[MMurphy37]

While I do have equivalent Pentax equipment, this is admittedly about Canon products, as the Pentax is decommissioned. Both are new however, so I what I learn about on is relevant for the other.

I just got a Vivitar 2x Macro Focusing Teleconverter. I also have two 2x Non-Macro focusing teleconverters (Vivitar and Rokinon). The lens I am using is a 50mm f/1.8 lens.

I noticed today that if I put a teleconverter behind the macro-teleconverter, I have more magnification than the other way around, but which combo would have the best picture, that is, with the least lens aberrations. For instance, I noticed If I put the Rokinon teleconverter on the body, and Vivitar on top of that, out of focus areas have a multicolored outline to them. When it is the other way around, the picture has less of this problem.

For the record, the Pentax also has a Macro-focusing telconverter, as well as three extension tubes, a 28-50mm lens, a 170mm lens, and an 80 - 200mm lens.

For both cameras, which combos would produce the highest magnification, the best clarity, and the most optimal distance from subject?

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Last edited by MMurphy37; 09-17-2012 at 06:17 PM.

[MMurphy37]

Side question - I realize that a 2x teleconverter will make any lens 2 stops smaller, but I am confused about how that works. Will my camera simply not be able to shoot at f/1.8 with its max aperture at ~4? What about the open-aperture metering? Will my cameras "know" there is a teleconverter make adjustments accordingly, or do I have to know and manually make the aperture wider, like changing it to 5.6 if it says 11?

[baro-nite]

Side question - I realize that a 2x teleconverter will make any lens 2 stops smaller, but I am confused about how that works. Will my camera simply not be able to shoot at f/1.8 with its max aperture at ~4? What about the open-aperture metering? Will my cameras "know" there is a teleconverter make adjustments accordingly, or do I have to know and manually make the aperture wider, like changing it to 5.6 if it says 11?

You can still shoot at f/1.8, but the effective exposure will be 2 stops slower (for each 2x TC you add to the optical path). Think of it this way: you're blowing up the center part of the image produced by the lens, but you aren't boosting the intensity of the light, so the result is dimmer -- you can't get something (magnification) for nothing.

As to image quality, experimentation as you've been doing is the best method, because it just depends on the particular lens and TC combination you're using. As a rule, stacking TCs is not recommended for best image quality.

[desertscape]

You ask a lot of questions at once. First, try not to use converters if possible. Most add aberrations. For macro work use extension tubes to gain the required magnification. A 2x converter will reduce speed by two stops because: If you had a 40mm lens with the diaphragm set with a 10mm opening, the focal ratio would be 4 to 1 (f/4). If you now add the 2x to it, the focal length now becomes 80mm. Your 10mm opening remains the same, so the ratio is now 8 to1 (f/8). That is a 2 stop difference.

If you are seeing color fringing with your stacked converters, that means there is visible chromatic aberration and not something you want on your images. Stacking converters is generally not done because of problems like what you have described.

[MMurphy37]

Lol, I have noticed that picture quality goes to shit when stacking them, although this might be good for something abstract, or possibly B+W where color fringing would be less noticable.

Let me see if I understand what has been said so far. Say I meter an object with a 2x teleconverter on, and the camera gives me a reading. That image, in reality, is going to be 2 stops darker, requiring me to either manually make the aperture wider or the shutter speed slower so bring the brightness back to the optimal average as calculated by the cameras light meter?

Or, because of the TTL metering, the camera will still operate normally with the tele-converter attached, with the exception that I will have longer exposures?


P.S. This site is awesome. I get responses VERY quickly.

[baro-nite]

Or, because of the TTL metering, the camera will still operate normally with the tele-converter attached, with the exception that I will have longer exposures?

That's it.

[MMurphy37]

That's it.

Excellent, but what if I still want a shallow DoF from the widest aperture? Is the teleconverter just making the image darker, or is it physically changing the aperture so that I simply can't have an aperture of f/1.8 with a 2x TC on a f/1.8 lens?

[baro-nite]

Don't worry -- if you're at macro distances you're going to wish you had greater DOF, not less.

[MMurphy37]

Don't worry -- if you're at macro distances you're going to wish you had greater DOF, not less.

Well yeah, but will that DoF be possible? Does the teleconverter make the widest aperture ~f/3.6?

[baro-nite]

Yes, the effective aperture becomes 3.6. Again, if you're shooting a macro magnifications, you're not going to have a problem getting shallow DOF. Here's a shot that's not even macro (just a close-in shot with the 18-55 kit lens), and is at f/8:

[MMurphy37]

Ok, so, and I know this might be frustrating but I like things to be crystal clear...what you are saying is that even though my camera on full-auto may read f/1.8 at whatever shutter speed, the actual aperture will never be that wide, correct?

[baro-nite]

The diaphragm on the lens is still going to be wide open. You've added optical elements behind the aperture, so we have to talk about effective aperture values (and effective focal length). My (limited) understanding is that this is a complex topic; a 2x TC increases the effective aperture value by roughly, but not exactly, a factor of 2 (2 stops).

As a rule, when using a TC you should stop down the lens somewhat anyway, for better image quality.

[Lowell Goudge]

just to make things simple for you, and this is regardless of camera type,

unless a teleconverter had electronics internally to modify the aperture it reports to the camera (the SMC-F 1.7x does this for example) most just feed the lens data to the camera. As a result, regardless of the impact the TC has optically, i.e. doubling focal length, while not modifying aperture diameter hence reducing in theory the true aperture, the camera will report the native lens setting, but expose for the true light hitting the metering.

as for getting more magnification with a different sequence of teleconverters, that is unusual, and I would need to see it to believe it.

your comment about CA and black and white has gone un-noticed but should be commented on,. CA , especially lateral CA in the plane of focus, will cause reduced sharpness when rendered in black and white. lets for example consider blue-green cast on one edge and red cast on the other of an object. since the red and blue green will both convert to roughly the same greyscale value, they will cause the image to blur. if you want the edges blurry (and lateral CA will do this) then convert to greyscale and dont worry, but if you want the edges sharp even in B&W, use a lateral CA correction that rescales the color layers (CA is just really a different magnification for each color to begin with) before converting to greyscale

[MMurphy37]

Would just the 2x Macro Focusing Tele-Converter with the 50mm f/1.8 create and accentuate enough lens aberrations to effectively ruin a photo? What else can reduce these problems other than using a more narrow aperture? It seems with the 2xMFTC that the macro-mode looks sharp as far as I can I tell, but when used in "normal" mode I can see a slight but noticeable reduction in sharpness. Could there possibly be an "optimal" range, where focal length, focus distance, and aperture all meet to produce an image with the least amount of any kind of distortion?

[newarts]

Would just the 2x Macro Focusing Tele-Converter with the 50mm f/1.8 create and accentuate enough lens aberrations to effectively ruin a photo? What else can reduce these problems other than using a more narrow aperture? It seems with the 2xMFTC that the macro-mode looks sharp as far as I can I tell, but when used in "normal" mode I can see a slight but noticeable reduction in sharpness. Could there possibly be an "optimal" range, where focal length, focus distance, and aperture all meet to produce an image with the least amount of any kind of distortion?

For any lens system there is at least one optimal combination of parameters. There is an aberration limit at the wide open end of the aperture range (if you open the aperture more the image gets worse) and a diffraction limit as the aperture gets smaller and smaller.

When you add auxiliary optics or reverse the lens (or go beyond the minimum focus distance) I don't think there is a way to predict how aperture changes affect the image. Experiment is the best way to find out.

Dave in Iowa