VERY technical question about depth of field (you've been warned)

[Senor Hound]

Okay, so I was messing around with a depth-of-field calculator which I have linked here:

http://www.dofmaster.com/dofjs.html

It says that a digital camera with a 1.5 or 1.6 sensor will have less of a depth of field at any given aperture and focal length than the film camera (i.e. 50mm at f/1.8). I told someone else here this, and they disagreed with me, and said the same lens should have the same depth of field no matter what the camera is, cause they have the same lens and aperture, so there shouldn't be any possible way for it to have a different depth of field.

I thought it was because a digital sensor might be less capable of reproducing depth of field, but I found out this isn't the reason. So now I think its because the lens has to enlarge the image more than film does, so the tolerance for what looks in focus is higher (circle of confusion). Therefore, depth of field decreases. And if this is true, that means that when you blow up a photo, your depth of field actually decreases the larger you make it, until you get to a point where you have no focused depth of field and the whole image looks blurry.

Can someone figure this out for me? I've looked on three websites with depth of field calculators and they all tell me the same thing. And I'm not the best at science, so I'm in the dark.

Can anyone who has a PhD in optics help me out?

Edit: BTW, I found this:

http://en.wikipedia.org/wiki/Depth_of_field#DOF_vs._format_size

It tells me what I thought was true, but it doesn't tell me why that's the case. And it is open source, so maybe its wrong too...

 

[RyanLilly]

Maybe I'm not understanding the DOF calculators terminology, But If they Mean that Between the "Near limit" and the "far limit" is what is in focus. Then the thing seems to be completely wrong, or just backwards.

I can't think of the physics behind it right now, I'm sure I could figure it out if I drew some picture to help myself visualize it, but its late, But the smaller the sensor or film, the larger the DOF, or in other words more distance is in focus.

 

[RyanLilly]

OK, Now I had to think about it. The Wider the lens the the larger the DOF, The longer the lens, the smaller the DOF.

There is nothing wrong with that DOF Calculator. I was thinking that the focal length was 35mm equivilent, but it uses the actual physical size.

Smaller sensors or film only have larger DOF, because they require very short lenses to get a reasonably wide viewing angle. So to obtain the same angle as a 38mm on 35mm film, might require a lens of only 5 or 6mm, so optically you have extremely long DOF


Choose a canon 5D or even a crop body with a 38mm,
Then choose a random P&S, like a powershot 5xx and 6mm lens, This should be about 38mm equiv. same f-stop for both, and look at the total DOF for each.

 

[Alex_B]

In order to get the same field of view, you will have to use a longer lens with the film camera.

Hence you should compare 50mm at f/1.8 on a 1.6 crop sensor with 80mm f/1.8 on film or on a full frame digitial sensor.

Then the 1.6 sensor will have more DOF.

 

[Alex_B]

oh, it has all been said now .. I was too slow

 

[Senor Hound]

In order to get the same field of view, you will have to use a longer lens with the film camera.

Hence you should compare 50mm at f/1.8 on a 1.6 crop sensor with 80mm f/1.8 on film or on a full frame digitial sensor.

Then the 1.6 sensor will have more DOF.

I'm not talking about field of view, I'm talking about depth of field. Why does a digital camera have a shorter depth of field when using the EXACT same lens and aperture as a film camera?

 

[Alex_B]

I'm not talking about field of view, I'm talking about depth of field. Why does a digital camera have a shorter depth of field when using the EXACT same lens and aperture as a film camera?

Because to get the same final print size (or size on your computer screen or whatever medium we talk of), you have to enlarge more if you record on a smaller sensor than on a large one (or film).

Hence you enlarge also the "unsharpness" more, to put it in such handwaving words.

To understand why it is usually said that smaller sensors produce larger DOF, you need to talk about field of view and different focal lengths though.

 

[RyanLilly]

Choose a canon 5D or even a crop body with a 38mm,
Then choose a random P&S, like a powershot 5xx and 6mm lens, This should be about 38mm equiv. same f-stop for both, and look at the total DOF for each.

If I want to take the exact same picture, same framing proportion and field of view with a Full Frame camera and a P&S, Take a 5D, 38mm lens@f/3.2, and to take the same picture with a Powershot a560(about a 6x crop factor) you need a 6mm lens @ 3.2.

A Canon 5D DOF= 4.13 feet
Powershot 560= infinate DOF, even though the feild of view is exactly the same.

Now If you could stick a real 38mm lens on that powershot, the field of view would look like a 228mm on the 5D, and the DOF would be a mere .66 ft.

Basically there is a ratio between sensor size and focal length, the wider the ratio is, the smaller the DOF, The closer the ratio is the larger the DOF

 

[Senor Hound]

Because to get the same final print size (or size on your computer screen or whatever medium we talk of), you have to enlarge more if you record on a smaller sensor than on a large one (or film).

Hence you enlarge also the "unsharpness" more, to put it in such handwaving words.

To understand why it is usually said that smaller sensors produce larger DOF, you need to talk about field of view and different focal lengths though.

Okay, that's what I thought. With that being said, does that mean the depth of field on an 8x10 photo is less than the depth of field on a 6x4 if the film negative is the same size? If so, then that would explain why you can only blow up a photo so far, until you get to a point where the depth of field is so insanely small it all looks out of focus?

Thank you for being so smart and so patient with me. I find stuff like this interesting, especially when its (somewhat) applicable to my hobby.

 

[Alex_B]

Okay, that's what I thought. With that being said, does that mean the depth of field on an 8x10 photo is less than the depth of field on a 6x4 if the film negative is the same size? If so, then that would explain why you can only blow up a photo so far, until you get to a point where the depth of field is so insanely small it all looks out of focus?

Thank you for being so smart and so patient with me. I find stuff like this interesting, especially when its (somewhat) applicable to my hobby.

no image is tack sharp the more you enlarge the more you will realise grain and general unsharpness, if you view it from the same distance. But an 8x10 you will usually automatically view from a larger distance than a 6x4, hence the effect appears less pronounced or even becomes irrelevant. You usually fill your field of attention with the whole image, instinctively.

only if you go to very large print sizes, you will often not step that far back, and hence you realise the limited resolution.

 

[Sark]

Two identical focal length lenses when set to the same f-stop produce exactly the same DOF. I'm not sure why Alex-B is introducing print size into the equation, because it's not relevent to the DOF, only the image sharpness.

The confusion with DOF when discussing sensor size is due to the fact that a smaller sensor requires a shorter lens to match the field of view of a larger lens used with a larger sensor.

Sensor size alone has no effect on DOF.

Sark

 

[Sark]

Because to get the same final print size (or size on your computer screen or whatever medium we talk of), you have to enlarge more if you record on a smaller sensor than on a large one (or film).

A smaller 10MP APS sensor needs to be enlarged no more than a 10MP full frame sensor. If by small and larger your refering to MP, then yes, that effects image size, but not DOF.
Sensor size, or MP alone has no effect on DOF.

Sark

 

[Alex_B]

Two identical focal length lenses when set to the same f-stop produce exactly the same DOF. I'm not sure why Alex-B is introducing print size into the equation, because it's not relevent to the DOF, only the image sharpness.

define image sharpness without print size. what appears sharp subjectively depends on print size.
you can define sharpness of course with respect to the recording medium, but this in practice is of little use.

 

[Alex_B]

A smaller 10MP APS sensor needs to be enlarged no more than a 10MP full frame sensor. If by small and larger your refering to MP, then yes, that effects image size, but not DOF.
Sensor size, or MP alone has no effect on DOF.

Sark

we do not need to include MP in this discussion since they do not tell much about resolution or sharpness. Of course MP define a certain limit, just as grain does. but we can assume in what we are talking about here sharpness is not limited by MPs.

from a smaller recording sensor to a certain size on screen or in print means more enlarging than from a larger sensor to the same size. And by large and small i only refer to physical size of the recording medium. nothing else.

 

[Sark]

define image sharpness without print size. what appears sharp subjectively depends on print size.
you can define sharpness of course with respect to the recording medium, but this in practice is of little use.

Yes, I said that ..."it's not relevent to the DOF, only the image sharpness." ..but image sharpness is not the same as DOF.

It's also true that image resolution is not a factor of the pixel count, but again, image resolution ( optical, or otherwise) is not relevent to DOF, only image sharpness.

To clarify. Are you suggesting image sharpness effects DOF.

Sark