[bertwert]

So I was experiencing less light in the image using extension tubes than not. I'm wondering if this is normal? SOme people in another thread is saying it is not. I'm doing this with a Takumar 55/1.8 at F1.8, 1/50s, ISO 400 focused at infinity. Distance is distance from end of lens to wall. The room is reasonably bright. I also (double) checked before each photo the exposure and that the lens is set at f1.8.

First photo (5141) is from 6 feet with no extension tubes.
Second photo (5142) is from 6 feet with extension tubes.
Third photo (5145) is from 6 inches with no extension tubes.
Fourth photo (5146) is from 6 inches with extension tubes.

[mattt]

It is normal.

You are effectively changing the focal length with Extension tubes. You are not increasing the widest aperture... subsequently the real ƒ stop is less for each bit of extension you add.

Good article for reference:

http://www.cambridgeincolour.com/tutorials/macro-extension-tubes-closeup.htm

[bertwert]

It is normal.

I thought so.

Looking at my image EXIF, the focal length is 800mm... oops.

...

I'm going to quote the future!

I saw the title and I was like "yup... Bertwert"

I was waiting for you to respond...
So I can prove you were wrong

[Imp]

Extension tubes Takumar 55/1.8 test

I saw the title and I was like "yup... Bertwert"

[pentaxus]

Technically, you are not changing the "focal length" of the lens. I'm not sure if it is the inverse squared law that describes what happens but I do know that you lose 1 stop of light at 1/2 lifesize and 2 stops at lifesize no matter the focal length of the lens. Close up lenses ( the things that screw into the front of the lens ) don't change exposure because they do change the focal length of the lens.

[Canada_Rockies]

If I remember correctly, the aperture is moved further from the focal plane, and is thus optically smaller. If you think of this as the focal length of a lens being where the aperture is [optically] located, moving a lens forward by the focal length results in two stops light loss.

A 50mm lens with an aperture of f/2 at 50mm from the focal plane, with a 50mm extension, the aperture is now at 100mm from the focal plane and the area covered by the aperture (uncovered?) viewed from the focal plane is reduced to ¼, and therefor two stops.

[gofour3]

So I was experiencing less light in the image using extension tubes than not. I'm wondering if this is normal?

Yes you will loose light with any extension between the camera and lens. This is from the Pentax Auto Extension tube manual if you are using a 55/1.8 lens:

#1 tube exposure factor is 1.9
#2 tube exposure factor is 2.2
#3 tube exposure factor is 2.6
#1+2 tubes exposure factor is 2.9
#1+3 tubes exposure factor is 3.3
#2+3 tubes exposure factor is 3.8

The exposure facture roughly divided by two give you the loss in an EV value.

Phil.

[Not a Number]

Loss of light is easiest related to the magnification factor from the following formula where M = magnification, L = extension length, F = focal length:

M = L / F

effective f-stop will be marked f-stop × (1 + M). This is usually expressed as the bellows Exposure Factor where EF = (1 + M) ^ 2.

So a 50mm lens on 25mm extension would = 0.5× magnification. At f/8 would effectively be f/12 in terms of exposure ((1 + 0.5) × 8 = 12). The EF would be 1.5 squared = 2.25

EV multiplier is the square root of the EF.