Aperture and bokeh

[peety3]

I'm looking for a little enlightenment, or perhaps to generate some discussion. In reading Bryan's reviews, a lot of the lenses that have "many" aperture blades (8 blades for example in the 200 f/2 IS L) suggest that the number of aperture blades contributes to the bokeh quality. So here's my question: don't most of us tend to evaluate the blur quality at wide-open, maximum aperture? And as a result, isn't the aperture blade count irrelevant at that point, as the blades are now "in" the aperture housing? Certainly it'd be disappointing if the blur quality changed drastically as soon as we stop down 1/3rd of a stop, but...

[Keith B]

Aperture blade shape has to do with it also. Most of the wides have 7 blades but are curved, with the exception of the new 24 1.4L mkII it has 8 also curved.


Where it comes into play with your question is the shape of the out of focus elements. See Bryan's review of the 3 different 50mm lenses for great examples of this and aperture blade shape. The 1.2 and the 1.4 both have 8 blades but there is a significant difference in the bokeh.


Just like blurred out lights in the background everything out of focus gets smoothed and takes on the shape of the aperture blades. While not technical mumbo jumbo, the wider the aperture, the more space there is for things to get smoothed out, taking any fine hard edges away.

[Daniel Browning]

So here's my question: don't most of us tend to evaluate the blur quality at wide-open, maximum aperture?

No. If you always shoot wide open and never stop down, then it would make sense to only evaluate bokeh wide open. But most of us also shoot stopped down.

And as a result, isn't the aperture blade count irrelevant at that point, as the blades are now "in" the aperture housing?

Yes, the aperture blades are irrelevant to wide-open shots.

[peety3]

Thanks for saying no. That helped.

[Keith B]

Sorry I missed what you were asking. Wide open went in one ear and out the other.

[Sean Setters]

You mean in one eye and out the other? []

[Keith B]

No, but I am tired of the strange stares I get with my ear pinned up against the monitor.

[Jon Ruyle]

Okay. Here is my question.


Suppose two lenses have the same focal length and aperture and are both perfectly corrected. You shoot them both wide open. Do they have the same bokeh?


I guess what I'm asking is, what are the parameters that determine bokeh? Correction, aperture, blades, sure. What, if anything, else?

[samoksner]

The optical design is first and foremost the deciding factor being bokeh. The camera to subject and subject to background distance ratio also plays an important part. Lens coatings play a part, although minor and lastly when stoped down, even 1/3rd of a stop, the roundness of the aperture blades. The more blades, the easier it is to have it round.

[Jon Ruyle]

I don't think I was very clear when I asked my question. I guess what I'm getting at is that


1) it *seems* that given the image of point a some specified distance on both sides of focus, one should be able to determine bokeh exactly, modulo off axis aberrations (which are, as far as I know, always considered undesirable).


2) the well corrected out of focus image of a point (not counting diffraction) is the aperture itself.


This seems to indicate that the only way to modulate bokeh is by shape of blades, or by over/under correction. I've often heard, however, that the shape of the blades isn't really that important... that the design of the lens is more so.


It would be interesting to see a comparison of the 50mm f1.2 and 50mm f/1.8 stopped down to, say, f/2.8 using an external round aperture. I wonder how different, if at all, they would be then.