Dak on cam
Forum Pro
Ok, so juggling lens equations has become an annoyingly consistent pastime for recent months for me. The main issue being how to wrap my head around what to expect under which conditions.
The easier concept to get a hold of was "background blur". Here the equations amount to "entrance pupil diameter is f/a where f is (actual) focal length and a is aperture number. Place a disk of that size right in the focus plane, and how large it appears there determines the size of blurriness the background receives, or the size of the bokeh circles around distant light sources.
And for macro work, a lot of stuff is "distant", namely a multiple of the focusing distance away.
So what if it isn't distant? If it's close enough that we worry about where the depth of field ends? Then the blur disk is a percentage of the entrance pupil, and the percentage is given by how much of the focusing distance we are away from the focusing plane.
It turns out that if we mark the circumference of a cylinder placed with its axis in the focus plane, the respective blur circle diameters will correspond to equal angles.
So this tried using dioptres of total strength +16 (Marumi +5, Marumi +3, Raynox +8 hello vignetting) telling the camera to focus at +∞ for an actual distance of 62.5mm. We see that the specular highlights indeed more or less touch with the focus on the axis and keep touching (while getting smaller) as we go around. The angular resolution (measured in radians) turns out to be pretty much m/a with m being the magnification factor which is f/d. The EXIF tells us f is 71.5mm here, and d we calculated as 62.5mm, making for m=1.144. So with a=F4.8 we'd expect there to be a*2pi/m bokeh circles to go around that ear ring. Plugging in the numbers gives us 26. Going with the finger nail over its edge makes it feel more like 32 so numbers are not quite correct but in a similar ballpark. If you trace where the paper is sharp (indicating the focus plane) it would appear that we have slight front focus at the specular ring which would suggest that the bokeh disks should overlap a bit more than they do.
Exact numbers aside, what does it tell us? The larger the magnification, the flatter the scene (or the narrower the aperture) needs to be in order to have the same blur appearance. If we shrink a scene by half and compensate by doubling the focal length or halving the distance, the blurriness of scene parts outside of the focus plane will double even if all the relations within the scene stay the same.
Or: macro scene composition is hard. But then you probably knew before juggling numbers.
--
Dak
The easier concept to get a hold of was "background blur". Here the equations amount to "entrance pupil diameter is f/a where f is (actual) focal length and a is aperture number. Place a disk of that size right in the focus plane, and how large it appears there determines the size of blurriness the background receives, or the size of the bokeh circles around distant light sources.
And for macro work, a lot of stuff is "distant", namely a multiple of the focusing distance away.
So what if it isn't distant? If it's close enough that we worry about where the depth of field ends? Then the blur disk is a percentage of the entrance pupil, and the percentage is given by how much of the focusing distance we are away from the focusing plane.
It turns out that if we mark the circumference of a cylinder placed with its axis in the focus plane, the respective blur circle diameters will correspond to equal angles.
So this tried using dioptres of total strength +16 (Marumi +5, Marumi +3, Raynox +8 hello vignetting) telling the camera to focus at +∞ for an actual distance of 62.5mm. We see that the specular highlights indeed more or less touch with the focus on the axis and keep touching (while getting smaller) as we go around. The angular resolution (measured in radians) turns out to be pretty much m/a with m being the magnification factor which is f/d. The EXIF tells us f is 71.5mm here, and d we calculated as 62.5mm, making for m=1.144. So with a=F4.8 we'd expect there to be a*2pi/m bokeh circles to go around that ear ring. Plugging in the numbers gives us 26. Going with the finger nail over its edge makes it feel more like 32 so numbers are not quite correct but in a similar ballpark. If you trace where the paper is sharp (indicating the focus plane) it would appear that we have slight front focus at the specular ring which would suggest that the bokeh disks should overlap a bit more than they do.
Exact numbers aside, what does it tell us? The larger the magnification, the flatter the scene (or the narrower the aperture) needs to be in order to have the same blur appearance. If we shrink a scene by half and compensate by doubling the focal length or halving the distance, the blurriness of scene parts outside of the focus plane will double even if all the relations within the scene stay the same.
Or: macro scene composition is hard. But then you probably knew before juggling numbers.
--
Dak
