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10/28/2003 07:40:14 PM · #51 |
| This is a good, and very important thread. It should be kept around |
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10/28/2003 07:42:03 PM · #52 |
Originally posted by ScottK:
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. |
So these teleconverter attachments are 0 mm thick? They must save a lot on materials making them that way! I'd think adding a magnifying lens to the light path would be very likely to change the focal plane. |
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10/28/2003 07:42:43 PM · #53 |
DOF is increased with the smaller aperture, not decreased...
Originally posted by hortopth:
If you're not sure whether or not to believe Jimmy, just throw on a teleconverter and you'll see that DOF is reduced. |
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10/28/2003 07:57:50 PM · #54 |
Originally posted by jmsetzler:
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.
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I think this is where you start going wrong - yes the 100mm macro lens is still a 100mm macro lens, but when you combine it with a tele-extender, you change the focal length of the resulting combination. A teleconverter/extender has a physical size which moves the original lens further from the sensor/ film - changing the focal length. It also has optical elements which further change the convergence of the lens, correcting for this shift 'outwards'
This is why for example, a 2x teleconverter is physically larger than 1.4x teleconverter. It also explains the light fall-off when adding these devices. |
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10/28/2003 08:11:07 PM · #55 |
Alright, I'll cry uncle, admit ignorance, and stand back to see if anything useful comes out of this.
I always asumed "focal length" refered to a distance from some point within the lens to some point behind the lens - like the focal plane.
Now I find these different definitions of "focal length":
from photo.net
...the distance from the lens to a point where parallel rays are focused to a point (diverge)...
From what point on the lens? To what point where parallel rays are focused? What does "diverge" mean here?
from dpreview
Q2. What is the meaning of focal length? In other words, what about
a 50mm lens is 50mm?
A. A 50mm lens produces an image of a distant object on the film that
is the same size as would be produced by a pinhole 50mm from the film.
See also Q5 below.
Huh? So focal length has nothing to do with a physical distance on the lens?
The most frustrating thing about technical discussions of photography is there seems to be no authoritative guidelines for the science behind it. I've seen at least 4 different calculations for DOF, from seemingly knowledgable sources. Argh!
Message edited by author 2003-10-28 20:20:01. |
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10/28/2003 08:18:34 PM · #56 |
Originally posted by GeneralE:
Originally posted by ScottK:
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. |
So these teleconverter attachments are 0 mm thick? They must save a lot on materials making them that way! I'd think adding a magnifying lens to the light path would be very likely to change the focal plane. |
See my post above, but specifically, in my first post I speculated that the aspect of focal length that affects DOF is in the relationship of the lens (i.e. the glass) to the diaphram, and that adding the TC behind that doesn't affect that relationship. I was taking a big leap in guessing that whatever the TC does to magnify the image after the light passes through the diaphram may not have any affect on the DOF. If it does change the focal length in such a way as to affect the DOF, then it doesn't seem right that you can use the TC's multiplier to determine the true focal length, because the physical distance that the lens moves from the focal plan is not related to the multiplier.
But what the hell do I know... |
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10/28/2003 08:22:31 PM · #57 |
sorry, you know what i mean
Originally posted by jimmythefish:
DOF is increased with the smaller aperture, not decreased...
Originally posted by hortopth:
If you're not sure whether or not to believe Jimmy, just throw on a teleconverter and you'll see that DOF is reduced. |
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10/28/2003 08:41:04 PM · #58 |
The correct answer is that the depth of field on the 100mm lens with the 2x converter will be greater than the 200mm lens with both lenses set at f/2.8.
The correct answers have all been hit on here in various places.
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10/28/2003 08:45:23 PM · #59 |
You could just stop looking for technical advice on the internet. There is plenty of hard scientific background texts on the theory of optics and how it applies to photography. In fact, for photography its all pretty basic optics anyway.
For example, try:
Basic Photographic Materials and Processes, Second Edition
Leslie D. Stroebel (Editor), Richard D. Zakia, Ira Current, John Compton
It is a pretty straightfoward book, though quite technically detailed. I found it relatively readable, but I have a telecommunications background (which covers a lot of optical theory for fibre lines and so on) |
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10/28/2003 08:45:56 PM · #60 |
Originally posted by jmsetzler:
The correct answer is that the depth of field on the 100mm lens with the 2x converter will be greater than the 200mm lens with both lenses set at f/2.8.
The correct answers have all been hit on here in various places. |
How can this be the correct answer when it is physically impossible ? |
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10/28/2003 08:48:48 PM · #61 |
Originally posted by Gordon:
Originally posted by jmsetzler:
The correct answer is that the depth of field on the 100mm lens with the 2x converter will be greater than the 200mm lens with both lenses set at f/2.8.
The correct answers have all been hit on here in various places. |
How can this be the correct answer when it is physically impossible ? |
I was playing devil's advocate to get an explanation of why :)
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10/28/2003 09:09:23 PM · #62 |
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10/28/2003 09:18:58 PM · #63 |
the physical properties of the lens do not change with a teleconverter, no matter how you look at it.
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10/28/2003 09:20:29 PM · #64 |
| So was I somewhere in the ballpark? |
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10/28/2003 09:23:58 PM · #65 |
Originally posted by ScottK:
So was I somewhere in the ballpark? |
The difference in the depth of field is because of the physical size of the aperture mainly. The 200 prime has an aperture twice the size of the 100 prime at the same setting.
The two photos in question could be made the same by shooting the 100mm lens with the 2x converter at f/2.8 and the 200mm lens at f/5.6.
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10/28/2003 09:28:17 PM · #66 |
| The original question was "will the depth of field be affected by the 2x converter?" And the answer is...? |
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10/28/2003 09:43:15 PM · #67 |
Originally posted by ScottK:
The original question was "will the depth of field be affected by the 2x converter?" And the answer is...? |
I tossed that question out and reworded to:
Let me rephrase the question:
We have a 100mm f/2.8 lens with a 2x converter. We also have a 200mm f/2.8 lens. We make the same photo with both lenses. The camera settings are f/2.8 and 1/125". The subject is the same distance from the camera for both photos.
Will the depth of field be different?
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10/28/2003 09:45:29 PM · #68 |
Actually, I think my last post may be incorrect.
I said the image would have the same DOF if:
the 100mm lens w/2x converter @ f/2.8
the 200mm lens @ f/5.6
Is this wrong? I think it may be.
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10/28/2003 09:55:17 PM · #69 |
Originally posted by jmsetzler:
Actually, I think my last post may be incorrect.
I said the image would have the same DOF if:
the 100mm lens w/2x converter @ f/2.8
the 200mm lens @ f/5.6
Is this wrong? I think it may be. |
Nope, that looks right John. Although for best clarity I would say
"the 100mm lens @ F2.8 + 2x converter"
Message edited by author 2003-10-28 21:56:44.
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10/28/2003 09:57:19 PM · #70 |
I don't think the converter itself plays a role in the depth of field difference.
I think the aperture size does.
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10/28/2003 09:58:15 PM · #71 |
I think the same depth of field would be produced with the following:
100mm w/2x converter @ f/2.8
200mm lens @ f/4
But I'm wrong... it's 200mm lens @ f/5.6 :)
Message edited by author 2003-10-28 22:01:32.
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10/28/2003 10:00:17 PM · #72 |
Originally posted by jmsetzler:
I think the same depth of field would be produced with the following:
100mm w/2x converter @ f/2.8
200mm lens @ f/4 |
Nope, f/5.6 is correct, the aperture on the 100mm is half the size @f/2.8, so 1/4 the area, so two stops slower.
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10/28/2003 10:02:41 PM · #73 |
So that sorta sums up the fact that the converter does not play a role in the depth of field difference.
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10/28/2003 10:04:25 PM · #74 |
Originally posted by jmsetzler:
So that sorta sums up the fact that the converter does not play a role in the depth of field difference. |
Well of course it does play a role, without it the 100mm lens would be just that.
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10/28/2003 10:06:40 PM · #75 |
It plays a role in modifying the focal length, but does it make any difference in the depth of field when comparing the two?
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