Sunday, 8 March 2009

Depth of field

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I received this question during the week.

David,

I'm struggling with the concept of aperture used at long focal lengths. Close-up I understand how wide apertures demand accurate focusing, but what about at a long distance? It seems much more forgiving. Another layer to my question is how a long focal length will compress the background - does this have an impact on aperture use too?

Thank you!

Matt

This presents a good opportunity to reflect on the factors that affect depth of field (DOF), the distance range from the camera (or more accurately the focal plane, marked on your camera's body by a circle with a single line through it) where subjects appear to be in focus. I shall stick to the headlines in this blog post but, for a more in-depth view on this topic, check out Sean McHugh's fantastic Cambridge in Colour website.

The most important variables in controlling DOF are:

1. The aperture of the lens. As the aperture becomes bigger, the DOF becomes smaller.
2. The magnification of the subject. As the subject increases in size in the image, the DOF becomes smaller. You can easily illustrate this by picking up an object and moving it closer to your face - watch the background blur as your eyes' DOF shrinks. You can increase the magnification of your subject by either moving closer to it (reducing the focal distance) or by using a telephoto lens (increasing the focal length).

The magnification of the subject depends upon the focal distance (distance between subject and the focal plane of the camera) and the focal length of the lens. Note the key difference between focal distance and focal length! It is possible to keep a constant magnification of the subject, and hence DOF, whilst changing the focal length of the lens by making changes to the focal distance. This may seem like nit-picking but it's worth getting your head around.

You can convince yourself that DOF depends on magnification, rather than focal length, by taking a series of shots of a subject at various focal lengths and keeping it the same size in the frame. The DOF appears independent of focal length.

Let's say you're using Canon's 24-105mm f/4.0 L IS zoom lens. You focus on your model at a focal length of 105mm in aperture priority mode at f/4.0. You take your first shot, which with these settings will have a modest DOF. You zoom out 10mm to 95mm, making your subject smaller in the frame, which means you have to move in closer to restore your model to their original size. You repeat this until you're at a focal length of 25mm. If you were to estimate the DOF in each shot by eye you'd say it was constant.

It isin't exactly the same, however. At shorter focal lengths the DOF does increase minimally, even if the subject stays the same size, which means that DOF is not completely independent of focal lengths. The impact of this is small compared to the impact of magnification though.

The following is also worth noting. Most photographers are interested in a shallow DOF for blurring backgrounds. DOF extends in front of and behind the focal distance, but the ratio between the two changes with focal length. At shorter focal lengths the DOF extends approximately 1/3 in front of the focal distance and 2/3 behind. At longer focal lengths this changes to 1/2 in front and 1/2 behind. Therefore longer focal lengths have an advantage for blurring the background.

It's important to note that moving closer to a subject will affect its appearance due to changes in perspective. It is possible to have a blurred background when shooting a model at shorter focal lengths, but the reduced focal distance will distort their features due to the impact on perspective. If you want to shoot people with a blurred background then use a telephoto lens.

To address Matt's comments directly.

1. Using a wide aperture at a greater focal distance gives you a greater DOF and therefore makes accurate focusing easier.
2. A longer focal length will magnify the subject and therefore reduce the DOF. Wider apertures always give less DOF - this is not dependent upon focal distance or focal length.

Questions and comments (due to confusion or errors on my part) are most welcome!

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7 comments:

Samuel said...

David,
This is a great read. How do you handle long focal lengths in low light. For example, at 135mm F2, in a dim light environment, I still need a very high ISO to increase the shutter close to something reasonable to avoid camera shake. BUT, in doing so i am also killing the ambient light. What is the best thing to do here?

Thank you...

David said...

Hi Samuel,

I'm glad you enjoyed it.

Like you I go to high ISOs. On the 5D Mark II I frequently use ISO 6400.

I'm not sure what you mean by 'killing the ambient light' at high ISO though. Provided you expose correctly a high ISO will capture the right amount of ambient light.

Have I misunderstood you? Feel free to elaborate.

Yours,

David

Samuel said...

Hi again,

To elaborate, in AV mode putting up the ISO increase the shutter speed, the faster the shutter speed the less ambient light? Or does high ISO balance out the exposre at high shutter speeds.

Last I tried, adjusting the ISO made no difference to my exposure.


Thanks David

David said...

Hi Samuel,

You've hit the nail on the head there. When you're in aperture priority mode the camera will give you what it thinks is the correct exposure. As you increase your ISO speed this allows you to access faster shutter speeds (minimising camera shake) whilst maintaining the same exposure.

Yours,

David

Samuel said...

Yes I did answer my own question there a bit.lol

My confusion was also considering there might be a relationship between focal length and stopping motion. Clearly I can be fast enough on the shutter to stop motion, but not fast enough to stop camera shake on a telephoto lens. Hence, if I have space to move, easier to work with a shorter focal length lens to frame the same subject, giving me a shutter speed more likely to correct camera and subject motion in one go.

Why do canon not offer IS in prime lenses btw?

David said...

Hi Samuel,

Again, you're spot on.

At weddings my most used lens is my 16-35mm f/2.8 L. This can be handheld at 1/15 sec - the limiting factor is subject movement rather than camera shake.

I imagine that Canon have given zoom lenses IS because f/2.8 is their widest aperture and they're big sellers. The market for primes is probably a lot smaller and perhaps doesn't warrant the investment to create primes with IS. Plus they can go as wide as f/1.2 which means it's easier to achieve adequate shutter speeds in very low light!

Yours,

David

Samuel said...

Yes , I'd love to get the 16-35mm but the current prices are just bonkers.

http://www.camerapricebuster.co.uk/prod319.html