Depth of Field and Zoom

Barry Goyette · July 13th, 2004, 07:49 PM

Aaron....magnification is a bad term...we all use it, but it actually refers to things like microscopes...things with a ratio of 1:1 or greater. That said magnification is not really a relative term in the sense you are talking about, a 20x microscope would magnify its subject by a specific amount relative to the viewer. A video lens that is a 20x has no such specificity to it. Its simply a term to describe the RANGE of the zoom, or the relationship between its widest setting and its longest setting. I think you are using the word magnification to refer to this relationship, and that's where you're going wrong.

If we were to apply the word magnification to a video camera lense (we'd be incorrect, but in the same sense as we would a microscope) then the longer the maximum setting, then the greater the "magnification".

Am I getting somewhere?

Barry

Jeff Donald · July 13th, 2004, 07:53 PM

Aaron, no, if everything is equal, the lens with the longest focal length (highest mm) produces the highest magnification. Smaller chips do not inherently have higher magnification or greater DOF. But if you choose to have the image on the smaller chip match identically the image on the larger chip, then the focal length must be changed or the distance from subject to chip plane must be changed. Changing distance to subject or the focal length changes DOF.

Nick Hiltgen · July 13th, 2004, 07:54 PM

I follow you barry, I don't follow you don, can you explain further, is anyone else impressed by hwo quickly the xl2 forum took off... a lot of talk for a camera 99% haven't even SEEN in person!

Jeff Donald · July 13th, 2004, 07:56 PM

The number of blades will not change the DOF, but will effect the Bokeh, or how out of focus highlights will appear.

Rob Lohman · July 14th, 2004, 03:09 AM

Nick: that's because a lot of questions are asked and a lot can
be answered as well due to careful inspection of the specs and
other information (like the XL2 watchdog) that IS available. And
a lot of this still depends a great deal on knowledge and experience.

Like the topic we are discussing now. It is a well known fact
that video zoom factors are not the same as microscopes for
example. It is also well known that the properties of DOF will
change if you change the chip size (or the READOUT size)
without changing the lens. This also "changes" the focal length
of your lens etc.

Nick Hiltgen · July 14th, 2004, 08:39 AM

ROb totally understand, I just think it's great that we can do all this now before most of us even touch the darn thing, quite a resource.

Jeff Donald · July 14th, 2004, 12:15 PM

Quote:

This also "changes" the focal length of your lens etc.

No, changing chip size does not change the focal length of a lens. A millimeter is still a millimeter, nothing has changed.

Rob Lohman · August 1st, 2004, 07:17 AM

Indeed, I worded this wrong. Thanks for catching this. Nothing
will change on your lens, ofcourse. What would be a better way
to describe this Jeff? Apparent focal length or something?

Jeff Donald · August 1st, 2004, 08:48 AM

In most digital still photography communities the term "crop factor" has been adopted, to indicate a change in image size. The reason being is that the image isn't magnified, it's just a smaller portion of the image. Video communities don't really think in those terms and generally refer to it as a magnification factor. For example the magnification factor of EOS EF lenses on the XL1 is 7.2X.

Barry Green · August 1st, 2004, 04:48 PM

<<<-- Originally posted by Rob Lohman : Apparent focal length or something? -->>>
The Field of View is what changes when you change the imaging size. A smaller chip will have a narrower field of view.

Jeff Donald · August 1st, 2004, 06:20 PM

This is where the debate, or perhaps confusion is a better word, starts. Barry is 100% correct in his thinking, except chips (CCD, CMOS etc.) don't have Fields of View. Lenses have FOV, for example, zoom a lens and the FOV changes, not the chip. The FOV is related to the diagonal of the chip, but FOV changes with change of change of focal length. It basically boils down to FOV being a characteristic of the lens and the chip is the device which displays the FOV.

Field of View is measured from the rear nodal point to the two opposite sides of the chip, when the lens is focused at infinity and the rear nodal point is one focal length from the chip.

Barry Green · August 1st, 2004, 09:39 PM

Yeah, it's overwhelmingly difficult to say it all and cover all bases.

What I meant was: given the same lens and all other things being the same, if you substitute in a smaller chip, you will have a narrower field of view as your resulting image. The reason being, the lens projects the same image at the same focal plane, but the smaller chip can (obviously) only record a smaller subsection of that same image. DOF and other such characteristics do not change, as those are all factors determined by the lens and are all set in stone by the time the image gets projected on to the chip. So Chip size determines how much of the lens' frame is seen. Larger chips = more of the image displayed, which means for an equivalent lens, you would see a wider field of view when using a bigger chip.

Jeff Donald · August 1st, 2004, 09:47 PM

Quote:

which means for an equivalent lens, you would see a wider field of view when using a bigger chip.

Correct, which is why 50mm is "normal" on 35mm film, 80mm to 105mm is normal on medium format and 210mm is normal on 4x5 format. A 50mm lens would be extremely wide on a 4x5 camera.

Aaron Koolen · August 1st, 2004, 09:49 PM

I think we're getting nit picky here, and talking mainly semantics.

Your "perceived" FOV changes with chip size but Jeff is probably being technically correct saying the chip doesn't change the "real" FOV, the focal length does.

Aaron

Jay Gladwell · August 2nd, 2004, 05:10 AM

<<<-- Originally posted by Aaron Koolen : Your "perceived" FOV changes with chip size but Jeff is probably being technically correct saying the chip doesn't change the "real" FOV, the focal length does. -->>>

Aaron, with all due respect, Jeff isn't being nit picky, he's being 100% factual. Would a 4x5 negative have a greater FOV than a 35mm negative? No! Why? Because the FOV of the light sensitive medium--negative, transparency, CCD--regardless of its size, is determined by the focal length of the lens. That's not being nit picky, it's being accurate.

Jay

July 13th, 2004, 07:53 PM	#17
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	Aaron, no, if everything is equal, the lens with the longest focal length (highest mm) produces the highest magnification. Smaller chips do not inherently have higher magnification or greater DOF. But if you choose to have the image on the smaller chip match identically the image on the larger chip, then the focal length must be changed or the distance from subject to chip plane must be changed. Changing distance to subject or the focal length changes DOF. __________________ Jeff Donald Carpe Diem Search DVinfo.net for quick answers \| Where to Buy? From the best in the business: DVinfo.net sponsors

July 13th, 2004, 07:56 PM	#19
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	The number of blades will not change the DOF, but will effect the Bokeh, or how out of focus highlights will appear. __________________ Jeff Donald Carpe Diem Search DVinfo.net for quick answers \| Where to Buy? From the best in the business: DVinfo.net sponsors

July 14th, 2004, 03:09 AM	#20
Rob Lohman RED Code Chef Join Date: Oct 2001 Location: Holland Posts: 12,514	Nick: that's because a lot of questions are asked and a lot can be answered as well due to careful inspection of the specs and other information (like the XL2 watchdog) that IS available. And a lot of this still depends a great deal on knowledge and experience. Like the topic we are discussing now. It is a well known fact that video zoom factors are not the same as microscopes for example. It is also well known that the properties of DOF will change if you change the chip size (or the READOUT size) without changing the lens. This also "changes" the focal length of your lens etc. __________________ Rob Lohman, visuar@iname.com DV Info Wrangler & RED Code Chef Join the DV Challenge \| Lady X Search DVinfo.net for quick answers \| Buy from the best: DVinfo.net sponsors

August 1st, 2004, 07:17 AM	#23
Rob Lohman RED Code Chef Join Date: Oct 2001 Location: Holland Posts: 12,514	Indeed, I worded this wrong. Thanks for catching this. Nothing will change on your lens, ofcourse. What would be a better way to describe this Jeff? Apparent focal length or something? __________________ Rob Lohman, visuar@iname.com DV Info Wrangler & RED Code Chef Join the DV Challenge \| Lady X Search DVinfo.net for quick answers \| Buy from the best: DVinfo.net sponsors

August 1st, 2004, 08:48 AM	#24
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	In most digital still photography communities the term "crop factor" has been adopted, to indicate a change in image size. The reason being is that the image isn't magnified, it's just a smaller portion of the image. Video communities don't really think in those terms and generally refer to it as a magnification factor. For example the magnification factor of EOS EF lenses on the XL1 is 7.2X. __________________ Jeff Donald Carpe Diem Search DVinfo.net for quick answers \| Where to Buy? From the best in the business: DVinfo.net sponsors

July 13th, 2004, 07:49 PM	#16
Barry Goyette Trustee Join Date: Oct 2001 Location: San Luis Obispo CA Posts: 1,195	Aaron....magnification is a bad term...we all use it, but it actually refers to things like microscopes...things with a ratio of 1:1 or greater. That said magnification is not really a relative term in the sense you are talking about, a 20x microscope would magnify its subject by a specific amount relative to the viewer. A video lens that is a 20x has no such specificity to it. Its simply a term to describe the RANGE of the zoom, or the relationship between its widest setting and its longest setting. I think you are using the word magnification to refer to this relationship, and that's where you're going wrong. If we were to apply the word magnification to a video camera lense (we'd be incorrect, but in the same sense as we would a microscope) then the longer the maximum setting, then the greater the "magnification". Am I getting somewhere? Barry

July 13th, 2004, 07:54 PM	#18
Nick Hiltgen Trustee Join Date: May 2003 Location: Atlanta GA Posts: 1,427	I follow you barry, I don't follow you don, can you explain further, is anyone else impressed by hwo quickly the xl2 forum took off... a lot of talk for a camera 99% haven't even SEEN in person!

July 14th, 2004, 08:39 AM	#21
Nick Hiltgen Trustee Join Date: May 2003 Location: Atlanta GA Posts: 1,427	ROb totally understand, I just think it's great that we can do all this now before most of us even touch the darn thing, quite a resource.

August 1st, 2004, 04:48 PM	#25
Barry Green Barry Wan Kenobi Join Date: Jul 2003 Location: North Carolina Posts: 3,863	<<<-- Originally posted by Rob Lohman : Apparent focal length or something? -->>> The Field of View is what changes when you change the imaging size. A smaller chip will have a narrower field of view.

August 1st, 2004, 06:20 PM	#26
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	This is where the debate, or perhaps confusion is a better word, starts. Barry is 100% correct in his thinking, except chips (CCD, CMOS etc.) don't have Fields of View. Lenses have FOV, for example, zoom a lens and the FOV changes, not the chip. The FOV is related to the diagonal of the chip, but FOV changes with change of change of focal length. It basically boils down to FOV being a characteristic of the lens and the chip is the device which displays the FOV. Field of View is measured from the rear nodal point to the two opposite sides of the chip, when the lens is focused at infinity and the rear nodal point is one focal length from the chip. __________________ Jeff Donald Carpe Diem Search DVinfo.net for quick answers \| Where to Buy? From the best in the business: DVinfo.net sponsors

August 1st, 2004, 09:39 PM	#27
Barry Green Barry Wan Kenobi Join Date: Jul 2003 Location: North Carolina Posts: 3,863	Yeah, it's overwhelmingly difficult to say it all and cover all bases. What I meant was: given the same lens and all other things being the same, if you substitute in a smaller chip, you will have a narrower field of view as your resulting image. The reason being, the lens projects the same image at the same focal plane, but the smaller chip can (obviously) only record a smaller subsection of that same image. DOF and other such characteristics do not change, as those are all factors determined by the lens and are all set in stone by the time the image gets projected on to the chip. So Chip size determines how much of the lens' frame is seen. Larger chips = more of the image displayed, which means for an equivalent lens, you would see a wider field of view when using a bigger chip.

August 1st, 2004, 09:49 PM	#29
Aaron Koolen Trustee Join Date: Apr 2002 Location: Auckland, New Zealand Posts: 1,727	I think we're getting nit picky here, and talking mainly semantics. Your "perceived" FOV changes with chip size but Jeff is probably being technically correct saying the chip doesn't change the "real" FOV, the focal length does. Aaron __________________ My Website Meat Free Media

August 2nd, 2004, 05:10 AM	#30
Jay Gladwell Join Date: Jul 2002 Posts: 1,244	<<<-- Originally posted by Aaron Koolen : Your "perceived" FOV changes with chip size but Jeff is probably being technically correct saying the chip doesn't change the "real" FOV, the focal length does. -->>> Aaron, with all due respect, Jeff isn't being nit picky, he's being 100% factual. Would a 4x5 negative have a greater FOV than a 35mm negative? No! Why? Because the FOV of the light sensitive medium--negative, transparency, CCD--regardless of its size, is determined by the focal length of the lens. That's not being nit picky, it's being accurate. Jay