Today we are looking for an experimental entertainment 🙂 The best way to learn something is to try and see.
There was a lesson called optics in high school. Concave mirror, convex mirror, lens, refractive index, these are the topics I hardly remember. Let’s do an experiment that we didn’t do in those days. Our topic is “lenses”.
Magnifying glass
It’s one of my earliest memories. My parents were teaching in a village school. We used to go to the district center for the National Education seminars, which were organized frequently during the holidays, and I would have to attend hours of boring meetings as a child at the age of 5 years.
First, some shopping was done, such as stationery and magazines, which were not found in the village. Milliyet Kardeş, the famous Turkish children’s magazine of the 80s, had a new issue that day. The giveaway toy was a plastic magnifying glass.
Besides the science cabinets that I tamper with using the advantage of having teacher parents, this magnifying glass can be considered as my first step in the optical world. During the meeting, I thoroughly examined the magazine using that magnifying glass. Being a child was super cool 🙂
When I had another magnifying glass I found recently at home, it took me to those days. So I decided to play with it.
Holding a magnifier in front of a white sheet and fixing it at a distance that corresponds to the focal length will focus the outside image on the paper in an upside-down position. The trees swaying in the wind, the cats wandering in the garden are very fun to watch and dream while they are dancing on paper as tiny figures.
As you can see in the diagram above, a convex lens bends light to the center and turns the top of the image to the bottom and bottom to the top. Of course, the same goes for the right and left. In the diagram, what we show as paper can really be paper, or a sensor of the camera, or a retinal region with optical nerves that detect light in our eyes.
A camera and our eyes see the world upside down. Since our eyes have been like this since our birth, our brain has adapted and it doesn’t feel like something’s wrong with us. On the camera, it is easy to show the image by turning it over again. Or maybe they just reverse-mount the sensor! 🙂
One way or another, if you want to clean the sensor one day, remember that the stains you see in the upper left corner of the photos are actually physically located on the bottom right corner of the sensor 🙂 Do not clean the wrong place.
Lens defects
A perfect lens can only be found in textbooks and diagrams. In real life, lenses have a wide range of optical flaws. They have been making different lens designs for over 100 years to reduce these flaws. You’ve heard of a lens with “three groups, seven elements.” This statement describes the use of 7 different lenses in three groups in design. Zoom lenses are even more complex. For example, the Canon 24-70mm F2.8 MKII has a formula that contains a total of 18 lenses in the form of 13 groups!
The work doesn’t end with design, it takes a lot of physical and chemical processes to manufacture and position the elements correctly.
Why are they trying so hard? Our eyes are just fine with a single lens, for example!
We have a very important design advantage. It has a sensor that is curved at an angle that is compatible with its own lens. The eye nerves are placed on a spherical surface within the eyeball.
The flat sensors of the cameras cause various kinds of optical problems as the pixels move away from the center. Hard working lens manufacturers are also trying to solve these problems. But stay tight, sensor patents from manufacturers, especially from Sony, are coming. Curved sensors will follow. Simple, cheap and very sharp lenses will come with them.
Chromatic aberration
Now I will share a diagram. It consists of 3 parts. At the top, we see how a perfect lens perfectly focuses the parallel rays to the focal point. But there is no such reality.
I’ll explain the middle and bottom parts below.
Light is not actually a whole. It is in the form of a spectrum that contains different wavelengths simultaneously. “Why would that concern us?” I’d like to say, but when it passes through a lens, each wavelength bends at a different angle!
There are separate dots that measure RGB (red, green, blue) colors in our camera. Several of them come together and create a pixel.
CA is not just a color problem.
Let’s look at the middle figure in the above diagram. There’s a big problem. The blue light of the same pixel is focused in front of the sensor. I mean, “front focused”. The red, on the other hand, is less bent, focused on the back of the sensor, meaning “back focused”! In short, that pixel is grossly blurry. It’s almost spread all over the sensor surface.
Even with modern design lenses, there is sometimes a chromatic aberration (ca) problem. This is not a simple color deviation, usually seen in high contrast regions as purple/green/red rainbows. You can clean that purple or red halos using Photoshop, but it can only solve some of the problem. What we call detail is light, and when different tones of light are scattered around, even if we delete the tones that disturb the eye we can’t recover the detail. Because we’ve just erased an important part of that detail.
When the diaphragm is closed, we will be limiting the angle of the incoming light and the separation of the light spectrum will decrease. In other words, we will improve the photo in terms of both color and sharpness. You can see this again in the bottom section of the above diagram.
Caution: Closing the aperture too much will damage the sharpness.
Photographing with a magnifying glass
After all this theory, let’s go to fun. If a camera lens is a lens that focuses light, a magnifier can do the same. Now let’s put this magnifier on our camera.
There will be no other lens attached. We will only use a magnifying glass as an optical device.
Let’s start with getting to know our lens a little bit. It’s a magnifier, but what’s the focal length? What is the aperture? We’d better know them. I would like to be able to use the Sony IBIS vibration suppression system in particular. Sony is able to block vibration on all lenses. If we are going to use IBIS, we have to select the focus value of the lens in manual lenses. If we choose wrong, we’ll make things worse while trying to be helpful.
I measure the distance between the magnifier and the paper using a ruler by focusing the ceiling lamp on a sheet of paper.
Magnifying glass stands at a distance of about 12.5cm from the paper. So our lens is 125mm. So I’m going to choose 125mm for IBIS. This value is often used in portraits. I can shoot portrait as subject.
We need to measure the diameter of the glass to determine the diaphragm. Since it is a single lens structure, detailed formulas are not required. The diameter of the glass is 47mm and we just have to put it in the formula.
Focal length / Aperture value = Aperture diameter
Aperture value = 125 / 47 = f/2.6
Well, the values of our lens are revealed. Let’s move on 🙂 But how are we going to mount the lens? To focus the lens to infinity, the sensor-magnifier distance should be 125mm. We need to increase this distance to focus closer. It should be adjustable. So I need a bellows. Let us forget the mounting part, it is not possible to make a real adapter. Adhesive can be used, I don’t want that either. I’m gonna hold the magnifier with my hand in front of the bellows!
As a result of my first test indoors, I get an image on the camera, but the situation is not plausible. There is a sense of excessively applied dreamlike “glow” effect. I don’t know what to expect. It’s not a quality magnifying glass. An ordinary stationery product. But there are two things I can do.
- Since there is no coating on the glass, light enters from every angle and reaches to the sensor. There is a general fogging issue in the image. I’m going to make a hood to prevent this.
- Wide open aperture almost always gives soft results. Even real lenses sometimes produce unacceptable images with wide open aperture. I can improve the image by using an external diaphragm right behind the magnifier.
It’s an M42 bellows. Since the external diaphragm is also M42, I attach it directly to the front of the bellows.
Paper cups are suitable for producing hood. I cut the bottom of one and tape it on the magnifying glass. It’s not gonna work because the cup is white. So I cut a piece of the black cardboard, roll and fill it in the cup. It’s not perfect, but it fits nicely in the cup. After correcting the edges using scissors, the magnifier starts to look like a lens.
I will hold the magnifying glass in front of the bellows as shown above during the shooting. I have experience with setups like this, although it is a bit troublesome to change the size of the bellows for focusing and keep the magnifier there at the same time. Looking at the viewfinder, I close my diaphragm to an average. I have no idea about the value of the aperture.
Yes, that is it 🙂 We managed to use our magnifying glass as a lens. It gives a soft image that looks like old webcams or movies we watch from VHS video tapes. But it works. We did our own free lens.
There are a lot of thoughts, plans, etc. in my head, but if I turn this into a race it won’t come to an end. Also it’s going to cost a lot. For now, it’s just enough to enjoy my magnifying glass a little more. Let’s work on a cat.
Now we can end the day here. We’ve reached our goal, had fun, learned. And if we have other ideas in our minds, what else do we expect? 🙂
