Why SLRs have shallower DOF than point-and-shoot cameras

Started Jul 25, 2012 | Discussions thread
Kerry Pierce
MOD Kerry Pierce Forum Pro • Posts: 19,757
Re: Why SLRs have shallower DOF than point-and-shoot cameras

egrivel wrote:

the "The depth of field is the region where the CoC is less than the resolution of the human eye (or of the display medium)." The resolution of the display media (sensor) for the D70 and D300 is different, so how can the DOF be calculated with the same value for the CoC?

No, where you have the display media = sensor, that isn't correct. Display media is the computer screen, a print, a TV screen, etc.

I'm still thinking that all of this stuff about DOF being constant is based on the assumption that you print the photo in the same size. Of course the DOF is being constant then, but all you're doing is taking the 12Mpix D300 image and downsampling it to the 6Mpix D70 image before looking at it (or probably downsampling both to whatever you use to print). I cannot imagine that when you look at the original image at 100% the DOF is the same.

You're right, display size/print size is a key factor, which is basically what you quoted above about the display media. I haven't looked at a DOF calculator thing in quite a while. But, one of the things it should tell you is that magnification is a key to DOF. Magnification can come in various forms, normally the lens, but the print or computer screen can also magnify an image, which results in different DOF. The larger you print, the more the image is magnified and the smaller the apparent DOF.

Then think about the physical size of the sensors. A DX image requires more magnification to print at 8x12, than does an FX image, because of the slightly larger size of the FX sensor. But, Large Format film, IIRC, requires almost no magnification at all, when printed to 8x12, because the negative is almost the same physical size as the print. But, a "normal" lens on DX is about 30mm, FX is about 50mm, and for Medium Format film, a normal is around 100mm, IIRC. So, the smaller the sensor, the shorter the lens (less magnification) to achieve the same FOV.

In the end, focal length, distance to subject and display size determine DOF, until you change formats, which then adds format size to the mix.

Is that still clear as mud, Eric? I'm not sure that I have the ability to explain it better than that.

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