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Alex's Gallery: DOF, Hyperfocal distance, and Bokeh

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Depth of Field

Depth of Field : a good site with some technical information ^*
Outsight Photography Depth of Field Calculators nice and easy to use site ^**

If you want to dig a little bit deeper into the math behind this concept, look at this article by Michael K. Davis. There he deals with DOF, circle of confusion, hyperfocal distance, and other related topics.

I won't go into formulas here but would like to point to one of the interesting things about DOF. DOF depends on magnification and aperture only. It does not depend on the lens's focal distance. In other words, if you use a 200mm lens to make a head and shoulder portrait of a person at f4, the DOF will be the same as if you used a 50mm at f4 lens to make the same portrait. However, rendering of the out-of-focus areas might be different. It is a lot harder to quantify, therefore I plan to conduct some tests.

The most current discussion about COC and correct viewing distance could be found in PDML archives (look for subject: " Circle of confusion/print viewing revisited... ").

Hyperfocal Distance

Knowing about this concept and being able to use it will save your day when you have to use one of your favorite manual setups in a situation when an autofocus system seems to be in order (when subject = children). To calculate this value, you might use online calculators, like
Outsight Photography Depth of Field Calculators

Depth of Field of "teleconverter + lens" combo

1. TC is responsible for the loss of light in the TC+lens setup. For example, 2xTC "makes" an 100mm f8 lens out of 50mm f4 lens.
2. 2xTC does not really make a 100mm lens out of 50mm. 2xTC is a collection of lens elements that magnify the central area of the image that is produced by the lens that is sitting on that TC, and that makes it look like the total "mm" of the setup is doubled.
3. TC also doubles COC (circle of confusion) values for each individual "sharp detail" in the picture that is produced by the lens in front. This means that the resulting number of details that are "still sharp" (whose COC value is still within the limits of sharpness) is decreased by half, which is the same as saying that the resulting DOF is half of the original.
4. DOF of the 100mm f8 lens (2xTC+50mm) combination is similar to that of the 50mm f2 lens. To maintain the DOF of 50mm f4 lens lens, you'd need to stop down the TC+lens combo down to f16. This means that you can "make" a 100mm lens with the same DOF as the original 50mm f4 lens at the expense of 4 stops of light.

Bokeh

"Bokeh refers to the quality of the out-of-focus "blur" surrounding the in-focus objects in a photograph. Bokeh is pretty important in portraits, as a rule, where usually you want only your subject in focus, while the rest of the background you want to be out offocus. In such photos, that background blur can appear smooth and pleasing("good bokeh"), or it can look harsh and distracting ("bad bokeh"). Fred (CetusPhoto@aol.com)

Now that we know what it is, you might want to see some examples of the good and not so good bokehs (I just made up a plural for bokeh) produced by various Pentax lenses. In fact, bokeh has been one of the major reasons that started PDML's subjective lens testing project. You may review it here (this site is owned by Valentin - one of the founders of the Pentax cave society).

It appears that there are many factors that contribute to the "good" bokeh, such us:

• number of aperture blades - important to keep the out-of-focus blur round and centered
• shape of the aperture at wide open - same reason
• lens's focal length - the longer it is, the easier it is to create a blurred blob on the image (see below)
• lens's overall design - seems to be the most important factor. There are lenses that seem to have everything they need to produce a good bokeh, but they don't. there are lenses that have irregular aperture shape, yet they produce very pleasant bokeh
• object itself - such us thin dark lines on a light background, etc.

Since bokeh is obviously affected by the lens's design, could there be a lens with adjustable bokeh? If you look at the Nikon web page ( this URL specifically ) , you'll find a DC lens, where DC stands for Defocus Control. It seems that this lens gives you an option of selecting a pleasing bokeh for foreground objects or background ones. Is this really an adjustable bokeh lens? Decide for yourself. Do you need to switch to Nikon system to be able to use it? I think not. Pentax system has enough lenses with good bokeh of both foreground and background objects to keep one happy.

I apologize for not remembering the name of the PDML member, who suggested this Nikon lens link.

Here is a good example that shows how bokeh varies with focal length in a normal lens:

I always explain the change of bokeh with different focal lengths to my friends this way: Imagine that a 50mm will give you a image very similar to the one you see with your naked eyes. Now take a picture of a bottle next to you with the moon behind it. The Bottle and the Moon will have almost the same size. Now use a 300mm telephoto, and change the distance in order for the bottle to maintain the same size (magnification) in the frame. Now the moon is much larger than the bottle. The circle of confusion (actual DOF) may be the same (given the same f.stop), but the relative proportions of the objects change, so the bokeh maybe different with the two lenses, leaving the *impression* that the DOF changed. You have to choose the one that best pleases your eyes. I myself prefer using a light tele, say my AF105 2.8D, because it gives more abstract backgrounds, and makes the main subject to pop out in focus. Of course, with backgrounds close to the subject this effect is almost null. And be careful, because using too long teles may induce lens shake, so the pictures tend to be not as sharp anymore, unless with tripods (well, you already know that...). Have a nice weekend Nelson Kao (nelson_k@mandic.com.br)

There seems to be another meaning to this Japanese word: