For the final part of our Know Your Camera assignment we looked at chromatic aberration.
Chromatic aberration is also sometimes known as colour fringing and is the result of light rays not meeting at one focal point, due to limitations and defects in the lens. Light is defracted by different amounts depending on its colour and is accentuated when it hits a positive or convex lens. It can be countered by having a negative or convex lens close to the positive lens but the more glass within a lens makes it heavier, more expensive and more complicated to make.
I took this photograph of holly in the garden and at this size it looks like a perfectly ok photograph. But, on closer inspection, particularly where the dark leaves are next to the bright blue sky, you can see a blue line around some edges of the leaves and a purple line around other edges.
This could have been corrected by stopping down the lens and is also much more obvious on wide aperture prime lenses.
Chromatic aberration can be corrected in Lightroom using the defringe sliders.
Vignette is defined as anengraving,drawing,photograph,orthelikethatisshadedoff graduallyattheedgessoastoleavenodefinitelineattheborder.
In digital photography a vignette is also referred to to as ‘light fall-off’ where the edges of the photograph are darker than the centre. It can be caused by the lens, increased by filters or lens hoods or can be added as part of the post production process.
Vignetting can occur in a number of ways. One of the main ways though is caused by the light being obscured by the barrel of the lens so that light rays that enter the lens at an acute angle ‘falls off’ at the edges. It can also affect the shape of the lens bokeh to make it look more like cat’s eyes than pretty circles.
Another thing to remember is that light coming in at an angle at the edges of the lens travels further than light heading straight into the lens. This means that light is weaker at the edges, also causing vignetting.
Another cause is that light is hitting a flat sensor at different angles. In the centre, the light is hitting the sensor head on but around the edges it meets the sensor at a slight angle also causing vignetting.
I understand that some cameras have built in software that can correct vignetting in jpegs but this information is not stored in RAW files and is ignored by Lightroom and Aperture. Both Lightroom and Aperture have features that can help to correct or enhance vignetting.
I wanted to check how much vignetting my camera and lens has. So I took three photographs, all of the same thing at different apertures in order to compare the vignetting.
Fortunately even at the widest aperture the vignetting is very slight and almost imperceptible.
For the slow shutter speed part of the Know Your Camera Assignment I had planned to take photos of some of some of the house Christmas lights that are around the town at this time of year. However after rooting around in the garage for something I thought I would take my camera in there and have a go at some slow shutter speed photography.
I saw this welding mask hanging from the roof in front of a row of heat exchangers. It was in front of a window, the blind was closed but there was a shaft of light coming in from the edge of it.
I wanted the depth of field to be fairly wide and I was several feet away so I set the aperture to f11 and the shutter speed to 20s.
In Lightroom I changed the RAW file to black and white and increased the contrast a fraction.
Although this is my favourite of the images I took, I spent some time taking other photos too, some at even slower shutter speeds.
In the next part of the Know Your Camera assignment we have looked at depth of field.
Depth of field is the term used to describe how big or small the focus point is in a photograph. A wide or deep depth of field is where all or most of the photograph is in focus and a shallow or narrow depth of field is where a small portion either side of the focal point is in focus.
Landscape photographs often use a wide depth of field in order to keep all the detail in the photograph sharp. When you want to focus in on a detail in an image and keep the surrounding area blurred a shallow depth of field is more appropriate.
Generally a shallow depth of field is achieved by using a wider aperture and being closer to the subject. Therefore a wider depth of field is achieved by using a smaller aperture and being further away from the subject.
You may also need to change the ISO in order to reduce shutter speed times.
I took these two photographs of coloured glasses. The first photo was taken at 20s at f29 in order to ensure that, even though I focussed on the dark blue glass, all of the other glasses are also in focus.
The second photo was taken at o.6s at F5 in order to make the blue glass the only glass that is totally in focus.
Clearly, at these shutter speeds I needed to use a tripod to steady the camera. I could have achieved an even wider depth of field if I had been able to move further away from the glasses.
Another part of our Know Your Camera assignment was to successfully photograph an object against a bright sky with the camera on an automatic setting.
It was getting late as I was wandering around the church yard and the sun was low but bright so I chose a statue and positioned myself with the sun behind.
At first the camera wouldn’t focus at all because the face on the statue was so dark.
I also thought it was a bit of a dull picture so I moved position slightly so that the brightest part of the sky was to one side. this made it much easier for the camera to focus. I also thought that the way the tree frames Jesus’ head looking like a halo and making the photograph much more interesting.
For this part of our Know Your Camera assignment we have been concentrating on focal length.
In the days of 35mm cameras, all lenses worked in a similar way and everyone know what it meant to have a focal length of 50 mm or 100mm. Now that we are in the digital age, lenses affect cameras differently depending on the size of the camera’s sensor. For example a 50mm lens on a DSLR is one thing but because a camera phone has a much smaller sensor a 50mm lens would be very much more zoomed in.
I had always thought that my Sony a77 had a full sized sensor and that my lenses worked the same as they did when I had a 35mm camera but in doing this exercise I see that it isn’t.
I started by taking a photograph with my lens on its widest setting then taking three more photographs at different focal lengths, trying to keep my subject at the same size and position in the viewfinder. The last image was taken with my lens at its most zoomed.
I have labelled each photograph with the focal length of my lens and the 35mm equivalent.
In Lightroom I changed the image to black and white and increased the contrast. In black and white you are not distracted by the very green grass around the graves.