[baw]

Yes, this is true. Same lens cropped the same yields the same DOF independant of sensor (Though in the real world you'd have very different resolutions after the crop)

Not sure which resolution you refer to. The resolution of the screen doesn't change by the crop.

Like I said, those 4 factors define DOF depending on which ones you hold constant.

The whole discussion was about wether sensor size has influence on DOF. I still feel it doesn't.

Sensor size matters primarily due to the magnification factor to print.

Not sure what you mean here.

Acceptable CoC size varies with sensor size.

Imo CoC is just a number you select indicating what you feel is acceptably sharp.

The whole calculation of DOF is not very relevant imo. It's the DOF in the viewed image that counts, wether on screen or in print.
You might find this article by Harold M. Merklinger interesting. He has a different take on DOF.

[Lowell Goudge]

Not sure which resolution you refer to. The resolution of the screen doesn't change by the crop.
The whole discussion was about wether sensor size has influence on DOF. I still feel it doesn't.
Not sure what you mean here.
Imo CoC is just a number you select indicating what you feel is acceptably sharp.

The whole calculation of DOF is not very relevant imo. It's the DOF in the viewed image that counts, wether on screen or in print.
You might find this article by Harold M. Merklinger interesting. He has a different take on DOF.

Now you see why I stopped making posts in this thread, the whole thing gets repeated over and over, and no one listens.

For depth of field and depth of field only, you are correct. What screws things up is people are confusing resolution with focus.

The way forward for the non believers is to compare the image of a 6MP ASP-C sensor to a 14 MP full frame using the same lens, and the FF image cropped to capture only the central 6MP (i.e. the ASP-C crop)

compare those 2 images and they will be identical in every way because they will also have the same exact resolution.

[simons-photography]

right admitting the two images are not exactly the same size (about 10 % out) but they are the same picture, the first on my fuji (crop factor4.5) with the lens at 66.7 mm (300mm 135 equiv.) and F5.6 the second is the K10D at 220 mm (330 mm 135 equiv.) also at F5.6 you can see a huge difference in DOF can't you ? the camera with a smaller sensor and therefore a shorter lens has much more DOF.

[Lowell Goudge]

right admitting the two images are not exactly the same size (about 10 % out) but they are the same picture, the first on my fuji (crop factor4.5) with the lens at 66.7 mm (300mm 135 equiv.) and F5.6 the second is the K10D at 220 mm (330 mm 135 equiv.) also at F5.6 you can see a huge difference in DOF can't you ? the camera with a smaller sensor and therefore a shorter lens has much more DOF.

You still don't get it do you.

I said take a photo with the same focal lenght lens on two different sensor sizes, crop them the same print them to the same size and they will have the same depth of field.

You are comparing a 66.7mm lens with a 220 mm lens

Take the same shot again with a K10D and a 67 mm lens and it wil look the same as the P&S.

it has nothing to do with "equivelent focal length" but actual focal length.

[simons-photography]

well nobody seems to be reading the original post, what my point is a camera with a smaller sensor will have more DOF because to achieve the same results it uses a shorter lens I mean its that simple... thats all I was saying. the feild of view with a 50 mm equi lens will be more on the P&S with a 4.5 crop factor versus the K10D with a 1.5 crop factor (and coresponding sized lenses)

[falconeye]

I know I will fail, but let me try nevertheless

-- To make a complicated story simple --


DoF (Depth of Field, in meters) depends on:
- FoV (Field of View, in degrees) and,
- Aperture (lens opening diameter, in mm, not f-stops)
ONLY !



This seems to be independent of pixel sizes (because is is based on the human eye's capabilities), sensor sizes, and so on.

But here comes one more bit to be understood:

For a given FoV and for a smaller sensor, you get a smaller focal length. Hence a smaller f-stop value for a given aperture in mm. And ONLY because the smaller focal length lens with smaller f-stop value may not exist in the wide-angle market place, you end up with more DoF (and less light). Of course, you could have always got the same DoF from the larger sensor by making the aperture in mm as small as for the small sensor camera, i.e., by stopping down (and again less light).

Or to make it really short:

The available range for DoF for large sensors is a superset of the available range for DoF for a small sensor. (*)


(*) One more word... because based on amount of light, this already automatically takes larger pixel sites (same noise at higher ISO) into account, i.e., when stepping down with a larger sensor, you increase ISO too (same shutter speed).

[Geekybiker]

Not sure which resolution you refer to. The resolution of the screen doesn't change by the crop.

Well we taking pictures with a equal MP camera but varying the sensor size you end up with different resolutions when you crop. Lowell covered this precisely in the next post.

The whole discussion was about wether sensor size has influence on DOF. I still feel it doesn't.

It does if you want to take the same picture, but mainly due to the fact that you need to change the lens to maintain FOV.

Not sure what you mean here.

Normally DOF is determined based on what is subjectively sharp reproduce in a 8x10print. A smaller sensor needs increased magnification to reach 8x10, so the CoC on sensor must be smaller to achieve a similar print at 8x10 compared to FF.

Imo CoC is just a number you select indicating what you feel is acceptably sharp.

Right. There is a small range of values people normally use. It doesnt really matter what you pick as long as you're consistent though.

---

Last edited by Geekybiker; 04-07-2008 at 10:53 AM.

[baw]

Well we taking pictures with a equal MP camera but varying the sensor size you end up with different resolutions when you crop. Lowell covered this precisely in the next post..

If you want to compare apples to apples you don't keep the Mp count equal, but the pixel density.

Think Nikon D3, full frame sensor, that can also shoot using part of the sensor, to accomodate "digital" lenses (for APS-C sized sensors)

Keeping everything else the same, I fail to see how switching to using part of the sensor will change anything in DOF.

[simons-photography]

smaller sensor smaller circle of confusion so for a different sensor size the same F number will have the same effect and the lens is proportionatly shorter and being shoerter will have more defpth of focus as we use shorter lenses with smaller sensors a camera with a smaller sensor will have more dof is as simple as that

[falconeye]

smaller sensor smaller circle of confusion so for a different sensor size the same F number will have the same effect and the lens is proportionatly shorter and being shoerter will have more defpth of focus as we use shorter lenses with smaller sensors a camera with a smaller sensor will have more dof is as simple as that

This sentence is hard to read. It may be false as well.

In all honesty, may I propose my own statement two postings above then?

I feel it may be easier to read and it automatically takes care of the "smaller circle of confusion" argument as well.

[falconeye]

FoV also has no place in the arguement. DoF is a function of:
Focal Length
Aperture
sensor size
[...]I again suggest that for this discussion - the terms equivalent and FoV have no place, they only confuse.

Hi PDL,

while I can see why to avoid FoV, my own post uses it -- for one reason:

DoF for given FoV only depends on aperture (in mm). So, by introducing FoV one effectively eliminates 2 out of 3 variables which cleans up the discussion a lot!

[Geekybiker]

If you want to compare apples to apples you don't keep the Mp count equal, but the pixel density.

Think Nikon D3, full frame sensor, that can also shoot using part of the sensor, to accomodate "digital" lenses (for APS-C sized sensors)

Keeping everything else the same, I fail to see how switching to using part of the sensor will change anything in DOF.

You're right, it won't change if you're just cropping. But I don't think that is the comparison that makes sense. Comparing FF to crop means comparing the WHOLE frame IMHO.

[baw]

You're right, it won't change if you're just cropping. But I don't think that is the comparison that makes sense. Comparing FF to crop means comparing the WHOLE frame IMHO.

I compare two sensor sizes: FF and APS-C
By keeping everything else the same it should be obvious that DOF doesn't change with sensorsize alone.
You change a second variable everytime and then conclude that DOF changes with sensorsize???

[Geekybiker]

I compare two sensor sizes: FF and APS-C
By keeping everything else the same it should be obvious that DOF doesn't change with sensorsize alone.
You change a second variable everytime and then conclude that DOF changes with sensorsize???

*sigh* Yes. For taking the same picture. Optically it has nothing to do with it, but sensor size affects what lens you need to get identical framing. Thus for practical purposes when taking photos it does affect DOF vs taking about it on a message board. But whatever, I'm done with this.

[baw]

*sigh* Yes. For taking the same picture. Optically it has nothing to do with it, but sensor size affects what lens you need to get identical framing. Thus for practical purposes when taking photos it does affect DOF vs taking about it on a message board. But whatever, I'm done with this.

Sorry, but if you don't like to reason straight then just don't participate in a discussion, iso confusing things with irrelevant comparisons