90mm f2.8 lens bokeh- no such thing as a stupid question...but

As I understand it (deep breath, stand back, wait for corrections :P )the bokeh is a function of the depth of field, so a lens configuration that gives you a narrow depth of field gives increased bokeh. As the monkey says, shorter focal lengths at the same aperture will give less bokeh, but I hope it's because of what I said.

It's possible to get nice bokeh from th 17-50, and shallow DOF, if you wish. The 2.8 aperture does that, not the focal length.



The 90mm will most likely have better overall IQ, being a prime. But as for DOF ... already been answered really.

Thecageyone wrote: »

It's possible to get nice bokeh from th 17-50, and shallow DOF, if you wish. The 2.8 aperture does that, not the focal length.



The 90mm will most likely have better overall IQ, being a prime. But as for DOF ... already been answered really.

No. None of those things are anything to do with bokeh.

Otherwise, focal length is a factor when considering depth-of-field (which should be brutally obvious given that aperture is expressed in terms of focal length).

melekalikimaka wrote: »

bokeh can mean alot of things to alot of people. you seem to be looking for depth of field over bokeh I 'think'.

Bokeh:
In photography, bokeh is the blur, or the aesthetic quality of the blur in out-of-focus areas of an image, or "the way the lens renders out-of-focus points of light."

Courtesy of Wikipedia.

From here on, that's all it means.

Thecageyone wrote: »

Personally I prefer circular. So long as it's not teddy bears or love hearts it's no big deal.

Starts making one right away.....

Fajitas! wrote: »

Bokeh:
In photography, bokeh is the blur, or the aesthetic quality of the blur in out-of-focus areas of an image, or "the way the lens renders out-of-focus points of light."

Courtesy of Wikipedia.

From here on, that's all it means.

As much as I ♥ Wikipedia (they've sent me Christmas cards more than once (although I was a little upset as I thought the money I donated to them could've been put to better use)), it's not prescriptive.

The issue I have with the term "bokeh" being used to describe blur or "quality" of the blur as it is often talked about (in the sense of the shape of out-of-focus points of light or the size of the blur disc it forms) is that it's a pointless term. All lenses can generate circular blur discs. You can take photographs with a digital P&S with a wide angle lens that have bigger blur circles than other photographs taken with a telephoto lens on an 8x10 film camera.

In this sense, it's not a meaningful term.

Also, using "bokeh" as a literal replacement for the term "blur" strikes me as an American peculiarity of trying to sound all fancypants at the expense of clarity and accuracy; c.f.: "filet", "entree", "a la mode".

dinneenp wrote: »

Thanks for the reply.
I'm talking about taking pictures of (mainly) my young kids. And when asking which would give more blurred background I mean under the same conditions and in same setting (e.g. Garden, indoors etc) and not vert low light.

I know the 50mm should be quicker and is a lot smaller and that they're different lenses but I'm wondering hick would be better for this kind of shot.

Cheers
Pa.

The lens you're selecting for this will most likely depend most heavily on the working distances you think you'll be using.

Set your 17-50 to 50mm and try using it that way for a while. If you're happy with the focal length for your intended purposes: get the 50mm, if you'd rather something a little longer: get the 90mm.

Trying to decide on which lens of two different focal lengths to get on the basis of which one will produce shallower depth-of-field is pointless. If you really want to do that, get the lens with the longest focal length, largest aperture, and lowest minimum focusing distance you can then always take photographs at the minimum focus distance wide open.

It may be helpful to stop thinking in terms of bokeh and more think about depth of field.

To do that you need to consider primarily - focal length, distance to subject and aperture (this being 100% accurate or not is nominally not relevant -- it works for me :cool:).

See worked examples below with constants of aperture and distance to subject and varying the focal length, which I think is what you are getting at in your question.

At 90mm and 20ft distance to subject @ f/2.8 you have 1.68ft of "in focus" depth of field.
At 50mm and 20ft distance to subject @ f/2.8 you have 5.58ft of "in focus" depth of field.
At 17mm and 20ft distance to subject @ f/2.8 you actually technically get infinite "in focus" depth of field from 9.13ft distance away from you.

Outside of your "in focus" areas starting at the near limit (see below) and running to the far limit you will be out of focus.

So, if I understand what you are asking, then yes at 90mm and assuming all other things being even, you will get more out of focus than at 50mm which in turn will be more than at 17mm.

But none of this says anything about the quality of the out of focus area - known as good or bad bokeh which will depend on attributes such as lens aberrations and the shape of the aperture (think quality of optics).

Worked examples calculated from DOF Master:

Assume Subject distance: 20 ft @ 90mm and f/2.8

Depth of field
Near limit 19.2 ft
Far limit 20.9 ft
Total 1.68 ft

In front of subject 0.81 ft (48%)
Behind subject 0.88 ft (52%)

Subject distance: 20 ft At 50mm f/2.8

Depth of field
Near limit 17.6 ft
Far limit 23.2 ft
Total 5.58 ft

In front of subject 2.41 ft (43%)
Behind subject 3.17 ft (57%)

Subject distance: 20 ft @ 17mm f/2.8

Depth of field
Near limit 9.13 ft
Far limit Infinity
Total Infinite

In front of subject 10.9 ft
Behind subject Infinite

As an exercise, stick your 50mm into it's lowest aperture and shoot from 20ft focussed on the subject, then 10ft focussed on the subject, then 5ft focussed on the subject, and just pay attention to what happens the background our out of focus area -- make sure you have elements in the scene that will allow you differentiate. This exercise will show the effect of distance to subject on the depth of field "in focus" and consequently the "out of focus" area. The quality of the out of focus area however should remain the same.

Oh, and by the way you will get far better bokey on 400iso neopan film than you will on your digital SLR sensor, and poleroids are better again.



(the last statement may not actually be true )

charybdis wrote: »

As much as I ♥ Wikipedia (they've sent me Christmas cards more than once (although I was a little upset as I thought the money I donated to them could've been put to better use)), it's not prescriptive.

The issue I have with the term "bokeh" being used to describe blur or "quality" of the blur as it is often talked about (in the sense of the shape of out-of-focus points of light or the size of the blur disc it forms) is that it's a pointless term. All lenses can generate circular blur discs. You can take photographs with a digital P&S with a wide angle lens that have bigger blur circles than other photographs taken with a telephoto lens on an 8x10 film camera.

What!?

The word bokeh implies the quality of the "out of focus" area. Obviously you can take photos with a digital P&S with a wide angle lens that have bigger 'blur circles' than an 8x10 camera. That has nothing to do with the camera or the focal length of the lens. It's moreso concerned with the physical lens and physical aperture diameters.

Bokeh comes in different forms, on review, it's "good bokeh" and "bad bokeh" and in special circumstances "odd bokeh". Good is usually defined as being soft, naturally blurred and providing creamy colours that don't have a solid form. Bad bokeh is hard, angular, unnatural bokeh. It's nasty, you notice it straight away. Odd bokeh produces cirlces. Or lines. Or doughnuts. Or flying monkeys.

The word bokeh is describing the quality of what's being blurred. It is a meaningful term. In the same way as food can be good food, bad food and odd food.

Assume Subject distance: 20 ft @ 90mm and f/2.8

Depth of field
Near limit 19.2 ft
Far limit 20.9 ft
Total 1.68 ft

In front of subject 0.81 ft (48%)
Behind subject 0.88 ft (52%)

Subject distance: 20 ft At 50mm f/2.8

Depth of field
Near limit 17.6 ft
Far limit 23.2 ft
Total 5.58 ft

In front of subject 2.41 ft (43%)
Behind subject 3.17 ft (57%)

Subject distance: 20 ft @ 17mm f/2.8

Depth of field
Near limit 9.13 ft
Far limit Infinity
Total Infinite

In front of subject 10.9 ft
Behind subject Infinite

Correct me if I'm wrong but this should state 'apparent depth of field'. The depth of field should be the same if it's shot at 2.8, it's the magnification of the lens that will show more or less depth of field. If you're shooting at 90mm, you're going to need your focus to be a lot more accurate than you will at 17mm at 20ft from the subject - Try shooting the exact same composition with a wide angle and a telephoto, at the same aperture. Lens distortions and characteristics aside, you'll be getting the same image, with same depth of field.

Corrected by Daire!

No, the word 'bokeh' can be used to further describe the background blur. Good bokeh is considered to be smooth and unnoticable, the circles you're describing are nicknamed 'circles of confusion'. What I'd consider really nice bokeh from a lens would have no 'hard' circles or shapes, just a natural blur that won't take over or distract from the actual content of the image.

My tuppence worth;), I have the 17-50 tammy and had the 90mm which I just got rid of last week. I used it about 5 times max Id say, one being to take pictures to sell it, I didnt like the lens, the guy who took it off me loved it but as I explained to him it was of no use to me. Being a macro it was quite slow on focus and the noise of focus was quite loud, the actual noise not image quality. On the other hand I wouldnt change the 17-50 for the world and just got a tokina 50-135 2.8 which I am loving tbh.

Image from 90mm wont attach but pm me your email and Ill send one on to you. Image from the 50-135 below. Only a couple of hundred euro in the difference of price so on experience I would look around if I was you.

Fajitas! wrote: »

Correct me if I'm wrong

Consider yourself corrected then
What ACD posted is correct. If you're shooting at the same position, with the subject at the same distance in every case, for a constant aperture, then of course the DOF will change as the focal length changes, there's nothing 'apparent' about it.

Try shooting the exact same composition with a wide angle and a telephoto, at the same aperture. Lens distortions and characteristics aside, you'll be getting the same image, with same depth of field.

Assuming by 'exact same composition' you mean trying to get the subject the same size in each case then this is correct. If you're shooting something at 10 feet with a 50mm, then shooting the same subject at 20 feet with a 100mm will give you the same subject size, and exactly the same DOF in both cases. General composition of course won't be the same because the FOV will be different but that's a whole different ballywhack.