephoto, and still get a sense of which is better at gathering light.

Theory
The main series of f-numbers goes as follows (a sequence of root 2): f/1.8, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32

This seems rather complicated and counter-intuitive at first, but the basic thing to remember is this:

lower f/number = larger hole = faster lens = better in low light
higher f/number = smaller hole = slower lens = better in bright light

Most SLR lenses will have their maximum aperture written on the side. This is the largest size of the hole, when the iris is fully retracted.

More expensive lenses will have a lower f-number, such as f/2.8, which means they have a larger maximum aperture - and will be better for taking photos in low-light conditions.

Things get complicated with zoom lenses - which often have a different maximum aperture at each end of the zoom range, eg. 28-105mm, f/4-5.6. This is saying:

at 28mm end, max aperture is f/4
at 105mm end, max aperture is f/5.6 (ie smaller than f/4.5)

How this affects your pictures - Depth of Field
The other thing which needs considering at this point, is something called Depth of Field. This means what proportion of the image is sharp, in front of and behind the actual point where you focus. I will cover depth of field more fully in another post, but for the purposes of apertures, in general, we need to remember the following:

lower f/number = smaller depth of field = less of image in focus
higher f/number = larger depth of field = more of image in focus

Examples
Let's compare the effects of using two extremes of f/number:

f/2.8

• Large aperture
• Good in overcast or low-light conditions
• Will tend to require using faster shutter speeds
• Smaller depth of field - less of the overall image in focus
• Better for portaits or closeups, because backgrounds become blurred and do not intrude

[large aperture - in low-light conditions, giving small depth of field (only the first couple of blooms are in focus). The background is unobtrusive]

f/16

• Small aperture
• Good in bright, sunny conditions
• Will tend to require using slower shutter speeds
• Larger depth of field - more of the overall image in focus
• Great for landscapes, where you want everything, from a few meters to infinity, to be sharp

[small aperture - in sunny conditions, giving a large depth of field (almost everything is sharp), great for landscape work]

As good practice, why not turn your camera to AP (aperture-priority) mode and take a few pictures of the same subject, but using different aperture settings. This will show you exactly what the effects of controlling the aperture can do for your photographs.