Day 185: Crab Spider

This little fellow (and he was small, as you’ll see if you skip forward a bit) was trucking across the floor of my home today. I relocated him to the sink to see just how close I could get, sticking a +3 dioptre on to the front of the lens, and using a flashgun to negate the daft exposure times required for the apertures I was using.

If you are going to use the Tamron 18-270 PZD for this level of close-up work, even casually, then a flashgun is really a necessity. If you’re going for maximum magnification, then there’s quite a difference between the usual 100mm macro lens and a 270mm lens in terms of depth of field. There’s about 2mm of depth to this shot.

Nikon D300; Tamron 18-270mm PZD @ 270mm: Distance of 59.6m recorded (due to dipotre); ISO 400; F/22; 1/160s. VC off.

More about using dioptres below…

Nikon D300; Tamron 18-270mm PZD @ 270mm: Distance of 39.8m recorded (due to dioptre); ISO 400; F/32; 1/160s. VC off.

Actually, this raises quite an interesting point about the engineering behind the Tamron 18-270mm PZD. You’ll notice that both of the shots were taken at 270mm. However, there’s quite a difference in scale. Both were taken with the same dioptre in place, and neither has been cropped. So why the difference?

In order to adapt the 18-270 for close-up use (which is quite important for an all-purpose lens), its designers have adopted a little trick. When you focus at close ranges, the focal length reduces slightly. This means that the “unused” portion of the lens barrel basically becomes an extension tube, increasing magnification. At its closest focus distance, the focal length is actually about 150mm, although the distance recorded in your camera’s EXIF data won’t reflect this.

When you use a dioptre, you can see this quite clearly. That’s because when you focus at “infinity” on your distance scale, you’ll actually be only a couple of feet away from your subject. (For this lens, “infinity” is denoted by a figure of 59.6m on the EXIF data. Beyond that, the lens can’t differentiate focus under normal use). At that mark, you’re using the true focal length as shown on the lens barrel.

If you focus closer, you’ll be only a few inches away. However, you’ve changed your focal length by doing that too. So now you might be shooting at 180mm, which will not give you the same magnification. Which is exactly what’s happened here. Two different shots, taken with the lens physically at its longest reach, but with a slight turn of the focus ring. What you need to be aware of is that this little quirk affects all focal lengths, every time you focus. The Tamron isn’t the only lens that focuses in this way. Most internal focus lenses operate in this way. But the staggering range of the 18-270 does mean that the effect is more pronounced here than with many other lenses.


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