I'm mostly concerned with portraiture here. When making a portrait, the choice of focal length is determined largely by the perspective effect of different lenses. Too short a focal length and people look like pinnochio with noses exagerated. Too long a focal length and they look like a cardboard cutout. However, for environmental (as opposed to studio) work, you also want to be able to blur out the background, and so the effect of the lens on blur is also important.
So here is my bold statement: The focal length has almost no effect on background blur as long as the subject is kept the same size in the frame at all focal lengths (i.e. the magnification is constant). The final caveat is that the magnification is small i.e. that you are not in the 'macro' region.
How come? Surely people dont spend $000s on long fast lenses for nothing? No they dont but they want the maximum magnification possible for wildlife shots etc, and shallow depth of field is a side effect. In portraits the subject is always the same size whether at 50mm or 500mm - that means on a longer lens you step away from the subject until the person is the right size. For shooting aminals you get as close as you dare and if you use a longer lens then great, your subject is bigger in the frame - probably they are always too small already!
Are you sure? have you tried it out?
I have tried two things:
Take a 50mm lens and a 200mm lens. Take something with writing on it like a news paper and stand it up on something. The place another object that is easy to focus on about 100mm (4 inches) in front of it. Now put on the 200mm lens wide open , stand far enough away that the newspaper fills the frame from top to bottom when you focus on it. You should be about 2m away or thereabouts. Now focus on the other object and observe how out of focus the newpaper is. You maybe cant read the text, but some of the smaller headlines are just legible.
Now put on the 50mm lens and stop it down to the same aperature as the 200mm lens (if its a zoom then this is easy!) and make sure you look through the viewfinder with the lens stopped down in DOF preview mode. Make the newspaper the same size again so that it fills the frame. Refocus on the object in front. You will be much closer this time. Observe the blur - it will be very similar. Probably ever so slightly less blurry than the 200mm
I have tried this in an optical raytrace program with 210mm and 105mm lenses (only 2:1 this time) Set the object distance and field angle so a 100mm object 2m and 210mm gives a 11mm image height. Now shift the focus by 200mm (bigger shift this time) and ask the program to draw the spot diagram. An out of focus spot is drawn with a radius of 0.315mm. Now switch to 105mm and 1m subject distance which gives the same image height of 11mm. Shift the object distance by 200mm and now the spot size has radius 0.423mm.
You will notice that the blur spot is only marginally bigger and definately not 2x. The reason it is bigger is because 200mm is 10% of 2m but 20% of 1m and therefore the conjugate point of the new object distance is disproportionately further from the original image plane. But it isnt much. So to a good approximation the lens focal length is irrelevant in determining the magnitude of the blur of out of focus areas.
So what can you do to control selective focus?
There are two things:
(1) Increase the F-ratio of the lens: this increases the angle of the marginal rays and so the blur is bigger in due proportion to the opening of the lens. Faster lenses have a shallower depth of field and a shallower depth of focus (i.e. thickness of the acceptable image in the camera).
(2) Make the sensor/film bigger. This decreases the other magnifiaction that everyone else forgets about. This is the magnification between the sensor/film and the print/monitor image. The same rules apply as the lens magnification. If the picture is very small then it will all look sharp. Take the same picture and make it fill the screen and then you can see the blur in the image and the perceived DOF looks narrower. Go on try it with a picture on your computer!
If you use an APS-C camera, your DOF is 1.6 times wider than those with a full frame camera at the same conditions of distance, fstop and subject size because the focal length is 1.6x shorter. This means that the magnification at the sensor is 1.6x less and also the blur distance ratio (as above) is also 1.6x less because of the reduced focal length. You then magnify the image by 1.6x more to make the print. So overall the depth of field is 1.6x deeper (you know that point and shoot cameras with microscopic sensors have everything is in focus! the lens is 2mm focal length, so 1m is infinty to it!). So you can see that DOF and OOF blur is proportionall to sensor size. You can think of this as affecting the 'effective' Fstop of the lens in the same way as you have an 'effective focal length' as people do when comparing lenses to 35mm. So compared to 35mm, some formats have the following effect:
Consider an f2.8 lens, used for portraits such that the subject always fills the frame:
|Format||Size Ratio||Effective Aperature||Aperature for f2.8 blur||Aperature for f1.4 blur|
The first column is the size ratio of the sensor or film. This assumes that the print has 10x8 aspect ratio and that you crop to get best use of the format.
The second column is how the blur compares to a 35mm format f2.8 lens. This is really revealing. How much would you pay for an F0.7 lens on a 35mm format camera?! And yet there are several 5x4 lenses out there at affordable prices with a maximum aperature under f2.9. Food for thought!
The other columns show what fstop is needed to mimic an f2.8 lens on each format. So on APS-C, my 50mm f1.8 plastic fantastic is like an 85mm f2.8 lens (just like my old tamron manual focus f2.8!) . Also even that hyper costly f1.0 50mm lens on your APS-C samera wont get you even near to F1.4 on full frame.
On 4/3rd's, you will obviously struggle to get good selective focus at portrait distances. On 5x4 most standard lenses wide open are about 4.5 - 5.6 giving f2.8 performance easily and cheaply.Becuase the demands on magnification are so little, even 1950's 4 element lenses are good enough wide open.
For medium format, the advantage of 6x4.5 film seems to me not so great. Alot of extra hassle for a small improvement in image quality and selective focus. 6x9 or 6x7 are a better choice, although pricer gear than 5x4.
So for portraits my favourite rigg is my 5x4 speed graphic with an Aero Ektar 178mm F2.5 or better still my Pentac 210mm f2.9 (which is smaller, lighter and longer despite being not so pin sharp, but thats fine for portraits - also the Bokeh is better than the aero wide open at the edge of the field)
Thats enough rambling for one night - sorry it got so techie. Exec summary - in film/sensor formats, bigger is always better, at least for portraiture.