How does focal length contribue to bokeh?

[Troman]

Hi, I'm trying to compare various lenses by bokeh, by their amout of blurness to be precise. I understand that bigger focal length and smaller f-number contributes to bigger blurness, but how do you compare both? For example "20mm is 1 f-stop".

The reason I'm wondering is because I want the biggest separation possible between the background and the object/subject and I'm trying to find a lens for my Sony Nex-5T with APS-C sensor that would be comparable to a full-frame 200mm lens at F3.2 because I know it would work for me. Or at least I want to be able to compare how inferior or superior will be the amout of blurness with a specific APS-C lens with a different focal length to the FF 200mm f3.2. For example I was thinking of Samyang 85mm F1.4 for E-Mount, but I don't know to compare it.

 

[mclewis]

Hi, I'm trying to compare various lenses by bokeh, by their amout of blurness to be precise. I understand that bigger focal length and smaller f-number contributes to bigger blurness, but how do you compare both? For example "20mm is 1 f-stop".

The reason I'm wondering is because I want the biggest separation possible between the background and the object/subject and I'm trying to find a lens for my Sony Nex-5T with APS-C sensor that would be comparable to a full-frame 200mm lens at F3.2 because I know it would work for me. Or at least I want to be able to compare how inferior or superior will be the amout of blurness with a specific APS-C lens with a different focal length to the FF 200mm f3.2. For example I was thinking of Samyang 85mm F1.4 for E-Mount, but I don't know to compare it.

You are forgetting that the blur effect is all to do with how far the camera is from the subject and how far the subject is from the background and the aperture of the lens. Focal length has nothing to do with it. The greater the distance the subject is from the background the more background blur there is. The greater the ratio between the camera subject distance and subject background distance the greater the blur.

 

[Chris R-UK]

Hi, I'm trying to compare various lenses by bokeh, by their amout of blurness to be precise.

Just to avoid confusion, although it is often misused, strictly speaking "bokeh" refers to the aesthetic quality of the out of focus areas of the image and the not the amount of out of focus blurring. So you can have good bokeh and bad bokeh but you can't have "a lot of bokeh" or "not very much bokeh". You can compare the bokeh of two lenses but not by comparing the amount of blurring

Good bokeh is a desirable quality in a lens and many serious lens reviews discuss bokeh. For example, this lens review from photozone.de discusses the quality of the bokeh about two thirds of the way down the page.

--
Chris R

 

[Christof21]

Hi, I'm trying to compare various lenses by bokeh, by their amout of blurness to be precise. I understand that bigger focal length and smaller f-number contributes to bigger blurness, but how do you compare both? For example "20mm is 1 f-stop".

The reason I'm wondering is because I want the biggest separation possible between the background and the object/subject and I'm trying to find a lens for my Sony Nex-5T with APS-C sensor that would be comparable to a full-frame 200mm lens at F3.2 because I know it would work for me. Or at least I want to be able to compare how inferior or superior will be the amout of blurness with a specific APS-C lens with a different focal length to the FF 200mm f3.2. For example I was thinking of Samyang 85mm F1.4 for E-Mount, but I don't know to compare it.

For a given framing, the amount of background blur (at infinity) depends on the ration focal/f#.

200/3.2=62.5 > 85/1.4=60.7

Very close but with the 200mm you have more background blur while having much more dof (for a given framing), which is interesting.

But 200mm might be a too long focal length in practice. ..

 

[Mark S Abeln]

For a given framing, the amount of background blur (at infinity) depends on the ration focal/f#.

200/3.2=62.5 > 85/1.4=60.7

Very close but with the 200mm you have more background blur while having much more dof (for a given framing), which is interesting.

But 200mm might be a too long focal length in practice. ..

To the OP:

Dividing the focal length of the lens by the aperture setting gives you what's called the 'front pupil diameter' of the lens, which is the optical width of the lens and is a cause of background blurriness.

As mentioned, a larger value of this will give you more background blur.

However, there are some caveats:

The background needs to be at least ten times the distance beyond your subject as the distance between camera and subject. The closer the background is to the subject, the more a long lens will have the background in focus compared to a shorter lens.

For this reason, many photographers prefer a somewhat shorter lens with wider aperture, because you don't always need to find a really distant background. Also, you don't need to stand so far from the subject.

This assumes the framing is the same, that is, the subject is the same height in the image with both lenses. Having the same distance means that less of the subject is seen with the longer lens; however, the background will appear blurrier.

This also assumes that you actually focus on the same subject, which isn't guaranteed with automatic focus. Focusing behind the main subject will decrease background blur.

 

[AdriaanMeijer]

Hi, I'm trying to compare various lenses by bokeh, by their amout of blurness to be precise. I understand that bigger focal length and smaller f-number contributes to bigger blurness, but how do you compare both? For example "20mm is 1 f-stop".

That would be nice, but it won't work that way. It is not about adding and subtracting, but about multiplication and division. While I'm quite a betha person and quite good with math, I'm not good at arithmatic. So I just have some main ratios in mind.

Distance camera - subject - background is most important.

walking, framing and focal length are in a way ways of working with those.

The (lack of) DOF is also a function of the size of the lens opening, as is the aperture. A double aperture (1 stop, say from f/2.8 to f/4.0, remember to quadrate the figures as we are talking surface ratios) will make the 'circle of confusion' twice as big.

I find playing with this site helps me to get the picture (pun not intended)

http://dofsimulator.net/en/

--
/All in my humble opionion of course!
/If I seem to talk nonsense or you can't understand me, it's probably my English

 

[CptAmerica]

How does focal length contribue to bokeh?

It doesn't. Bokeh is a quality determined by the build of the lens.

 

[AdriaanMeijer]

How does focal length contribue to bokeh?

It doesn't. Bokeh is a quality determined by the build of the lens.

Yes, Bokeh is the quality.

Yet focal length can contribute to the amount of this effect. I'd rather speak of selective focus as some kind of contrary to DOF (what speaks of what is in focus) or Bokeh (what mainly speaks of the shape of these circles of confusion)

 

[cookedraw]

You can not compare it like this and get a 100% correct result, bokeh is too complicated to calculate .

200mm f3.2 will give the same background blur on all formats. If you have the lens and that is what you want, use an adapter.

--
" Use the shutter button on the headset cord " - Leonardo Da Vinci

 

[Troman]

If you have the lens and that is what you want, use an adapter.

No, I don't have it, I only own Sony 50mm 1.8 E-Mount and Soligor 75-205mm, 3.8-4.8. But I want much better background separation for portraits, so I'm thinking about Samyang 85mm F1.4 and Samyang 135mm F2.0 right now. My budget is about $500-$600.

 

[CptAmerica]

How does focal length contribue to bokeh?

It doesn't. Bokeh is a quality determined by the build of the lens.

Yet focal length can contribute to the amount of this effect.

Nope. There is never an "amount" of bokeh. You simple misunderstand what it is. You are confusing "Bokeh", with "Out Of Focus"

 

[cookedraw]

If you have the lens and that is what you want, use an adapter.

No, I don't have it, I only own Sony 50mm 1.8 E-Mount and Soligor 75-205mm, 3.8-4.8. But I want much better background separation for portraits, so I'm thinking about Samyang 85mm F1.4 and Samyang 135mm F2.0 right now. My budget is about $500-$600.

The Soligor lens is an excellent portrait lens.

Wide open aperture, minimum distance to focus and background far away will do the job.

Stop it down for awsome hexagonal blur spots . Look no further.

--
" Use the shutter button on the headset cord " - Leonardo Da Vinci

 

[AdriaanMeijer]

How does focal length contribue to bokeh?

It doesn't. Bokeh is a quality determined by the build of the lens.

Yet focal length can contribute to the amount of this effect.

Nope. There is never an "amount" of bokeh. You simple misunderstand what it is. You are confusing "Bokeh", with "Out Of Focus"

It seems we disagree on this one.

 

[Chris R-UK]

How does focal length contribue to bokeh?

It doesn't. Bokeh is a quality determined by the build of the lens.

Yet focal length can contribute to the amount of this effect.

Nope. There is never an "amount" of bokeh. You simple misunderstand what it is. You are confusing "Bokeh", with "Out Of Focus"

It seems we disagree on this one.

But the bokeh does change as the degree of OOF/blurring increases. Most lenses have their best bokeh when the background is completely blurred.

 

[cookedraw]

How does focal length contribue to bokeh?

It doesn't. Bokeh is a quality determined by the build of the lens.

Yet focal length can contribute to the amount of this effect.

Nope. There is never an "amount" of bokeh. You simple misunderstand what it is. You are confusing "Bokeh", with "Out Of Focus"

It seems we disagree on this one.

But the bokeh does change as the degree of OOF/blurring increases. Most lenses have their best bokeh when the background is completely blurred.

I don't agree. Bokeh looks best when the CA is very low and it is not the same as f1.2...

People confuse background separation with a big mess , background separation is when subject is in focus and background is out of focus.

 

[CptAmerica]

How does focal length contribue to bokeh?

It doesn't. Bokeh is a quality determined by the build of the lens.

Yet focal length can contribute to the amount of this effect.

Nope. There is never an "amount" of bokeh. You simple misunderstand what it is. You are confusing "Bokeh", with "Out Of Focus"

It seems we disagree on this one.

--
/All in my humble opionion of course!
/If I seem to talk nonsense or you can't understand me, it's probably my English

Disagree all you like, you are wrong. You can never have more, or less, bokeh. Bokeh can change for the better or worse as the DOF increases or decreases...(always subjective) but you can't get more or less bokeh. If you think something different, you don't understand what it is. It's ok, I won't lose any sleep.

 

[cookedraw]

let's call it out of focus blur. foreground blur and background blur or something.

What OP mean is that focal length is important for a sharp border of blur between subject and background. I think OP prefer the Samyang 135mm f2.

--
" Use the shutter button on the headset cord " - Leonardo Da Vinci

 

[Troman]

The Soligor lens is an excellent portrait lens.

I wish it was faster at 205mm.

I think OP prefer the Samyang 135mm f2

I do but I went for the Samyang 85mm f1.4 for now.

 

[Mark S Abeln]

I went for the Samyang 85mm f1.4 for now.

Good lens at a good price. May all your backgrounds be blurred!