What is Barrel Distortion?

The world of photography is full of strange terms. Pin-cushion effect, bokeh, depth of field, and white balance are just a few that take a moment to get to grips with.

The one I'm blogging about today is barrel distortion. This is the curious effect that reminds you a picture was taken using a circular lens, because the straight lines curve slightly.

It's particuarly obvious in photographs of buildings or other man-made structures because, not surprisingly, they tend to have a lot of straight lines.

The photos in the this blog entry demonstrate the effect. They are all stunningly unexciting pictures of the wall of my house, which comprises courses of bricks laid in a 'stretcher bond' pattern.

The top photo was taken with the Sigma 50mm macro lens. If there is any distortion it's difficult to see - the lines look good to me.

The second shot is with the kit lens that came with my Sony A350, at maximum zoom, or focal length of 70mm. Again, any distortion is invisible.

The final picture is with the same lens but at the other extreme of its focal length, 18mm. This is a wide-angle setting, which is where barrel distortion tends to occur. It's visible in this picture - which looks as if something is pushing the centre of the shot forward slightly, curving the lines of the brickwork.

Effects like this give the lie to the claim 'the camera never lies'. Incredibly sophisticated though they are, cameras are still not a match for the human eye.

Using a macro for holiday snaps

It's holiday time. This year we had the opportunity for a week by the sea, in Swanage.

This raised the question of whether to take my new Sigma macro lens. I naturually assumed that I would mainly use the kit lens that came with my Sony Alpha 350. It's a DT18-70mm F3.5-5.6, which means it's a basic zoom lens.

However, my Sigma macro was already on the camera and I decided to leave it there. I'm not certain how much dust enters a camera body whenever the lens is changed, but currently I'm being careful not to change too often.

On arrival in Swanage I decided to start taking pictures with the Sigma lens. It's supposed to

give good landscape results. After a little while I got used to the fact that it wasn't a zoom and if I wanted to get closer to a subject I had to physically move. It also did not give me the same wide-angle options as the zoom.

But you can learn to live with that. I downloaded the results, and I was impressed.

What clinched it was a comment by my thirteen year old daugther. She observed that the pictures looked more realistic because the buildings did not have curving walls. She was referring to

what's called barrel distortion, which results in straight lines becoming curves in photographs.

Barrel distortion tends to occur using wide angle lenses, or the wide end of a zoom lens. Apparently it's relatively easy to fix with software, but I prefer the pictures to come out of the camera without the need for post-processing.

I decided to stick with the Sigma macro lens for the rest of the week. I took some great portrait shots and landscapes, and it allowed me to get really close to subjects. It suited my style of photography.

There's a lot more that I want to write about. But the message of this post is simple - a macro lens works perfectly well as a general-purpose lens; it's not just for close-up work.

All the pictures in this post were taken with my Sigma 50mm macro.

Sigma 50mm macro lens reviews

I've been using my new Sigma 50mm macro lens for a couple of weeks now. It's giving great results, as seen in some of the pictures on this blog.

However, my journey into the world of macro photography has barely begun and I'm confident there are much better pictures ahead, waiting to be taken once I've learned more.

Today I thought I'd post links to some reviews of the Sigma 50mm macro lens. It was originally announced in 2004, as seen from this article on the Digital Photography Review site. However, they don't appear to have a subsequent review of the lens when released.

The first review is from the ePhotoZine website, and is dated January 2006. The one difference I've spotted is that the lens reviewed had a switch to move between auto-focus and manual focus. This is absent from my lens, probably because the same switch is built into my Sony A350 camera.

In general the review of the lens is positive, although it does pick out a couple of minor issues with slow auto-focus and the lack of a soft case. The former I've already encountered when trying to take pictures of bees on clover. The bees spend a few seconds on the clover head - plenty of time of a picture - but they're constantly moving as they forage for nectar. This seems to prevent the lens from achieving focus for long enough to shoot.

As for a soft case - I've long accepted that camera and lens manufacturers no longer supply any sort of cases with their products. Irritating, but it's a commercial reality that accessories push up produce prices.

The internet give a voice to ordinary people, particularly when it comes to product reviews. Amazon is a great source for these, and reviews for the Sigma lens are extremely positive. I've linked to the reviews for the Canon mount lens as there are more of these than for Sony.

Other reviews you may be interested in are dphotojournal and dyxum.

Why I chose the Sigma 50mm DG macro

At the end of the day it was about cost. My budget just wasn't going to stretch above £300 (approx. $450) and this was the only lens that I could afford.

I started my search for a lens by going after the Tamron 90mm Di. It reviewed well, but proved difficult to get hold of. I'm not experienced at buying lenses online and I'm wary of buying high-value items from suppliers I don't know.

One or two sites claimed to be able to supply it for a good price, around the £330 mark. But I sought out reviews of those sites and the feedback was generally negative.

I then broadened my search to include the Sigma range of lenses. They do a 105mm and 70mm macro, but both were priced at around £400. The Sony macro lenses were even more expensive.

My desire for a longer lens was driven by the desire for a solution that allowed me to stand further back from the subject matter. However, as I considered the cost of these lenses I thought about the sort of macro photograhy I wanted to do.

Some of my subjects will be living creatures, such as insects. Being able to keep my distance would avoiding disturbing them. However, a lot of my macro work will probably be with static objects (such as the peas) and in relatively controlled environment.

This is why I opted for the 50mm lens from Sigma. It's not the ideal, but I'm hoping that it will be a good compromise between length and price. Time will tell.

Peas please

This is one of my first macro photographs, taken yesterday. I've just bought a macro lens for my Sony A350 - the Sigma 50mm F2.8 DG Macro - and I was keen to try it out.

The peas were freshly picked from my garden, the first of this summer's crop. I wanted to re-create other photos that I've seen of fresh fruit and vegetables, making them appear fresh and appetizing.

The process was simple enough. I washed the peas and put them into a bowl. Washing left tiny droplets of water on them, which helps to give that fresh appearance.

The camera was hand-held for the shot. Other details are:

- 1/25 shutter speed
- f4.0 aperture
- 100 ISO

Most of the settings were automatic, although I focused manually. I'm already sensing a preference for manual rather than autofocus.

I take photos in RAW and JPEG format, performing post-processing on the RAW file. In this case the peas were not actually as green as the picture suggests, so I altered the hue to improve the quality of the colour.

I'm quite pleased with the result. It's not as sharp as I'd like; my preference would be to have more in focus. I need to work on depth of field and learn the best combination of ISO and aperture to get better results.

The full picture is available at my Flickr account.