Pondria wrote:
But here I saw and heard something different.


You've been fooled.

loudtiger wrote:
i don't think the 35/F2 is USM at all.

It doesn't and it's painful to use in low light. How much hunting can any one lens do?

RGS65 wrote:
That may be so, but there is an obvious DOF difference to me comparing the images. Charles is right. Just look at the detailing on the white sheer curtain in the background on thr 1.8, mush more in focus to me. And the red can, If the 1.8 red can does not look more in the DOF to you than the others you need to get new glasses. Anyone explain it?

I see what you mean. At least two possibilities:

1. Slight differences in focusing among the three lenses, i.e. the true plane of focus shifted slightly forward or back; and

2. Maybe the f/1.8 isn't truly f/1.8, but more like f/2. That would explain its slight underexposure compared to the other two, and give it slightly more DOF.

Also, the f/1.0 appears to be slightly longer compared to the other two. That would give it slightly *less* DOF.

Nill
~~
www.toulme.net

DavidP wrote:

Because it's not physically possible for three TRUE 50mm lenses that are TRULY at f/1.8 and TRULY focused at the same point to have different DOFs.


Sure it is, if the physical position of the front lens element is different. If this camera was mounted on a tripod, then the front of the 50 f/1.0L would be considerably closer to the subject than the others (just refer back to the photo of the three lenses).

Naturally, if you look at the equations for hyperfocal distance and DOF, you'll find that there are three critical variables: focal length, aperture, and distance to subject. And distance to subject is by far the most heavily weighted of the three (the hyperfocal distance relates to the square of the distance, giving it exponentially more importance than the aperture).

So we stipulate that the aperture and focal length are identical. If we assume 50mm focal length, f/1.8, a full frame sensor, and a camera positioned 2 feet from the subject, the total depth of field is less than an inch. So even if the front of the 50mm f/1.0L was an inch farther forward than the others, the field of focus will be considerably different than the other lenses, assuming that all other variables are the same.

Nill Toulme wrote:

I see what you mean. At least two possibilities:
2. Maybe the f/1.8 isn't truly f/1.8, but more like f/2. That would explain its slight underexposure compared to the other two, and give it slightly more DOF.
Nill

I bet you are correct.

DrPablo wrote:
DavidP wrote:

Because it's not physically possible for three TRUE 50mm lenses that are TRULY at f/1.8 and TRULY focused at the same point to have different DOFs.


Sure it is, if the physical position of the front lens element is different. If this camera was mounted on a tripod, then the front of the 50 f/1.0L would be considerably closer to the subject than the others (just refer back to the photo of the three lenses).

Naturally, if you look at the equations for hyperfocal distance and DOF, you'll find that there are three critical variables: focal length, aperture, and distance to subject. And distance to subject is by far the most heavily weighted of the three (the hyperfocal distance relates to the square of the distance, giving it exponentially more importance than the aperture).

So we stipulate that the aperture and focal length are identical. If we assume 50mm focal length, f/1.8, a full frame sensor, and a camera positioned 2 feet from the subject, the total depth of field is less than an inch. So even if the front of the 50mm f/1.0L was an inch farther forward than the others, the field of focus will be considerably different than the other lenses, assuming that all other variables are the same.


What would happen if you mounted the UV filter 12” in front of the lens? Would it change your DOF calculation? I thought the distance to subject was measured from the sensor or film where the image is actually focused and not from the piece of glass closest to the subject.

MikeZ wrote:
I thought the distance to subject was measured from the sensor or film where the image is actually focused and not from the piece of glass closest to the subject.

You are absolutely correct, DOF calculators are based upon the distance from film plane to subject.

Thanks for the test.

The relevant point for me is the light fall-off (vignetting). Which one (50/1.4 or 50/1.8) is better at 1.8?

Thanks again!

MikeZ wrote:
What would happen if you mounted the UV filter 12” in front of the lens? Would it change your DOF calculation? I thought the distance to subject was measured from the sensor or film where the image is actually focused and not from the piece of glass closest to the subject.

You are correct in thinking that subject distance, as practiced in photography, is measured from the image plane (film or sensor). Some cameras mark this position with a symbol: circle bisected by a line.

DOF calculations, however, use subject distance to the front principal plane (equal to the position of the entrance pupil for symmetric lenses). In extreme cases, the object plane may lie within the outermost element of the lens! I achieve this with my CZ fisheye and 40mm of extension. When used, filters are part of the optical system, and affect DOF to a small degree - think about swimming pools as a magnified example.

Refs:

http://www.vanwalree.com/optics/dof.html
http://www.vanwalree.com/optics/dofderivation.html