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Catadioptric or mirror lens Bokeh, how to create it


John Jovic

Catadipotric and mirror lenses are characterised by their harsh ring bokeh which, although not always desirable, is a notable point of difference and can be used for creative effect. But you don't need a 'cat' or mirror lens to achieve this bokeh as a very similar effect can be created using a clear filter, such as UV filter, with a 'spot' in the middle.

The spot can be made with a small round label or sticker, such as those you'd find in any office supply store. It doesn't matter how you make the spot as it only needs to be opaque (i.e. it blocks light totally) and in the examples below several stickers have been placed on top of each other to make sure they block all light. But use common sense. Don't damage an expensive filters coatings by putting a permanent and difficult to remove sticker on it, so use a cheap filter and/or make sure the sticker is not 'too' sticky (the oil from your fingers and handling could do the trick).
The lens used for these tests has a cheap filter with a spot stuck in the middle.  

With a small spot in the centre of the lens all of the bokeh is dramatically effected, not just the highlights.


This is how the bokeh would normally look, ie without a spot filter.

The ring effect in bright highlights is just the most noticeable effect of a spot in a filter (and cat or mirror lenses). However, all of the Bokeh is effected in all parts of the image so all the OOF areas will be rendered differently and this can also be used for creative effect. In the examples below the otherwise smooth bokeh of the test lens has been made quite harsh and dramatic with the use of a spot filter. Harsh bokeh is often avoided and scorned but it is just another optical quality which can be used to great effect in the right circumstances. The important point here is that you can control it as long as you are starting with a lens that already has relatively normal/soft bokeh.

Harsh bokeh is created in the OOF areas with the spot filter.  

Smooth OOF areas with no filter.

The degree of the effect will vary with the size of the spot and the lens used. Some experimentation will quickly tell you what works for you and your chosen lens. If the spot is too large then you'll soon know it as vignetting and flare will become apparent, especially as you stop down. A small spot will permit you to stop down to some extent before flare and vignetting become objectionable.

The 80mm lens used in these examples typically has an unobtrusive bokeh.   In this example no spot was used so this is how the bokeh would normally look with most lenses. The out of focus (OOF) highlights here show nothing unusual.
Small spot. Unless stated otherwise, this small spot was used in most of the sample images on this page.   This is an example of the effect from the small spot. The effect is mainly seen in the OOF highlights as a bright ring. There is a small increase in veiling flare.
Large spot, too large in fact because it caused a great deal of flare and some vignetting however this same spot may have worked well on another lens.  

This is an example of the effect from the large spot. The effect is mainly seen in the OOF highlights as a narrower bright ring and in more veiling flare than compared to when the small spot was used.

Further examples are shown below, all with and without the small spot filter.



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