Background blur and focal length

I've been reading and lurking a lot of the photo forum sights and understand quite a bit, but have one question in regards to background blur.

Would the background blur change if shooting at the same aperture, same framing size, but different focal lengths?

For example, if I was using say the 70-200 lens wide open at F2.8. If I were to frame a picture at 70mm focal length, and then take another picture while framing the picture at a 200mm focal length, would the background blur be the same?

There would be significantly more background blur at 200mm - as the focal length increases, the depth of field decreases. I think.

Never mind, I got background blurring and depth of field confused for a moment...

Actually, this has me thinking. Wouldn't it be useful to have a calculator that could graph out the amount of blur vs. distance for a given focal length and f stop? Sure all the graphs would loosely be a v shaped graph of a gaussian profile

I think something like that could be very helpful for people to visualize just how steep the transition from in focus to out of focus vs. distance something would be.

Not graphical, but useful: http://www.dofmaster.c​om/dofjs.html
Mobile phone (Java) version: http://www.curved-light.net/software/

You do hear many assign DOF as the main contributor to bokeh but in fact, as far as the technical aspects of a sho,t it is only half of the recipe. Any two lenses of different focal lengths can have the EXACT same DOF using the required apertures. The equations to calculate the DOF are readily available. So does that mean the bokeh will be the same? Of course not. If a shot is composed with the two lenses so that the subject is the same size on film, the longer lens will be further away and the angle of view will be more narrow, covering less background. The shorter lens will have a wider angle of view and get more background information in. The result? It will appear that the background in the shot taken with the shorter lens is more in focus when , in fact, it is not, but it does contain more of the background infirmation. Of course what the background actually is or has in it is the other major component.
I saw a site before that diagrams this out nicely and has sample images but I couldn't find it again. I'll keep looking.

Cheers.

You can assess the degree of background blur for any shot by creating a blur disk calculator. This tells you how big a single point will appear (on the sensor) at a given distance from the plane of focus. Comparing this to the circle of confusion gives a degree of blurriness for a given situation.

LordAlex wrote in post #5966889
You do hear many assign DOF as the main contributor to bokeh but in fact, as far as the technical aspects of a sho,t it is only half of the recipe. Any two lenses of different focal lengths can have the EXACT same DOF using the required apertures. The equations to calculate the DOF are readily available. So does that mean the bokeh will be the same? Of course not. If a shot is composed with the two lenses so that the subject is the same size on film, the longer lens will be further away and the angle of view will be more narrow, covering less background. The shorter lens will have a wider angle of view and get more background information in. The result? It will appear that the background in the shot taken with the shorter lens is more in focus when , in fact, it is not, but it does contain more of the background infirmation. Of course what the background actually is or has in it is the other major component.
I saw a site before that diagrams this out nicely and has sample images but I couldn't find it again. I'll keep looking.

Cheers.

There is a background blur comparison for macro lenses here:
http://www.the-digital-picture.com …SM-Macro-Lens-Review.aspx

LordAlex wrote in post #5966889
I saw a site before that diagrams this out nicely and has sample images but I couldn't find it again. I'll keep looking.

Cheers.

You mean this I think:
http://www.luminous-landscape.com/tutorial​s/dof2.shtml

Basically, if the object in focus is the same size in the frame and the apertures are the same, the DOF will be the same.

The DOF is the area in front and behind the focal point that SEEMS to be in focus.

When you zoom in, the background elements appear to get "closer" to the focus point. Therefore the elements that are more blurred (ie further away) APPEAR to get closer. Therefore it APPEARS you get less DOF... which you don't it just APPEARS that way.

WA does the opposite, it distorts distance. Therefore the elements further away (ie further from the DOF and therefore more out of focus) APPEAR to be further away. You can see more of the stuff near the DOF (which seems more in focus) so therefore it APPEARS your DOF has increased... but it hasn't.

Clear?

Clear as the bokeh from a 800mm lens.

Stealthy Ninja wrote in post #5967055
You mean this I think:
http://www.luminous-landscape.com/tutorial​s/dof2.shtml

Not the same page I saw before but he makes it clear as well.

Cheers

sebr wrote in post #5967003
There is a background blur comparison for macro lenses here:
http://www.the-digital-picture.com …SM-Macro-Lens-Review.aspx

Yes! That's the one ! Exact same DOF but very different background due to AOV. Thanks, I wanted to bookmark that.

Cheers