SLR Geek

How do you know if a lens will produce more Bokeh? The longer the lens focal length divided by the wider the aperture opens divided by the minimum focus distance and hence closer you are to the subject tells the whole story. The focal length to maximum aperture is easily obtained by simply dividing the focal length by aperture. For instance, the ratio of a Canon 85mm F/1.2 II L lens when shot wide open would be 85 divided by 1.2 giving you a ratio of 70.8.

On the other extreme is the Canon 15mm F/2.8 fisheye. This lens produces no Bokeh, dividing 15 by 2.8 only gives you 5.3 when shot wide open (see photo below). As you can see there is no Bokeh!

Example of no Bokeh Canon 15mm Fisheye

Now of course a lens can be stopped down giving a whole range of numbers but for the purposes of this comparison we are hunting for maximum Bokeh effect of a lens so you always run the fastest aperture and the longest focal length in your equation if it’s a zoom.

But what about the minimum focus distance you say? Haven’t forgotten you. Let’s take the 200mm F/2 lens 200 divided by 2 with ratio of 100. Now lets say that lens has a minimum focus distance of 10 feet. 200 divided by 2 for a 100 then divide by the minimum focus distance of 10 and we get a 10. Now if we have this same lens the 200mm F/2 and it only had a minimum focus distance of 15 the numbers would be 200 divided by 2 for 100 divided by 15 for a number of only 6.6 which would be less then half the Bokeh.

In these equations the higher the ending number the more Bokeh you get. Just remember this equation and you can run a comparison against any group of lenses. Focal length by minimum F stop by minimum focus distance (200mm / F/2 / 10 = 10). What about sensor size you say? Simple divide the final number by how much your sensor is cropped. Example (200mm / F/2 / 10 / 1.6 = 6.25). Why you ask does a full framed sensor have more Bokeh isn’t it the same lens? Yes, but in order to frame the subject the same exact amount on a cropped sensor you have to back up more vs the full framed sensor. Remember the higher the minimum focus distance the less Bokeh.

Speaking of zooms let’s take a familiar lens range such as the Canon 24-70 F/2.8 L zoom. At 70mm and 2.8 we get a ratio of only 25 and then at 24mm it’s even lower 24 divided by 2.8 is only 8.5. Of course if you get close enough to your subject you can get some Bokeh but it’s nothing like the 85mm F/1.2. If you only have a ratio number of 8.5 sometimes you end up with a distracting background in the Bokeh like in the photo below of a beautiful bride taken with the Canon 24-70mm F/2.8 L lens at 70mm which ends up being distracting because the cars behind her are not ouf of focus enough (see photo below).

Example of Distracting Bokeh

Why not use a longer lens you ask? Well you can for some subjects and a 70-200 F/2.8 zoom at 200mm would be much better with a ratio of 71 which is very similar to the 85mm F/1.2 at 70.8. And an even longer focal length would produce even more. But when shooting a portrait it’s nice to be able to talk to your model without screaming or using a megaphone. A 600mm F/4 lens with a ratio of 150 will produce a solid wall of Bokeh at it’s minimum focus distance but you would be so far away from your model for a full portrait you would need to talk on the phone to direct her.

Enough talk of theory, what do photos we are talking about above look like?

The opening photo taken at F/1.4 a near minimum focus distance with a 85mm F/1.2 is an example of fantastic Bokeh with the background blurred to the point it looks like a very pleasing painting.

No Bokeh! Yes, absolutely! A 15mm Fisheye at F/2.8 is only 5.3 in ratio and when stopped down to F/4 like in the photo below it’s almost impossible to get any Bokeh at all with a ratio number of only 3.7.

Bad Bokeh? Yes, there is such a thing. Some lenses like the Canon 100-400 IS L Zooms and some other zooms can at times produce distracting or bad Bokeh when stopped down to say F/8 at a focal length of 135mm. Tree limbs can leave crazy outlines. On the other hand open the 100-400 up to F/5.6 and shoot at 400mm and you will produce a wall of Bokeh.

I hope this information has sparked your interest in Bokeh. In the future I am going to talk about depth of field or DOF and just how hard it can be to be your entire subject in focus when you are trying to get the minimum DOF.

Below is another good example of great Bokeh!