DoF on digital versus film

[Finder]

You can't take depth of field or depth of focus in isolation. Both are related to each other and very importantly both are related to obtainable resolution on film or sensor.

Yes, yes, yes. Film is better than digital and digital is better than film. Now that we have that out of our system.

A 100mm lens at f/4 will have the same depth of field as a 25mm lens at f/1. However, the F/1 lens, baring aberrations, will have a smaller depth of focus and higher resolving power. So much for relationships.

 

[Finder]

The simple reason why the myth is incorrect is that depth of field is set by aperture, focal length, and a criterion for spatial resolution, and if one keeps aperture of the larger camera the same as that in the smaller camera, the two cameras record the same image with the same signal-to-noise ratio and the same depth of field with the same exposure time."

That is true if by aperture you mean the entrance pupil and not the exit pupil (f-number). If you break down the criteria you will also find that while the theoretical exposure time is the same, the actual exposure is not as the exit pupils are not equal (different f-numbers). However, the author is simply fixing a different set of parameters than the usual ones photographers use (and making a few assumptions like the pixel response is only a function of pixel area and the system really falls apart when subject distances are shorter than infinity). Both are methods are "true," but one (the usual one photographers use) tends to be more practical than the other. Clark's kind of thinking has led to stupid claims such as the f-number is not actually a factor in exposure in digital imaging, however when you do the math, the exact same results can be made with Clark's criteria as with f-numbers. It is just people having fun with numbers.

 

[palec]

"The myth, simply stated, is: smaller digital cameras have a larger depth-of-field than larger digital cameras."

Panasonic LX-3, 24mm (35mm equiv), f/2.8

EOS 5D, 24mm, f/2.8

Where's the focus point on LX-3 sample?

 

[NickTrop]

Dr Thunder? Great illustration. I don't even understand why there is even debate here.

 

[StaaleS]

Probably because people overlook that when you change the sensor size you must also change something else; either focal length or subject distance, in order to maintain the same(ish) composition. It is easy to compare 100% crops (which do, indeed, show exactly the same DOF) and ignoring the fact that the photos they are cropped from are actually very very different.

 

[antiquark]

"The myth, simply stated, is: smaller digital cameras have a larger depth-of-field than larger digital cameras."

Panasonic LX-3, 24mm (35mm equiv), f/2.8

EOS 5D, 24mm, f/2.8

Thanks so much for posting that. Often these DoF debates are devoid of any sort of real world examples.

 

[Finder]

The debate is because of Clark's use of vocabulary. The Dr. Thunder picture is NOT illustrating what Clark means--don't get me wrong, I think Clark's ideas are far from practical, but he is not wrong either.

To illustrate Clark, you need to know when he says "aperture," he does not mean the f-number of the lens (exit pupil), but the entrance pupil of the lens. For example, a 100mm lens @ f/4 has the same entrance pupil as a 25mm lens @ f/1--25mm to be exact. So in object space, the DoF should be the same with those optics. Clark will also say each pixel in a 6x9 12 MP sensor will receive the same number of photos from the 100mm f/4 and the same for a pixel in a m4/3 12 MP sensor behind the 25mm f/1 optics. This use of sensors also freezes the spatial limits of the format. If focused at infinity, both images from both systems will appear the same.

Convoluted? Yes. Wrong? No. His system is pretty laborious to use and will start breaking down by simply focusing closer (and all we need to do is find an f/1 25mm lens).

 

[Roger Hicks]

I've always found that d-o-f is best determined empirically; it sometimes seems to vary in practice (though not in theory) between lenses of the same focal length and aperture, but different optical designs, on the same medium.

There's almost certainly something in the 'soup' analogy, but personally, I learn more from just using my lenses.

Cheers,

R.

 

[NickTrop]

Probably because people overlook that when you change the sensor size you must also change something else

Everything is driven by "sensor size", which I would call "the plane" be it a frame of film or a CMOS/CCD sensor - from the physical size of the camera and lenses, to DoF, to overall/general quality. This is why I think the whole "film vs digital" debate is silly. Has nothing to do with "megapixels", which if finally starting to dawn on people. With prime lenses, especially in the "standard focal length" range there isn't all that much difference... any decent one made by a competent supplier will do. Large format contact prints look like you cut out reality with an Exacto knife and magically transported it onto a piece of paper. This is obviously due to the size of the area the lens is resolving the image on - and there are vast differences from large format down to teeny tiny cell phone sensors. Even most Leica users will tell you that granny's 6X6 folder (which she hopefully handed down to you...) "takes better pictures" than their M-whatever with a 'Cron. The relevance of film? From a purely pragmatic perspective - inexpensive upfront equipment costs and a more "doable" pay-as-you-go way, from an out-of-pocket expense stand-point (for many, especially non-pros), to get "full frame" picture quality over the digital alternative.

 

[flyalf]

Depth of Field, Depth of Focus have enough variables in the real world that worrying about the difference between film and sensor seems to me to be wasted energy. Perhaps in some specialized photography it is of consequence, but not in shooting wide open in bars.

Well, then do not worry . Sorry, as a matter of fact I have for many years photographed with 135 film, and various optics on SLR. Im used to using hyperfocal (with tripod). When I started using FF DLSR recently it seemed that the DoF was "thinner" than expected. If this is just a matter of perception because the workflow is different I dont know. Thats why I ask, and eventually will adjust my photo habits (stop down a bit more).

 

[Nikon Bob]

Alf

I have been using the same Nikon lenses on my D700 as I had been using on film Nikons and have not noticed any difference in DOF. The FF D700 sensor is so very slightly smaller than 135 film frame that the charts do show a tiny difference in DOF. I am just not sensitive enough to actually notice it say using my 50/1.4 wide open. I guess you are a much keener observer than I am to actually notice it. It is there but I just never thought it would be noticeable to any great degree.

Bob

 

[flyalf]

Bob, others,

Thanks for sharing experiences. Im beginning to doubt if the DoF are different, or if its my perception of DoF . Until further evidence comes up I will blame my "observations" on the difference in handling photos, e.g. using PC screen and various SW, as opposed to the old analogue print. I have to few prints from digital to conclude. And anyway as soon as I print another factor is introduced affecting the perceived DoF.

But I will probably stop down a bit more on digital...