Boketh, I’m a little confused about this subject of boketh.

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Tim Tucker
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Boketh, I’m a little confused about this subject of boketh.
6 months ago

Boketh
I’m a little confused about this subject of boketh.

Now I’m quite new to digital photography. I held out because of my love of black and white film. I first had a Canon ASP-­‐C which I didn’t get along with as I found it more of a point and shoot compared to 35mm. So I sold it and bought a Nikon D600, with the added advantage that I could use my old Nikkor lenses from my F2.

Back in my day, and I’m old enough to be able to say that, we used to worry about getting the subject matter in focus, now I see constant questions about out of focus performance of a lens wide open. Is this really a big issue, or are we forgetting some basic image management principals?
Using a lens wide open is not something I found to produce good results. Even the best lenses are a little soft at the edges wide open, so are you measuring a true strength of the lens or its inherent weakness?

Nikon F2, Nikkor PC 105mm f2.5 close focus at f4 or f5.6. Note the shallow depth of focus, it only just covers the eyes.

Now with my D600 and the 105mm lens the depth of apparent sharpness (I don’t like the term depth of field) wide open is too narrow to be of any use. I use at lease f4 or f5.6 as a minimum. Here are some rules of thumb:

1)Double the f-­‐number and the depth of apparent sharpness is doubled.

2)Double the distance to the subject and the depth of apparent sharpness is four times deeper.

3)Half the focal length and the depth of apparent sharpness is four times deeper.

So if I decrease the distance to the subject by half then the depth of apparent sharpness is reduced by a factor of four. Close up on the head and shoulders and your depth of field is reduced so much that f4 or f5.6 is the minimum I need to keep both eyes in focus. Keep the subject to background distance as large as possible and hey presto – boketh.

Nikon F2, Nikkor PC 105mm f2.5 close focus at f5.6

Place the subject in context with the surroundings and you can see the diminishing focus as you recede into the background. I’m still using f5.6.

Nikon D600, Nikkor PC 105mm f2.5 mid focus at f5.6, note the diminishing focus.

The background plays an important role. I was going to use a backlit tree as an example of bad boketh on any lens, until I realised I posted that photo as a good example! So I guess it’s just image management, some things look good, some bad. It just depends how you handle them, and how you want the finished image to look.

Nikon F2, Nikkor PC 105mm f2.5 close focus, probably set to f5.6.

However, even in my day (here we go again), there were lenses with bad boketh. Mainly mirrors and mediocre zooms, and there were a lot of the second category. We use a lot of zooms these days, with their multiple elements and tortuous light paths through enhanced coatings to reduce flame and aberration. Perhaps that adds an acutance to a boketh.

With ASP-­‐C sensors you are using focal lengths of half that of full frame so your depth of apparent sharpness is four times deeper in any situation. You will always be fighting it. To reduce your depth of apparent sharpness by a factor of four using the aperture is a change from f11 to f2.8. Back to a wide open lens again.

I still judge my lenses by how they keep thing in focus, not their out of focus wide open. Zoom lenses may have changed a lot, but not fixed primes. So am I missing something? Seriously, there are a lot of aspects of digital photography that I don't fully understand yet and are different to film photography.

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