Posts by Category: Getting Technical

More tutorials dealing with the technical aspects of photography.

High ISO Noise Comparison

Posted by Caz on Monday, 12th November 2007

I have already talked a little about the effects of high ISO noise in a previous post. And one of the reasons I wanted to upgrade from my Canon EOS 300D to my new 30D was it’s alleged better performance at high ISO settings, plus the ability to choose ISO 3200 in really dark situations, whereas the old camera would only go up to ISO 1600.

So I thought it might be useful to take a closer look at some photos taken with each camera, and see if things really are any better in dim conditions.

EOS 300D – Sigma 135-400mm EF f/4.5-f/5.6 – ISO 1600

Back in December 2005, I went to see Saracens play Wasps away in High Wycombe. It was a fairly late kickoff, and being December, it wasn’t long before the floodlights were on and things were getting pretty dingy on the pitch. Here’s a picture of ref Ashley Rowden supervising the scrum:

[The ref’s head is highlighted above]

The picture doesn’t look too bad resized for viewing on the web. But looking at it at full resolution, you’ll see what happens:

[The highlighted section shown at Actual Pixels (100%) size]

You can see that the skin tones show noticeable coloured noise, and the black areas are also very mottled. The whole image is fairly soft and not very saturated.

EOS 30D – Canon 100-400mm EF L f/4.5-f/5.6 – ISO 1600 / ISO 3200 (HI)

Yesterday, I was back to see Saracens play Glasgow at home in Watford. Admittedly with a different lens, but it’s the camera body itself which is the biggest factor in image quality in low light. By the second half, the floodlights were on again, and this is the first shot we’ll look at taken at ISO 1600:

[The Scrum Half’s head is the piece to be enlarged]

Again, the photo above is good quality at web resolution, but I think there’s a definite improvement at full size:

[The second highlighted section shown at Actual Pixels (100%) size]

This time, the blacks are cleaner and there is less obtrusive coloured noise in the skin tones. It looks noticeably sharper, and the colours are more vivid.

Here’s another one, as it was getting even darker. In order to keep the shutter speed up, the only option was to increase the ISO setting, since I was working at the lens’ maximum aperture, f/5.6. So I used the ISO setting on HI – which basically means 3200:

[The Prop’s head is the bit we’re interested in]

I’m impressed with the web resolution image – certainly crisp, clean and good colours. And closeup?

[The third highlighted section shown at Actual Pixels (100%) size]

Pretty impressive! I’d say 3200 with the 30D looks better than 1600 with the 300D. So I reckon it’s been a worthwhile upgrade for that alone. Plus the fact the 30D is much quicker a writing images to the Compact Flash card – vital when taking a sequence of high-speed action photos. With the old camera, I would often be left frustrated after following the action up the pitch – I’d miss a try because the camera was still unloading it’s buffer to the card!

Needless to say, rugby in the dark days of winter push the capabilities of a camera quite hard. The speed of action and low light are both factors which probably won’t bother you too much if you just take a few holiday snaps or family pictures with your SLR.

Lenses & Perspective

Posted by Caz on Friday, 23rd March 2007

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.

White Balance & Colour Temperature

Posted by Caz on Wednesday, 21st March 2007

Background
Have you ever wondered why the photos you take indoors sometimes come out looking very yellow? It’s a common mistake to make, and this tutorial should help you understand why it happens, and how to get it right!

Theory
The human eye is very good at adjusting to different light sources, and the brain compensates without you noticing, when going from outdoors to inside. If you look at a piece of white paper under both sorts of light, you won’t see any difference - but a camera will.

This effect is due to the colour temperature of the light. Here are some examples:

  • Cadle flame – noticeably orange/yellow [1850K]
  • Tungsten lightbulb – slightly yellow [2800K]
  • Average daylight – white light [5500K]

You don’t need to worry about the degrees Kelvin values of the light, just that they appear to be different “colours” to film or a CCD sensor. It’s this difference which affects the white balance of a picture – what the camera records as being white. Most films were biased to record white properly in daylight (daylight balanced film). Others were designed for use indoors with tungsten bulbs – tungsten balanced film.

In the digital world, we have the same issues, but most cameras will let you select the appropriate white balance (as well as guessing it with Auto WB). These are common symbols and what they mean:

daylight [daylight] shade [shade] cloudy [cloudy]
tungsten [tungsten] fluorescent [fluorescent] custom [custom]
Auto [auto white balance]

Most times, AWB makes a good guess, but if you find the picture has a colour cast, try selecting the correct white balance for the lighting. Custom lets you calibrate the camera to the lighting (for instance, if you don’t know what sort the light source is). Point the camera at a piece of white paper and hit the calibrate button – then the camera will “know” what true white is supposed to look like and adjust accordingly.

Examples
So, what happens when we get it wrong? Here are some comparisons.

Tungsten Lighting
Yellow cast because “daylight” was selected under tungsten lighting:

[Tungsten light incorrectly balanced for daylight shows a yellow colour cast]

[Tungsten balance shows much more natural colours (although there’s quite a lot of yellow in the picture anyway!)]

Fluorescent Lighting
These three were taken in my office (under fluorescent lighting), but deliberately selecting three different white balance settings. Using Photoshop to sample the proportions of Red, Green and Blue of the “white” paper (highlighted with the red box in each picture, just to the right of the monitor), shows which colour is difficient, causing the shift in colour. Peak white, under neutral lighting, should have a value of R: 255, G: 255, B: 255. Numbers less that that (but still pretty equal) show shades of grey – ie neutral hues.

[Daylight setting gives R:236 G:227 B:182]

Blue is much lower, hence the picture is a bit too yellow (blue and yellow are opposite each other in the colour spectrum). Although this isn’t nearly as bad as the previous example, tungsten light balanced for daylight.

[Tungsten setting – gives R:220 G:228 B:246]

Now there is too much blue in the image, and the whites do have a blueish cast.

[Fluorescent setting – gives R:229 G:228 B:228]

Now we’ve got it right and the white paper is neutral in tones. Notice the picture on the monitor now has “blue” highlights instead of white – this is because the LCD emits light at a different colour temperature to the fluorescent lights overhead – and why we have to be careful when we take pictures with multiple light sources of differing colour temperatures. You can never get them all right in the same picture!

Even outdoors we can run into similar problems when some areas of the photo are in bright sun and others are in the shade.

The image on the left shows a player in the shady part of the pitch. The whites in the background are neutral, but his “white” shirt has a disctinct blue tint to it. Shadow areas will always show a blue hue compared to the full sun conditions (that’s why many cameras have a separate setting for it). So if you’re taking a portrait in the shade and want natural skin tones, switch to the Shade setting for best results.

Why not experiment with the White Balance settings on your camera. Afterwards, it’s very easy to forget and leave tungsten selected, so next time you take pictures in daylight, they will come out too blue!

More on Depth of Field

Posted by Caz on Tuesday, 20th March 2007

Background
I’ve already mentioned Depth of Field briefly, in my tutorial on Apertures and F-Numbers. But it’s not just the aperture you are using which determines how much depth of field your pictures have.

Theory
Three factors influence the depth of field of an image. They are:

  • Aperture of lens – the f-number you use at the time of taking the image
  • Focal length of lens – eg, 28mm for a wide angle, 300mm for a telephoto.
  • Distance to subject – how far you are from the main focus of the image.

Again, this might seem complicated, but the important things to remember are these:

smaller aperture = greater DoF than larger aperture
with the same focal length and distance to subject

wide angle lens = greater DoF than telephoto lens
with the same aperture and distance to subject

greater distance to subject = greater DoF than smaller distance
with the same focal length and aperture

Examples
The following show three pairs of pictures, each where two of the three parameters (aperture, focal length, distance to subject) have been kept constant, and the third is varied. Hopefully, this will demonstrate the effect of each. They were taken at a recent rugby match (not specifically for this tutorial, but they illustrate the points well). Whilst I will quote the shutter speeds and ISO setting used, they actually bare no relevence to the depth of field, so are just included for interest.

Changing The Aperture
Same focal length = 400mm
Similar subject distance = about half way across the pitch

[small aperture - f/10, 1/800th @ ISO 400 – most of the image is in adequate focus, giving quite a busy appearance]

[larger aperture - f/6.3, 1/400th @ ISO 400 – background is more blurred, giving better separation from the players]

Changing The Focal Length
Same aperture = f/7.1
Similar subject distance = close to the nearest touchline

[shorter focal length – 130mm, 1/250th @ ISO 400 – everything from touch judge on left (green) to furthest player is pretty sharp]

[longer focal length – 375mm, 1/400th @ ISO 400 – nearest player is blurred (he’s about the same distance as the touch judge was in the above) and the background is also very soft]

Changing The Distance To Subject
Same aperture = f/5.6
Similar focal length = about 250mm

[further away – far touchline, 1/4000th @ ISO 400 – most of the image is in sharp, including much of the background. The action was about 70m away]

[closer distance – near touchline, 1/400th @ ISO 400 – background is more blurred, giving better separation. The action was about 10m away]

Checking Depth of Field
Some cameras allow you to check the depth of field you will get before you take the picture. This is usually called Depth of Field Preview. Usually, when you look through the viewfinder, the camera is using the maximum aperture of the lens to show you the image. Pressing the DoF Preview button shuts the lens down to the size of aperture it will use at the time of taking – this often results in a darker view in the viewfinder (the lens is letting in less light) but you can visibly see just what will be sharp.

Maximizing Depth of Field
There is one special point in the image, which if you focus on it, will maximize the depth of field you get with any given combination of focal length of lens and aperture. This is known as the Hyperfocal Distance – and is approximately 1/3rd of the way into the picture, from where you are standing to infinity. There’s another complicated formula telling you what will be sharp, but basically, if you set focus at the hyperfocal point, everything between half way to that point and infinity will be in focus. There is more sensible reading on the subject here.

As good practice, why not experiment with apertures, focal lengths and distance to subject, to see how you can go about controlling the depth of field in your pictures.

Putting It All Together – Exposure

Posted by Caz on Monday, 19th March 2007

Background
I’ve already explained the basics of ISO speed, apertures and shutter speeds. Now it’s time to put them all together and see how they relate to the overall exposure of an image. The EV or Exposure Value for any given scene and lighting is always a constant.

Theory
The correct exposure for an image is governed by a complicated formula. However, all you really need to know is what effect altering any of the three options has on your pictures. So, for a particular scene and lighting conditions, your camera might suggest this exposure:

ISO 200, 1/250th @ f/5.6

If we were to change the ISO setting to 400, the film or sensor is now twice as “quick” at gathering the same amount of light, so the exposure time can be halved:

ISO 400, 1/500th @ f/5.6

Or, the same amount of light will be let in using 1/250th of a second as before, but decreasing the size of the aperture (increasing the f-number):

ISO 400, 1/250th @ f/8

Assuming we leave the ISO setting as it was originally, the following are also equivalent exposures to the first:

ISO 200, 1/125th @ f/8

ISO 200, 1/500th @ f/4

To see more equivalent values, you might like to look at this interactive EV calculator.

What does this mean for your pictures? Well basically, the combination of shutter speed, ISO and aperture is always a trade-off, depending on your priorities – or what you can change. Sometimes you may have low light and hit the widest aperture of your lens, which will give you slower shutter speeds. If you still want pictures which don’t suffer from camera shake, you will need to increase the ISO setting.

Other times, you might have plenty of light (on a sunny day), so use a slow ISO setting, and choose the aperture/shutter speed to either freeze movement (fast shutter speed) or maximise depth of field (small aperture).

Examples
Here are some shots I took of the moon, in October 2004. With this sort of subject, you often don’t know what will be the “correct” exposure, so by keeping two of the three parameters the same, and just changing the third, you can bracket the exposure to see which is best.

In this case, I used ISO 200 (to preserve fine detail) and the lens was set at it’s widest aperture, f/5.6. The only option left was changing the shutter speed accordingly. Apart from the blue hue, see what effect this has on exposure:

1/250th

This version looks a little bit too bright, it’s overexposed.

[1/250th @ f/5.6 with ISO200 – overexposed. Loss of detail in the highlight areas, bottom right]

1/400th

This version looks about right, it’s correctly exposed.

[1/400th @ f/5.6 with ISO200 – nicely exposed. Enough detail in the highlights and shadows]

1/640th

This version looks a little bit too dark, it’s underexposed.

[1/640th @ f/5.6 with ISO200 – underexposed. Loss of detail in the shadow areas, centre]

As good practice, why not take a few pictures of the same subject, but using a combination of different shutter speeds, apertures and ISO settings. Change one at a time and take a series of shots, then see what effect this has on your subject, and the way the camera records it.