The Dance Of The Numbers

on October 07, 2018
I am surprised that Albert Einstein and Isaac Newton were not keen digital photographers - because anyone who attempts to do photography these days needs to be a mathematician - and it might have been the other way round. We're all familiar with the off-sets and multiplication factors that are needed for understanding focal lengths when people are assessing what big or small sensors do - but we're not that familiar. And it's still possible to flummox us with science when it comes to equivalences in exposure between f stops, T stops, and other ratios. Fortunately for most of us TTL metering and Automatic flash can put most of the hard work at a distance while we take the actual pictures. I was brought to this thought when i checked out two tele-converters from the Stirling Street storeroom this week, looking to tes them out with the lenses I own. I was prevented from this by the construction of the tele-converters themselves and by not owning the right lenses. I'm not alone in this confusion, though, and it is a recurring them that has been back and forth over the counter for most of the major camera and lens brands. Tele-converters sound like a good idea; you clap one on and multiply the focal length of your lens for a price that is cheaper than that to buy a longer lens. Plus you needn't carry a large lens - the tele-converter is going to be very short and compact. What's not to like? What's not to like is that for many of these products in the past, they have been dedicated to one particular lens whilst seeming to fit a whole range. Some pretty awful optical mismatches have occurred, and have given tele-converters a bad name. Fortunately the makers have now specified the lenses to which these can be applied and improved the optical performance greatly. A person who uses a fresh T/C from Fujifilm, Canon, Nikon, or any other modern maker will be getting a good result. But I won't. I would need one of the following lenses to take advantage of either of these Fujifilm converters:
  1. Fujifilm 80mm f:2.8 Macro lens
  2. Fujifilm 50-140mm f:2.8 lens
  3. Fujifilm 100-400mm f 4.5-5.6 lens
The result with the 2 X is to increase each focal length by 2...the 1.4 X does it by 1.4. Simple maths. But remember that these function on a APS-C sensor so you then go for another round of multiplication to arrive at the equivalent to full-frame focal length and angle of view. In both cases the T/C chops of either 1.4 or stops of light while it boosts the focal things might seem a little dimmer. Fortunately, on modern EVF cameras like the Fujifilm, there can be circuits in place to increase the brightness in the finder even when you lose it on the sensor. You pick up a little noise or graininess on some finders as this happens but you still get to see what you're aiming at. It's the element and mounting sticking up in the centre of the converter - the three lenses listed are all hollow enough in the back-end to mate with this converter, but the shorter lenses and the other zooms are all too full near the rear to accept them. It's the same with the other maker's tele-converters. They are made to do a task well, but only with selected lenses. Note: There are universal tele-converters but do not expect the same sterling optical performance that you can get with the dedicated ones. And do not expect the autofocus mechanism in the main lens to work at all - you'll be focusing on your own.