Heart Shaped Bokeh In Background

Joseph S Wisniewski wrote:

adso2 wrote:

So recently, I got a few of the bokeh shapes that you can put over your lens.

As you are finding out, that really doesn't work.

In order to control bokeh, you have to have your heart shaped aperture at the point inside the lens where the aperture blades already live. At anyplace else, you get complex interactions that may be pleasing or may not, depending on the relationship of foreground and background, the design of the lens, and the phases of the moon.

You're in luck. This is actually about shaping the OOF PSF (out-of-focus point spread function), and I happen to be an expert in measuring and manipulating OOF PSFs... publishing scholarly research on this topic for several years now.

For this to work, the heart-cut-out must form the aperture -- but that doesn't mean it must be inside the lens where the blades that normally form the aperture are.

Basically, placement needed in front of the lens to form a new aperture depends on the optical design of lens. Most non-ultrawide lenses have designs in which, at the position the lens filter threads are at, the effective aperture diameter is approximately (usually not exactly):

aperture_diameter = focal_length / lens_f_number_set

Thus, to form a new aperture, the disc you put at the filter position must have a hole in it that has a diameter smaller than the aperture_diameter calculated above. If the hole is too big, you'll simply get vignetting. Yes, I said that right: too big a hole causes vignetting.

Here's an example. Suppose you have a 50mm f/2 lens. Well, that means we'd expect the aperture_diameter to be about 50/2=25mm at the filter position. That means the cut out portion of the heart would have to be narrower than 25mm or you'll get vignetting no matter what you do. Ok, suppose the heart cut-out is 10mm... good to go, right? Well, sort-of. If you accidentally let your lens stop down to f/8, you'll get vignetting if the heart is larger than 50/8=6.25mm... so a 10mm cut-out would vignette! Basically, the lens would have to be open wider than f/5 to not vignette with the 10mm cut-out on your 50mm lens. By the way, the larger the cut-out is without vignetting, the bigger each out-of-focus heart image will be in your photos, so you actually have some control over the size of the hearts too.

Back to your 35mm SLR lens. Shorter focal length lenses often use designs that don't place the entry pupil where we expect it, so the cut-out might need to be even smaller, or it might need to be closer to the front element of the lens than the filter threads. It's easy to find out by simply looking at the live view as you bring the cut-out closer to the lens front -- you want the point at which it stops vignetting. Incidentally, if that's a 35mm f/2.8, the cut-out needs to be less than 35/2.8=12.5mm... which I bet yours isn't. A longer focal length and/or faster lens will allow a larger cut-out to be used.

One last note to anyone trying to do this with a compact camera: this bokeh-shaping trick can still work, but the actual focal length of the lens is what matters -- not the equivalent field-of-view focal length. So, your compact might have a lens marked as f/2.8 with an effective field of view like a 35mm on a full-frame camera, but if the sensor is small enough that this field of view comes with a 4mm lens, then your cut-out would need to be less than 4/2.8=1.4mm. Good luck trying to cut that out with a pair of scissors!