Am I doing something wrong?

Started Dec 5, 2012 | Questions thread
Wally626 Senior Member • Posts: 2,312
Re: Am I doing something wrong?

Barrie Davis wrote:

adrianandmel wrote:

This is not unhelpful as I agree. I am slowly realizing that I need a shorter focal length,

No! NOT a shorter focal length. Least Depth of Field occurs at longest focal length...

.... (wide aperture and greater background seperation is correct, though.)

ISO is only incidentally involved. For best image quality you should always use the lowest ISO you can get away with in the lighting conditions prevailng.

greater background separation and obviously a large aperture to achieve a narrow depth of field. I am also understanding that I am able to manipulate an image based on my lens through focal length and iso to achieve a good blurry background effect.

The optimum focal length for getting shallow Depth of Field is not always straight forward. In general longer focal lengths, closer subject distances and larger apertures result in less DoF. If going from 35mm to 70mm causes the aperture to become too small then the advantages of a longer focal length may be offset by the smaller aperture, longer focal lengths also mean you need to be further away for the same framing.

One example is if you have the same object size and the same aperture, the DoF will be the same with a short lens and close distance compared to a long lens and long distance. With the typical zoom lens you cannot maintain the same aperture as you move so you may get some change in DoF.

Example 16-85mm f/3.5 to 5.6 zoom with a 2 meter object diagonal, CoC 0.0094 mm

16mm, f/3.5, 1.1 m distance, DoF 0.34 m

85mm. f/5.6, 6.1 m distance, DoF 0.54 m

So you do a little better at 16mm than at 85mm, if you keep the object size the same.

If you keep the same distance, 1.1 m, then the 85mm f/5.6 drops to 0.02 m DoF but the object size drops to 0.34 m.

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