TP: Bokeh Studies

Bokeh Studies

This article was prepared in draft form by Turerkan, and has been proofed by a couple of moderators. Please comment on it if you think there are any major inaccuracies.

Introduction

Photographic lenses are primarily made to deliver best possible image quality within the depth of field commanded by magnification ratio. However, achieving this was not enough for some. A niche among Japanese photographers got interested in the image quality in out of focus areas of their pictures. Provided that out of focus areas cannot be sharp, it must be something else that they were after. They called it boke, a Japanese word with the literal meaning fuzzy. The spelling bokeh became widespread due to ease of pronunciation (see this topic for pronunciation of 'bokeh'). In photography world, bokeh stands for the way out of focus areas are rendered by a lens.

Initially lenses were designed for sharpness within the possible depth of field. Good or bad bokeh was just a random outcome of the design process. As demand for good bokeh spread among photographers, lens makers became interested in the concept too. One of them was Minolta, which took on the Leica design philosophy that preferred an overall pleasing look instead of maximum sharpness. Unmeasurable qualities of resulting photos were judged more important than straight MTF test scores. Minolta pioneered bokeh research and created several special lenses with enhanced rendering of out of focus areas.

Most A-Mount photographers show interest in bokeh. Some chose A-Mount for its lenses with good bokeh while others discovered the secret magic of the out of focus areas as they acquired classic Minolta lenses.

What is good bokeh?

We understand that every lens renders out of focus areas differently. However, judging which lenses have good or bad bokeh is complicated. That is because bokeh is a matter of taste, there is no exact measure of how well a lens renders out of focus areas.

Fortunately there is a general consensus among photographers, who prefer bokeh that is soft, smooth and not distracting. Lenses having these qualities are popular among photographers and generally command higher prices.

Bokeh Samples

Here are some bokeh samples with evaluations. After closely examining these pictures, you will have a general idea on what to expect from lenses in terms of their bokeh.

Photograph by Dirk, using 85/1.4 RS. This shot demonstrates excellent bokeh. There are no visible disks, even the highlight in the background perfectly blends into the silky smooth bokeh.

Photograph by Sony, using 70-200 SSM A very hard situation bokeh-wise. Harsh highlights lead to visible circles in the bokeh. Also there are very fuzzy details overlaid on sharp details. Despite the less than ideal conditions, the lens did its job remarkably well. There are no distracting effects; the bokeh is not ‘busy’ as some would say.

General Wisdom On Bokeh

~ Prime lenses tend to have better bokeh than zooms.
~ Macro lenses tend to have better bokeh than regular lenses.
~ Telephoto lenses tend to have good bokeh
~ Retrofocal wideangles tend to have bokeh problems (A-Mount lenses below 50mm are retrofocal)
~ Under-corrected lenses have good bokeh (old lenses, Russians etc.)
~ Well-corrected long range zooms tend to have bokeh problems
~ Mirror lenses have tend to have very distinctive bokeh

Optical Knowledge on Bokeh

The technical background of bokeh is really complicated and a comprehensive explanation would be well beyond the scope of article. Hence, we will simplify the complex details before starting. Suppose that we have a small bright dot that we want to photograph - a single shining point of light. Our lens has 2 elements, and has a round aperture. Our camera is a simple box. The following diagram shows this setup in two test cases, in the first, the point of light is in focus, but in the second it is out of focus. Now we will analyze what happens through the process of taking a photograph.


Image formation on Sensor/Film plane.

The image of our dot is formed by a cone of light rays from the lens, the rays converge towards the sensor, and form a single point on the sensor/film plane when the lens is in focus. Notice that the image on the sensor has colour chromatic aberrations (CA), this is illustrated to show how optical aberrations affect bokeh.

In the second diagram the lens is focused on a different point further away. Our original point object is out of focus, the rays coming from it still form a cone, but this time the cone meets the sensor/film plane before it is a clear point. The cross-section of a cone is a disk, so we get a disk shaped image on the sensor, the disc is dimmer than the point because the light is spread over a larger area. If the aperture of the lens is hexagonal, the image will also take a hexagonal shape

What happens if we shoot a larger object? Just consider your object as a group of infinitely many small points. Each of them projects a circle shaped image on the sensor, and the overlapping of those images creates a smooth bokeh blur. Bright objects and light sources will get their circles exposed more than other circles around them, so their circles will not blend in, and be more noticeable. You can observe this effect in most real world shots where depth of field is shallow.

We can also see how optical aberrations affect bokeh. Optical aberrations are kinds of imperfections in the image projected by a lens. They are usually caused by laws of physics, the nature of the glass used and compromises made in lens design. All lenses display varying degrees of optical aberrations; usually a more expensive lens will be expected to have less aberrations. However those 'imperfections' are typically corrected within depth of field; for the out of focus, there is no strict line between good and bad. For instance Spherical Aberrations that decrease sharpness for in-focus objects may result in smoother OOF area rendition. The following illustration displays effects of various aberrations on bokeh.


Optical Aberrations and their effects on bokeh.

Exceptional Cases

a. Catadrioptic Lenses
Catadrioptic, Reflex or Mirror lenses as we usually call them, use a combination of reflective and refractive optics to deliver high focal lengths in a small and cheap package. Such lenses gain their optical power from curved mirrors, glass lens elements may be incorporated to provide corrections. Although there are different design principles, all reflex lenses have a mirror in their optical path, blocking the centre part of their front element. You can imagine this as a regular glass lens with a donut shaped aperture. As blur disks take on the shape of the aperture, mirror lenses produce distinctive donut shaped blur disks. These donut shaped blur disks tend to result in distinctive backgrounds. Care is needed to make sure they do not become too distracting and this can be a limiting factor with these lenses. The following picture shows the donut shapes from a mirror lens which can easily become distracting.


"Ebony and Ivory", a photograph by our member Dirk, using 500/8 Reflex lens.

b. STF
     
STF (Smooth Transition Focus) is a special lens design developed by Minolta. Currently there is only one lens of this type, the Sony/Minolta 135/2.8 STF. The unique point of this lens is the apodization filter placed next to the aperture. This filter gradually darkens towards the edges, creating a semi transparent region between the aperture opening and aperture blades. The effect this gives is that the blur disks from this lens gradually dissolve into the background, creating a smoothness that is impossible with conventional lenses. Read the Sony 135/2.8 STF Review for more information.

Choosing Lenses

In photography there is a lot to know and so much to buy. It's easy to go crazy in pursuit of bokeh, but one has to remember that good bokeh does not suddenly make you a better photographer. Using bokeh in your artistic expression requires awareness, a lot of experience and quite a bit of thinking. Be careful not to over-spend in your pursuit. There are many good lenses with good bokeh that do not break the bank, and deliver that creamy smoothness you have been longing for! Once you become confident in using bokeh as a tool in creating a good image, you will know what you need and how much you are willing to pay for it.

Also see Bokeh, bokeh and more bokeh.
     
Below you will find a small list of lenses with budget friendly prices that deliver good bokeh quality.
     ~ Minolta 70-210/4 “The Beercan”
     ~ Tamron 90/2.8 Macro
     ~ Minolta and Sigma 50/2.8 Macro
     ~ Sony/Minolta 50/1.4
     If you are fine with manual focusing, and want to get there very cheaply, you might consider M42 lenses. Two well-known examples with good bokeh are the Takumar 50/1.4 and the Jupiter 85/2.

External Resources on Bokeh
Vanwalree "Optics" - advanced articles on optics.
Luminous Landscape - on bokeh in general
Rick Denney - bokeh comparison of several old and modern lenses.
Ken Rockwell on bokeh
Flickr: Bokeh Smooth and Silky
You are a bokeh freak if you... -humorous look at the subject
The 135 STF -- a website by Dyxum member fother


The following additional examples were collected from the "Sample Images" forum. They are reproduced with kind permission of the photographers.

Kitlens showing bad bokeh




Lensbaby showing good bokeh




135 STF showing good bokeh




Cosina "plastic fantastic" showing good bokeh



Tamron 90 showing good bokeh




Sigma 17-70 showing good bokeh




Min 28/2 showing bad bokeh




KM 28-75 showing moderately good bokeh




Beercan showing good bokeh




Min 24-105 showing bad bokeh



Dirk's 135 STF -- great bokeh



Dirk's 85mm f1.4 again (good bokeh)





Comments are welcomed




Edited by Kiklop - 15 April 2010 at 23:09

Cosina "plastic" what is it?
and i think 28 f/2 is better than 24-105, Cosina and somewwhere 28-75

qwz wrote: Cosina "plastic" what is it?

It's the Cosina 100/f3.5 macro. Great performer, low price but feels cheap, hence its nickname "plastic fantastic".

Thanks for this article. I was never looking at bokeh, but you guys here from dyxum, have affected me with this stuff.

That is an interesting article, but it doesn't explain what causes the 'double line' (not an effect I had been aware of) or 'bright-edge' bokeh... I don't agree with him about roundness of aperture not being an issue, but I'll acknowledge that it isn't the only one...

Edited by Bob J - 03 March 2008 at 16:38

A very nice article on bokeh overall.

Here are a few thoughts/comments I have on the article.

The large image sizes, and or borders with text, makes the article challenging (and frustrating) to read with the horizontal scrolling required. You have to find the end of the message, scroll to the right, then go back up to find where you were reading, etc... Repeat for every image/text section in the article.

Laptop with 1280 x 800 screen resolution.

Some questions/comments on the second image in the article; specifically the following excerpts from the text with that photo.

photo quote - "Harsh highlights lead to visible circles in the bokeh."
Aren't visible circles inherit in all bokeh. Sounds like what you're implying is bokeh must be smoothed out (OOF background) with little detail.

photo quote - "Also there are very fuzzy details overlaid on sharp details."
Isn't that foreground bokeh which is brought about by the composition of the photo rather than the performance of the lens. How else would OOF foreground objects render?

Additionally, can you explain the difference between the kit-lens "bad bokeh" versus the beercan "good bokeh" photos. The bokeh looks pretty similar to me.

One last item. In the article I didn't see any mention of "points of light" as a bokeh technique, which can be used to advantage in low light scenes, cityscapes, etc...

@ glad2badad
Do you know you can get rid of the left hand column by clicking on the icon to the right of the RSS feed button? This column does not appear in the photo forums, but the subject is about lenses and we have to "suffer" this layout.

As stated, the author of the article was Turkerkan -- the opinions expressed are his, not mine, and he is not currently inhabiting Dyxum for personal reasons. But I have noted your comments and we will look at some editing.

Edited by brettania - 05 March 2008 at 04:19