# Do we still need a TC?

Started Nov 11, 2013 | Discussions thread
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 Re: Basic error In reply to James O'Neill, Nov 12, 2013

James O'Neill wrote:

Mark Ransom wrote:

A teleconverter works by magnifying the image that it gets from the primary lens. You could get the same effect by simply cropping, but the assumption is that the magnified image will be higher in quality than the cropped image.

No that's not the assumption. The TC can't possibly out put an image of any higher quality than went into it. So why do we have one ?

Sorry, I was unclear. I was talking about the final image quality, comparing the two results side-by-side. I would never claim that a TC could magically improve the lens.

If you record 1/2 the image with all the pixels (or all the film area) you will get better results than if you record all the image with all area and print half the result.

The net resolution is normally given as 1/net = 1/medium + 1/lens
for simplicity assume sensor and lens have equal res if the smallest the medium can resolve is 1/100th mm and the smallest the lens can resolve is also 1/100th mm the dot you can get when you put them together is 2/100mm. -i.e. 1/50th .

So lets say your lens can manage 1000 lines per image width and so can the sensor, net you get 500 lines per width. Crop to half the width you get 250 lines.

With a TC you're putting 500 of the 1000 lines onto 1000 lines of sensor res - net you 333 lines with the TC (less any loss from the TC itself)

I'm familiar with that formula for combining resolutions and it makes sense to me. There are a few assumptions that need to be questioned though. First, is it reasonable to assume that the resolution of the lens and sensor are approximately equal? That just happens to be the point where the formula works most dramatically; once one gets much lower than the other it tends to dominate. They may be equal, I just don't know how to tell. You alluded to another problem at the end, "less any loss from the TC itself". How much is the loss? I suspect it can be significant.

Mark Ransom's gear list:Mark Ransom's gear list
Pentax K-7 Pentax K-01 Pentax smc DA 15mm F4 ED AL Limited Pentax smc FA 50mm F1.4 Tamron AF 70-300mm F/4-5.6 Di LD Macro +2 more
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