Food for Thought: Depth of Field

10/28/2003 06:12:21 PM · #26

Originally posted by jmsetzler:

But a 2x converter still does not change the size of the aperture :)

Exactly. With a 100mm F2.8 lens, I start with a 35.7mm aperture. With a 200mm F2.8 lens, I start with a 71.4mm aperture.
When we add the converter to the 100mm lens, we have a 35.7mm aperture in a 200mm lens, for F5.6 efectively (200/35.7).

10/28/2003 06:16:01 PM · #27

Originally posted by kirbic:

Originally posted by jmsetzler:
But a 2x converter still does not change the size of the aperture :)

Exactly. With a 100mm F2.8 lens, I start with a 35.7mm aperture. With a 200mm F2.8 lens, I start with a 71.4mm aperture.
When we add the converter to the 100mm lens, we have a 35.7mm aperture in a 200mm lens, for F5.6 efectively (200/35.7).

The focal length of the 100mm lens did not change. The lens elements have not gotten further apart by adding the 2x converter.

10/28/2003 06:21:31 PM · #28

The total focal length of the [100mm lens + converter] system is 200mm.
Here are some questions:

1.) Is F number of the [100mm lens + converter] system F 5.6?
2.) Is the F number of the 200mm lens F 2.8?

If answers to 1 & 2 are "yes", then the aperture diameter must be different, which we can also conclude from the definition of F number.
If the focal lengths are the same, but the apertures are different, the DoF will be different, in this case larger for the 200mm F 2.8 lens.

10/28/2003 06:26:57 PM · #29

Originally posted by kirbic:

Your first statement is incorrect. A 100mm focal length lens is 100mm regardless of a converter being in place. It's physically 100mm and that does not change.

I stated in the reposted question that the aperture settings were f/2.8 on both lenses. Adding a 2x converter does not change the aperture from f/2.8 to f/5.6. What it DOES do is reduce the amount of incoming light by two stops, which would be the equivalent of f/5.6 where SHUTTER SPEED is concerned.

Canon EF-S 17-55mm f/2.8 IS USM

10/28/2003 06:28:20 PM · #30

Originally posted by jmsetzler:

But a 2x converter still does not change the size of the aperture :)

cvt_

Canon EOS-20D

10/28/2003 06:29:26 PM · #31

being 2x conversion the outside edge of the image comming in from the lens has been effectivly cropped and the rest magnified 2x.
so theres 1/4 the light getting through (2 stops).
the dof at the same focal length of the lens (not effective length) will show the same apature and DOF, just larger image.

(prolly wrong.. but my interpretation?)

10/28/2003 06:30:17 PM · #32

Originally posted by jimmythefish:

Not true. Aperture isn't the size of the hole..it's the ratio of the size of the hole and the focal length. With the teleconverter you're changing the focal length of the lens, and hence changing the aperture.

Originally posted by jmsetzler:
But a 2x converter still does not change the size of the aperture :)

However you look at it, the aperture represents an area (size) of the opening.

10/28/2003 06:32:50 PM · #33

Keep in mind that we are using these two lens configurations to produce two identical compositions.

10/28/2003 06:33:36 PM · #34

Well I'm looking at it from a physics standpoint. Take a 1.4x teleconverter, a 200mm 2.8L lens and attach it to your 10D. What you'll get is a 280mm F4. The hole made by the leaf aperture is the same size, yes, but the ratio of that opeing to focal length has changed.

What's the point here? This isn't anything new...

Originally posted by jmsetzler:

However you look at it, the aperture represents an area (size) of the opening.

Message edited by author 2003-10-28 18:35:48.

Canon EF-S 17-55mm f/2.8 IS USM

10/28/2003 06:34:53 PM · #35

Originally posted by jmsetzler:

Originally posted by kirbic:
The total focal length of the [100mm lens + converter] system is 200mm.
Here are some questions:

1.) Is F number of the [100mm lens + converter] system F 5.6?
2.) Is the F number of the 200mm lens F 2.8?

If answers to 1 & 2 are "yes", then the aperture diameter must be different, which we can also conclude from the definition of F number.
If the focal lengths are the same, but the apertures are different, the DoF will be different, in this case larger for the 200mm F 2.8 lens.

Your first statement is incorrect. A 100mm focal length lens is 100mm regardless of a converter being in place. It's physically 100mm and that does not change.

I stated in the reposted question that the aperture settings were f/2.8 on both lenses. Adding a 2x converter does not change the aperture from f/2.8 to f/5.6. What it DOES do is reduce the amount of incoming light by two stops, which would be the equivalent of f/5.6 where SHUTTER SPEED is concerned.

I believe we must use the focal length of the entire system, e.g 200mm for the 100mm + 2.0x converter. The Dof WILL change, this I can assure you. I am not an optical engineer, but I bleieve if we consulted one, the answer would be "use the combined focal length".

cvt_

Canon EOS-20D

10/28/2003 06:38:56 PM · #36

then your moving away from the object, will make the DoF appear twice of what it is.
so to create the identical image you'd need to use f1.4

10/28/2003 06:43:00 PM · #37

Originally posted by cvt_:

then your moving away from the object, will make the DoF appear twice of what it is.
so to create the identical image you'd need to use f1.4

No.. You are the same distance from the subject with both lens configurations.

10/28/2003 06:47:16 PM · #38

If you're going to use these two lenses to make the same composition, you can simply stop the 200mm 2.8 down to 5.6 and use the same subject distance. Or, if you're a freak and you wanna match the 100 plus telecon exactly when using the max. apertures of both lenses, back up to the distance determined by the equation I posted above with the 2.8 (given 2.8 for one aperture and 5.6 for the other calculate the distance to subject variable) set the aperture to 2.8 and then crop the image to match the field of view produced by the 100 and the teleconverter.

Message edited by author 2003-10-28 18:48:14.

10/28/2003 06:48:04 PM · #39

I'm going to take a stab at the answer: I think the DOF of the 100mm lens won't be affected by teleconverter. You specified that the teleconverter was between the lens and the camera. As far as my limited understanding of DOF goes, it's affected entirely by the relationship of elements that occur (in physical location) between the diaphram and the end of the lens. The teleconverter simply magnifies the resulting image (this I'm primarily guessing at). So, if my understanding is remotely correct, the DOF is determined before the light ever reaches the TC.

Is that even close?

10/28/2003 06:56:31 PM · #40

Originally posted by ScottK:

It might seem that way, but it is not true. Remember that the image is in focus only at the sensor plane. No fucussed image is formed by the 100mm lens alone in this configuration, the paths of the light rays are affected by the converter, and this WILL have the same effect as using a lens of 200mm focal length.
As a thought exercise, consider how many different configurations of elements can be used to come up with a 200mm lens. The possibilities are endless, including some configurations with no lenses at all (just mirrors). All of these configurations obey the DoF equations, as laid out in a previous post.
I have the 2.0X converter, anyone willing to lend me a 200/2.8 L lens for the test? :^)
O, I think I'll need the IS version so we can critically judge the DoF.

Message edited by author 2003-10-28 18:57:30.

glimpses

Sony DSC-F707

10/28/2003 06:58:25 PM · #41

Are we doing this to look professional? =)

10/28/2003 07:00:56 PM · #42

I'm just hoping to avoid looking stupid. :)

10/28/2003 07:01:10 PM · #43

Originally posted by glimpses:

Are we doing this to look professional? =)

I'm doing it at the moment because I'm an anal engineer, or because I love a good techie debate, take your pick :^))

10/28/2003 07:04:32 PM · #44

I'm doing it to prove that I really do need that 300 F4L instead of simply buying the 1.4x to go with my 70-200 F4L, which despite what some may think will turn it into a 98-280mm F5.6L. When you start buying L lenses with your credit card you tend to learn why you're spending that money.

If it's a lesson, it's a lesson on how to confuse a lot of people with misinformation...

James.

Originally posted by kirbic:

Originally posted by glimpses:
Are we doing this to look professional? =)

I'm doing it at the moment because I'm an anal engineer, or because I love a good techie debate, take your pick :^))

Message edited by author 2003-10-28 19:07:18.

10/28/2003 07:11:30 PM · #45

I just can't believe there's no one out there jumping at the chance to loan me a 70-200 2.8 L IS...

10/28/2003 07:12:46 PM · #46

iwouldificouldbuticantsoiwont

Originally posted by kirbic:

I just can't believe there's no one out there jumping at the chance to loan me a 70-200 2.8 L IS...

10/28/2003 07:26:51 PM · #47

I'll take a crack at another aspect of this: I don't think (think being the operative term) that adding the teleconverter changes the focal length. Lets say you have a 100mm lens. You add a 2x TC, so you get the equivilent of 200mm. Now you had that same 2x TC to a 200mm lens and you get an equivilent of 400mm. The "effect" of the TC is to "add" 100mm to the first lens, and 200mm to the second lens. But the length of the TC hasn't changed. So I don't think you can say that the focal length had doubled (or, conversely, as was stated earlier, that the aperature has halved) by adding the TC.

But I'm just a humble PnS operator, so what do I know? :)

10/28/2003 07:29:12 PM · #48

Trust me, it does.

Originally posted by ScottK:

hortopth

Canon EOS-D60

10/28/2003 07:35:23 PM · #49

Originally posted by jmsetzler:

But a 2x converter still does not change the size of the aperture :)

No, but I believe (now correct me) that moving the original lens that much further away from the camera will marginally change the depth of field, regardless of whether any magnification has occurred.

If you're not sure whether or not to believe Jimmy, just throw on a teleconverter and you'll see that DOF is reduced.

Message edited by author 2003-10-28 19:46:00.