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Question about Depth of Field and Bokeh
I've been reading a lot of the threads on this forum discussing Depth of Field and bokeh and I had one question I wasn't sure of the answer to..

As far as I understand, to get a shallower depth of field you can1.) Increase the aperature (lower f-stop)2.) Move closer to the subject.

By doing this, you get a shallower depth of field and because of that you get more bokeh in the background. (more fuzzy backgrounds, isolating the subject of focus more.)..

My question is this: If, for example you use the same f-stop (we'll say f2.8 for this example) on a subject, but different zoom length, how does that affect the depth of field and bokeh..

For example. I have a subject that fills a frame at 50mm and f2.8. They are in focus and the background has x amount of bokeh..

If I take the same subject, use a 200mm at f2.8 and back up until that subject fills the frame in the same way as the 50mm example above, will I have the same depth of field and the same bokeh? Or will one be more extreme?.

I hope the question makes sense...

Comments (10)

Mike Duncan wrote:.

I've been reading a lot of the threads on this forum discussing Depthof Field and bokeh and I had one question I wasn't sure of the answerto..

As far as I understand, to get a shallower depth of field you can1.) Increase the aperature (lower f-stop)2.) Move closer to the subject.

By doing this, you get a shallower depth of field and because of thatyou get more bokeh in the background. (more fuzzy backgrounds,isolating the subject of focus more.)..

My question is this: If, for example you use the same f-stop (we'llsay f2.8 for this example) on a subject, but different zoom length,how does that affect the depth of field and bokeh..

For example. I have a subject that fills a frame at 50mm and f2.8.They are in focus and the background has x amount of bokeh..

If I take the same subject, use a 200mm at f2.8 and back up untilthat subject fills the frame in the same way as the 50mm exampleabove, will I have the same depth of field and the same bokeh? Orwill one be more extreme?.

I hope the question makes sense..

It does make sense..

Well, to get the theory right search for Depth of Field and bokeh on Luminous Landscape website. It's quite well explained..

Now, to your question..

You have learned that DoF depends on the magnification. So, if you shoot at f2.8 and the subject fills the frame, you'll have the same DoF, no matter the focal lenght..

Bokeh is dependant on the optical properties of the lens. So, theoretically, it does not vary with focal lenght..

However, the amount of background in your image is not the same, because perspective changes, too..

Suppose you shoot a portrait with a tree as background..

If you shoot at f2.8 and same magnification, you'll have the same DoF, say 2 cm..

However, at a focal lenght of 24 mm, you'll have the whole tree in the background, and it's gonna be quite visible. At 300 mm, you'll have maybe a part of a branch as background..

So, the longer the focal lenght, the less cluttered the background will be..

Hope this helps..

/d/n..

Comment #1

Ok. I think I understand. I'll go check out that Luminous Landscape website too..

Thanks a lot for the answer!..

Comment #2

There is a simple d-o-f calculator here that allows you to see the effect of different focal length, aperture, distance etc..

Http://www.dofmaster.com/dofjs.html.

Best wishesMike..

Comment #3

And a depth of field table here:.

Http://www.dofmaster.com/doftable.html.

I print a few of these off and keep them in my bag with me..

P.S. Thank goodness for someone that uses the Subject line intelligently! I rarely bother to read those that simply say "Help please" or the like ..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #4

Mike Duncan wrote:.

My question is this: If, for example you use the same f-stop (we'llsay f2.8 for this example) on a subject, but different zoom length,how does that affect the depth of field and bokeh..

Depth of Field varies with image size, and it is deepest when image size is SMALL, and smallest (shallowest) when image size is LARGE..

Therefore, any means of decreasing image size will tend to increase apparent DoF, (aperture remaining the same)...

Shooting with a wider lens decreases image size, (so wides have 'more' DoF) shooting from further away making small prints instead of big ones..

Naturally the corollary holds true. Increasing image size reduces apparent DoF (aperture remaining the same)...

Shooting with a tele increases image size (so teles have 'less' DoF) shooting from nearer making big enlargements..

Now, if your subject happens to be the same size in BOTH your tele AND in your wide shot (because you moved closer to fill the frame and compensate for zooming wider) then the DoF will be the same in the subject areas of the image. However, the background image elements will be smaller in the wide shot, because they are still relatively further away, so they will appear more sharp in the wider angled view..

Now that you know what to look for, perhaps you can verify this through the viewfinder of your own camera and zoom lens..

Please note, none of this has anything to do with "bokeh". Bokeh does not describe the *amount-of-out-of-focus-ness* in the fuzzy areas of a photograph..

The term "bokeh" is used to describe the *nature* of the out of focus zones whether they are smooth and creamy (good bokeh) or rough and wirey (bad bokeh).. and is a matter of how the lens is contructed, optically, and how many blades are in the diaphragm..

I hope this was helpful.Regards,Baz..

Comment #5

And in fact shows a lot of insight..

DOF is a function of not only the aperature and subject to sensor distance, but also focal length..

So yes, a longer focal length would give less DOF and generally speaking, better bokeh..

That's why I love my 70-200 f2.8 lens. Portraits at 150-180mm and f4 come out so nice..

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

Chefziggyhttp://www.pbase.com/chefziggy/lecream.

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window..

Comment #6

Mike Duncan wrote:.

My question is this: If, for example you use the same f-stop (we'llsay f2.8 for this example) on a subject, but different zoom length,how does that affect the depth of field and bokeh..

For example. I have a subject that fills a frame at 50mm and f2.8.They are in focus and the background has x amount of bokeh..

If I take the same subject, use a 200mm at f2.8 and back up untilthat subject fills the frame in the same way as the 50mm exampleabove, will I have the same depth of field and the same bokeh? Orwill one be more extreme?.

In this example where the subject framing is the same at both focal lengths, you will most likely get the same DoF (assuming the subject distance is not near the hyperfocal distance of the 50mm lens) on both. However, the background blur will be significantly different. As for bokeh, as others have said, that term refers to the quality of the OOF area, not the quantity..

See the following link about the rule of thumb where the DoF is only dependent on f-stop when magnification/subject framing is constant (it breaks down with WA lenses and subject distances near the hyperfocal distance)..

Http://dofmaster.com/dof_imagesize.html.

As for what you are really asking (what lens will blur the background more?), the answer is the longer lens. I've copied an earlier post that I wrote on this - sorry for the awful WB, but it just set on AWB, and didn't bother to correct, since they were just to show the OOF area..

50mm at f5.6.

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

100mm at f5.6.

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

200mm at f5.6.

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

400mm at f5.6.

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

Note that all these have the same DOF. Since the following are true:1) the focusing distance was much smaller than the hyperfocal distance2) the subject was kept the same relative size.

3) and the same f-stop was used then the DOF will be the same regardless of focal length..

I hope the question makes sense..

It does to me. I hope my answer makes sense...

Comment #7

Wow. Thanks a lot for the detailed and easy to understand answers everyone..

I now think I understand the difference between DoF and bokeh and how they they relate and differ to each other..

I will definitely print out some of those depth of field charts and use that DoF calculator too..

And finally Dave, thanks for the visuals. Those pictures make it very easy to see the differences between the zooms...

Comment #8

Devnull wrote:.

Bokeh is dependant on the optical properties of the lens. So,theoretically, it does not vary with focal lenght..

A lens's spherical aberration is important with respect to it's bokeh. For example, the Nikon 105mm f/2 allows the photographer to vary the degree of correction of spherical aberration to produce the desired appearance of out-of-focus areas..

Now, if you're making a zoom lens you have a lot of problems to deal with, and it's real likely that you will go from over- to under-correction in your spherical aberration as the focal length changes. So I would expect that the bokeh of any zoom lens will vary with focal length..

Leonard Migliore..

Comment #9

Great response. I just started figuring this out myself this past week fooling around in a similar way as you..

Thanks for posting...

Comment #10

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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