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09-07-2010, 10:25 AM   #1
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Sensor size vs. DOF

I know that a larger sensor means less depth of felid, but why is that exactly?

Is it because smaller sensors force you to use shorter focal lengths to get similar fields of view? creating an illusion of decreased DOF, where the equivalent focal lengths would simply seem to have different DOF because the actual focal lengths vary so much, but the actual focal length's DOF would remain unchanged regardless of sensor size

or would a 50mm lens on Pn'S and on a DSLR have different DOF's because of some law of optics I don't understand

did that make sense at all?

Through my reading and research I haven't been able to find out why these phenomenons happen, just that they happen, and now I'm wondering because I just got thrust back into the world of small sensors with my new video camera

09-07-2010, 10:31 AM   #2
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A larger sensor means a larger FIELD OF VIEW. The depth of field is a property of the lens (and, to some extend, resolution).

If you consider effective focal lengths in relation with the crop factor, it messes things up. A 50 mm remains, in fact, a 50 mm lens, whatever the sensor you're using.
09-07-2010, 10:44 AM   #3
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I know that, that's the basis of my question

on my K7, my 50-200 4-5.6 has much less DOF at 200mm f/5.6 than 50mm f/4, maybe I'm using the wrong term, I've noticed it's much shallower, with far more blurry OOF rendering at 200mm, so much so that I can't even replicate it with my 50mm f/2

so let's say that I want to take a shot with the same field of view on a dslr and a point and shoot, I'd have to use a much shorter focal length on the point and shoot right? - wouldn't that lead to the dslr having less DOF simply because the actual focal length of the lens is so much longer than the point and shoot (since they have to achieve the same field of view)

(I see you're an optical designer, I explained this scenario further to clarify my question not because I don't think you understand the concept)

Now my main question is: does depth of field change with sensor size (given the the actual focal length and aperture are constant, with the sensor size being the changing variable)

or is it perceived to change because on two sensor sizes, two different actual focal lengths can provide similar fields of views, but different DOF's
09-07-2010, 10:51 AM   #4
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DOF is smaller on full frame given same framing and focal length because you have to stand closer to your subject. Your 50-200mm, depending on the settings, may be giving you the illusion of less DOF due to the field of view. Basically, you're seeing a smaller angle of the blurred background, which makes it SEEM like you have less DOF, all things being equal.

09-07-2010, 10:57 AM   #5
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oh geeze, I thought that DOF decreased as focal length increased

I don't know where I read that but I must have somewhere, now that screws up my whole understanding
09-07-2010, 12:11 PM   #6
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Circle of Confusion..

Circle of confusion - Wikipedia, the free encyclopedia

Circle of Confusion

Basically and very simplistic, everything else being equal (focal length, f stop, distance to subject, etc), the smaller the recording medium, the smaller portion of the center of the image circle is used. Being the sharpest point, the other stuff (Out of focus) gets cropped off.

09-07-2010, 12:50 PM   #7
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QuoteOriginally posted by future_retro Quote
oh geeze, I thought that DOF decreased as focal length increased

I don't know where I read that but I must have somewhere, now that screws up my whole understanding
If you keep the same f-stop, it does. If you keep the same real aperture, it doesn't (it's the same as cropping and using a TC)

I wrote a pretty good (imho) explanation here:

what makes a good cameras (like DSLR) can take a good bokeh and the other cameras can't? - Photography - Stack Exchange

Last edited by Eruditass; 09-07-2010 at 01:22 PM.
09-07-2010, 03:40 PM   #8
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QuoteOriginally posted by future_retro Quote
I know that a larger sensor means less depth of felid, but why is that exactly?

Is it because smaller sensors force you to use shorter focal lengths to get similar fields of view? creating an illusion of decreased DOF, where the equivalent focal lengths would simply seem to have different DOF because the actual focal lengths vary so much, but the actual focal length's DOF would remain unchanged regardless of sensor size

or would a 50mm lens on Pn'S and on a DSLR have different DOF's because of some law of optics I don't understand

did that make sense at all?

Through my reading and research I haven't been able to find out why these phenomenons happen, just that they happen, and now I'm wondering because I just got thrust back into the world of small sensors with my new video camera
Most cameras, from cell phone cameras to point & shoots to dSLRs, show focal lengths and apertures in terms of 35mm equivalents. But if you look very closely at the spec sheet or the lens you'll see that the actual focal lengths and apertures are much smaller. For example, for the Canon G10 P&S that I have on my desk:

Actual focal length range of zoom lens:
6.1mm - 30.5mm

35mm equivalent focal length range of zoom lens:
28mm - 140mm

So on this particular camera, a 50mm focal length (in 35mm equivalent) is actually just 10.89mm.

Now take this information and apply it to apertures. The aperture F-number, after all, is simply the ratio of the focal length to the aperture diameter. On a 35mm camera, for example, F2.0 on a 50mm lens is:

50mm / 2.0 = 25mm, or about an inch.

On the G10 P&S, however, the math is:

10.89mm / 2.0 = 5.44mm, or a much smaller opening.

Ultimately, the smaller the opening, the greater the depth of field. A pinhole camera, for example, has a nearly infinite depth of field, and the G10 described above, with it's 5.44mm opening is much closer to the pinhole camera than the 35mm camera described above with an opening close to an inch in diameter.

Hope this helps...

09-08-2010, 05:44 AM   #9
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QuoteQuote:
I thought that DOF decreased as focal length increased
It does. But it doesn't for "effective" focal length. An APS sensor just does what you would get if you took a picture with a film camera and then cropped the sides.
09-08-2010, 07:46 AM   #10
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QuoteOriginally posted by bdery Quote
It does. But it doesn't for "effective" focal length. An APS sensor just does what you would get if you took a picture with a film camera and then cropped the sides.
Yes whenever I say focal length I mean actual focal length of the lens, I usually refer to cropping as equivalent focal length or something similar
09-08-2010, 02:35 PM   #11
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QuoteOriginally posted by future_retro Quote
oh geeze, I thought that DOF decreased as focal length increased

I don't know where I read that but I must have somewhere, now that screws up my whole understanding
AFAIK indirectly you are right.
DOF is a combination of 2 factors:
f stop and magnification.
f stop was explained above.
and the magnification?
with the same focal length, the closer you are to the subject without changing f stop the thinner DOF you'll get.
With changing focal length, from the same spot and with the same f stop the longer focal length you use, the thinner DOF you'll get....

my 2p
09-14-2010, 06:35 PM   #12
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Depth of field is a function of three things predominantly:

Focal length ("mm" of lens)
f/ stop (Aperture)
Focal distance (how far into the scene are you focused).

f/2 at 28mm has a much wider depth of field than f/2 at 200mm, for example. And in general, the closer you are focused the less your depth of field. That's why macro photographers generally have to stop their lenses down; focused so closely DOF is generally razor thin.
09-14-2010, 07:52 PM   #13
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QuoteOriginally posted by JeffJS Quote
Circle of Confusion..

Circle of confusion - Wikipedia, the free encyclopedia

Circle of Confusion

Basically and very simplistic, everything else being equal (focal length, f stop, distance to subject, etc), the smaller the recording medium, the smaller portion of the center of the image circle is used. Being the sharpest point, the other stuff (Out of focus) gets cropped off.

JeffJs is on the money with this answer. The Circle of Confusion is effected by the sensor/film size. In addition to the focal length, aperture and focus distance,the CoC is also used to calculate the Depth of Field.


http://www.dofmaster.com/dofjs.html
09-14-2010, 07:53 PM   #14
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QuoteOriginally posted by brofkand Quote
Depth of field is a function of three things predominantly:

Focal length ("mm" of lens)
f/ stop (Aperture)
Focal distance (how far into the scene are you focused).

f/2 at 28mm has a much wider depth of field than f/2 at 200mm, for example. And in general, the closer you are focused the less your depth of field. That's why macro photographers generally have to stop their lenses down; focused so closely DOF is generally razor thin.
True, but the Circle of Confusion is used in the calculation and the C of C changes depending on format.

http://www.dofmaster.com/digital_coc.html
09-15-2010, 06:59 PM   #15
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Guys. You are all missing a point

DOF is a function of lens aperture and the ratio of image size including the enlargement of the print related to subject size

If you take a picture of something with a 200 mm lens for example and shoot with both formats ( film and ASP-C) and print the subject to the same size the depth of field will be the same

Where people vet crossed up is they use shorter lenses and move closer to get the same image size on the two different sensors and then enlarge them with different print ratios from the sensor to print

The net result is comparing apples and oranges
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