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!

All About ISO

Background
In the early days of film photography, the International Organization for Standardization (ISO) defined a standard to determine how sensitive the film was to light. This was known as the ISO speed. It is still relevent in digital photography today - the ISO equivalent speed is still a measure of how sensitive the CCD inside the camera is - and thus how quickly it will capture the image it "sees" through the lens.

Theory
ISO speeds normally follow the pattern: 50, 100, 200, 400, 800, 1600. A "slow" ISO speed is at the lower end, ISO 25, 50 etc. A "fast" ISO speed would be 800 or 1600. Most digital compact cameras work in the 100-400 range, although some go higher. Most digital SLRs will work in 100-1600, and some can even cope with 50-3200.

Examples
What does this mean for your pictures? Here are some advantages and disadvantages of two extremes of the ISO range:

ISO 100

• Best in sunny conditions outdoors
• Gives good colour saturation
• Best at recording high-contast scenes
• Images have good definition (sharpness)
• Shadows and highlights reproduced in good quality
• File size (for JPGs) will be lower - more images on a memory card
• Leads to slower shutter speeds - can cause camera shake (blur)

[ISO100 - there is good colour saturation and sharpness in the picture]

ISO 1600

• Best in dim conditions - overcast days or indoors without flash
• Gives reduced colour saturation
• Best at recording low-contrast scenes
• Images have reduced definition (sharpness)
• Shadow areas likely to suffer from ISO "noise"
• File size (for JPGs) will be higher - fewer images on a memory card
• Leads to higher shutter speeds - often avoiding blurry pictures

[ISO1600 - there is less colour saturation and sharpness in the picture]

In the above images, I have not done anything to alter the quality apart from cropping and resizing for the web. But this doesn't show the full story - here's a full-size comparision between the photos - showing the centre portion round the doorway:

[Left, ISO100 - good quality, colours and sharpness. Right, ISO1600 showing poor shadow detail and lots of "ISO noise"]

Of course, these effects go gradually from one end to the other as you go through 200, 400 and 800. It's always a trade-off between being able to take (any) picture (in dim conditions) and the technical quality that will result from using a high ISO speed. And the beauty of digital is that you can change the setting from one shot to the next if you need to - in the days of film, you were stuck with it for the whole roll!

So, if your camera will let you set the ISO speed manually, why not have a go at experimenting with different speeds next time you are out shooting? You will then learn what gives you the best results in various lighting conditions.