Understanding Bokeh Variation Between Lenses

I was wondering if someone could help us understand what causes the variation in Bokeh (i.e. blurring of background) between various lenses. This is an area of lens physics that I know nothing about.

Mike Wooding and I were shooting from a blind yesterday. He was shooting with his Nikon 200-500 f5.6 lens and I was shooting with a Nikon 500 f4 VR-G lens. From the same distance, both shooting at 500mm using the same body (D810), same aperture with the subject on the same perch, the blurring of the (same) BG was very different between the two lenses. With my 500 it was much smoother and much more pleasing, like I was shooting at an aperture at least one stop faster (i.e. him at f8 and I at f5.6) We were both wondering why any 500mm (even if it is a zoom lens) would not render the BG exactly the same with all the same settings. I am sure there is a logical (physics) explanation for this effect, but we were at a loss for what it is. Can somebody explain why this happens? Is it because one lens is a zoom and the other a prime?

Much appreciated

There are many factors from the number aperture blades, to the type of glass elements, to the grouping of those elements, to the coatings - just many many variables to what an out of focus area looks like. It is not surprising that an $8000 lens would outperform one selling for a small fraction of that, especially a zoom. Don't confuse depth of field with Bokeh - at the same settings the two should have the same depth of field but how it renders the out of focus areas could be and would be very different. For example, an extreme case that illustrates the point would be to shoot an extremely OOF specular highlight with your 300 f/4PF and the older 300 f/4D. The OOF specular highlight will be very different, one will have an chopped in half onion look (PF) while the other will be relatively smooth (D). This is due to the very different optics used between the two. Another example would be to shoot the same OOF highlight with a lens with just 5 aperture blades and the other with one that has 9 aperture blades. The OOF highlight will have a somewhat pentagonal look with the 5 aperture blade lens and almost round with the 9 blade one.

Okay, thanks EJ, and I understand what you are saying and that the design of the two lenses is different enough to cause this. Any idea what it might be about the Nikon 200-500 that would be so different with blurring the BG? Would the same apply to any zoom; i.e. Tamron, Sigma etc.or is each lens design so different that it could vary greatly?  This is definitely another factor that I had never really thought about before this when deciding on a lens purchase or switch. The BG was so noticeably nicer with the 500 VR f4G. Could it also vary between brands; i.e. Nikon 500 F4 vs Canon 500 f4 vs Sigma 500 f4?

Interesting question. Looking forward to following this. It seems that bokeh is not being discussed frequently in zoom lens reviews? Is prime vs. zoom the biggest factor in the quality of the out of focus areas?

Richard

Richard B. wrote:Interesting question. Looking forward to following this. It seems that bokeh is not being discussed frequently in zoom lens reviews? Is prime vs. zoom the biggest factor in the quality of the out of focus areas?

Richard

No I wasn't saying that at all,  was just saying that when comparing a $1400 consumer zoom vs an $8000 profesional prime, there is bound to be more care in the optical design and manufacturing towards something like how the OOF part of an image looks.  here is an article that delves much deeper into the factors affecting this:
http://bokehtests.com/styled/index.html

And here is a shorter simplified article on it:
http://www.whatdigitalcamera.com/techno ... sign-93535

Thanks for the links EJ. So if I understand this correctly, it has little to do with zoom vs prime and all to do with the quality and design of the lens. If that is correct, then it is possible that the Sigma 150-600 (or any other quality zoom) could have very nice bokeh because the lens design my be better or different for that result? And equally possible that another similar prime, but with slightly different design, may have noticeably worse bokeh? This is definitely something I have not thought much about when considering the purchase of a lens. Since most of my photography is bird photography, this is a very important factor for any lens choice, and definitely something I will be paying more attention to in the future.. 

So did you notice any difference between the Sigma 500 f4 and your older Nikon 500 f4 in this regard?  The difference between the 200-500 and the 500 prime was not insignificant . . . . . it was very apparent!

Perhaps another comparison question would be the Nikon 200/500 versus the Sigma 150/600S ? I recognize that there are multiple other factors in evaluation of a lens, but as Tim stated I have not considered bokeh in lens decisions in the past. I would think it would be a valid factor to compare zoom to zoom as well as other lenses. And a real basic question, how do you measure/compare Bokeh?

Richard

I still think a zoom would be more prone because you have to make more compromises in lens element design to insure that you can project a sharp image at all focal lengths. The shape of those elements are probably not perfectly optimized for a single focal length but rather a compromise to be sharp at all focal lengths. So I think for any given focal length you could always design a prime lens with better bokeh balls than you could design a zoom and still get great sharpness through the whole focal length range.

The people that most consider bokeh are portrait shooters and you will see that most 85mm, 105mm, 135mm portrait primes take painstaking measures to control bokeh. there are even lenses with apodization elements built in specifically to smooth out bokeh. Both Sony and Fuji have lenses like this and they are much loved by portrait shooters, especially outdoor portrait shooters where you can completely defocus the background.

Google apodization lens if you want to learn more.

Just ran across this interview with the designer of Laowa lenses this AM, there is a question and good answer about the look of bokeh balls and onion ringing in it. It's between the pictures of the Bosch building and the bokeh balls if you don't want to read the whole thing:
https://phillipreeve.net/blog/the-man-b ... zero-d-fe/