Telephoto lens versus tele-converter?

[deepakram]

hello, group-members!
i was looking at a tempting offer for a sigma 170-500 telephoto lens, when my friend came up with the idea of a teleconverter - a 2x converter would make my 28-200 work as a 56-400!

since the cost of the converter is less than a quarter of the cost of the telephoto lens, i wanted to know - is there any reason why i should be opting for the latter? are teleconverters known to have any problems?

 

[j_mcquillen]

Teleconverters are certainly a cheap and easy way to get extra 'length' from your lenses, and there's absoloutely no reason why you shouldn't use one...

It all depends on how seriously you're taking your photography... If all you want is a bit of extra zoom to help capture your local sports team, or the birds in your back garden, then a teleconverter is going to do the job. If you're wanting something for more serious sports or wildlife photography, then a longer lens will be best - teleconverters always cause a loss of light, so even with a fast lens, you're going to have to use longer exposures. I think teleconverters can also limit the size of aperture you can use on certain lenses, restricting your choice of depth of field.

Hope this helps

 

[Big Mike]

Adding a teleconverter to your 28-200 would be adding more glass for the light to pass through, quite possibly degrading the image quality. Of course the image quality of a 170-500 lens might not be that great either.

Also, A 2X teleconverter will probably cost you two stops of light. So unless you have a fast lens (F4 or better) it might be hard to focus and auto focus may not work at all (if you have an auto focus camera).

I'm sure some people use teleconverters with all sorts of lenses, but typically they are used with high end fast telephoto lenses.

 

[deepakram]

my god! big mik's signature took a while to digest!
thanks, guys, for that instant reply!
however, i guess i must expose my ignorance quickly! what do you mean by "lens speed"?
i have a nikon f65, with a tamron 28-200 aspherical macro. this combo has served me well, and i was happy - till i saw some stunning bird-snaps taken at a lake near my house - now i want to do that too!

 

[Soulreaver]

Lens speed is given by by its largest aperture, that is its F stop number.
Fast lens, or bright lens , are those that have F 1.2 to F 2.8 usually.So they pass more light.A slow lens will be an F 4 and forward.
The converter will cost you some stops.For instance, most 200mm lens will have F 5.6 at its largest aperture.With a converter that will beequivalent to a F 11, so not much light will come through.
Its not a big problem in daylight, and you will have to use a tripod most likely anyways.

 

[Rob]

Teleconverters are lenses in themselves, and some are better than others. The most extreme you can really go is 2x, with 1.5x being a slightly safer option.

Sadly with photography there's no cheating... if it looks really good and it's cheap, that's usually because it's a dark setup. You can get 500mm f8 mirror lenses very reasonably, but that's another argument.

To answer your question, find out what the f number is of your 28-200 zoom at 200 and add on the number of stops which the t/converter will lose you. I suspect your lens will be in the order of f5.6 at 200mm, so lets call it that for the example. Lets also say that the t/c will lose you 3 stops of light (reasonably likely for 2x). Your end result will be an f22 lens at 400mm - virtually useless for wildlife or sports.

If your proposed Sigma is (at a guess f4-f8) then it's going to be f8 at 500mm.

That shoddy calculation would indicate that the Sigma will give you four times the light, as each f-stop is double the light. They are listed in the FAQ (it's difficult to remember the numbers off by heart without a lens handy!)

Generally, t/c are designed for long prime lenses with a low f-number (lots of light). A t/c which loses three stops of light on a f2.8 lens is not a problem, but if you're pushing f5.6 or f8 on the top end of your telephoto zoom, it probably will lose you way too much light (which you didn't have much of anyway).

Let us know your f-numbers of the lenses and the t/c and we can work it out properly.

Rob - professor of extremely dodgy photographic maths.

 

[deepakram]

My lens is specified as a tamron AF28-200MM F3.8/5.6XR Aspherical (IF) macro.
so, as you rightly guessed, it's 5.6. if i lose two stops, i'll get about f8, i guess. how did you arrive at the figure of f22? you mean i should extrapolate 5.6@200mm to 400mm?
That would indeed be a very big loss!
do let me know what you feel!
it's just that i'll get a teleconverter at a sixth of the cost of the 170-500 - that's a very huge difference!

 

[ksmattfish]

A x1.4 teleconverter needs an extra stop. A x2 teleconverter needs 2 extra stops. Keep in mind that many consumer auto-focus SLRs start having problems auto-focusing around f/8.

As has been mentioned, teleconverters are designed to be used on longer focal length lenses. You will have better results using it at the 150mm to 200mm end of your lens.

I think a lot of freelance nature photographers use teleconverters with good results. Would they rather have a big, white Canon 1000mm+ lens? Sure, but they can't afford it

I've even seen work by one photog who stacks 3 and 4 teleconverters to get outrageous focal lengths.

 

[Rob]

how did you arrive at the figure of f22?

Oops, for three stops, it should have been f16, not 22. Although the quoted loss for 2x t/c is 2 stops, I have found it to be nearer 3 with Nikon AF and t/c gear. Sorry for bad maths, but it's Friday and I have been in the pub. :blushing:

 

[deepakram]

Hi,
i have always used the fully-manual mode in my camera, and plan to continue doing so - that's the only way i enjoy shooting! that said, and also with the fact that i will only use the teleconverter if i plan to use my lens at the higher end of the focal length, does it make sense to go for the teleconverter instead of the telephoto lens?

 

[walter23]

At least in the case of canon gera, a f/2.8 maximum aperture is recommended to get your autofocus to work properly with a 2x teleconverter. Canon's 1.4x teleconverter is supposed to work with a f/4 max. aperture though.

 

[kfoster]

I think your decision should depend on how much you want to spend on a t/c. I think if your buying a Sigma/Tamron t/c which is realatively inexpensive, then why not give it a try. If you are unsatisfied then you aren't out of a lot of cash and you can always use it if you decide to get the lens later.

Of the people I know that use t/c's they always use them on big telephotos like the 400mm and 600mm. None of them ever use them on lower end telephoto zooms. They say that after the lens is stopped down from f/5.6 to f/11 that you end up with a dark viewfinder and a very slow shutter speed. Thats why they always use lenses that are f/4 or better.

A good way to check to see what you will be getting for light at f/11 is to set your lens to f/11 and then push the DOF button on your camera. This will close the diaphram down to f/11 so you will actually see how much light you get there. Just a suggestion.

K

 

[deepakram]

i guess i do have the money required to purchase a second-hand sigma 170-500 - will give it some thought! thanks a ton for all those excellent analyses!