Let's make a camera lens from a magnifying glass
We’re after some experimental fun today :) As they say, the best way to learn something is to try it yourself.
Back in high school physics we had a unit called optics. Convex mirror, concave mirror, lenses, refractive index, etc.—we may not remember much. Let’s do now a little experiment we didn’t do back then. Our topic is lenses.
Magnifying glass
This is among my earliest memories. My parents were teachers at a village school. During the holidays we often went to the district center for Ministry of Education seminars, and as a five-year-old I had to sit through hours of boring meetings. Or rather, even if they left me with a babysitter I wouldn’t stay put—I’d tag along anyway.
Before the meeting, we’d buy stationery and magazines we couldn’t find in the village so the child (me) could be kept busy during the presentations. That day I had the new issue of the famous 80s preschool magazine Milliyet Kardeş in my hands. The free toy of the month was a plastic magnifying glass.
Unless we count the science cabinets I poked around thanks to being a teacher’s kid, this magnifying glass was probably my first step into the world of optics. I spent the whole meeting enchanted, inspecting the magazine cover to cover with that magnifier. Being a kid is awesome :)
The other day I randomly found another magnifying glass in a drawer, and it instantly took me back. I decided to play with it a bit.
If you hold a magnifying glass in front of a white sheet of paper and fix it at the distance equal to its focal length, it will project the outside scene onto the paper upside down. It’s great fun to let your imagination run wild while trees swaying in the wind and cats wandering in the garden dance in miniature on the paper.
As you can see in the diagram above, a convex lens bends light toward the center, flipping top to bottom and bottom to top. The same applies left-to-right. The “paper” in the diagram could literally be paper, or it could be a camera sensor, or even the retina in our eye where the light-sensing nerves sit.
Both a camera and our eye see the world inverted. Because we’ve seen this way since birth, our brain adapted and it doesn’t feel “wrong.” In a camera it’s easy to digitally flip what the sensor reads. Maybe they even just mount the sensor upside down! :)
Either way, if you ever decide to clean your sensor, remember: the speck you see at the top-left of your photo is physically on the bottom-right of the sensor :) Don’t keep cleaning the wrong spot.
Lens imperfections
A perfect lens exists only in textbooks and diagrams. In the real world lenses have all sorts of optical aberrations. Designers have been creating different lens formulas for over 100 years to reduce these. You’ve probably heard “3 groups, 7 elements.” That means 7 separate lenses arranged in 3 groups. Zooms are even more complex. For example, the Canon 24–70mm f/2.8 mkII has a formula with 18 elements in 13 groups!
It doesn’t end at design; manufacturing those glass pieces perfectly and positioning them perfectly requires a host of physical and chemical processes.
Why all the fuss? Our eyes get by just fine with a single lens!
Our eye has a huge design advantage: its “sensor” is curved to match its lens. The optic nerves sit on a spherical surface inside the eyeball.
Camera sensors are flat, which causes all kinds of optical issues as you move away from the center. Poor lens makers are forced to solve these issues. But brace yourself: patents for curved sensors—led by Sony—have started to appear. Curved sensors are coming. Simple, cheap, very sharp lenses will come with them.
Chromatic aberration
I’ll share a diagram now. It has three parts. At the top we see how a perfect lens would bring parallel rays to a perfect focus. Of course, that’s not reality.
I’ll explain the middle and bottom parts below.
In reality, light isn’t a single thing. It’s a spectrum that contains different wavelengths at once. I wish we could say “So what!”—but as this spectrum passes through a lens, each wavelength refracts at a different angle!
Our camera has separate photosites measuring R, G and B. A few of these together form one pixel.
CA isn’t just a color problem.
Look again at the middle part of the diagram above. There’s a big issue. The blue light for the same pixel focuses in front of the sensor—i.e., front focus. Red, refracting less, focuses behind the sensor—i.e., back focus! In short, that pixel is badly blurred—its light is smeared over nearly the whole sensor surface.
Even in modern lens designs we sometimes see chromatic aberration (CA). It usually reveals itself as purple/green/red fringes in high-contrast areas, but it’s not just a simple color shift. If you clean that purple or red band in Photoshop, you’ve only solved part of the problem. Detail is light, and when different tones of that light get scattered, removing the unwanted tones won’t bring the same detail back—because you’ve also removed an important part of that detail.
In the black-and-white era, they limited the spectrum with single-color filters and thus limited CA. For example, if we mount a blue filter and shoot only blue, there’s no color fringing—all blues come to the same plane. On B&W film it still looks monochrome, but it actually contains only the blue channel. Since we don’t want an all-blue photo today and we can’t redesign the lens, the easiest thing is to stop the aperture down a bit.
Stopping down narrows the incoming light cone and reduces dispersion across the spectrum. So the photo improves in both color and sharpness. You can see this “fix” in the bottom part of the diagram.
Heads-up: stopping down too much hurts sharpness.
Photography with a magnifying glass
Enough theory—let’s have some fun. If a camera lens is a device that focuses light, a magnifying glass can do the same job. Let’s mount this magnifier on our camera.
No other lens will be on the camera. Our only optical element will be the magnifying glass.
First, let’s get to know our “lens.” It’s a magnifier, but what’s its focal length? What’s the aperture? It’s good to know these—especially focal length so I can use Sony’s IBIS stabilization. Sonys can stabilize any lens. With electronic lenses the body reads the data, but with a manual lens we must enter the focal length ourselves. If we enter the wrong value, we might end up doing more harm than good.
I focus the ceiling lamp onto a sheet of paper and measure the magnifier–paper distance with a ruler.
To determine the f-number we need the glass diameter. Since it’s a single element, no fancy formulas needed. The glass is 47 mm across; plug that into the relation:
Focal length / f-number = aperture diameter
f-number = 125 / 47 = f/2.6
Great—now we know our lens specs. Let’s shoot :) But how do we mount it? For infinity focus, sensor–magnifier spacing must be 125 mm. To focus closer we need to increase that distance. In short, it must be adjustable—so we need a bellows. As for mounting, let’s forget it—there’s no ready adapter. I don’t want to glue anything either. I’ll hand-hold the magnifier in front of the bellows!
My first indoor test produced an image, but it wasn’t exactly encouraging. The dreamy “glow” effect looked cranked to eleven. I’m not sure what I expected—this isn’t a quality optic, just an ordinary stationery-store magnifier.
Spherical aberration
That dreamy effect, softness, and low contrast in the first shot aren’t solely the fault of chromatic aberration. There’s an accomplice: spherical aberration.
The word “spherical” points straight to the source. Simple lenses are made as parts of perfect spheres because they’re easiest to manufacture. But physics throws us a curveball: rays hitting the outer zones of a spherical surface bend more than those near the center.
Rays from the outer edge focus just behind the lens, while central rays focus farther back. The upshot? Light never forms a single tack-sharp point. The “best focus” is really a blurry disk. That signature dreaminess and loss of crispness come from this!
Spherical aberration is sharpness’s archenemy.
This isn’t unique to our homemade setup; all simple spherical lenses have it. What to do? Same fix as before: stop down!
By closing the external iris we physically block the problematic outer zones and let light through the better-behaved center. Because central rays focus to much closer planes, the image instantly gets sharper and punchier.
This is also why modern pricey lenses include special aspherical elements: their shapes bring rays from all zones to the same plane, nearly eliminating spherical aberration.
Improving the image
I can do two things.
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Because there’s no special coating on the glass, light sneaks in from all angles and reaches the sensor. There’s a general haze. To prevent this I’ll make a hood.
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Wide open almost always looks quite soft. Even real lenses can be unusable wide open. I can improve the image by placing an external iris right behind the magnifier.
My bellows is M42. The external iris is also M42, so it screws right onto the front. Aperture: check.
Paper cups are perfect for DIY hoods. I cut out the bottom of one and tape it to the magnifier. Because the cup is white, that would throw things off. I roll some black card into a tube and line the inside. It isn’t perfect, but it fits nicely. After trimming the excess with scissors, our magnifier starts to look like a lens.
As shown above, I’ll hand-hold the magnifier in front of the bellows while shooting. It’s a bit of a hassle to both change bellows length for focusing and hold the magnifier there, but I’ve got experience. Looking through the viewfinder I stop the iris down to an average setting. No idea about the exact f-number.
Yes, that’s it :) We managed to use our magnifier as a lens. It gives a soft look reminiscent of old webcam footage or movies watched from VHS tapes—but it works. We’ve made our own free lens.
My head is now buzzing with ideas and plans, but if I turn this into a race, there’ll be no end—and it’ll get pricey. For now, just enjoying this magnifier a bit more will do. Let’s try some cat shots too.
