Using teleconverters

[dreed]

When using a teleconverter, there is a focal length conversion factor (1.4x, 2x, etc) and an accompanying f-stop drop f/4 -> f/5,6, etc.

Does the f-stop drop relate only to drop in light intensity (making it more of a T-stop adjustment) or does it also represent depth of field?

[Colorado David]

DoF is determined by the lens, not the converter.  So if you put a 2 stop converter behind an f/2.8 lens, you'll have the DoF of the f/2.8.

[Petrus]

DoF is determined by the lens, not the converter.  So if you put a 2 stop converter behind an f/2.8 lens, you'll have the DoF of the f/2.8.

No.

The LENS becomes a f/4 or f/5.6, so DOF will also be f/4 or F/5.6 (or whatever). How would the sensor know if you are shooting with a 200mm f/2.8 + 2x converter, or a 400mm f/5.6??

[Ray]

You can't sensibly compare depths of field if the fields are different.

There are some comments on the internet that claim, for example, that a 100mm lens at F2.8 will have the same DoF as the 100mm lens used with a 2x converter at F5.6, or a 200mm lens without converter used at F5.6.
What is meant by such statements is, if you crop the 100mm F2.8 image to the same Field of View as a shot taken with a 200mm at F5.6, then you will get the same DoF.

This is the same principle that applies when comparing the equivalent focal length and F/stop of images taken with full-frame and 4/3rds format.

Disregarding minor discrepancies due to different aspect ratios, a 4/3rds-format camera used with a 100mm lens at F2.8 produces the same field (FoV), and the same 'depth of field', as a 200mm lens on a full-frame camera used at F5.6, shooting from the same location and distance to subject, of course.

However, there will likely be minor discrepancies due to resolution differences. A poor teleconverter will possibly result in a slightly greater DoF, and an excellent teleconverter in a slightly shallower DoF. But that's pixel-peeping. 

[Bart_van_der_Wolf]

When using a teleconverter, there is a focal length conversion factor (1.4x, 2x, etc) and an accompanying f-stop drop f/4 -> f/5,6, etc.

Does the f-stop drop relate only to drop in light intensity (making it more of a T-stop adjustment) or does it also represent depth of field?

Hi,

The DOF will be roughly (!) the same because you change both focal length and effective f/stop. It will not look exactly equivalent because of optical design (pupil factor).

Cheers,
Bart

[Petrus]

OK, let's start afresh: 400mm f/5.6 and 200mm f/2.8 with 2X teleconverter will produce exactly the same optical result full open. That is really all there is to it.

[dwswager]

When using a teleconverter, there is a focal length conversion factor (1.4x, 2x, etc) and an accompanying f-stop drop f/4 -> f/5,6, etc.

Does the f-stop drop relate only to drop in light intensity (making it more of a T-stop adjustment) or does it also represent depth of field?

Assuming the same subject distance and focus distance, you end up with basically at the same place.  That is YES.  A 400mm at f/5.6 and a 200mm with a 2x teleconverter at f/5.6 gives the same DOF.  Note that modern cameras/lenses/TCs handle the apertures automatically.  A 200mm f/2.8 with the 2x TC will have a minimum aperture of f/5.6.

A good TC primer:

Teleconverters (Multipliers) - A technical overview

[Telecaster]

Since you're changing the actual focal length by using the TC, the focal length / aperture diaphragm size ratio also changes. Given Bart's caveat you've thus turned your lens into a longer one with a smaller f-stop.…which impacts DOF.

-Dave-

[Bart_van_der_Wolf]

OK, let's start afresh: 400mm f/5.6 and 200mm f/2.8 with 2X teleconverter will produce exactly the same optical result full open. That is really all there is to it.

Yep (give or take some pupil factor effect due to optical design). The 200mm f/2.8 with 2X teleconverter effectively becomes a 400mm f/5.6.

Cheers,
Bart

[jjj]

However, there will likely be minor discrepancies due to resolution differences. A poor teleconverter will possibly result in a slightly greater DoF, and an excellent teleconverter in a slightly shallower DoF. But that's pixel-peeping. 

What about 5 teleconverters!

[Ray]

What about 5 teleconverters!

The greater the magnification, the poorer the quality of the equivalent focal length of lens that results. My owns tests have confirmed that even a very modest 1.4x converter provides little resolution advantage over an image without converter, cropped to the same FoV as the image with the converter. Any resolution advantage is often negated by the higher ISO required, or the slower shutter speed required due to the higher F/stop number of the lens when converter is attached.

Converters are a waste of time, in my very, very humble opinion. 

[dwswager]

Any resolution advantage is often negated by the higher ISO required, or the slower shutter speed required due to the higher F/stop number of the lens when converter is attached.

That is assuming you are using an inapporpriate shutter speed for the magnified focal length or increasing the ISO.  I shoot daytime sports with the AF-S Nikkor 70-200mm f/2.8 VR II and 1.4x TC-14EIII and find the results acceptable.  And since I shoot a D810, I find a 1 stop increase in ISO not a problem, when I might need it, as I might have when I was shooting the D300.

[Ray]

That is assuming you are using an inapporpriate shutter speed for the magnified focal length or increasing the ISO. .

Inappropriate? Do you mean, if conditions were such that one would use an unnecessarily fast shutter speed at base ISO without the converter, then one could reduce shutter speed instead of increasing ISO when using a converter which requires a stopping down of F/stop?

I'd be wary about this approach, especially when using a 36mp camera. Maximum resolution is often dependent upon using a reasonably fast shutter speed, despite the benefits of VR. The higher the pixel count, the faster the shutter speed needs to be, except when using a tripod.

I shoot daytime sports with the AF-S Nikkor 70-200mm f/2.8 VR II and 1.4x TC-14EIII and find the results acceptable. And since I shoot a D810, I find a 1 stop increase in ISO not a problem, when I might need it, as I might have when I was shooting the D300.

Even with a so-called ISO-less camera, such as the D800 or D810, increasing ISO by one stop whilst doubling shutter speed, results in approximately a 1-stop reduction in DR and a 1-stop reduction in SNR at 18%. You can't get away from this, except by using Canon cameras where DR at low ISOs is approximately equally bad from ISO 100 to 400 and only about 1/2 a stop down at ISO 800.

You might well find the results acceptable when using a high quality zoom with teleconverter, such as the 70-200/F2.8. I would think the main advantage would be the ability to see the action more clearly because of the greater magnification. However, if you were to compare two shots of the same scene, with and without teleconverter, either using the same ISO but a faster shutter speed for the shot without converter, or using the same shutter speed but a higher ISO for the shot with converter, I doubt that you would find any meaningful benefit outside of pixel-peeping.

You should see a marginal increase in resolution when using the converter, if the lens is good. However, if the image without converter was shot at a lower ISO, it can be sharpened more, thus narrowing any resolution differences. If the image without converter was shot at the same ISO but using a shutter speed twice as fast, there will likely be some increase in resolution due to that faster shutter speed.

[dwswager]

I mean shooting at an appropriate shutter speed for the magnified focal length.  And yes, you do incur a small penalty for increasing ISO.  Since I usually don't require all the DR the D810 has at base ISO for sports, that isn't even a penalty and the images up to about ISO 800 are Excellent and up to about 3200 are still very good. 

What ISO do you suspect indoor sports shooters use, though they do not usually need the extra reach of a TC or super telephoto lens.  Everything in photography is about trade offs.  Give something to get something of higher value.  The trick is making smart choices.  The good shot you get is always better than the great shot you didn't!

Inappropriate? Do you mean, if conditions were such that one would use an unnecessarily fast shutter speed at base ISO without the converter, then one could reduce shutter speed instead of increasing ISO when using a converter which requires a stopping down of F/stop?

I'd be wary about this approach, especially when using a 36mp camera. Maximum resolution is often dependent upon using a reasonably fast shutter speed, despite the benefits of VR. The higher the pixel count, the faster the shutter speed needs to be, except when using a tripod.

Even with a so-called ISO-less camera, such as the D800 or D810, increasing ISO by one stop whilst doubling shutter speed, results in approximately a 1-stop reduction in DR and a 1-stop reduction in SNR at 18%. You can't get away from this, except by using Canon cameras where DR at low ISOs is approximately equally bad from ISO 100 to 400 and only about 1/2 a stop down at ISO 800.

You might well find the results acceptable when using a high quality zoom with teleconverter, such as the 70-200/F2.8. I would think the main advantage would be the ability to see the action more clearly because of the greater magnification. However, if you were to compare two shots of the same scene, with and without teleconverter, either using the same ISO but a faster shutter speed for the shot without converter, or using the same shutter speed but a higher ISO for the shot with converter, I doubt that you would find any meaningful benefit outside of pixel-peeping.

You should see a marginal increase in resolution when using the converter, if the lens is good. However, if the image without converter was shot at a lower ISO, it can be sharpened more, thus narrowing any resolution differences. If the image without converter was shot at the same ISO but using a shutter speed twice as fast, there will likely be some increase in resolution due to that faster shutter speed.

[Ray]

I mean shooting at an appropriate shutter speed for the magnified focal length.  And yes, you do incur a small penalty for increasing ISO.  Since I usually don't require all the DR the D810 has at base ISO for sports, that isn't even a penalty and the images up to about ISO 800 are Excellent and up to about 3200 are still very good. 

What ISO do you suspect indoor sports shooters use, though they do not usually need the extra reach of a TC or super telephoto lens.  Everything in photography is about trade offs.  Give something to get something of higher value.  The trick is making smart choices.  The good shot you get is always better than the great shot you didn't!

If you have to increase ISO, whether to get a shutter speed appropriate for the effectively longer focal length, or merely to compensate for the change in f/stop, then regardless of DR considerations, you still get a noisier image across the whole range, which is likely to be noticeable for skin tones, which I think would feature in most sports shots.

I'm just trying to be clear here as to the real benefits of the teleconverter. It seems to me that in the interests of reduced weight and cost, one is accepting the performance of a poor quality, or at best a mediocre quality lens.

In other words, an excellent 200mm lens used with a 2x converter becomes a mediocre 400mm lens, and a mediocre 200mm lens used with a 2x converter becomes a poor quality 400mm lens.

If what I write is true, and I believe it is as a result of my own tests, then the real benefits of the teleconverter are reduced weight and cost, and the facility to see any action more clearly through the viewfinder, due to the greater magnification.

If this results in one capturing 'better' moments during sports events and the like, then the use of the teleconverter is justified.

[Ray]

I mean shooting at an appropriate shutter speed for the magnified focal length. 

Actually, there's an interesting aspect of this perceived need to increase shutter speed due to the longer, effective focal length of a lens used with teleconverter. The concept is, the enlargement of the scene unavoidably includes the enlargement of any movement in the scene, and/or the movement from camera shake, therefore, one should increase shutter speed to compensate for this.

However, if one is comparing the technical quality of images taken with and without converter, the image without converter is cropped and enlarged through a different process of interpolation, but enlarged nevertheless.
in order to achieve the maximum 'freezing of movement' in the image without converter, I would speculate that one should use the same shutter speed that is appropriate for the image produced with the converter, but I'm not certain about this.

If I'm wrong on this point, then in order to get an image with both equivalent DoF and equivalent (or better) sharpness when using a 1.4x converter, one not only has to raise the ISO one stop because of the increase in F/stop number, but also raise ISO another stop in order to get the faster shutter speed required to freeze the enlarged movement. That's a 2-stop difference in ISO. Quite significant I'd say.

If one is using a 2x converter, then in order to achieve maximum sharpness and the same DoF, one might have to increase ISO by 4 stops. Am I right or wrong on this point? 

[dwswager]

This evening, I was shooting high school soccer.  ISO ranged from 64 at the beginning of the game to 6400 by the end of the game.  Was shooting with D810 with the 70-200mm f/2.8 w TC-14EIII (1.4x).    Without the TC , the ISOs would have been 64-3200.  Not much you can do about it!   A 80-400mm f/4 lens would be nice.

Because the angle of view is narrowed with the TC on the same sensor, you have an apparently longer lens.  Hence, any camera shake is also magnified.  If you set the Auto ISO setting for minimum shutter speed to AUTO, it will automatically compensate and require and equivalently faster shutter speed than without the TC.

Actually, there's an interesting aspect of this perceived need to increase shutter speed due to the longer, effective focal length of a lens used with teleconverter. The concept is, the enlargement of the scene unavoidably includes the enlargement of any movement in the scene, and/or the movement from camera shake, therefore, one should increase shutter speed to compensate for this.

However, if one is comparing the technical quality of images taken with and without converter, the image without converter is cropped and enlarged through a different process of interpolation, but enlarged nevertheless.
in order to achieve the maximum 'freezing of movement' in the image without converter, I would speculate that one should use the same shutter speed that is appropriate for the image produced with the converter, but I'm not certain about this.

If I'm wrong on this point, then in order to get an image with both equivalent DoF and equivalent (or better) sharpness when using a 1.4x converter, one not only has to raise the ISO one stop because of the increase in F/stop number, but also raise ISO another stop in order to get the faster shutter speed required to freeze the enlarged movement. That's a 2-stop difference in ISO. Quite significant I'd say.

If one is using a 2x converter, then in order to achieve maximum sharpness and the same DoF, one might have to increase ISO by 4 stops. Am I right or wrong on this point? 

[telyt]

I'm just trying to be clear here as to the real benefits of the teleconverter. It seems to me that in the interests of reduced weight and cost, one is accepting the performance of a poor quality, or at best a mediocre quality lens.

In other words, an excellent 200mm lens used with a 2x converter becomes a mediocre 400mm lens, and a mediocre 200mm lens used with a 2x converter becomes a poor quality 400mm lens.

Some lenses are good enough that they don't become mediocre when using teleconverters (assuming a good teleconverter).  I recently saw a comparison of the Leica 400mm f/4 APO with the Leica 280mm f/4 APO plus Leica APO 1.4x teleconverter.  The 280+TC was spectacular (my own experience as well) and the Leica 400mm f/4 APO didn't catch up to the 280+TC until about f/8.

[ErikKaffehr]

Hi,

I would say that a case may be made for a converter matched to a lens. In that case the converter can actually be designed so it further reduces some aberrations in the lens it has been designed for.

Alternatively a lens can be designed to be used with a given extender. So a single 1.4X extender may be shared between several lenses.

Best regards
Erik

Some lenses are good enough that they don't become mediocre when using teleconverters (assuming a good teleconverter).  I recently saw a comparison of the Leica 400mm f/4 APO with the Leica 280mm f/4 APO plus Leica APO 1.4x teleconverter.  The 280+TC was spectacular (my own experience as well) and the Leica 400mm f/4 APO didn't catch up to the 280+TC until about f/8.

[telyt]

Hi,

I would say that a case may be made for a converter matched to a lens. In that case the converter can actually be designed so it further reduces some aberrations in the lens it has been designed for.

Alternatively a lens can be designed to be used with a given extender. So a single 1.4X extender may be shared between several lenses.

Best regards
Erik

The Leica 1.4x TC was designed in the mid-1980s and intended originally for the pre-modular 280mm f/2.8 APO and 400mm f/2.8 APO lenses.  The 280mm f/4 APO was designed ten years later.