[Enkhbayar]

It's a common mistake that the smaller DOF mean the more blurry background, actually DOF can't directly represent how blurry is the background in your photo, but Bokeh size does.


So I did some calculation, made this BOKEH SIZE TABLE to indicate some popular portrait lens's blurriness of various far of background.


In the table,

The first column mean distance between background and subject to be taken.

“Bokeh size” is the diameter of circle of confusion on a sensor.
“Vertical resolution on an APS-C sensor” mean how many circle can fit into image's height(shorter side, not width). This is not the exact resolution of the blurry background, but you can refer this value as a measurable number of blurriness of the background. The lesser resolution the more blurry bokeh.

for full body portrait




for half body portrait





The value is calculated using a simple formula, assuming the lens is a simple thin lens. May be not accurate enough for lens specific or industry level consideration.

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Last edited by Enkhbayar; 01-30-2012 at 09:05 AM.

[altopiet]

Hi Enkhayar, this seems to be your first post to Pentax forums.

As an amateur photography hobbyist, all this info seems very impressive, but to be honest, I haven't got a clue what you're talking about.

It's a common mistake that more small DOF mean more blurry background, actually DOF can't directly represent how blurry is the background in your photo, but Bokeh size does.

As far as I've seen on photo's taken by myself, no matter what you call it, if I use smaller DOF, the background is more "blurry". I do know how to set the aperture on my lens to achieve more "blurry" background, but as my K-x do not have a specific "bokeh" setting, can you please explain to a layman, how your calculations affect me, and how I should apply it to make me take better photo's?

[causey]

Interesting, Enkhbayar--thanks.
That's why everybody needs a 85mm f1.4. I'll have to get that Vivitar 85/1.4.

[Enkhbayar]

thanks for your reply altopiet,

DOF is depth of field, the sharp and clear range in your photo, not directly mean blurriness.
"Bokeh size" in my table directly mean blurriness.
sometimes, different lens have same DOF but it is not true always have same blurring on background.

sometimes shallow DOF can't achieve a strong blurry background, as shown in table, the blurriness will significantly increase by the background getting farther, but if the background is too near to the subject to be taken, the background can't be very blurry.

to let the background be more blurry do:
1.make the subject close to camera;
2.choose farther background( as shown in table, blurriness of farther than 100m background is almost equal to infinity background);
3.use bigger aperture;
4.longer focal length can make background blurry easier.



the tables are showing, background blurriness for the same full body portrait(first table, 2*1.3m of view) or half body portrait(second table, 1.5*1m) with 50mm, 85mm and 135mm lenses at different aperture setting.
you can compare those lenses on blurriness of same distance of background:

such as in the first table, 50 F/1.4's background is slightly blurry than 85mm F/1.8 before the background is near than 5m, but 85 F/1.8's blurriness get far stronger as distance increases.

[altopiet]

Thanks for the more detailed explanation Enkhbayar. Just another Q. As you do not mention any specific make of lens in your calculations, but in your intro, you talk about

some popular portrait lens's

I take it that you mean all lenses of popular focal length used for portraits, at similar f stops, would produce the same results?

Edit, might be a good idea if this is moved to https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/132843-bokeh-club.html by the mods?

[Enkhbayar]

Thanks for the more detailed explanation Enkhbayar. Just another Q. As you do not mention any specific make of lens in your calculations, but in your intro, you talk about I take it that you mean all lenses of popular focal length used for portraits, at similar f stops, would produce the same results?

ideally, YES.
in fact there are slight complications, such as pupil shape is not always a circle, and varies between different lenses and apertures.
i did not take them in to consideration, but I think those do not effect too much to my calculation.

[altopiet]

Thanks again, it is very interesting

[Ubuntu_user]

Wow. This is interesting.

I wonder though when I'd use this in the field??

[JinDesu]

I assume that larger "bokeh size" is generally more preferred - the greater blur to isolate the subject and give that 3-D effect, and also to reduce the "busy" background.

Whenever shooting for bokeh effect, I try to put the furthest background to my subject when possible. I guess this table helps to show that in order for me to get equal or better bokeh effect on, say the 135 F2.8 compared to the 50 1.7, the background has to be at least 5 meters behind my subject.

In practice though, we're limited by where our subject can stand and the preference for extremely far backgrounds is preferred anyways.