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05-03-2011, 11:44 PM   #1
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What is IQ and bokeh?

Before I came on this site there were a few terms that I'd never come across.

Can someone please explain to me what 'bokeh' refers to?

Secondly, what is the 'IQ' of a lens? What does it define?

Thanks

05-03-2011, 11:59 PM   #2
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IQ is the Image Quaility
Bokeh, is the quality of the Out of focus (OOF) blur.
05-04-2011, 12:01 AM   #3
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05-04-2011, 02:24 AM   #4
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And a Bokeh is controlled by the size of the aperture. The bigger the size the more bokeh and the smaller the size the bokeh effect is minimal. Also it has to do with the distance of the background to the subject.

Senior experts, please correct me if I am wrong. I just mentioned above what I've been learning from you.

05-04-2011, 05:03 AM   #5
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QuoteOriginally posted by sany Quote
And a Bokeh is controlled by the size of the aperture. The bigger the size the more bokeh and the smaller the size the bokeh effect is minimal. Also it has to do with the distance of the background to the subject.

Senior experts, please correct me if I am wrong. I just mentioned above what I've been learning from you.
Those are both true, quantitative aspects of out-of-focus features but are not what's commonly thought of as Bokeh which is the quality of out-of-focus features.

Good Bokeh is usually considered an out-of-focus spot with soft edges and bad Bokeh when such spots have hard of bright edges.


Bokeh sampler page

Sometimes hard edged Bokeh adds to the photo and sometimes not.

Bokeh is mostly a characteristic of a lens. Neutral Bokeh (out-of-focus disks smooth to the edges) is what's usually sought by lens designers.

If Bokeh is not neutral (soft or hard edges) it will often flip from one type to the other for underfocus vs overfocus spots (a result of a lens defect called spherical aberration.)

Finally, Bokeh will vary with f-stop; the size of the out-of-focus spot is directly proportional to the actual size and shape of the aperture and as a result will often be a funny shape for wide open apertures.

Here's a great example of bad Bokeh detracting from a photo by Just1moreDave* https://www.pentaxforums.com/forums/pentax-beginners-corner-q/141924-other-th...l#post1488235:

Hard-edged Bokeh leading to double images this severe is common with mirror lenses.

You were absolutely correct in your comment that the out-of-focus features shown in the above examples would have been smaller had they been caused by subjects closer to the plane of focus but their nature would have been pretty much the same.

* this is a marvelous photo example - perhaps the most extreme example I've ever seen of bad bokeh overwhelming a photo to the extent that one can barely make out the photo's intended subject.

Last edited by newarts; 05-04-2011 at 06:24 AM.
05-04-2011, 05:25 AM   #6
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QuoteOriginally posted by JimD Quote
Before I came on this site there were a few terms that I'd never come across.

Can someone please explain to me what 'bokeh' refers to?

Secondly, what is the 'IQ' of a lens? What does it define?

Thanks
Actually, a good idea for this site would be a glossary of commonly & less commonly used terms posted somewhere. Photography like all different disciplines does carry a lot of terminology which can be confusing. A list of general photography terms could be established along with a list of more Pentax specific ones.

Any thoughts? Unless of course this idea already exists and I can't find it.

Michael
05-04-2011, 05:42 AM   #7
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I second that. I think the wiki would be a good place for that, but if we could put pictures also illustrating the principle (where applicable). Like with Bokeh, show examples of bad and good bokeh. Same with depth of field.
05-04-2011, 08:39 PM   #8
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My bokeh photo is so bad, it's infamous!

I have a composite photo online that I did to compare some lenses, that shows some of the elements of Image Quality and Bokeh. You're going to have to really look to see the differences. Worry about these things after getting composition, exposure and focus right.

I set the camera up on a tripod at my local botanical garden and kept the same position throughout. I took these photos in a short period of time to have the same light. With each lens, I have three photos at different apertures. The photos were set to the same white balance, downsized, converted to 8 bit color and pieced together for a composite. It's probably larger than your screen so look at it full-size.

135mm Lens Test

You should be able to see the depth of field widen as aperture increases. You can also see differences between color and contrast on a good lens compared to ordinary ones. Most of the time, you can see those attributes change with aperture as well - in this case they don't change a lot. Sharpness will change greatly too, but I use a brick wall for those and the photos are really boring.

I find this type of side-by-side comparison makes the differences stand out more than a set of typical photos. You can create your own with the kit lens. It should be easy to demonstrate vignetting (darker corners) at 18mm or softness at 55mm, and see it change with different apertures.

If you're dying to see brick walls at 135mm, the rest of the test is here:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/68032-135mm-le...imes-zoom.html

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