Depth of field - effect of focal length

[Henryti]

Hi,

Grateful if any of you technical, engineering sorts out there can help with this. My question is - how is depth of field affected by the focal length of a lens if you keep the subject (eg. a person) the same size in the frame?

My understanding is that, all else being equal,

(i) the longer the focal length the shallower the depth of field and

(ii) the closer you are to the subject, the shallower the depth of field.

Using a 85mm and a 50mm lens as an example, in order to keep a subject the same size in the frame, you would have to be closer to the subject with the 50mm lens compared to the 85mm. So at the same aperture on the same camera, which would have the shallower depth of field? Is there any kind of formula or guide if you want to compare different focal lengths?

Thanks.

Henry

 

[Brett8883]

So my job at work can be described in its simplest form as translating between technical and non-technical people. This is necessary because god love them non-technical people never ask the question they really want answered and god love them the technical people almost never answer just the question that was asked. I’m the buffer.

So how id answer this question (based on the responses from the technical people) is that to compare the background blur of different focal length lenses and apertures from different distances while keeping constant framing you would take the focal length divided by the f-number. Higher the number will give you more background blur. Any objections?

No objection, but your answer can be refined. It is correct only for very distant backgrounds. In practice, this means that the background must be several times as far away as the subject or more.

If the background is very close to the subject (e.g. a head and shoulders portrait and the background is a couple of feet behind the subject), then the blurring changes very little with focal length (if the f-number and framing are constant).

In between situations can be worked out using the formula:

size of blur = (focal length / f-number) x distance from subject to background / distance from camera to background

Yea that’s a much more through and comprehensive answer to the question. Technical people are very good at giving comprehensive answers (that’s what makes them good at what they do) but where I come in is helping them condense their message into the lowest simplest form possible. It can be painful for technical people since they just aren’t wired to think that way.

The way I understood the question wasn’t how to find the exact amount of background blur, it’s how do you compare two systems. All non-technical people really want to know is which system provides more subject isolation. System A or system B. The exact amount is unnecessary/unhelpful to a large degree honestly.

Isn’t it true that focal length/f-number will always give the correct answer to this question when the subject framing and distance from subject to the background is the same? No matter the focal length, f-number, or sensor size combination? Is there any situation that wouldn’t be true? Given of course the background is far enough away from the subject that depth of field doesn’t extend into the background.

Yes, that is perfectly true. My post, that you have quoted above (in green), was simply refining your answer by explaining exactly the meaning of your last sentence above. For anyone interested, I explained exactly what happens if the background is not far enough away for your statement to be true.

Again I’m not adding anything to the technical discussion here, just taking the information you guys have laid out and trying to help condense down your message. You seem to be interested in getting the message out so people can understand it or I wouldn’t bring this up.

I think “size of blur = (focal length / f-number) x distance from subject to background / distance from camera to background” is still too complicated for non-technical people.

I am sure that is true. However, some readers here are quite technical and it was included for their benefit. I find that it is always difficult to decide how much of the mathematical details to include. We will probably always disagree on that one, but one of the merits of the dpreview forums is that they bring together people with widely differing points of view. You can have your approach and I can have mine. ;-)

Yea forsure

 

[Henryti]

Thanks everyone for your in-depth response, especially those who took extra time to take photos to illustrate the point. I learned a lot, including that my initial understanding on this subject was wrong.