Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Canon Lenses

Reply
 
Thread Tools Search this Thread
Old May 25, 2005, 9:41 AM   #1
Super Moderator
 
peripatetic's Avatar
 
Join Date: Nov 2004
Posts: 3,599
Default

I think there is much confusion about this.

I cannot see any logic to the claim that a lens with a maximum aperture of say f2.8 has better bokeh than a lens with a maximum aperture of f4.

Clearly DOF is reduced when the aperture is increased, but that's not what we're talking about. So how is one to evaluate a claim that a faster lens gives better bokeh? Or indeed exactly the opposite?

Mike Johnston's recent article on Luminous Landscape for example where he states:

for a fast lens it has pretty good bokeh

The full article makes it very clear that he believes that faster lenses usually have worse bokeh!

http://www.luminous-landscape.com/co...m-may-05.shtml

Another article claims:
There are a couple of myths about bokeh:

1. Bokeh is controlled by the roundness of the lens diaphragm.
2. Faster lenses have better bokeh.


The full article:
http://www.rickdenney.com/bokeh_test.htm

So at the very least there is considerable doubt, and a logical gap as to why two lenses with different maximum apertures would consitently display the characteristic that the lens with a wider maximum aperture would give better bokeh. In fact the second article allows one to decide whether agrees or not - there are lots of pictures, which seem to contradict the general claim that a faster lens gives better bokeh.

The only explanation that makes any sense to me is that in general, more expensive lenses are engineered to higher standards, and that includes the factors that lead to better bokeh. So as a general empirical matter it may well be true that faster lenses have better bokeh, but this has nothing to do with their aperture but rather that both wider apertures and good bokeh have a positive correlation with the higher quality lenses.
peripatetic is offline   Reply With Quote
Sponsored Links
Old May 25, 2005, 10:36 AM   #2
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

peripatetic wrote:
Quote:
I cannot see any logic to the claim that a lens with a maximum aperture of say f2.8 has better bokeh than a lens with a maximum aperture of f4.
FYI - http://www.wlcastleman.com/equip/rev...comparison.htm
NHL is offline   Reply With Quote
Old May 25, 2005, 11:00 AM   #3
Super Moderator
 
peripatetic's Avatar
 
Join Date: Nov 2004
Posts: 3,599
Default

NHL wrote:
Quote:
peripatetic wrote:
Quote:
I cannot see any logic to the claim that a lens with a maximum aperture of say f2.8 has better bokeh than a lens with a maximum aperture of f4.
FYI - http://www.wlcastleman.com/equip/rev...comparison.htm
That is not a logical explanation. It is a simple comparison between a fast and a slower lens showing that the faster has (marginally) better bokeh.

Which proves nothing towards a general claim of that principle. My claim is that you are falling foul of the fallacy of joint effect.

I am quite willing to accept that there may be a general correlation between fast lenses and good bokeh, but there is no causal relation. I have also been unable to find any possible theoretical explanation for the assertion.

How do you explain that in the second article I reference there are clear examples where the bokeh of a slower lens is better than a faster? If you claim all A's are B's then if someone finds a single A that is not a B then the claim is false.

peripatetic is offline   Reply With Quote
Old May 25, 2005, 11:57 AM   #4
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

peripatetic wrote:

Quote:
It is a simple comparison between a fast and a slower lens showing that the faster has (marginally) better bokeh.
Yeap - but this 'margin' is still visible when the f/2.8 is stopped down to f/4 as compared to the f/4 lens at f/4




Quote:
How do you explain that in the second article I reference there are clear examples where the bokeh of a slower lens is better than a faster? If you claim all A's are B's then if someone finds a single A that is not a B then the claim is false.
1. The test were done at f/4-f/11 to standardize the lenses - At their largest aperture f/3.5 is not that far off f/2.8

2. The distances subject/background are specific for this test which can all affect 'bokeh' if the distances were different the results will be different - i.e. theses examples are exceptions and not the rules... :?
NHL is offline   Reply With Quote
Old May 25, 2005, 12:21 PM   #5
daz
Senior Member
 
daz's Avatar
 
Join Date: Apr 2005
Posts: 436
Default

I think the question that first must be answered is what is bokeh followed by what is good or bad bokeh. I think this link may help. http://www.kenrockwell.com/tech/bokeh.htm This helped me to understand what is generally good or bad bokeh and how it is related but not totally dependent on what the aperture is set too. It shows how even two lenses with the same general specs i.e. same aperture and lens length can have "better" or "worse" bokeh. It is not just that something is out of focus and where (front or back) but in what way it is out of focus and if it is in front or back.

My two cents.

DAZ

daz is offline   Reply With Quote
Old May 25, 2005, 1:57 PM   #6
Super Moderator
 
peripatetic's Avatar
 
Join Date: Nov 2004
Posts: 3,599
Default

NHL wrote:
Quote:
peripatetic wrote:

Quote:
It is a simple comparison between a fast and a slower lens showing that the faster has (marginally) better bokeh.
Yeap - but this 'margin' is still visible when the f/2.8 is stopped down to f/4 as compared to the f/4 lens at f/4
Yes, that was clear from the link, and my point remains: So what? I don't deny your example, but it doesn't prove anything, you cannot generalise from the particular to the universal.

Quote:
Quote:
How do you explain that in the second article I reference there are clear examples where the bokeh of a slower lens is better than a faster? If you claim all A's are B's then if someone finds a single A that is not a B then the claim is false.
1. The test were done at f/4-f/11 to standardize the lenses - At their largest aperture f/3.5 is not that far off f/2.8
I think you need to make up your mind what you are trying to claim. Your previous point was that the faster lens gave better bokeh at all apertures. Now suddenly these examples are invalid because not comparing bokeh at maximum aperture. Is evidence is only admissible when it supports your argument?


Quote:
2. The distances subject/background are specific for this test which can all affect 'bokeh' if the distances were different the results will be different - i.e. theses examples are exceptions and not the rules... :?
Surely if your claim were valid the faster lens would triumph in all circumstances? From a logical point of view, a single exception disproves the universal. If there is an exception there is no rule.

So perhaps you believe there is a substantive claim to be made that faster lenses are better at particular distances? But that seems rather a weak claim to me, and we would then need to know what conditions and what distances.
peripatetic is offline   Reply With Quote
Old May 25, 2005, 3:09 PM   #7
Senior Member
 
BoYFrMSpC's Avatar
 
Join Date: Apr 2005
Posts: 339
Default

For all lenses > f2 , it is not the case that they have better bokeh. Sorry, couldn't help myself whenever you kept refering to universal proofs.

But discrete mathematics asides, you gave a link once upon a time about reading the MTF charts.

http://www.luminous-landscape.com/tu...ding-mtf.shtml

What seemed interesting was the statement around the end of the article. The closer the meridonial and sagital (dotted/dashed) lines are, the better bokeh.

When I read this, I was like :?:
So bokeh depends on the quality build of the lens??

And then I went to your second link, peripatetic, and I noticed that the picture with the Canon 70-200mm f4.0L looked really nice...

And then I looked at its MTF chart from canon, sure enough...

I would definitely agree that larger aperture is not (if it is, at all) the only factor to "good bokeh".
Attached Images
 
BoYFrMSpC is offline   Reply With Quote
Old May 25, 2005, 3:15 PM   #8
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

peripatetic wrote:
Quote:
Your previous point was that the faster lens gave better bokeh at all apertures.
Did I ever say that?

-> Who would close down their lenses to get better bokeh???
NHL is offline   Reply With Quote
Old May 25, 2005, 4:07 PM   #9
Super Moderator
 
peripatetic's Avatar
 
Join Date: Nov 2004
Posts: 3,599
Default

NHL wrote:
Quote:
peripatetic wrote:
Quote:
Your previous point was that the faster lens gave better bokeh at all apertures.
Did I ever say that?

-> Who would close down their lenses to get better bokeh???
So are you claiming that the bokeh of a lens is better wide open than stopped down? A cursory review of MTF charts shows that's not always the case.

The closer the meridonial and sagital (dotted/dashed) lines are, the better bokeh.

As BoYFrMSpC points out - with the the Canon 70-200mm f4.0L the bokeh (as indicated by the MTF) is clearly better at f8 than f4. So they would do well to close down the lens to get better bokeh.


Every other post you claim:

"Faster lenses give better bokeh."

And cite it as a reason for purchasing the Sigma 70-200 f2.8 rather than the Canon 70-200 f4 L for example.

I find it difficult to believe that you only use your lenses at maximum aperture, and even if you do, then encouraging folk to purchase a fast lens for better bokeh is misleading because some photographers frequently use their lenses at narrower apertures to get good DOF, that doesn't mean they don't want good bokeh however. There's a big difference between the bokeh of an L telephoto and a 500mm mirror even though both may be at f8.
peripatetic is offline   Reply With Quote
Old May 25, 2005, 5:20 PM   #10
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

Are you guys claiming the 70-200 f/4L has a better bokeh than the 70-200 f/2.8L by the look of the MTF chart?:



-> which is clearly not the case "The closer the meridonial and sagital (dotted/dashed) lines are, the better bokeh".


BTW the Sigma meridionial and sagittal are very close (regardless of focal lenght):



... at least closer than any of the Canon's :blah:



and yes I try to exploit the f/2.8 as much as I could (in full sun):

70-200 f/2.8 EX:

120-300 f/2.8 EX


... Are you guys smoking something???
This comparison clearly shows the f/2.8 has a better bokeh than the f/4: http://www.wlcastleman.com/equip/rev...comparison.htm and the MTF shows the Sigma closer meridionial and sagittal as well according to your own quotes from the luminous landscape.

Do you all have to hate Sigma that much? :idea:
NHL is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 8:49 AM.