[biz-engineer]

A while back, it was said that lens bokeh amount (blur) is a function of aperture diameter, or focal length divided by minimum f number.

Examples:

50mm @ f/1.4 => 50/1.4 = 35.7
100mm @ f/2.8 => 100/2.8 = 35.7
135mm @ f/2 => 135/2 = 67.5
70-200 2.8 zoom @ 200 f/2.8 => 71.43

Looking at those numbers, it looks like the 70-200 f2.8 zoom should be able to produce more blur than the 100 macro lens. But while using the D-FA 100 macro, I was able to produce a lot of background blur when getting close to subject. I realized the focal-length / aperture ratio might give a wrong estimate for lens blur ability. Minimum focus distance should be included but how?

[Sandy Hancock]

Focal distance is totally part of the equation.

What none of the mathematics accounts for is bokeh quality. To me at least, that matters much more.

[kypfer]

A while back, it was said that lens bokeh amount (blur) is a function of aperture diameter, or focal length divided by minimum f number.

... plus a whole bagful of other parameters, like how many elements in the lens, how well these elements are corrected for working under conditions that produce the required effect, distance from lens to subject, distance from subject to background ... and probably even whether the tide is in or if it's a new moon, depending on which article you read

Even the term 'bokeh', as with so many other hijacked buzz-words, can mean different things to different people!

To my mind, the only comparison that can be valid would be to set up a circumstance that produces an acceptable effect with one lens, then, without changing any other factor, replace the lens with another and compare the results then produced. The results you prefer then define the combination that works best FOR YOU. If that configuration with that lens can be used to produce acceptable images under different circumstances then you've found a set-up that works for what you want. Doubtless there'll be others that prefer working closer or further away from the subject than you do and will have their own preferred combination of lens and conditions.

Lots of fun and frustration can be had just trying to find one of these "holy grails"!


Enjoy the search

[GUB]

I guess these fellows have got it figured out !
How much blur? - A visual background blur calculator

[GUB]

Don't think "quality" is the right word here. Different bokeh suits different subject. Perhaps "attributes"?

[steephill]

The maths are usually used the other way round i.e. maximum aperture = focal length/front element size. Bokeh is more of a quality than a quantity and as has been said is much more than just depth of field which (although a calculated factor) embodies a lot of assumptions and is little more than a useful general guide.

[Sandy Hancock]

The maths are usually used the other way round i.e. maximum aperture = focal length/front element size. Bokeh is more of a quality than a quantity and as has been said is much more than just depth of field which (although a calculated factor) embodies a lot of assumptions and is little more than a useful general guide.

^THIS.

Depth of field (a quantifiable optical property) and bokeh (a qualitative artistic opinion) are not the same thing.

It's a bit like comparing track and field athletics to ballet.

[Serkevan]

^THIS.

Depth of field (a quantifiable optical property) and bokeh (a qualitative artistic opinion) are not the same thing.

It's a bit like comparing track and field athletics to ballet.

Wait, are you telling me that a ballet dancer doesn't win by spinning faster? I have been looking at it wrong all these years!

[BigMackCam]

A while back, it was said that lens bokeh amount (blur) is a function of aperture diameter, or focal length divided by minimum f number.

Examples:

50mm @ f/1.4 => 50/1.4 = 35.7
100mm @ f/2.8 => 100/2.8 = 35.7
135mm @ f/2 => 135/2 = 67.5
70-200 2.8 zoom @ 200 f/2.8 => 71.43

Looking at those numbers, it looks like the 70-200 f2.8 zoom should be able to produce more blur than the 100 macro lens. But while using the D-FA 100 macro, I was able to produce a lot of background blur when getting close to subject. I realized the focal-length / aperture ratio might give a wrong estimate for lens blur ability. Minimum focus distance should be included but how?

Yes... At any given focal length and aperture combination, depth of field reduces and increases with focus distance - which is why macro photography is typically performed at narrow apertures, and even then depth of field can be so shallow that stacking of multiple images (each one taken with minor focus adjustments) is required to get enough of the subject in acceptable focus.

[biz-engineer]

Yes... At any given focal length and aperture combination, depth of field reduces and increases with focus distance - which is why macro photography is typically performed at narrow apertures, and even then depth of field can be so shallow that stacking of multiple images (each one taken with minor focus adjustments) is required to get enough of the subject in acceptable focus.

Ok but how to factor in min focus distance when comparing different lenses?

[Serkevan]

Ok but how to factor in min focus distance when comparing different lenses?

Minimum focus distance has no effect unless you're focused at that distance. It appears in the equations as the subject distance, nothing more. It's just that some lenses cannot focus down to that distance.

[UncleVanya]

Some lenses have awful and busy bokeh while another lens of identical f stop and focal length is smooth as butter. This is far more complex than the formula given...

Appeture shape, aspheric lenses, etc all play a role.

[biz-engineer]

I guess these fellows have got it figured out !How much blur? - A visual background blur calculator

Yes, back ground distance also plays a role.

[Serkevan]

Some lenses have awful and busy bokeh while another lens of identical f stop and focal length is smooth as butter. This is far more complex than the formula given...

Appeture shape, aspheric lenses, etc all play a role.

I understood Biz was talking about the amount of blur more than its quality.

Quality, of course, is a completely different matter.

[biz-engineer]

Minimum focus distance has no effect unless you're focused at that distance. It appears in the equations as the subject distance, nothing more. It's just that some lenses cannot focus down to that distance.

Sure. Now if I use my 100 macro to make a brenizer (bokeh pano), I can produce more melted background compared to doing the brenizer with my 70-200 f2.8, although the 70-200 appeared at first to be a better choice for a brenizer.

---------- Post added 11-03-20 at 12:43 ----------

I understood Biz was talking about the amount of blur more than its quality.

That's right.