Barrel Distortion

 tutorials  No Responses »
Jul 262009
 

Seaweed covered rocks, St Bees Beach

After taking this shot I was quite looking forward to seeing how it came out. I was using my Hoya Circular Polariser at the time and I’d rotated it to bring out the best of the colour both in the sky and on the rocks – the green of the seaweed was really emphasised by it and it was one of those rare moments when I was really glad I’d bought it.

Sadly, when I viewed the shot on the computer, this is what it looked like…

Seaweed covered rocks, St Bees Beach

Notice the sad face being pulled by the horizon – I’d forgotten that the Sigma 18-200 has really noticeable barrel distortion (where straight lines are bent away from the centre of the image) at the wide (18mm) end. That’s not to say that it’s a bad lens – for the price it’s a pretty good all rounder.

Fortunately it’s possibly to remedy this (with a bit of trial and error) with The GIMP.

Versions prior to 2.4

Open the file and select “Distorts” from the “Filters” menu. Select “Curve Bend” from the sub menu that appears. You should then see something similar to the following:-

The GIMP, Curve Bend Screen

To correct the barrel distortion we need to build a curve which will be used to modify the image. To do this I first placed a straight line roughly where the horizon was in my shot by clicking on the left and right sides of the grid. Then I placed a point (by clicking in the desired place on the grid) in the centre of this line which I then I slowly dragged downwards, changing the straight line into a slight curve. Once the results of the preview looked acceptable I pressed the “Ok” button and waited for The GIMP apply the changes (which took about 15 seconds).

Versions 2.4 and up

A new filter was added in version 2.4 – Lens Distortion. Open the file and select “Distorts” from the “Filters” menu. Select “Lens distortion” from the sub menu that appears. You should then see something similar to the following:-

The GIMP, Lens Distortion Screen

As the distortion on my original image was convex (bulging outwards) I needed to apply some concave distortion. To do this I tweaked the “Main” value and eventually settled on a value of -12.

From the manual…

Main – The amount of spherical correction to introduce. Positive values make the image convex while negative ones make it concave. The whole effect is similar to wrapping the image inside or outside a sphere.

More documentation on this filter can be found here – http://www.mirrorservice.org/sites/docs.gimp.org/en/plug-in-lens-distortion.html

Spring Afternoon

 textures  No Responses »
May 022009
 

Spring Afternoon

Earlier in the year I set myself a challenge in order to improve my photography and broaden my horizons a bit – I’d seen this concept mentioned in some old magazines that were passed on to me. I find it can get terribly difficult to think of things to photograph so I decided to combine two of my favourite past times, cycling and photography.

The idea was that I would attempt to find something to take pictures of each time I went out – it didn’t matter what – just anything that took my fancy.

On this occasion I was taking a short cut across some fields. It was early spring and despite the sunshine the ground was a little too wet for cycling, let alone lying on, so I got pretty soaked getting this shot.

Spring Afternoon

Despite having the features I wanted, the shot didn’t capture my feelings at the time – the warmth of the first sunny day of the year, the freedom of being out doing what I wanted to do. It needed to be more fluffy and dreamlike and fortunately I knew exactly what I was going to do.

A friend on Flickr produces some fine textures for layering and I already had one in mind that would add the tone I was looking for to the picture.

Purchase print at RedBubble | imagekind | Photo4me

Railings, Mevagissey Harbour

 landscape  1 Response »
Apr 262009
 

Railings, Mevagissey Harbour

Purchase print at RedBubble | imagekind

In a number of previous posts I’ve mentioned just how much difference a few minutes can make to a scene. The above shot was taken on the first day of a recent holiday in Cornwall whilst we waited for our accommodation to become available.

I’ve become more interested in black and white than colour photography just recently and it seemed the perfect style to capture the old harbour. The sunlight was patchy at the time and I got the shot I was looking for, looking out to sea through the railings (see below), however just as I was preparing to move on the sun broke through the clouds and lit the scene perfectly. It was impossible to ignore the combination of colours in front of me so out came the red filter I’d been using and in went the ND grad to improve the sky.

Railings, Mevagissey Harbour

Harbour Wall, Mevagissey

 b&w, filters  No Responses »
Apr 192009
 

Harbour Wall, Mevagissey

It seems I’m currently “very into” black and white photography – although to be fair, it’s something that I’ve always liked when other people have done it but never really bothered with it myself.

With the advent of digital photography I think B&W has got a bit of a bad name for itself – tools which can greyscale an image mean that it’s an easy way to attempt to salvage a flawed colour image (not that I don’t convert my own colour images sometimes also). The shot above however was achieved using a Cokin P003 red filter, with the image then being desaturated and the contrast tweaked slightly – which took about 30 seconds at most – the ideal amount of processing time when you’ve got a lot of photos to work through :)

Why desaturate? Why not just shoot in black and white mode on the camera? Well, if I was shooting in jpeg mode it might be a good idea but as I’m shooting in raw mode, when I get the images into Lightroom they will be red again anyway so there seems little point. It doesn’t really help for reviewing the shots on the camera either as the black and white preview displayed on the camera seem quite different to what I see in Lightroom. Besides, I’ve got used to previewing the red images now – once you’ve learned to trust your manual exposures it’s not that much of a problem.

Using an external filter isn’t without its problems though – the main drawback with the red being that you lose 3 stops worth of light so you end up having to make sacrifices in either ISO, shutter speed or aperture (or a combination of the three). Shooting at ISO100, f/11, 1/250s isn’t going to be an option unless you are taking a picture on the surface of the sun so I usually end up at ISO200, f/8, 1/100s – not ideal, but as an exercise in learning to take control of your camera in its manual mode I can heartily recommend it.

Fortunately many DSLRs now have the ability to simulate the affect of certain colour filters built in to their Monochrome picture options – certainly the Canon range do. These can be used without any loss of light, but only have an effect if shooting in JPG – raw images remain unaffected by them.

Winter Tree and Photomatix

 hdr, tutorials  4 Responses »
Apr 022009
 

The Winter Tree shot was created using 3 bracketed exposures, shown below. Most DSLRs can be set up to take exposures like this – the camera will take 3 shots (or more depending on the camera) each at different exposures. In this instance they were taken at 0, -2EV and +2EV so that I end up with 3 shots that are all 2 stops apart.

I found the idea of “stops” the most complicated part of getting to grips with photography, but basically a stop is the doubling or halving of the amount of light hitting the sensor – so if we had a 1 second exposure, the next stop down from that would be 1/2s and the next stop up would be 2s. ISO and aperture settings also affect the amount of light hitting the sensor and therefore their adjustment can also be measured in stops, however when taking bracketed exposures these two remain constant – only the shutter speed changes. (A more detailed explanation can be found here – http://www.andystafford.co.uk/exposure-control-stops/)

Having set the camera up to take my 3 shots, I pressed the shutter release and this is what I got…


1/125s
1/500s
1/30s

The numbers underneath each shot are the exposure times – see how the second is 4 times the first, and the third is 1/4 of the first (give or take some rounding). This is due to the 2 stops difference between each shot.

You’ll notice that the shots (apart from the final one) look quite dark. There are three reasons for this:-

  • I wanted to keep as much cloud detail as possible so I metered off the brightest part of the sky to make sure I didn’t get any blown highlights.
  • It was slightly windy and I didn’t want too much tree movement in the shot so I tried to keep the shutter speeds as fast as possible. (1/30s is still quite slow really considering there was moving foliage but I got lucky in this instance). Photomatix can align movement but better to try to limit the problems at source.
  • Accident. I would also have taken a brighter set but I was running out of time.

Once home I loaded the images into Photomatix as follows (Note: if you want to try this there is a link to the source images at the bottom of this article):-

These aren’t the default settings – I find that different combinations of alignment methods work better for different shots. I left Align Source Images set to “By correcting horizontal and vertical shifts” just in case the wind blew the tripod at all and turned off “Attempt to reduce ghosting artifacts”. Movement, especially clouds and running water, can cause problems in Photomatix and I’ll often try different options in these instances.

Once Photomatix had finished its initial processing I pressed the Tone Mapping button and tweaked the settings until I got something that I was happy with.

If you’ve not used Photomatix before don’t be alarmed by how the image looks at the moment – what’s on the screen is only a rough guide. The finished product will look much better :)

The settings that made the major difference to the image were those to do with saturation. With the main Colour Saturation slider set to 0 the image was black & white but changing the Saturation Highlights slider re-introduced colour in the the brighter elements of the shot – the clouds.

If you want to have a go yourself, the source images are available – Source images (3.2MB).

I’m afraid they are jpegs rather than the original raw files, due to space and bandwidth considerations, but they are good enough quality for trying out this example.

Visit http://www.hdrsoftwaretutorial.co.uk/ for tutorials and a discount code which will give you 15% off the purchase of Photomatix.