Teleconverters in macro

[Harald]

Hello gang,

I am wondering....

If I use an 1.4x teleconverter, it increases the focal length by 1.4

Meaning, when I use an 10x Mittu on my Canon 70-200 and one 1.4x converter, my magnification ends up at 14x?

I think 1.4x tele is a good choice since it only take 1 f/stop

Not sure if my calculations are correct

Bet there is at least 1 or 50 in this forum that can tell me the correct math

[Scarodactyl]

Yes, it will give you 14x. You will still be well within the reasonable magnification for the objective's numerical aperture.

[rjlittlefield]

That will also be effective f/25 as seen by the sensor. (Feff = m/(2*NA), m=14, NA = 0.28)

With most modern sensors that will be well into diffraction territory, so expect some softness when pixel-peeping.

--Rik

[Scarodactyl]

For sure, but it's never too early to adjust your expectations for sharpness when entering the micro arena

[AlP]

I quite frequently use a 1.4 extender with the MPE 65mm to give me 7x. I am very interested to see your results with the Mitutoyo 10x microscope objective, 1.4 extender and 70~200.

[Harald]

Hello,
Thanks for your great replies and time
It is so great to be a member in this great forum
I will post some results with my setup with 70-200 and 1.4x
Looking forward to test it out

All the best from Norway!

[bralex]

That will also be effective f/25 as seen by the sensor. (Feff = m/(2*NA), m=14, NA = 0.28)

With most modern sensors that will be well into diffraction territory, so expect some softness when pixel-peeping.

--Rik

Rik, is there a book you can point me too that discusses this type of calculation and the theory behind it? I'd like to understand better. If I plug the numbers for the Mitty 20x (NA = 0.42) I get f=23.8; does that suggest that I will not get sharp results using one? I've been thinking hard about whether to spend the $$.

[Harald]

Hmmmm,

I have a Mittu 20x
Do I dare to test it with 1.4x converter?

Well, what could go wrong and what do I loose?

[JKT]

You didn't mention which teleconverter. Don't go cheap there! EF 1.4x III or RF 1.4 will do and I can't tell about the best Sigma or Tamron ones. The others...

[JKT]

That will also be effective f/25 as seen by the sensor. (Feff = m/(2*NA), m=14, NA = 0.28)

...or is it actually f/24? Which is the accurate formula - m/(2*NA) or m*fwork?
Combining the latter with fwork = 1/2/tan(asin(NA)) gives the other value ... but have I gotten that formula right?
Yes I know - practical effect is minimal.

Anyway, with that formula the Mit 20 is f/21.6 without TC and f/30.3 with it. What that means in sharpness should be easy to test with any lens that stops that far down.

[rjlittlefield]

Rik, is there a book you can point me too that discusses this type of calculation and the theory behind it? I'd like to understand better.

I suggest to start with the "FAQ: How can I calculate effective aperture?".

Then if you have more questions, ask. I have not found any book that concisely explains what matters for our work. The material is in the books, of course, but it is scattered over many pages and expressed in ways that are often difficult to interpret and apply.

If I plug the numbers for the Mitty 20x (NA = 0.42) I get f=23.8; does that suggest that I will not get sharp results using one?

I agree with your calculation of effective f/23.8 at rated magnification, and yes, that will be strongly in diffraction territory on most modern sensors.

Or, you can put the same objective on a 100 mm tube lens so that it operates at 10X. In that case the effective aperture will be 10/(2*0.42) = f/11.9, and if you're using APS-C, the whole sensor will be covered by high quality image.

In both configurations the object-side NA is 0.42, so both configurations give the same potential resolution on subject.

The 10X configuration will be sharper on sensor because it packs the same information into half the distance. However, the 10X image will not be sampled as well because of sensor issues like pixel size, Bayer filter, and anti-aliasing. So in practice, on subject the 10X configuration may not show as much detail as the 20X configuration.

This tradeoff between "sharp" and "detailed" is a constant issue in digital photography. There is a long discussion in the old thread at viewtopic.php?t=2439 .

--Rik

[Harald]

You didn't mention which teleconverter. Don't go cheap there! EF 1.4x III or RF 1.4 will do and I can't tell about the best Sigma or Tamron ones. The others...

I have the Canon EF 1.4x II

[JKT]

You didn't mention which teleconverter. Don't go cheap there! EF 1.4x III or RF 1.4 will do and I can't tell about the best Sigma or Tamron ones. The others...

I have the Canon EF 1.4x II

Unfortunately that is not on the same level as III. It introduces considerable barrel distortion, some CA and softness near corners.

[Harald]

Hi,
Here I've used 1.4x II

http://www.photomacrography.net/forum/v ... 27&t=45700

Think that 1.4x III is on it's way

[JKT]

On that one you likely wouldn't see the difference as there is nothing near the corners. The rainbow colors you see are not due to the TC.