Extension Tube + Sensor Crop

[JaKS_Foto]

Hi all, Can you help?

If I attached my 100mm lens to my 60D, it equates to a 160mm. If I then place a 12mm extension tube in the 'middle', does it then become a 179mm ?

If I then place a 25mm extension tube in the middle, is it the equivalent to a 219mm?

Sorry for my lack of knowledge, hope you can help :)

[rajjayaraj]

Re: Extension Tube + Sensor Crop

As far as my knowledge, the extension tube will not increase the focal length. It does increases the magnification factor. The telephoto convertor would give you the extra reach focal lenght.
Raj

[Stan]

Re: Extension Tube + Sensor Crop

An extension tube just reduces the minimum focus distance of the lens enabling you to get closer to the subject, ie increased magnification

Stan

[S_J_P]

Re: Extension Tube + Sensor Crop

My understanding is that a 100mm focal length is 100mm no matter what sensor size you have.

A teleconverter will refract the lens' output to provide a spread on the sensor which is equivalent to a lens with a focal length of 1.4x, 2x or whatever the multiplier you have fitted.

An extension tube (without any optical elements) merely allows you to focus on closer objects (and hence achieve a higher magnification factor) by increasing the length of the light-path after the lens, providing more space for the rays from a close object to converge. The ability of the 100mm lens to focus on distant objects will be diminished.

When a 100mm lens is fitted to a crop-sensor camera, the smaller image-circle (occupying just the central portion of the image) gives the visual effect of a longer lens in terms of coverage, however the depth of field and other characteristics are as for a 100mm lens. When a teleconverter is fitted, the effective focal length is modified along will all the visual charactistics, when an extension tube is fitted all the optical characteristics remain the same as for a 100mm lens (but the effective f-stop will increase as the light has to travel further to reach the sensor)

So, to summarise,

100mm + extension tube = close-focus 100mm
100mm + 2x teleconverter = 200mm

[Hereford_EOS2]

Re: Extension Tube + Sensor Crop

There's a big difference between extension tubes and extenders.
Extenders increase the focal length of the lens by x1.4 or x2.0 and when added to say a 200mm lens gives 280mm and 400mm.
An extension tube only increases the distance between the camera and lens and basically puts the lens further away from the sensor.
This does not increase the focal length of the lens whatsover but does shorten the minimum focal distance.
Because the focal distance is shorter it allows the photographer to get very close to the subject.
The extension tube contains no optical elements; its sole purpose is to move the lens farther from the image plane.
Quite useful for macro photography where you want to get very close to the subject, but not ideal for creepy crawlies as getting that close with more often spook them.
You also need a slightly longer exposure when using extension tubes due to the loss of light.

[JaKS_Foto]

Re: Extension Tube + Sensor Crop

Many thanks indeed people, I thought I was getting something for nothing with an extension tube!

[colin C]

Re: Extension Tube + Sensor Crop

Many thanks indeed people, I thought I was getting something for nothing with an extension tube!

To an extent you are ..................... the ability to focus a lot closer without having to fork out for a very expensive lens.

Colin

[JaKS_Foto]

Re: Extension Tube + Sensor Crop

Many thanks for the replies. I was at work when I responded earlier, but I do mean, many thanks.

I have learnt a great deal on this forum, and it is appreciated.