Do we still need a TC?

Started Nov 11, 2013 | Discussions thread
Ian Stuart Forsyth
Ian Stuart Forsyth Veteran Member • Posts: 3,174
Re: Basic error

MightyMike wrote:

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 ?

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)

If the parent lens out resolves the sensor by a factor of 2 then adding a 2x TC will still net you more detail than cropping. I'd suggest that many lenses still out resolve even high pixel count sensors by a good margin.

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Mike from Canada
"I am not a great photographer! God is a great creator! All I do is capture His creation with the tools He has provided me."
'I like to think so far outside the box that it would require a telephoto lens just to see the box!' ~ 'My Quote :)'

I would have to agree with you Mike

Several people have been playing around with a Rasberry Pi camera board and installing them on 35mm and cropped lenses using 100mm and 200mm FL. You would be amazed with the detail that is being reached with these setups It would equate to a 225mp cropped sensor so I see no reason why you would not see an improvement in IQ with a 2X tc on a 60mp cropped camera body over just cropping

This is a worth while read once you translate the link on diffraction

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