Discussion
I was having a quick run-through with my new Sigma 70-200 f2.8 which arrived today, and comparing it with the Nikkor 70-300 f4-5.6.
Taking care to aim at the same point I got:
70-300 at 200mm: 1/640th at f4
70-200 at 200mm: ditto (as expected)
Then I added the 2x TC to the Sigma, making a 140-400 f5 lens, and repeated the process
70-300 at 300mm: 1/320th at f5.6
70-200 + 2xTC at 300mm: 1/250th at f5.6 (different)
Focal length the same, aperture the same - shutter speed different... odd.
Taking care to aim at the same point I got:
70-300 at 200mm: 1/640th at f4
70-200 at 200mm: ditto (as expected)
Then I added the 2x TC to the Sigma, making a 140-400 f5 lens, and repeated the process
70-300 at 300mm: 1/320th at f5.6
70-200 + 2xTC at 300mm: 1/250th at f5.6 (different)
Focal length the same, aperture the same - shutter speed different... odd.
simpo two said:
I was having a quick run-through with my new Sigma 70-200 f2.8 which arrived today, and comparing it with the Nikkor 70-300 f4-5.6.
Taking care to aim at the same point I got:
70-300 at 200mm: 1/640th at f4
70-200 at 200mm: ditto (as expected)
Then I added the 2x TC to the Sigma, making a 140-400 f5 lens, and repeated the process
70-300 at 300mm: 1/320th at f5.6
70-200 + 2xTC at 300mm: 1/250th at f5.6 (different)
Focal length the same, aperture the same - shutter speed different... odd.
It'll be the underfanged flingle hongle not quite catching I suspect.
simpo two said:Er, yup I'm with you so far...
Taking care to aim at the same point I got:
70-300 at 200mm: 1/640th at f4
70-200 at 200mm: ditto (as expected)
simpo two said:...right...I think, I'm still there...but the TC needs how many stops?
Then I added the 2x TC to the Sigma, making a 140-400 f5 lens, and repeated the process
F2.8 -> F4 -> F5.6 if it's two stops, for example.
simpo two said:differs by 1/3rd of a stop - is that significant? If your TC is (in practice) "eating up" two and one sixth stops, your F5.6 is (again in practice) nearer F6.3
70-300 at 300mm: 1/320th at f5.6
70-200 + 2xTC at 300mm: 1/250th at f5.6 (different)
simpo two said:What it "says" in theory and what's happening in practice still seem to be within about 1/3rd of a stop.
Focal length the same, aperture the same - shutter speed different... odd.
I think you need to go out, take a million and one photos and then tell us how good the new lens really is (I might even put aside my prejudice against these new-fangled zoomie thingies.....)
beano500 said:
I think you need to go out, take a million and one photos and then tell us how good the new lens really is (I might even put aside my prejudice against these new-fangled zoomie thingies.....)
That was a good analysis. I'd expected the exposures to be the same, but in the event of any difference I'd hoped the mega-lens might have come out in front.
Anyway, the problem is solved as I don't really eeed the TC and it will soon be on its way to a new PH home
the size of the objective lens doesn't really matter, since that is included in how the aperture is calculated, as far as I am aware. I think your difference could come from one of two things,
1) The apertures and focal lengths given on lenses are only approximations, they are not always 100% accurate. A small difference here could tip the metering from the 1/320 to the 1/250. Is there always this 1/3rd of a stop discrepancy? Basically the point here is that just because the lenses say they are the same spec, doesnt actually mean they are.
2) Using the aperture measurment to determine the speed of a lens is risky, as it is a theortical calculation of how much light could go through, in relation to the focal lenght and objective lens size primarily. But it does not take into account the transmission % of light through the actual glass. Consider that a 50mm f/1.4 made with black glass will still be f/1.4, but will it be faster than a "normal" 50mm f/1.8? In this case, the teleconverter could be taking up more light than it should be technically, or it could actually be the main lenses where the difference lies. again, it may only take a small difference to trip the camera's metering by that small amount.
1) The apertures and focal lengths given on lenses are only approximations, they are not always 100% accurate. A small difference here could tip the metering from the 1/320 to the 1/250. Is there always this 1/3rd of a stop discrepancy? Basically the point here is that just because the lenses say they are the same spec, doesnt actually mean they are.
2) Using the aperture measurment to determine the speed of a lens is risky, as it is a theortical calculation of how much light could go through, in relation to the focal lenght and objective lens size primarily. But it does not take into account the transmission % of light through the actual glass. Consider that a 50mm f/1.4 made with black glass will still be f/1.4, but will it be faster than a "normal" 50mm f/1.8? In this case, the teleconverter could be taking up more light than it should be technically, or it could actually be the main lenses where the difference lies. again, it may only take a small difference to trip the camera's metering by that small amount.
dcw@pr said:
But it does not take into account the transmission % of light through the actual glass. Consider that a 50mm f/1.4 made with black glass will still be f/1.4, but will it be faster than a "normal" 50mm f/1.8? In this case, the teleconverter could be taking up more light than it should be technically, or it could actually be the main lenses where the difference lies. again, it may only take a small difference to trip the camera's metering by that small amount.
I think that must be it - the extra glass in the TC must be affecting it; as you say, f'5.7' might be enough to tip the balance.
dcw@pr said:
no need to put the 5.7 in '...'s. You wouldn't say "I drove at '78'mph" just because 78 wasn't a speed limit
I did that so you'd know it was deliberate and not a typo. Otherwise someone might say 'Don't you mean 5.6?' I was trying avoid this problem and a second post, but my cunning plan failed.
simpo two said:
no need to put the 5.7 in '...'s. You wouldn't say "I drove at '78'mph" just because 78 wasn't a speed limit
I did that so you'd know it was deliberate and not a typo. Otherwise someone might say 'Don't you mean 5.6?' I was trying avoid this problem and a second post, but my cunning plan failed.
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