Doug Kerr
Well-known member
This is to revisit a topic I discussed a little while ago.
There is often interest in the matter of how does the format size of a camera affect our ability to cause a certain degree of blurring on background or foreground items as part of the artistic vision of the image.
Let be first dispose of certain grounds for objection to my observations on this matter:
• Yes, there is more to the matter of bokeh than the size of the blur figure created in the image by a point on an out-of-focus object.
• Yes, the impact of spherical aberration, coma, obliquity of the pupils, and other matters mean that, even for an essentially circular aperture stop, the computation of the "size" of the blur figure using the classical geometric equations is potentially inexact.
That having been said . . .
If we consider the blur performance of two cameras with different format sizes, under these conditions:
• Camera focused at the same distance in both cases.
• Illustrative foreground or background object of interest at the same distance in both cases.
• Focal length in the two cases such as to yield the same field of view.
• Size of the blur figure considered with respect to the size of the image, or the size of the main subject (for example, considered as they would appear to viewers looking at prints of the same size viewed at the same distance in both cases).
Then, to attain the same size of the blur figure in both cases, the f-numbers would have to be very nearly proportional to the sizes of the format (diagonal dimension, for example).
Thus, if in a full-frame 35-mm (ff35) format size camera, we would need to use an aperture of f/2.8 to get the blur performance we sought (insofar as size of the blur figure is concerned), then on a Four-Thirds System camera (format approximately 1/2 the ff35 size), to attain a comparable blur figure size, we wouild need to use an aperture of approximately f/1.4.
Of course, your mileage may vary.
Best regards,
Doug
There is often interest in the matter of how does the format size of a camera affect our ability to cause a certain degree of blurring on background or foreground items as part of the artistic vision of the image.
Let be first dispose of certain grounds for objection to my observations on this matter:
• Yes, there is more to the matter of bokeh than the size of the blur figure created in the image by a point on an out-of-focus object.
Actually, I do not plan to speak of bokeh at all, but this has always been a popular axis of attack.
• Yes, a non-circular blur figure does not have a diameter.• Yes, the impact of spherical aberration, coma, obliquity of the pupils, and other matters mean that, even for an essentially circular aperture stop, the computation of the "size" of the blur figure using the classical geometric equations is potentially inexact.
That having been said . . .
If we consider the blur performance of two cameras with different format sizes, under these conditions:
• Camera focused at the same distance in both cases.
• Illustrative foreground or background object of interest at the same distance in both cases.
• Focal length in the two cases such as to yield the same field of view.
• Size of the blur figure considered with respect to the size of the image, or the size of the main subject (for example, considered as they would appear to viewers looking at prints of the same size viewed at the same distance in both cases).
Then, to attain the same size of the blur figure in both cases, the f-numbers would have to be very nearly proportional to the sizes of the format (diagonal dimension, for example).
Thus, if in a full-frame 35-mm (ff35) format size camera, we would need to use an aperture of f/2.8 to get the blur performance we sought (insofar as size of the blur figure is concerned), then on a Four-Thirds System camera (format approximately 1/2 the ff35 size), to attain a comparable blur figure size, we wouild need to use an aperture of approximately f/1.4.
Of course, your mileage may vary.
Best regards,
Doug