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ahutty said 4 years ago 10/3/2010 12:20:43 AM EDT

Depth of field, also called depth of focus, refers to the region of an image that appears in sharp focus. It's a property that is quite subjective, and your focal length can significantly influence how you perceive it.

If you've read many tutorials on depth of field (or DoF), you may have come across the statement "that for a subject of the same magnification, DoF is independant of focal length". While this statement isn't true for all circumstances (i.e. macro photography), it does apply to most camera to subject distances for "normal" photography. This may not agree with your own perception of DoF, especially if you've worked with lenses with long focal lengths. This tutorial aims to demonstrate why your experience may differ from what you'd expect from reading the theory.

DoF depends on your aperture, the distance to your subject and your camera format (or crop factor). It also depends on the focal length, but this effect can be ignored at most distances.

Or can it?

While this might be true in theory, you would be ignoring at your peril. Dum dum dum
:-D
Two images of the same subject, taken at the same magnification (so they appear to be the same size in each image) demonstrates why quite nicely.


Focal length 53mm, aperture = f/8
.

.


Focal length 300mm, aperture = f/8
.

The batteries in the top image appear to be the same size as the batteries in the bottom image. The magnification is the same. The depth of field is essentially the same too, but appears to be greater in the top image.

Why?


Well I think it's obvious in an example like this one. When the focal length is greater, the background is magnified more, so you can see the background in greater detail and therefore see just how out of focus it is.

ahutty said 4 years ago 10/3/2010 12:44:33 AM EDT

If you don't believe that the DoF is the same in both of the above images, it can easily be demonstrated too. I've copied the square to the left of the batteries from both images and pasted them side by side in the image below. The square from the 300mm shot has be resized to match the size of the square from the 53mm shot.


Yep, that looks pretty much the same to me.

Satisfied?

But what does it all mean?
Well technically the DoF is the same for the same given magnification of a subject and the same aperture, even when the focal lengths differ. But this is somewhat irrelevant because the out of focus effect is exaggerated by lenses with longer focal lengths; this is because the out of focus background is magnified more.

The DoF being the same in both cases may only really be relevant in technical applications. Most of us here are concerned about the visual impact of the image. If you want to isolate your subject visually from the background, a longer focal length does work better in most circumstances because it will show a smaller portion of the background.

Technical details:
Camera: Canon 400D (crop factor of 1.6)
Lens 1: Canon 17-85mm IS
Lens 2: Canon 70-300mm IS
Photo 1: f=53mm, aperture=f/8, subject distance= 0.26m
Photo 2: f=300mm, aperture=f/8, subject distance= 2.21m
Subject - background distance = 0.28m

Further Reading
Depth of Field Tutorial, Cambridge in Colour
Depth of field, Wikipedia

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