MTO 500mm f/8 - a fine example of a catadioptric lens

over 1 year ago (Edited)

What do you imagine when you think of a photographic lens? A silly question, isn’t it? Everybody knows what a lens is – an elongated object holding glass elements. One of its end is adapted fit a camera body.

Here is one of the special lenses in my collection - the MTO 500mm f/8.

Wait, what is this shiny piece in the centre of this lens? How will the camera take the photo with that in the way?

Well, seemingly it can, but the results are unusual and have their own character. Because of the obstruction in the middle of the lens, the bokeh pattern is not circles but rings.

I posted some similar (and I would say better) abstract photos like the one above some time ago.

https://ecency.com/hive-194913/@taimen/abstract-photos-using-very-shallow

Catadioptric (mirror) lenses

This kind of lens is called a catadioptric or mirror lens. It resembles some telescopes. The reason why this weird construction is useful is that it allows light to travel towards the camera, reflect from a mirror at the back of the lens, then reach a second mirror at the front of the lens (on the back of the central obstruction) which finally directs light towards the camera sensor or film. Essentially, light travels twice inside the tube of the lens which makes it possible to produce lenses with longer focal length that are physically shorter. How willing would you be to carry around a lens that is half a meter long? For the users of imperial units, that is 1.7 feet. I would need a huge hiking backpack and a helper to carry all my gear…

Here is my attempt at drawing the construction of the lens. A better picture can be found, for example, in ref. [3].

In this picture, you can imagine that the camera body is on the right, coloured in gray, and the sensor or film lies at the black line. The other big black rectangle is the lens. The glass elements are shown in cyan. The blue surfaces are actually mirrors. The dashed gray line is the optical axis of the lens. I drew two light rays entering the lens, passing around the obstruction and reaching the primary mirror - the two fragments with mirror surfaces on the right-hand side. The primary mirror reflects the light to the secondary mirror situated on the back of the front element. It reflects light back towards the camera. Additional glass elements are responsible for directing the light towards the sensor or film, and for removing optical aberrations.

The MTO lens

While quite heavy at 1.2 g = 2.7 pounds (according to ref. [2]), this MTO 3M-5A 500mm f/8 lens is still acceptable to carry around and use handheld for a while. Your hands get tired if you hold it for long, so you need to take breaks. Having a tripod is not a must on a sunny day but you should be prepared to use high ISO if you decide to go to the forest.

Use cases

Astrophotography

Obviously, a lens with a long focal length is suitable for objects that are far away. The Moon is a great subject, and so are sunspots. Eclipses are also nice to shoot, though, timelapses are tricky because the Moon or the Sun get out of the frame too quickly. The field of view is only five degrees on a full-frame body. My APS-C sensor has a crop factor of 1.5x which restricts it further. Somebody with a tracking telescope mount would be able to just set it up and enjoy the eclipse.

I have captured enough frames for timelapses of both lunar and solar eclipses, but so far, they have been sitting in my digital archives because I need a program to recognise and align the frames together. Please let me know if you have used one yourself!

Lunar eclipses are beautiful, producing copper tones in the Moon.

Solar eclipses are even more exciting. Sadly, I was only able to observe a partial eclipse from my country at that time.

Bird photography

Long lenses and bird photography is a classic. Some of them are fairly brave and tend to come close to people.

Especially if you bribe them.

Others prefer keeping distance.

This particular species has been quite hard to capture.

I like its extremely bright and saturated beak. I did not blow up the saturation slider in Lightroom, this is its natural colour.

As you can see, the background that this lens produces can sometimes be restless. If you take a modern 135mm lens and set a wide aperture, there will be a perfectly smooth background behind the birds and make them pop. Still, I would sacrifice the smoothness for magnification. Nowadays, there already are iOS apps that use AI to recognise the subject in the photo and simulate a wide aperture. The results are amazing.

Macro photography

The closest focus of the lens is 2 meters, but with an extension tube, you can gain some working distance. Still, I would not really use this lens for tiny details because I have much sharper ones. This MTO 3M-5A is among the sharpest mirror lenses available (and much better than modern ones), though the central obstruction causes diffraction and lowers the contrast and sharpness.

Abstract and other

I already showed some abstract examples, but here is a slightly more generic photo, nothing special... except for having a huge ring. I wish it was brighter.

Sometimes the character of the lens can actually be masked. I would not have believed this one was taken with a mirror lens if it were not mine.

Final words

I am a proud owner of this marvelous lens. Aparently, it usually stays home, but I rarely regret the occasions when I took it along. Maybe my shoulders would complain a bit. I would say the biggest issue for me is that the aperture is just f/8. Obviously, the central obstruction reduces the amount of light that can enter the lens, so inevitably it is a slow one.

P.S. I own two more mirror lenses, the bigger brother MTO 1100mm f/10.5 and a faster one, Rubinar 500mm f/5.6. Should I make a short review of them, too?

Acknowledgement

This post was inspired by a discussion with @qwerrie a while ago.