bokeh - aperture - focal length chart

[jamiebonline]

hi everyone

i am curious about something. is there any kind of chart that shows how the bokeh changes relative to aperture and focal length. for example, let's say you have a 50mm 1.8 lens and a 85mm 1.8 lens. you take a picture at 1.8 on the 50, what would you have to increase the aperture to on the 85 to get the same amount of background blur? is there some formula?

thanks

J

 

[Alastair]

If you mean "depth of field", then yes there are websites such as DOFmaster that will provide tables. By the way, this has been discussed before and the answer may not be what you expect (and depends on a bunch of other variables - are you keeping the distance to subject the same or compensating to to frame identical subject frame dimensions?).

But.. "bokeh" is something else, and a qualitative, subjective thing that's difficult/impossible to measure and even more difficult to get two people to agree on. I suspect it's just a terminology/wrong word thing and what you mean is depth of field..

 

[Kodiak Qc]

•

DoF will not make your picture sharper but clearer
as it will remove the bokeh in a deeper field.

is there some formula?

•
No, no formula but some basic understanding.

Situation dependent aspects

• bokeh increases as you're getting closer to subject
• bokeh increases as the distance subject : BG increases too
  
• bokeh increases as the aperture is bigger

Gear dependent aspects

• the quality of bokeh increases with the number of aperture
  blades in higher contrast situation
• bokeh will come apparent quicker the longer the focal length

 

[Nawty]

And also, depth of field is only half the story, long lenses take a smaller piece of the background for the same subject framing so although the DoF may be the same/similar, the background can look much more blurred with the longer lens (and also, of course, the distance to the subject has changed to match the framing which changes things too).

On one hand it is very simple and on others very difficult to be precise as there are so many inter-related variables and you are never really comparing like for like.

 

[Canon Bob]

This might give you some idea.... blur calculator

Bob

 

[sk66]

hi everyone

i am curious about something. is there any kind of chart that shows how the bokeh changes relative to aperture and focal length. for example, let's say you have a 50mm 1.8 lens and a 85mm 1.8 lens. you take a picture at 1.8 on the 50, what would you have to increase the aperture to on the 85 to get the same amount of background blur? is there some formula?

thanks

J

Everything else remaining the same, double the FL gives you 1/4 the DOF. It would take four stops of aperture to return the DOF back to the 50/1.8 equivalent.

But, to keep the same composition 2x FL would require double the distance... which gives you ~ 4x the DOF. So, for the same composition the DOF will remain essentially constant by aperture (when DOF is shallow).

Aperture is pretty easy, 2 stops is 2x or 1/2 the DOF...

 

[snerkler]

There's sites on the web that have virtual images on them allowing you to select different FL's and apertures and seeing the differences. But still it's only a guide.

Don't forget about compression too.

 

[HoppyUK]

hi everyone

i am curious about something. is there any kind of chart that shows how the bokeh changes relative to aperture and focal length. for example, let's say you have a 50mm 1.8 lens and a 85mm 1.8 lens. you take a picture at 1.8 on the 50, what would you have to increase the aperture to on the 85 to get the same amount of background blur? is there some formula?

thanks

J

There's no formula. Leaving aside the subjective qualities and considering just the amount of background blur, there are several variables. As Nawty/Ned mentioned, it's not only about depth of field - focal length (angle of view) has a big impact on perceived blur even when DoF may be technically similar.

For maximum background blur, use a long lens with a low f/number. Move the main subject as far from the background as possible and make sure you fill the frame.