September 25th, 2011

First some practical rule I use: I call it the 10/10 rule:

At f/1, 10mm the hyperfocal distance is 10m.

To get to real situations:

  1. Divide 10m by the f-number you want to use (example f/5.6 gives ~1.8m)
  2. Compute the square of your focal length and divide by 100 (example 28mm * 28mm ~ 800, hence 8 )
  3. Multiply the two results (example 1.8m * 8 = 9.6m)

This rule is reasonably exact, you can use it as well to find the right aperture (the near-field is roughly half of the hyperfocal): Say, you have a 50mm lens and need everything from 10m to infinity in focus.

  1. Multiply the min focus distance by 2 (example: the hyperfocal is then 20m).
  2. Divide the lensfactor as computed in (2) above by the hyperfocal distance (example 50*50/100=25, 25/20 = 1.25m)
  3. Multiply by 10 to get the required aperture (example 1.25*10 ~ f/13)

You can shorten this by dividing the hyperfocal distance by 10 and execute only step 2, this gives numbers that are easier to compute.

This approximation is designed for “normal” APS-C cameras with a pixel-pitch of ~5µm. To be fair this makes sense down to f/8 - for smaller apertures the diffraction will get you unreasonably long hyperfocal distances. I recommend to fix the algorithm by setting the start distance of 10m to 8, 6 or 4m for f/11, f/16 or f/22.

What is hyperfocal distance?

Glad you ask, without much ado, hyperfocal distance is the focus distance from there even objects at infinity will still appear sharp. It is computed as

H = f*f/N/c + f

where f is the focal length, N the f-number and c the diameter of the circle of confusion (CoC)

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Tele lens subject separation

September 11th, 2011

A common knowledge item in photography is that tele-lenses allow you better subject separation. I had been interested in how much we are talking about.

You have for sure noticed that the subject separation (aka DoF, Depth of Field) is more pronounced at close distances. To make things comparable we have to define comparable scenes and from a photographer’s point of view the common denominator is the magnification. In common portrait scenes you have a magnification of 1:10 to 1:100. This means that that 1cm (or inch) on the sensor will represent 10 to 100cm (or inches) of the subject.

The basic optical formula tells us that the magnification is the relation of the subject distance and the focus distance. The constant coupling these lengths is the focal length of the lens. Short: Double the focal length and all other lengths will have to double too. If you shot your friend so that the face fills the whole frame with a 50mm, your will have to step back four times as far to achieve the same with a 200mm lens.

Here two images shot at 70 (right) and 210mm (left). The magnification of the subject is roughly the same and both pictures had been taken at f/5.6:


This comparison makes it easy to see the difference: The diameter of the bokeh circles on the water is three times wider. In other words: The blur that causes the subject separation is proportional to the focal length - at constant magnification and aperture!

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