You want to blur the background?


Senior Member
There is a new Depth of Field page at Want to blur and hide the background? Comparing Depth of Field of Two Lenses

It has a new DOF calculator, but it was not done to be general purpose. DofMaster is already a very good one.
Specifically it computes Circle of Confusion at the background behind the subject, in concern about blurring and eliminating the background. The calculator compares situations of two lens regarding this (CoC at the background, and DOF at the subject).

It could be said to be an anti 50mm f/1.8 article. :) Actually it just shows a much better plan. DOF also depends on other factors than aperture, also focal length and subject distance. We can use that too.

It does make the point that the 50 mm f/1.8 is NOT best plan. Novices get the notion it is, but we won't see many pros suffering that way (they tend to want their work to sell). There will be better results to stand back with a longer lens. Instead of 50mm f/1.8 at 6 feet, instead maybe 200mm f/4 at 24 feet. Or even 100mm at 12 feet with f/2.8. Generally we're outside and have lots of room so this is no issue. But there may not always be enough room indoors.

200mm is 4x longer than 50mm, so if standing back 4x farther, it has exactly the Same Field of View as the 50 (at the subject). And if at the same aperture there, the Same DOF too (at the subject) - the adage about the "same image" has same DOF, etc. (at the subject, it does).

But the background is certainly NOT the same then... most of it is zoomed and gone missing (you want it excluded right?), but what's left is in more blurred focus (assuming that is a plus here)... due to focal length and distance to the background. In regard to the background, the longer lens standing back has a vastly larger Circle of Confusion (the blur circle diameter) at the background, computed by this new calculator. The actual CoC number isn't so useful, but it being LARGE is the point and this benefit. And then if opening the 50 to f/1.8 to help blur the background, the subject DOF goes to pot too.

Here is the default and the example shown in images.


Anyway, it makes the point that standing back with the longer lens will be better results, sometimes much better,
in regard to blurring the background, (your plan is to blur it, right?)
and in regard to simply excluding most of the background, (you want the clutter eliminated, right?)
and in regard to better DOF at the subject. (f/1.8 certainly needs better DOF at the subject, right?)

A major point is that since the telephoto background is so much smaller area, we can easily and simply slightly move the camera sideways to select the best (least objectionable) part of it to be shown in the picture.

We could add that that standing back with a longer lens is much better portrait perspective too... 50 mm is too short for head and shoulders work.

You should try it. It works.
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Senior Member
Challenge Team
Nice explanation. It is just another illustration why 85 or 105mm lens make good portrait lenses. (At least I have read they are good for that on here. Ha!)


Senior Member
Nice explanation. It is just another illustration why 85 or 105mm lens make good portrait lenses. (At least I have read they are good for that on here. Ha!)

Thanks. Yeah, the 105mm lens was always the classic standard for portraits with 35mm film. That view would be 70mm for DX. I've always been pretty sure it was because of the perspective. Assuming a 2x3 foot field of view is necessary for a head and shoulders shot, then (for full frame 35mm), a 50mm has to be up at only 4 feet. That's simply too close for good perspective. Distorts noses and things. The ladies don't like the way they look, but probably can't tell you quite why. The 105 is forced to be back at about 8 feet (just to include 2x3 feet of view). And farther for any wider view. Hence, always good perspective.

I don't know any one prescribed subject distance number for proper portrait perspective, but my own notion is at least 6 or 7 feet. I normally try for 10 feet, maybe 120mm (FX), typically waist up work. I mean, if you have room, why not? That would be 80mm for DX. It was not uncommon for shooters to prefer 135mm lenses (35mm film), for the flat (undistorted) perspective of models.

The perspective of course comes only from where the lens makes them stand, it is not from any other property of the lens or focal length. The lens does not matter, but where you stand does. Perspective is what we see from there. And of course, full length or groups need a wider lens, in lieu of standing so far back. But the Minimum distance always applies, to any face anywhere.

I do know 4 feet is too dang close. 5 feet almost works, it is subtle, but the ladies do prefer their shots taken at a bit longer. This could fail, but look in the Exif of those shots your wife likes best of her, and see what the focal length was. :) Can't go wrong on perspective out at 8 or 10 feet. The perspective rule always was "For whatever format, use whatever lens gives the view you want from at least 6 or 7 feet" (and more is considered good insurance).

A couple from at Google under Portrait Perspective feet:

CommonSensePhotography: 10 feet is a good distance for all lenses for portraits

Ken Rockwell: Therefore we want to be at least about 15 feet away when photographing people in order to achieve realistic proportions.

Chuck Gardner: He's Canon, but I always thought he was pretty level headed otherwise. :) He has useful articles, one page is Perspective and Distortion

He doesn't give a number exactly, but says "When I do a portrait I look at the face from various distances from 7 to 12 feet through a lens and note how the appearance of the face changes. When the camera gets close the nose starts to look bigger relative to the eyes and ears further back, especially in an oblique view."

I'm not sure I am able to see it then and there at the time, but I do get better results standing back more.

This is just classic portrait knowledge about perspective.
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You need a "calculator"................. sheesh................... this was a basic thing we learned from the scale on lenses before such aps were even thought of