Beginner Extention tubes

[horse.collier]

Have started using macro extension tubes just recently and came across an article about them creating greater magnification, and how to calculate what it is. An example went like this.

If you use a 20mm extension tube with a 50mm lens, the magnification increase is 20mm / 50mm = 0.4x.

I am using a Nikon D7500 with a crop factor of 1.5. Do i have to add this to the calculation, if so where please

 

[Garry Edwards]

No, it makes no difference.
The calculation relates to the extra distance created by the extension tube(s) divided by F (the focal length of the lens) when focussed at infinity. a

But, although accurate, that calculation is theoretical because if the lens isn't focussed at infinity there will be extra distance, and a change in magnification

Using a smaller sensor doesn't change this, the effect of the smaller sensor is that it effectively crops out part of the image.

 

[hunnymonster]

Also the usual equation is only valid for a "thin lens" ie. Not a compound lens as almost all camera lenses are. Works to a first approximation but isn't exact.

A zoom lens - all bets are off

 

[Garry Edwards]

Also the usual equation is only valid for a "thin lens" ie. Not a compound lens as almost all camera lenses are. Works to a first approximation but isn't exact.

A zoom lens - all bets are off

Yes, there are various factors at play, but extension tubes remain one of the easiest and least expensive ways of getting close to the subject, and I can't really see many situations where we would need to actually know the technicalities. I've had a set of auto extension tubes for years, one of my main reasons for using the smallest one is simply to allow me to shoot reasonably close when using a lens that simply doesn't focus closely enough. If that isn't enough then I simply switch to a a bigger one, or combine two or more together. I've never actually used the entire set, probably because I'm not into macro photography, but extension tubes are a versatile tool, the only real downside is that they sometimes work too well, and this is due to the fact that they aren't adjustable.

The next step up is of course bellows, which allow for infinite length adjustment, and other options include using a second (reversed) lens, using a "diopter" close-up lens (usually with a substantial quality loss) and using a macro lens, which apart from some clever optics is basically just a lens with a built-in extension tube, i.e. with a much greater focussing range than is found on most other lenses.