"Exakta" was originally and for many years a registered trademark of the Ihagee-Kamerawerke Frankfurt/Main. After the bankruptcy of Ihagee-Kamerawerke AG, the brand name "Exakta" was sold to Japan. Cosina has mainly produced the optical calculations of old lenses under this name and sold them all over the world. For a number of years, the photo-shop "Porst" has also offered lenses under the name "Exakta" on the German market.
Especially this lens was manufactured in the mid-80s by Cosina and sold under various brands, like Vivitar, Exakta and Carl-Zeiss Jena. It was produced according to my research with various bayonets: M42, Olympus OM, Minolta SR, Canon FD and Pentax PK. In addition, it is the same as Cosina MC 24mm f/2.8. More information can not be found on the Internet about this lens.
Let us first come to the statistical data:
Mount: various, like M42 or PentaxPK a.s.o.
Lens construction: unknown
Closest focus distance: 12 cm (in Macro-position), otherwise 19 cm
Macro magnification 1 : 4
Number of diaphragm blades: 5
Weight: 161 gramm / 5,7 oz.
Length: 43mm / 1,7 inch
Filter size: 52mm
Apertures: 2.8 to 22
The lens feels very valuable, it is made of metal and solid. Aperture and focus ring work smoothly and without mechanical problems. Without an adapter, the lens is compact and lightweight. On my Sony a6000 it works with the massive PK adapter already slightly topheavy.
You get this lens under different brands, but the same optical quality often buy on flea markets or on Ebay. The prices are very different and usually vary between 20 and 60 euros. This is very favorable for a wide angle. For use on different system cameras you can buy simple adapters on the Internet. I have been using adapters from China for many years. These are very cheap - about 10 to 20 euros - and of excellent quality.
With a focal length of 24mm you do not have to worry about the possibility of shaking. Even with exposure times around 1/30s the photos are sharp. Provided the optics are good and perfectly calculated. This is exactly the case with the Exakta 24mm f/2.8.
When the aperture is open, the sharpness in the center is already very good, even if the picture impression has a little contrast and milky. To the edges of the picture, the photo becomes quite blurred and uneasy. The image quality improves with a slight stopped down. The sharpness is outstanding from aperture 5.6. The optical performance is above all doubt for this price.
Distortions are not too disturbing despite the wide angle and can easily be corrected in Photoshop or Lightroom.
Stopped down to f/11 can produce very nice star effects. Note the resolution and sharpness at this 100% crop.
One disadvantage is the Exakta 24mm however: it is extremely sensitive with backlight. I am not aware of whether the lens originally enclosed a lens hood. But it should not be difficult for the filter size 52mm to find a lens hood to screw in.
The Bokeh is very quiet and pleasant. Overall, a truly recommendable and inexpensive wide-angle lens that can be used as an excellent walk-around lens on an APS-C sensor.
Here are some pictures taken with the Exakta 24mm f / 2.8 in the last few weeks: