The light macro quietly takes
At life size your f/8 behaves like f/16 — two stops gone before anything else. Pick what you are photographing and see what it costs, and what an extension tube actually gives you.
What are you photographing?
Magnification is how big the subject lands on the sensor. Life size — 1:1 — means a 10 mm insect covers 10 mm of sensor.
Step 2 — the aperture you set—
How close a tube gets you
A hollow tube between camera and lens, with no glass in it. It costs light and takes away infinity focus, but it is the cheapest way to get closer.
Why close focus costs light
effective aperture = f-number × (1 + magnification) · stops lost = 2 · log₂(1 + magnification)
Focusing close moves the lens away from the sensor. The aperture opening stays the same size, but the light from it now spreads over a longer distance, so less of it lands — exactly as if the aperture had been stopped down. At life size the lens sits twice as far from the sensor as at infinity, which is two stops.
Through-the-lens metering already sees the darker image and compensates. A handheld meter or a studio flash calculation does not, which is where this figure earns its keep.
Why macro is so hard to light
Two stops vanish into the extension at 1:1, and to get any depth of field at all you are stopped down to f/11 or f/16 — which is really f/22 or f/32. Add a shutter speed fast enough to beat the subject moving in the breeze and there is almost no light left. This is why macro photographers use flash for work that looks like it was shot in daylight.
The other half of the problem is depth of field. At 1:1 on full frame at f/8 it is around a millimetre, and stopping down to fix it runs straight into diffraction — at an effective f/32 the whole frame softens. That trade has no good resolution in a single frame, which is why focus stacking exists.
Ways to get closer, and what each costs
A macro lens. Corrected for close range, usually 1:1, no light lost beyond the extension itself. The expensive, correct answer.
Extension tubes. Empty spacers with no glass, so nothing is added to degrade the image. You lose infinity focus and the light calculated above. Cheap and surprisingly good.
Close-up filters. Screw-on lenses. No light lost, but they add glass in front and single-element ones soften the corners noticeably. Achromatic doublets are much better and cost accordingly.
Reversing the lens. A wide-angle mounted backwards gives high magnification cheaply. The pupil magnification assumption above breaks down entirely here, so treat the effective-aperture figure as indicative.
Related
Focus stacking planner for the depth-of-field problem, and flash guide numbers for the light one.
Questions people actually ask
Why do I lose light when focusing close?
Because the lens moves further from the sensor. The aperture opening stays the same physical size but sits further away, so its light spreads over a larger area and less falls on any given point. At life size the lens is roughly twice as far out as at infinity, which costs exactly two stops.
Does my camera compensate automatically?
In aperture-priority or program mode, yes — through-the-lens metering measures the light that actually arrives, extension loss included. The correction matters when you are metering externally: a handheld meter, studio flash, or working from a guide number, none of which can see what the lens is doing.
Do extension tubes reduce image quality?
Not directly — they are hollow spacers with no glass. What they do is push the lens outside the range it was corrected for, so a lens designed for distant subjects may show more aberration up close. A short tube on a decent lens is usually excellent; a long tube on a wide-angle less so.
What magnification do I actually need?
For a whole flower, 1:4 to 1:2 is plenty. For an insect's eye, 1:1 or beyond. Above 1:1 the difficulties compound quickly — depth of field under a millimetre, four or more stops of light gone, and any vibration magnified along with the subject.
Why is the effective aperture only an approximation?
The formula assumes a pupil magnification of 1, true for a symmetrical lens design. Retrofocus wide-angles and telephoto designs depart from it, and the exact correction needs a figure manufacturers do not publish. For a purpose-built macro lens the estimate is close; for a reversed wide-angle it can be well off.