| Author | Thread |
|
|
10/28/2003 10:09:02 PM · #76 |
Originally posted by jmsetzler:
But a 2x converter still does not change the size of the aperture :) |
Of course not, but it does change the size of the aperture relative to the focal length. Which after all is what f-number is, and why we end up at f/5.6 with the combination of converter+100mm@f/2.8
|
|
|
|
10/28/2003 10:41:21 PM · #77 |
I think part of the problem is that you still think the macro lens with the teleconverter is still really a 2.8 aperture - this is incorrect. Aperture is a function of focal length as well. The teleconverter does change the actual focal length of the lens combination. For it to remain as a 2.8 aperture lens, the actual aperture would have to get larger.
This is why for example cheaper zooms are slower at the telephoto lens, because the aperture diameter actually remains constant.
Also why you can get a lot of 50mm f1.8 lenses and not many (!) 500mm f1.8 lenses. The actual size of the hole is a direct function of the f value and focal length.
So - the upshot is, when you put the teleconverter onto the 100mm lens it _is_ a f5.6 aperture, because the aperture has remained the same. |
|
|
|
10/29/2003 12:31:43 AM · #78 |
I don't see where anything's been proved about the effect of the TC.
And, according this calculator, to get the same DOF at the same focus distance as a 100mm @2.8, a 200mm lens needs to be set more like 11.5 (at a focus distance of 120 inches) to 12.2 (at a focus distance of 48 inches), as two examples.
As far as I can tell, you guys are confusing the fact that (according to John) the TC reduces the light/exposure by 2 steps, and the fact that f/5.6 is two stops below f/2.8. So to get the same exposure that the 200mm gets @ f5.6, the 100mm w/ TC needs an aperature of f/28. I don't think there's a connection between those 2 steps and the DOF calculation. (But I didn't look real hard...) |
|
|
|
10/29/2003 01:04:13 AM · #79 |
OK, I think I'm with you at least part way on this...
A lot of terms seem to be getting misused. In particular, aperature isn't the size of the opening (as you said below, but John keeps implying otherwise), but the ratio of the focal length and the diameter of the opening in the diaphram. So when John said "But a 2x converter still does not change the size of the aperture", what he really meant (or what it sounded like to me and, probably, everyone else) was that it doesn't change the size of the opening in the diaphram.
Now, when you add the TC, let's assume for a moment that the TC has a focal length of 100mm, that is the distance from the lens to the focal plane is 100mm, right? And, if that's the case, then when you put the 100mm lens in front of it, it adds up to 200mm, so with the same sized opening, you've cut your apperature in half - gone from 2.8 to 5.6, as you stated. If I'm following you right, then I think that's what your saying.
If that's the case, what happens when you use that TC with a 200mm lens? You aren't going to double your focal length, because the focal length of the TC is still only 100mm. And yet, the TC has doubled the magnification. So by the logic that the TC must increase the focal length, then the calculation of DOF would need to use a focal length of 300mm, not 400mm, would it not? And, if that's true, then I would assume that it's not likely that the focal length of the TC would be exactly 100mm. The length of my 2.4x converter for my A40 is about 50mm. Seems like a 2x converter would be somewhat smaller than that, but maybe the optics aren't comparable to an SLR's.
So, the point is, if the converter affects the focal length, it doesn't seem to me that it neccesarily increases it by the amount of the magnification.
follow-up: Just found a Sigma 2x converter which is 45mm deep - I'm not sure how this relates to focal length, but thought I would throw that out...
Originally posted by Gordon:
I think part of the problem is that you still think the macro lens with the teleconverter is still really a 2.8 aperture - this is incorrect. Aperture is a function of focal length as well. The teleconverter does change the actual focal length of the lens combination. For it to remain as a 2.8 aperture lens, the actual aperture would have to get larger.
This is why for example cheaper zooms are slower at the telephoto lens, because the aperture diameter actually remains constant.
Also why you can get a lot of 50mm f1.8 lenses and not many (!) 500mm f1.8 lenses. The actual size of the hole is a direct function of the f value and focal length.
So - the upshot is, when you put the teleconverter onto the 100mm lens it _is_ a f5.6 aperture, because the aperture has remained the same. |
Message edited by author 2003-10-29 01:25:10. |
|
|
|
10/29/2003 01:21:02 AM · #80 |
And, that's exactly what I said in my first post - so why was it so hard to just say that??? Or, when I asked the answer to your original question - you seemed to refuse to answer then, but now - you do??? You make my head hurt! ;)
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. |
|
|
|
|
10/29/2003 03:13:23 AM · #81 |
I think I failed this class. Being a newbie to this, you guys lost me on hello.
|
|
|
|
10/29/2003 03:41:51 AM · #82 |
Originally posted by timmi:
I think I failed this class. Being a newbie to this, you guys lost me on hello. |
Me too... I know there's a lesson in there someplace but reading the whole thread twice didn't help! Guess we're not ready for this class just yet...
|
|
|
|
10/29/2003 04:33:38 AM · #83 |
Originally posted by timmi:
I think I failed this class. Being a newbie to this, you guys lost me on hello. |
Anything in particular? DOF, teleconverters, the zen art of confusing object lessons wrapped in mysteriously vague suppositions? I might have some help on the first two - the third one is dragging me under. :)
BTW, maybe it's me, but the links in your sig don't seem to be working. They keep taking me to a dead folder in my own portfolio (probably some catch-all error trap in the web site's code). |
|
|
|
10/29/2003 06:23:21 AM · #84 |
hmm, well, i understood most of that.
when I mucked with my old 35mm slr, I never new of the DoF and apature, I basically viewed it as "the speed of the lens", so I could stick f/2.4 and run a fast shutter, or f/16 and slow shutter, I never knew anything more till I read the theory on it about a month ago.
I have played with it a fair bit now..
and now I've ordered a sigma telephoto, and the shop threw in a 2x for no extra charge (not that I really wanted it).. but will be interesting to try it and learn more!!
I knew a 2x would move it 2 stops in brightness (1/4 the area), but I never understood how it varied the dof/apature.
Very informative thread...
thanks
|
|
|
|
10/29/2003 08:16:18 AM · #85 |
This is my understanding of lenses and teleconverters. I'm not saying it is correct, just that this is my understanding.
A 2X TC will "convert" a 100mm/f2.8 into a 200mm/f5.6. Because the f-number is directly related to the physical size of the lens aperture and the lens focal length, and since the actual aperture opening is unaffected by the TC, the f-number must go up to compensate for the increase in the effective focal length.
The 2X TC does not really make a 200mm lens out of a 100mm lens. The teleconverter is a collection of lens elements that magnifies the central area of the image projected by the lens attached to the TC, and that makes it appear that the total "mm" is doubled.
Because of this magnification, a 2X TC also doubles the CoC (Circle of Confusion) values for each individual "sharp detail" in the image that is produced by the attached lens. Therefore the resulting number of sharp details that are "still sharp" (where the CoC value is still within the limits of acceptable sharpness, the definition of DOF) is decreased by half, which is the same as saying that with the 2X TC attached, the resulting DOF is half of the original. (The CoC factor is the reason why a 1.4X TC has less of an effect on sharpness than a 2X TC.)
So DOF of the "virtual" 200mm/f5.6 lens is similar to that of a 100mm/f1.4 lens. To maintain the DOF of 100mm/f2.8 lens, you'd need to stop down the "virtual lens" (the 100mm/2.8 + 2X TC) to f/11. This means that you can "make" a 200mm lens with the same DOF as the original 100mm/f2.8 lens at the expense of 4 stops of light.
Another way to look at it is that the 2 stop effective decrease in aperture (increase in f-number) doubles the DOF, but at the same time you are doubling the effective focal length, which reduces DOF by a quarter. Doubling a quarter gets you half.
Message edited by author 2003-10-29 11:57:17. |
|
|
|
10/29/2003 10:53:03 AM · #86 |
You guys are making it way too hard on yourselves.
THE TELECONVERTER CHANGES THE FOCAL LENGTH.
Teleconverters don't have a set focal length...they have a magnifying effect on the lens. You're just adding more lens elements to multiply the focal length by 1.4x or 2x, in Canon's case. Consider the lens and the teleconverter as a new, single entity. This increases the focal length because it moves the elements of the original lens further from the film plane and thus has a magnifying effect.
The aperture is reduced on the new lens combination for the same reason. The diaphragm opening hasn't changed in size, but the focal length has increased. The ratio between the diaphragm opening and the focal length is what determines aperture. If the aperture is made smaller, the depth of field is increased for a shot where the field of view remains constant. This is an optical side-effect of adding the teleconverter and is aperture-dependent in this case. It doesn't matter if the diaphragm was stopped down to F/5.6 on a 200 F/2.8 or if a 2x teleconverter was added to a 100 F/2.8. The result is the same. This holds true for the light gathering abilities of a lens, and whether you add the teleconverter to the front of the lens or the back. It doesn't matter. If the focal length is changed by the teleconverter's multiplication effect, the aperture is accordingly reduced. The light entering the camera is reduced. There is a direct and non-negotiable change to the DOF when aperture is changed, and it doesn't matter HOW the aperture was changed, just that it was.
If you want to disagree, I suggest heading on over to the DPReview Canon SLR Lens Talk forum, posting that the 1.4x or 2x teleconverter doesn't affect depth of field, and then ducking.
Message edited by author 2003-10-29 10:59:02. |
|
|
|
10/29/2003 11:37:38 AM · #87 |
Originally posted by jimmythefish:
It doesn't matter if the diaphragm was stopped down to F/5.6 on a 200 F/2.8 or if a 2x teleconverter was added to a 100 F/2.8. The result is the same. |
My understanding is different, which I tried to explain in my reply. Do you have references that say that the DOF will be the same? I think there would be a 50% difference in DOF for the examples you gave.
Message edited by author 2003-10-29 11:38:53. |
|
|
|
10/29/2003 11:45:58 AM · #88 |
Originally posted by EddyG:
This is my understanding of lenses and teleconverters. I'm not saying it is correct, just that this is my understanding.
A 2X TC will "convert" a 100mm/f2.8 into a 200mm/f5.6, and it is the teleconverter responsible for the loss of light hitting the film plane. The actual aperture of the lens is not effected by the TC, but because less light is being passed to the film plane after going through the TC, the camera must meter as if the lens aperture was f/5.6 to ensure proper exposure. |
As far as I understand it - this is where you start going wrong.
The f-number that we call aperture is related to the actual physical size of the hole in the lens (which is really the aperture) by the focal length. The f-number thus changes when you add a teleconverter - as the focal length of the lens has changed.
F-number relates the aperture size as a fraction of the focal length - so f2.8 on a 50mm lens is an aperture opening of 50/2.8 or approx 18mm
On a 500mm lens, f2.8 means a 180mm physical aperture diameter - this is why fast, long lenses have end elements the size of dinner plates.
The focal length of a lens is a measure from the film sensor to the middle of the lens - again the teleconverter changes this. It is inserted in between the film plane and the lens - moving it further away from the sensor and changing the focal length of the lens. The lens element in a teleconverter actually acts to spread the image, so that it will be focused further back than if the teleconverter was just an extension tube.
Extension tubes work in a similar way, but the lens loses the ability to focus on inifity - teleconverters have the lens to maintain this ability.
100/2.8 is the same physical aperture size as 200/5.6 (both equal a 36mm aperture diameter) so with the change in focal length, but the fixed aperture size of 36mm, the lens with teleconverter actually has a max f-number of 5.6 now.
Message edited by author 2003-10-29 11:48:25. |
|
|
|
10/29/2003 11:53:25 AM · #89 |
Originally posted by Gordon:
As far as I understand it - this is where you start going wrong. |
OK, your explanation makes perfect sense in that regard. That explains why an 100/2.8 becomes a 200/5.6 because of the 2X TC. I've edited that part of my reply accordingly.
Any thoughts on the rest of my "understanding" as it relates to DOF?
Message edited by author 2003-10-29 11:57:39. |
|
|
|
10/29/2003 12:04:59 PM · #90 |
Why would you expect the 100 f/2.8 + 2x teleconverter to behave any differently than a 200 f/2.8 when both are operating at f/5.6? Aperture is one variable in the equation I posted way above. If all else remains equal, aperture is going to affect depth of field.
Originally posted by EddyG:
Originally posted by Gordon:
As far as I understand it - this is where you start going wrong. |
OK, your explanation makes perfect sense in that regard. That explains why an 100/2.8 becomes a 200/5.6 because of the 2X TC. I've edited that part of my reply accordingly.
Any thoughts on the rest of my "understanding" as it relates to DOF? |
Message edited by author 2003-10-29 12:05:38. |
|
|
|
10/29/2003 12:31:15 PM · #91 |
Well I just found this page on the Nikon 2X teleconverter and it specifically states:
Depth of field: Half that of the lens in use
So it seems that at least Nikon agrees with my understanding.
|
|
|
|
10/29/2003 01:00:20 PM · #92 |
The problem is that DOF is related to aperture AND focal length. It's a relationship defined in the equation I posted above. It also varies according to subject distance. If you attach a 2x teleconverter to the lens and keep everything else the same you will reduce the depth of field by a factor of two, yes. This means that if you stopped the aperture down to f/5.6 on your 100mm lens and stood at the same distance from the subject with the same camera body (same format size) the depth of field will be double compared to the 100mm lens with the teleconverter attached, at f/5.6 (as wide as possible). However, the field of view will be different! Your photograph will not be the same. If you had the entire person in the frame with the 100mm, you'll now likely only have the torso in the frame with the teleconverter attached. Furthermore, the maximum aperture has been reduced from 2.8 on that lens to 5.6.
This means that if you were to compose the same image with the same field of view with the 100mm with the 2x attached vs. the 100mm alone, you'd have to be standing much further back from the subject with the teleconverter on, which means that your depth of field would increase again (as depth of focus is dependent on distance to subject).
The bottom line is that you can't get something for nothing. If you simply remember that the 1.4x adds a stop and the 2x adds two stops, you're gold.
Originally posted by EddyG:
Well I just found this page on the Nikon 2X teleconverter and it specifically states:
Depth of field: Half that of the lens in use
So it seems that at least Nikon agrees with my understanding. |
Message edited by author 2003-10-29 15:20:34. |
|
|
|
10/29/2003 01:08:32 PM · #93 |
But I'm talking about this example:
Originally posted by jimmythefish:
It doesn't matter if the diaphragm was stopped down to F/5.6 on a 200 F/2.8 or if a 2x teleconverter was added to a 100 F/2.8. The result is the same. |
Where you would stand in the exact same spot with the same aperture (5.6) on both lenses, same distance to subject, with the same field of view. The DOF on the 100mm with the 2X TC would be half of the 200/5.6 lens without a TC. My point being that the result is NOT the same, even though the focal length and apertures are effectively identical. |
|
|
|
10/29/2003 01:34:39 PM · #94 |
Hmm? Yes, a 200mm lens at f/5.6 is an identical performer to a 100mm plus 2x teleconverter wide open, which is f/5.6. The DOF is IDENTICAL with these two combinations. The 100 F/2.8 with the 2x teleconverter is no longer an f/2.8 lens. It's a 200 mm f/5.6. That's all you need to know.
Originally posted by EddyG:
But I'm talking about this example:
Originally posted by jimmythefish:
It doesn't matter if the diaphragm was stopped down to F/5.6 on a 200 F/2.8 or if a 2x teleconverter was added to a 100 F/2.8. The result is the same. |
Where you would stand in the exact same spot with the same aperture (5.6) on both lenses, same distance to subject, with the same field of view. The DOF on the 100mm with the 2X TC would be half of the 200/5.6 lens without a TC. My point being that the result is NOT the same, even though the focal length and apertures are effectively identical. |
|
|
|
|
10/29/2003 01:46:12 PM · #95 |
Originally posted by EddyG:
But I'm talking about this example:
Originally posted by jimmythefish:
It doesn't matter if the diaphragm was stopped down to F/5.6 on a 200 F/2.8 or if a 2x teleconverter was added to a 100 F/2.8. The result is the same. |
Where you would stand in the exact same spot with the same aperture (5.6) on both lenses, same distance to subject, with the same field of view. The DOF on the 100mm with the 2X TC would be half of the 200/5.6 lens without a TC. My point being that the result is NOT the same, even though the focal length and apertures are effectively identical. |
Yes, in your case, taking an entirely different picture, the depth of field would be half.
But - if you move back so that the same subject is rendered the same size in the image, the depth of field will be the same, but the angle of view will be different.
Like I said way back at the dawn of time when this question was posed, there isn't enough information in the question to provide a meaningful answer without a lot of qualification. |
|
|
|
10/29/2003 02:00:15 PM · #96 |
Hurray!!
Measurbation is alive and well on dpchallenge.com !!
There is a god ...
:-))
|
|
|
|
10/29/2003 02:18:10 PM · #97 |
From //www.faqs.org/faqs/rec-photo/lenses/faq/
Q34. How does a teleconverter affect exposure, focusing, depth of
field and image quality?
A. A lens of focal length f and f-number N with a teleconverter of
magnification K attached will behave in all respects like a lens of
focal length K*f and f_number K*N.
If the aperture diameter and focus are left untouched and an ideal
teleconverter is attached, the lens will focus at the same distance,
the image, including the diffraction effects and lens aberration
effects, will be K times as large, the exposure will need to be K^2
times longer, the hyperfocal distance will be multiplied by K and the
depth of field will be divided by K. A practical teleconverter will
also contribute some of its own aberrations. (See the technical
notes.)
On the other hand, if you open the aperture to keep the same effective
f-number and hence the same exposure time, the image will be enlarged
by K, the diffraction will be unchanged, the depth of field will be
divided by K^2 and the hyperfocal distance multiplied by K^2. The
aberrations are increased by three effects: the lens is opened to a
larger aperture, the teleconverter multiplies those (probably larger)
aberrations by K, and then combines them with some of its own.
Since the focusing is unchanged, the minimum focusing distance is the
same whether or not a teleconverter is attached. (See the technical
notes.)
(This is written and maintained by David Jacobson at HP - who also maintains //www.photo.net/learn/optics/lensTutorial so I would assume it has been fairly carefully reviewed/ debated to death) |
|
|
|
10/29/2003 02:25:47 PM · #98 |
Feel free to discuss all you want, but by the same token, here's my comment:
This is a pretty much totally useless thing to know. Instead of trying to prove who has the biggest physics phallus, just go out and take some great pictures.
:-))
Message edited by author 2003-10-29 14:25:58.
|
|
|
|
10/29/2003 02:32:32 PM · #99 |
Hallelujah Gordon, for providing what should be regarded as an authoritative source. Hopefully this will setle the issue. In reality, fat chance.
Mag, LOL.
|
|
|
|
10/29/2003 05:36:15 PM · #100 |
Originally posted by Gordon:
You could just stop looking for technical advice on the internet. |
Now my dilema is, which of these pieces of advice do I accept???
Just kidding. :)
Thanks for finding that. I did a little searching last night, but didn't find anything useful. And for all well-intentioned theorizing, I really like to see some coroborating source like this.
Mag, I hear you. This isn't the kind of thing where I can personally see using in a situation where I go out, prepare to take a shot, and start calculating which is the better setup to use. It does have a couple of positive applications: Like James mentioned, it might help in deciding what gear to purchase - knowing the limitations of going with a TC vs. a whole other lens. Also, for me it's nice to at least have a basic grasp of these concepts so that, when I really screw up some shots and don't get the results I expected, I have some clue later as to why - or at least know that it's been discussed, and I can come back later to try to get a better understanding.
|
|
