Home General Photography Let’s convert a camera lens into monocular/telescope

Let’s convert a camera lens into monocular/telescope

by Güray Dere

Last weekend when we went to visit the elders, I found my binoculars in the storage where my things used to be, the binoculars I had tried so hard to buy in middle school. I had saved up for a long time and then found it in Russian bazaars. It was already old. Now it is worn out. The eye adjustment doesn’t work. The axis is broken. I’m sorry to see it like this, but it hasn’t lost its old leather smell. It immediately reminded me of the excitement when I first touched it. I hadn’t put it down for years.

Even such a mid-spec binoculars was very useful for astronomical observations. It could make easy targets visible, such as the major nebulae, Andromeda, 4 of Jupiter’s moons.

Now that time has passed, my curious little girl is next. The day I got my new wooden tripod, she claimed the old one. I couldn’t break her, so I said, “Well, let’s make you a pair of binoculars on this tripod.” She asked for her binoculars almost every day until I ordered the lens and adapter from eBay and the postman brought them to her. She’s happy at last. She shows her gift everywhere. She even wanted to take it to her school and kindergarten class.

Monocular / Telescopic Converter

We can call them binocular or telescope adapters. You may remember the concepts of objective and ocular. In devices such as microscopes and telescopes, lenses consisted of two parts. The part that the eye looked at was called the ocular. This is the one. The other part is our lens as you know it.

The rays of light coming through the lens are gathered at the focal distance of the lens, just like in a magnifying glass. A lens creates a clear image on the sensor, but not in our eyes. Because there is an extra lens in our eye. This lens distorts the work! So we need to use another lens to bring those rays to an angle that is compatible with our eye. The ocular lens makes it compatible with the eye. The ocular focal value also affects the magnification.

M42 ve Pentax PK teleskop adaptörleri
M42 and Pentax PK telescope adapters

Telescope adapters must not be very popular because they are sold at affordable prices. If you want to buy brand new ones, there are not many alternatives and they are expensive. But old ones sometimes go for scrap prices. It is possible to find them for every camera mount on the market. Especially for mounts that are not used much anymore, both lens and adapter can be very cheap.

M42 ve Pentax teleskop adaptörleri
M42 and Pentax PK telescope adapters

So I looked around and saw different models and bought one mount adapter for my daughter (M42) and one for my own lenses (Pentax PK). The Pentax one is the original Asahi Pentax, so the quality is a bit higher. They also put a rotating focus adjustment on the eye piece, which allows you to focus much closer than the minimum focusing distance of the lens if you want.

Let’s talk about lens selection

Tokina 80-250mm Autozoom f4.5

After the adapter part was done, I had to find an M42 mount zoom lens. I chose M42 as a mount because they are not preferred and they are very affordable. I was looking for two things in terms of features: a wide zoom range and tele ability. And a tripod connection. Because it will be heavy to carry in the hand, and because it will give a high level of zoom, handheld use will be very shaky.

Telescope adapters usually use 10mm eyepieces. When calculating the magnification, the two lenses are proportioned to each other. So if we use the lens at 80mm, 80/10=8X, if we make it 250mm, 250/10=25X. I don’t know what these Xs are according to, not exactly according to what we see with the naked eye. Because a lens around 35mm creates the same effect as the naked eye. So it cannot magnify.

There were a lot of candidate lenses. So I followed them all for a while and saw the average prices. A few weeks later I bought a very clean looking Tokina 80-250mm that caught my eye at that moment. The prices are very reasonable. The postage costs more than the price of the lens. Customs must have been confused too. They called me for an inspection. They said something pithy like, “No way, these are the most expensive things.” I was expecting this and to prove that it was not a modern thing, I showed them the lens on an old Zenit film camera body I had taken with me. After weird comments like “Is this the biggest lens?” they were convinced and gave me the lens. I picked it up and came back.

Dürbünümüz hazır
Our monocular ready for observation

Since the lens has a tripod collar, we immediately placed it on the old Benro, attached the adapter to the back of the lens and our monoular is ready. “I will make the first observation.” We had a competition with the little one and I won!

Now it is a bit difficult to explain what it looks like when you look through it. But in order not to keep you in wonder, I did my best and I put the cell phone against the eyepiece and took photos. When we look at it with the eye, the image is not centered and vignetted, it is much bigger and much clearer. The cell phone lens doesn’t get very close to the lens of the eyepiece, so the image is only in the middle. Also, today is a dark day in Istanbul with a lot of snow. Due to the precipitation and lack of light, the cell phone camera had a hard time. I had to hold the phone with my hand, so I also struggled, especially at high magnifications. Also, I didn’t go outside in the freezing weather. I shot through double glass windows. The dust and reflections of the glass affected the sharpness. Anyway, you know, you have to look at it with your eyes.

The lens works from 80mm to 250mm, so you can zoom in and out nicely. By changing the aperture we can play with the light conditions and depth of field. I shot with Tokina at f11. f11 is the aperture at which this lens produces the sharpest image.

Let’s look at our landscape first. This is what it looks like when I shoot with my cell phone:

Test çekimi için manzara
Landscape for test

I chose the building ahead as my target. We will look at the marked area at different focal lengths. First I set the lens to 80mm.

80mm ile görünüm
View through 80mm

80mm is similar to the magnification of normal binoculars. A good value for general landscape viewing. It can be used comfortably in the hand as the shake is not very bad. Let’s take a look at 250mm.

250mm ile görünüm
View through 250mm

Now that’s what I call magnification! We can see distant details. But holding it in your hand is very tiring for the eyes. It shakes a lot. At this magnification you have to use it with a tripod. Once the lens is stabilized, the image is very good.

Hand scope

Tele zoom lenses are big and heavy. If we want to have a handheld scope to throw in our camera bag, I would recommend 135mm. The zoom power is good, but it is still suitable for handheld use. It is also convenient in size and easy to carry. Those who are into bird and nature photography can come up with such a solution if they don’t already have a pair of binoculars.

Pentax 135mm ile el dürbünü
Pentax 135mm with hand scope

The K series SMC 135mm f2.5 is one of Pentax’s sharpest lenses. When the adapter is also made of Pentax glass with SMC coating, I can’t help but think, “Who knows how much you have to pay to get a scope with such a sharp image?”

135mm ile görüntü
Image through 135mm

Using the 135mm with the scope adapter, the zoom is as shown above. In this shot I used the aperture at f8, which seems ideal to me. The sharpness is amazing to the eye. In dim light or in the evening there is a chance to use this lens with an aperture as open as f2.5. The characteristics of the lens directly determine the quality of our scope.

Sky scope

500mm teleskop
500mm telescope

Or a telescope. Long tele lenses are a bit more than scopes. There is no chance to use them handheld because of the vibration. Here I’m going to use a mirrored lens to bring it a bit closer to a telescope. Mirrored lenses are also known as Catadioptric or Maksutov designed telescopes. The lens I have is the Russian made MC 3M-5CA 500mm, which is also a good mirror lens with a m42 mount.

When we add the telescope adapter, it looks like the one on the side. Since I didn’t have a tripod mount this time, I tried to roughly fix it on the tripod with hair elastics. Some models also have tripod mounts, but then they are a bit heavier.

There are also 1000mm and above models of mirrored lenses. These are a bit difficult to use because they are large. But there are models between 250mm-400mm, which give a sharp image and are as small as a kit lens in size. But because they are rare, they are difficult to obtain and their prices are high.

500mm dürbün görüntüsü
500mm telescope

With the 500mm, we got closer to the target. With these optics, when I look at the birds in the trees around, I can get portrait-level images and examine their eye colors. Since the lens is f8, it requires light for normal photography. It gets along well with cameras that can give high ISO. When we use it as a scope, there is no problem. The ISO value of our eyes must be quite high 🙂

We could go a bit further and use a teleconverter. In fact, let’s do it now.

1000mm dürbün
1000mm telescope

The Pentax Rear Converter A 2X-S teleconverter doubles the focal value of the lens. So we will be using a 1000mm lens. You know there are disadvantages to using a TC. We lose a bit of sharpness and light. Well, let’s see.

1000mm dürbün görüntüsü

That’s a bit overkill. The sharpness is far from superb. So it’s hard to tell where the focus is. I held the cell phone in front of the lens and tried not to shake it and I barely got this picture, but I don’t know if I really focused there or not. It gets very sensitive. I don’t think we’ll need the 1000mm, but at least we’ve seen it.

And of course, I set up the mirror lens in the garden one night and focused it on the moon. Then I went inside, dressed the little one tightly and said, “Come and see what I have to show you!” She leaned over curiously and put her eyes to it and her jaw dropped. The excitement in her voice when she said, “Ooo! Dad, I can see the mountains above it!” made all these efforts pay off.


I think the telescope adapter is a great thing. It turns idle lenses into fun toys. Let’s compare it to a real pair of binoculars


  • We can have a wide range of zoom ratios. We can have flexible magnification with zoom lenses or very high magnifications with tele lenses
  • Because we can use aperture, we have control over the depth of field and the amount of light.
  • We can focus the closest focus of the lens (or even more). With real binoculars we cannot look close. Focusing starts at 15-20 meters.
  • And the price. Quality binoculars will cost much more.


  • Because we only look with one eye, it can be tiring after a while.
  • A real binoculars will be more ergonomic during use and transportation.
  • Real binoculars are more durable. Our lenses are often fragile.

I had difficulty at one point. Especially at high magnification, it is necessary to use it on a tripod. But then it becomes difficult to look around. It’s not easy to loosen the head, adjust the position, tighten it again and again on my ball-head tripod. Especially considering that I gave it as a gift to my daughter, it’s quite difficult for a 6-year-old. There are tripod heads called “fluid-head” that are mostly used for video. These heads, which have a long handle on the back, can be adjusted to the desired degree of rigidity and allow smooth rotation with that handle. It would be great to be able to observe a bird with binoculars in this way. I’m putting one of these on the buy list.

Caution: Never look directly at the sun through binoculars or telescopes. Do not even look through the optical viewfinder of your camera. It can cause permanent blindness.

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