Thursday, February 08, 2007

Super-Takumar 50 mm f/1.4

All test images were done using Fujicolour Superia ASA 200 colour negative film and scanned using a Fujifilm Frontier 340 digital minilab. Film cost: RM 9.50. Processing and scanning: AU$12.00.

Click here for large size image
Like all normal people, the first thing I do upon taking delivery of a lens is to admire the glass and aperture. The second is to mount it on the camera. The third, is to take photos of it.

The Super-Takumar 50mm f/1.4 lens is a very fast, sharp and widely available lens from the early 1970s.

By some (probably biased) accounts, it is the sharpest standard lens ever made.

The scene used for the sharpness test, with a $20 note for resolution tests and the paper as a metering and data-logging area. This particular shot was taken at f/1.4, where the lens is at its fuzziest. Note the brightness falloff towards the edge of the photo.

A close view of the centre of the test scene at f/1.4. The focus was probably incorrect, hence the incredibly soft image. As later tests would show, the focus is extremely sensitive at f/1.4.

A close view of the lower left corner at f/1.4.

The scene at f/5.6, where the lens is at its sharpest. Note the very even brightness across the frame.

A close view of the centre of the test scene at f/5.6.
"OMG, the sharpness can cut atoms!"

A close view of the lower left corner at f/5.6.
The slight gap between the $20 note and the white paper can be resolved, just barely.

There are more advantages to the f/1.4 than mere light-collecting ability. The huge aperture causes the depth of field to be extremely shallow; inconveniently shallow in some circumstances. This shallow depth of field causes objects outside the area of focus to be heavily blurred, which can be used to the photographer's advantage.

Depth of field at f/1.4.

A close up of the above image.
The region in focus is approximately between the 170th and 179th mm marks. The ruler is inclined at 45 degrees from the optical axis, thus the depth of field is 9 / sqrt(2) = 6.4 mm. The 175 mm mark is 47 +/- 3 mm away from the camera.

Depth of field at f/5.6.
Compare the clarity of the ruler's tick marks with the preceding scene.

Depth of field at f/16.
Note the sharpness of the blue photo frame on the right side. Compare with the above images to see the effect of shallow depth of fiels.

The lens mount is an M42, which is a 42 mm diameter screw thread. An adapter (usually inexpensive) is required for M42 lenses to fit onto other cameras mounts, such as the Canon EF, Nikon F, Yashica/ Contax, Pentax K and Olympus Four Thirds.

If autofocus and aperture-coupling is not a critical requirement, M42 lenses are excellent value for money. Well cared-for lenses are widely available on ebay at decent prices. The M42 era was before the age of plastics, and the lenses are (almost) completely constructed of metal and glass.

This might prompt some to say, “look at the quality of lenses in the past. Compare that with the cheap plastic cladding on those horrid Canon EF zooms. Where has all that quality gone?”

This is a completely misguided train of thought, as the original price of the old lens was not considered. Also, purchasing power in the 1970s is much much less than today. Buying a 50mm 1.4 was a major investment. And do you really want to bag full of metal prime lenses?

A major disadvantage of the Super-Takumar 50 f/1.4 is the use of thorium in one of the optical elements. The addition of thorium gave the glass some desirable optical properties (higher index of refraction) that made it require one less lens (hence better clarity), but the thorium glass is susceptible to browning.

The decay of thorium is radioactive in nature, although the emissions are negligible compared to background radiation. The decay causes the thorium lens to brown, sometimes severe enough to render the optical assembly useless.

This browning can be reversed by supplying energy in the form of UV radiation to the affected area. The following composite image compares the same lens before and after UV irradiation. The background is a sheet of white paper, suitably exposed to produce the same shade of grey in both cases.

Before and after UV irradiation.

As can be seen, the dark yellow cast in the lens has been greatly reduced by treatment. It is expected that further treatment will remove all visible traces of thorium decay products.


The Super-Takumar 50 mm f/1.4 is a fast lens with maximum aperture of f/1.4.
At maximum aperture, depth of field is extremely shallow, and vignetting is visible. The image is also appears to be slightly fuzzy.

The lens is extremely sharp at f/5.6. Sharpness at the image's edges is not compromised. There is no discernable vignetting, even on a sheet of paper.

The lens contains a thorium-glass optical element which darkens over time (in scales of decades). However, the process can be be rapidly reversed with UV irradiation.

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