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Explain to me a point-and-shoot's wide depth of field
June 14, 2014 8:38 AM   Subscribe

Digital point-and-shoots have a very wide depth of field. I've been explained this is because of the tiny sensor size. But film P&S have very wide depth of field too, despite being "full frame." My guess is because of the lens distance to film and small aperture. Is this correct? Say, if I frankenmount a film P&S lens to a DSLR would I get the same wide depth of field of a P&S?
posted by jgwong to Sports, Hobbies, & Recreation (13 answers total) 5 users marked this as a favorite
 
Oh, this is complicated. It has to do with angle of view.

50 millimeters of focal length is always 50 mm of focal length, it's a constant. If you think of changing the sensor size mounted the same distance back, you will see that what you are really doing is changing the angle of view.

So, what really happens, is that to get an equivalent angle of view on the tiny sensor of a (let's say) camera phone, they use an extremely short focal length. Shorter focal length = greater depth of field. (and the converse, this is why the lovely portraits one sees with the backgrounds dreamily out-of-focus are often taken with longer-focal-length lenses.)
posted by pjern at 8:52 AM on June 14 [1 favorite]


Pjern covered the focal length issue pretty handily. The other factor is a small aperture. Since most p&s cameras assume they'll use a flash in anything other than full bright sunlight, they also have a narrow aperture between the lens and the sensor, which also contributes to ling depth of field.
posted by colin_l at 9:00 AM on June 14


Um, I still don't understand about focal length. The aperture part is clear.

Let's talk in terms of film cameras. What is the difference between a P&S and an SLR, given both have a 35mm focal length and the same "sensor" size? That is, what makes a P&S produce such wide DOFs with ease?
posted by jgwong at 9:14 AM on June 14


I'm not sure where the 50mm or 35mm focal length entered into the discussion. Does your point and shoot advertise that length? 50mm is the "normal" length for 35mm film. I have no idea what the size of the ccd in your point and shoot is, but the distance from the plane of the ccd to the lens is tiny, probably less than 10mm in most point and shoots I can think of. Shorter focal length = longer depth I'd girls.
posted by colin_l at 9:21 AM on June 14


I mentioned 35mm as an example. Rephrasing my question, if both film size and focal length are equal on a film P&S and an SLR, what accounts for the P&S' wide DOF?
posted by jgwong at 9:43 AM on June 14


The only other major factor in that case would be aperture width. Narrower width = greater depth of field. A shallower or deeper focus range would be a matter of the photographer's choice or a result of the camera's auto exposure settings or priority.

It's possible one lens might have a narrower minimum aperture width than another but that would only affect photographs made in the aperture range available to one lens but not the other.

In your scenario if the focal length, sensor size, aperture width and other exposure settings were identical then the depth of field would be identical (notwithstanding any possible differences in sensor sensitivity).

I don't know what your basis is for saying that P&S cameras have greater depth of field than SLRs, but it may be that they are programmed in such a way that the aperture is generally set to produce a greater depth of field if the photographer does not make settings that override the auto settings. In other words, it may be that what you're seeing is not necessarily due to the capability of the camera, but rather due to the results of the settings that are used by default in P&S cameras.
posted by under_petticoat_rule at 9:59 AM on June 14


Given the same film or sensor size and aperature and focal length, SLRs and P&S cameras have the same depth of field. Totally the same.

P&S cameras often ship with fairly wide angle lenses though. I suspect that's what you're observing.
posted by chrchr at 10:04 AM on June 14 [1 favorite]


under_petticoat_rule: I was thinking on those cheap, plastic film P&S. They behave with that wide DOF.

chrchr: You are right, I forgot they usually have wider focal lengths. But I'm sure there are 35mm P&S. Anyway, I got my answer.
posted by jgwong at 10:38 AM on June 14


A lot of those cheap single-use cameras are fixed-focus for cost reasons, so the deep DOF is to ensure the subject is in focus.
posted by misterbrandt at 12:29 PM on June 14


Here's another way to describe it. The focal length of the lens determine the actual size of the image on the sensor or the film, e.g. if the image of a man is 1/2 inch on a camera with a large sensor, it will be 1/2 inch on a camera with the same focal length and a narrow sensor. The image from the small sensor is a cropped copy of the image from the large sensor. To get the same effective field of view, the camera with the small sensor will be given a lens with a shorter focal length.

But I thought you were asking a different question: why do P&S cameras have such a wide field of view so they are hopeless for photographing scenery? Its because they are optimized for taking pictures of groups of 2-4 people at a distance of a few feet. That's what most amateur photos are.
posted by SemiSalt at 12:43 PM on June 14


The OP refers to "wide" depth of field and focal length. The word "wide" seems a bit off in those contexts. It's possible there's a lack of agreement about the dimensions being discussed. Does the OP mean "field of view" rather than "depth of field"?

A point-and-shoot and an SLR that take the same film size and have lenses with the same focal length and aperture will have the same depth of field.

On preview: What chrchr said.
posted by JimN2TAW at 4:43 PM on June 14


JimN2TAW: No, I mean Depth of Field. As in how those fixed-focus film P&S get everything from 1.5m to infinity in focus.
posted by jgwong at 5:13 AM on June 15


Disposable cameras use 800 ASA film which is very sensitive for direct sunlight photography. Therefore, the fixed aperture can be made very small (like f/16) which means lots of DOF.
posted by nathancaswell at 10:42 AM on June 15


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