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I've used the Pentax Super Takumar 50 f1.4 and love the lens but notices that in certain lightning situations I get a yellow blur in the shape of the a lens blade. Using my hand I can sometimes stop the glare and other times it might not work. Below is an image of the problem.
I know lens have many different coatings but is there a way to know how good a coating is, how it compares with other coatings or at least stops the problem I have?
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Lens coatings vary on a lens model by lens model basis. There are quite a few different models of the Pentax Super Takumar 50mm f/1.4. Some had 8 lens elements, some had 7 lens elements. Some had 6 aperture blades, some had 8 aperture blades. Some had different coatings than others. Some had open aperture metering but most did not.
In the case of the Pentax Super Takumar 50mm f/1.4 that was made long before the digital era of consumer photography it probably has fewer internal lens surfaces that are coated at all than many similar lenses made more recently and optimized for digital cameras. Film is generally much less reflective than the filter stack that sits in front of the sensor in digital cameras. The increased reflectivity of digital sensors has forced lens makers to put coatings on the rear surfaces of lens elements as well as on the front surfaces that were coated in the film era. When lenses without the additional rear coatings are used with digital cameras, they tend to produce more flare - particularly a type of flare known as ghosting.
To further complicate things, some of the Super Takumar lenses used thorium glass that is slightly radioactive for one of the rear elements and it tends to yellow over time. Some owners claim that leaving the lens exposed to UV light (such as sunlight) will remove the yellow cast from the glass.
If your lens has a thorium element it would explain why the lens flare you are seeing has a yellow cast to it. If the flare you are seeing can be prevented by blocking stray light from outside the field of view a lens hood would be a solution you should consider.
