OT: Digital DOF?

[abufletcher]

Several times I've heard it said (and seen it demonstrated) that the

smaller sensor on most digital cameras results in extended DOF (i.e.

an inability to utilize selective focus). I'm having trouble getting

my mind around this idea. Would anyone care to try to explain this.

 

The way I see it, having a smaller sensor has the effect of

increasing the focal length, for example, by a factor of 1.5 on a

Nikon D100, and this would mean that, say, a 35mm lens would be

equivalent to about a 50mm. Wouldn't that mean less DOF not more?

Is it a whole different story with non-SLR digital camera (like my

CoolPix 5000) which use really short focal length lens (e.g. 8mm)?

 

As you can see I'm seriously confused here. Help!

[fotografz]

Hi Donald.

 

When the sensor is smaller, the lens has to be a wider angle

just to produce a normal view. Wide angles have a much greater

depth of field.

 

For example , the Canon G3 has a 7.2mm to 28.8mm zoom lens

just to produce a 35mm to 110mm (<I think) perspective in terms

of regular 35mm cameras. The effect is most pronounced with

point-and-shoot digital cameras with fairly small sensors.

 

So, using your example of a 35mm on a D100, you get the

magnification of a 50mm, but the DOF of the 35mm which is a

wider angle lens.

 

On full frame cameras such as the Canon 1Ds there is no

difference in DOF compaired to a 35mm film camera.

[terrestrikon]

The above reply got the basic answer, but the question actually gets more complicated.

 

It's very interesting. Not being an engineer, and having found discussions on this topic by engineers to be somewhat arduous to follow (I'm never sure whether they themselves know what they're talking about, let alone my understanding their verbal shorthand or not), I found the following quite useful, but not, by any means, a complete tour d'horizon:

 

http://www.sciencenews.org/20030329/bob9.asp

 

In any case reading this I could imagine, based on current technology, and proven designs moving from drawing board to production line, that in a year or two (at the most) some kind of pocketable P&S will have about 10 megapixels, negligible distortion, and very deep depth of field, outperforming in many ways current top of the line MF gear, and probably for a price point of about $500.

 

I guess the question then about DOF becomes especially interesting if what you want is shallow instead of deep DOF. Will digital lens/software combinations be able to 'artificially' approximate the effect of fast apertures wide open? Or even not-so-fast apertures wide open? To the extent they can't, as an amateur I expect to see film cameras remain attractive, at least as a craft pursuit, but if digital can successfully emulate normal MF lenses' DOF control, together with higher resolution and lower distortion, then, to be honest, I'm kind of worried about the future of film. This from consumer P&S, not to even consider professional grade digital gear...

 

Printer technology is something else. To my eye wet darkroom products are clearly better, at least in the sub-mega-thousand dollar range, but at some point possibly home printers will catch up as well.

 

Interesting times.

 

G.

[peter_white2]

Twas Written:

 

So, using your example of a 35mm on a D100, you get the magnification of a 50mm, but the DOF of the 35mm which is a wider angle lens.

 

Correct me if I'm wrong, but if you enlarge the image from a 35mm lens to the same magnification as a 50mm lens, aren't the circles of confusion the same size, all other factors equal? If that's correct, then there's really no difference in depth of field.

 

PJW

[hm]

Technically, this has nothing to do with digital, the same phenomenon is true for any format smaller than another format. These days "gobs of depth of field" is associated with digital point 'n' shoots because they have extremely small sensors, and thus are "really small format". The higher end DSLRs are fairly close to 35mm in size, so the effect is not so great, and cameras like the Canon 1Ds are the same size as 35mm, and so have identical depth of field.

 

If you grab your Leica and stand next to a guy with a 6x9 MF camera, and try to take the same composition of the same subject from the same distance, you'll find that you're using a much shorter focal length lens than he is, a natural result that different format sizes have on angle of view. Your shorter focal length lens will tend to yield far greater depth of field than his much longer lens, as depth of field increases exponentially as focal length drops. Now, to achieve the same on-print resolution, you'll need to enlarge much more than the MF guy will, so your CoC will need to be much smaller. This effect reduces depth of field for the 35mm camera, but only linearly, while the smaller focal length lens increases it exponentially, so the net effect is that the 35mm camera will see greater depth of field at a given aperture.

 

You'll see the same thing when comparing your Leica (or a 35mm full-frame digital) to a lower-end digital SLR with an APS-sized sensor, or when comparing that DSLR with a digital point 'n' shoot.

[wind.dk]

From the links on the Photo.net front page: <A HREF="http://www.photo.net/learn/optics/dofdigital/">Depth of Field and Digital</A>.

[abufletcher]

George, I read this article a couple of months ago and I remember thinking "YIKES! Soon we'll have all digital auto-everything cameras with +/- dial-in DOF compensation."

 

BTW, yesterday a friend just showed me how his D100 can automatically decide what is the closest object in the frame and focus on it in a sort of "proximity priority" AF mode. Scary is all I can say.

 

AF users take note (and Leica users take heart), however, a goodly number of the winners in the POYi competition include an out-of-focus foreground subject against a sharp background.

 

I think I bought my M just in time!!!

 

--Don

[fotografz]

Peter, I was thinking about that also. It would be interesting to do

a test, because I think you are right, when the lens is designed

to cover a full 35mm frame, the DOF should be the same no

matter what size the sensor receiving the image is. What is

confusing is that when you use a 35mm and crop to what a

50mm would produce, is the DOF the same as the 35mm or the

same as a 50mm? Lets take an extreme, shoot a 21mm which

has enormous DOF and crop the enlarged image to the

equivlant of a 90mm image size. At f/4 focused at the same

point, would the DOF of the 21 image be greater, or the same as

the 90 at f/4?

[abufletcher]

Mark, Peter, this is getting at the heart of my confusion. I have always believed that all other things being equal a 50mm slice of a 35mm view (or a 90mm slice of a 21mm view) would be the same as a photo shot with a 50 or a 90 -- the only difference being the (print) enlargement factor. Isn't this exactly what is being shown in those angle of view series at the back of every lens catalogue?

 

Also isn't DOF directly related to focus distance rather than being an "inherent" quality of any lens. In photographer's shorthand, we can and do say that wide-angles offer more DOF, but isn't this sort of like claiming that wide-angles alter perspective?

[hm]

Changing the enlargement factor does impact depth of field, it reduces it. But the real point is that if you crop the image (effectively creating a "smaller format"), but want to hold angle of view steady, you need an accompanying change to shorter focal length lens, which increases depth of field to a greater degree than enlargement decreases it.

 

So taking a shot with a 35mm focal length lens, but then cropping it to match the angle of view of a 50mm lens, only increases the enlargement factor, which decreases depth of field. If we then take a shot with a 50mm lens without changing our distance, and make a print from that frame without cropping, and compare our two prints (the 35mm cropped and 50mm uncropped), we'll see that we have identical perspective and framing, but the 35mm lens crop will display greater depth of field. In fact, the difference will be almost exactly one f-stop's worth of aperture.

 

This "cropping" is effectively creating a small format smaller than 35mm. The notion of "lens designed for 35mm" doesn't really have meaning in terms of depth of field, it just means that its image circle need be no greater than needed for a 36mm x 24mm frame.