Help me understand the effects of using a TC

I did a test a couple days ago to see just how much my TC20E hurts

performance on my 80-200 AF-S Zoom...And I'd like a bit of help

understanding the results!

 

First off the test: Zoom on tripod, Focused on a tree trunk about

15 feet away under bright sun, 200mm, f5.6, took first photo. Then

I mounted the 2X TC, zoomed back to '100' on the zoom, left aperture

at f5.6 (which is now the minimum aperture due to the 2X) refocused

and took second picture.

 

The results: Of course the first photo (no 2x) is sharper and has

more contrast, but the difference was much less than I thought it

would have been. what surprised me is that the two photos have

different DOF. The photo with the TC Had much less DOF than the one

without. Why is that? Obviously I don't understand what a

teleconverter does optically to do it's work.

 

Is this hypothesis correct: I understand that the camera reported

f5.6 because it is accounting for the 2 stop change, and the lens is

actually at it's max aperture (f2.8). So is the TC actually

magnifying 100 f2.8 and to do that 'costs' 2 stops of light?

 

I'd really like to understand what's going on here! I plan to go

out tomorrow and shoot again, offsetting the f stop by 2 to re-

compare picture quality. Perhaps I should do it at several

apertures.

 

For what it's worth, equipment used was D70, 80-200 afs, TC20E all

on sturdy tripod. I shot RAW mode under bright sunlight. In

photoshop I compared center and corner crops onscreen and then

printed with modest unsharp mask applied.

longer focal length equals less dept of field, by using a 2x converter you are

increasing the focal length of your lens. TC converter you are using does cost

2 stops, I think you have the jist of it, Nikons teleconverter while not cheap

maintain the sharpness and contrast very well of the lens.

Thanks Gary. I don't know if you noticed from my wordy post, but for my comparison I was at an effective 200mm for both shots (I zoomed back to 100mm when I added the 2x TC), which is what has me confused, I was using the same focal length, same f stop as reported by the camera, and got different DOF.

Your method is a good choice. A subject rendered the same size on the film plane with a lens of any focal length and the same f/stop will have the same depth of field.

Thank you Edward,

That is exactly what I believe. Yet there is clearly a different depth of field between these two photos, the one with the TC has notably less d.o.f. That's what has me confused!

 

New Hypothesis:

I just took another critical look at the two photos and maybe what I've assumed was reduced d.o.f. is actually different quality of bokeh?...I mean waaay different. I think the bokeh without the 2X actually looks quite a bit 'sharper' than the bokeh with it, enough so that it appeared like less DOF. Honestly, the 2X seems to really tame out of focus areas! I'll have to play a bit with this idea.

 

My subject, a tree trunk, added to the confusion. The trunk is in focus and the background grass is out of focus, there isn't really any area of transition from focus to non-focus which would have made it more obvious.

 

Which leads to another question: If it's truly a change in the quality of out-of-focus areas that makes them look less sharp, maybe this 2X tc doesn't deserve the lukewarm reviews it gets? That it just requires a very critical focus...

 

Here is an actual pixel crop to help with this...<div></div>

Maybe photographing something where it's easy to tell the DOF will clear the confusion about bokeh/DOF. A row of coins, beer bottles, a railing? I am interested in the explanation for what you are seeing.

 

-a

Greg, IMO, you are using a lens which is not renowned for its out of focus rendition. If you compare the images from the 80-200 with those of a lens like the 105mm f/2.5 or the 105mm f/2 DC you will see the creamy smooth OOF of the 105mm lenses. The 80-200 zoom gives what one would call a "cross eyed"OOF areas.

 

With the added TC this unpleasantness gets somewhat reduced as the distraction becomes less.

 

No 2X TC actually "increases"the sharpness of any lens.

Hi Greg,

 

What your TC is doing is optically magnifying a portion out of the centre of your image.

 

There is no free lunch - two reasons:

 

1 the image needs four times as much light (2 stops) as you are only looking at 1/4 of the original shot when you use the TC, so you need a slower speed, and

 

2 every glass element the light travels through will degrade the quality of the image. From what you posted the contrast of the shot with the lens at 100mm and the TC is lower than the contrast of the shot with the lens at 200mm, which is what I would expect.

 

For what it is worth, the TC20E is probably the best quality 2x TC you can buy anywhere. You are up against the laws of physics.

 

With DOF, the further something is away from the plane of focus, close and far, the less sharp (fuzzier) it will appear. Depending on the optical formula, there may be other effects like doubling of lines, etc with out of focus areas. The best bokeh (Japanese for the out of focus effect of a lens) is a smooth gradation away from full sharpness. The DOF guides shown on a lens (or on the old manual lenses anyway) assume a particlar circle of confusion (which doesn't mean a ring of Canon users ;-). Bob Atkins states on his site the CoC for 35mm work is usually taken to be 30 microns, which is 0.03mm. That is, anything 0.03mm across or less is taken to be a sharp point. The TC is doubling all of the fuzziness in the image, so your 0.03mm becomes 0.06mm, so the depth of field appears to be less. And the optical qualities at 200mm are different from those at 100mm and the light is passing through fewer glass elements, so the image appears sharper at 200mm than at 100mm x2. But it is better than missing the shot. So only use the TC to get longer than 200mm.

 

You are using some excellent gear, so the results you are getting with the TC and lens combo would probably pass most inspections.

 

Regards, Ross

Thanks, Ross, the CoC being magnified makes sense to me. So, that means that the zone of apparent sharpness will be narrower using the TC than without it, correct focus will be much more critical.

 

I think I'll take a shot at 100mm without the TC, f5.6 and compare the center crop of the image to the shot with the TC...that should be enlightening. I should see similar out of focus effects, right?

 

It seems pretty clear that while the contrast and sharpness suffers compared to the lens alone, it IS possible to get critically acceptible results with this combo.

Greg,

 

<p>

You might get some clarification by reading <a href="http://www.vanwalree.com/optics.html">Paul van Walree's depth of field article</a>.

 

<p>

--Bill

Bill, thank you. That is a fantastic link. Regards, Ross

Bill, that is a fantastic link...Definitely good background material, well written. Thanks.

Er, what Ross said!

If I understood correctly you had the same focal length in the two shots. So in the case of the TC the lens will be 5.6 instead of 2.8 and this will give more depth of field to the TC shot. That's about it.

I think your hypothesis is right. The diameter of your lense's aperture is set to 2.8, yet the camera reports that the amount of light that reaches the filmplane (should i say ccd-plane?) is reduced with 2 stops. Nevertheless the depth of field is determined by the diameter of the aperture, not by any numeric value assigned to it by your camera. You should get the same DOF using your lens without TC set to 200mm and F=2.8. Sounds logical to me at least:)

Unfortunately I've been down with a terrible cold for the past 2 days. Thanks for all your responses! I am going to do a bit more testing and post to a new thread my results...

In your example the two cases are zoom set long, or zoom set wide + TC, both at the same subject distance and same ultimate magnification. Press the DOF and look in the front of the lens. If the apparent diameter of the entrance pupil is the same in the two cases, the depth of field will be the same.