eos extension tube question

[wetWolger]

So I am going to get an extension tube for my 10d. I need a little bit more macro power. Anyways I have never used extension tubes. So I am wondering if I should get just this 25mm unit:
http://www.bhphotovideo.com/bnh/con...234&is=REG&addedTroughType=categoryNavigation
or this set:
http://www.bhphotovideo.com/bnh/con...102&is=REG&addedTroughType=categoryNavigation
Money is tight but I rather make the right purchase the first time. So I am asking you guys who have used these before to give me your input. How big of diffrence is 11mm going to make in a photo? Will I need the flexability in the set to get good focus ability?

Also someone said they wont work well with my lens because it can only go to an f-stop of 3.5 and thus it has fewer fins. And this will distort the image due to the lack of fin quantity. Can anyone confirm or deny this statment? Thanks

 

[reefrunner69]

You can usually find the kenko set for much cheaper on ebay from sellers with lots of good feedback. The ext tubes can be stacked so the difference is more that 11mm. Using a 50mm lens you can get 1:1 magnification with 50mm worth of extension. As extension tubes have no glass in them at all (it is just a hollow tube between the lens and the camera, they do not effect image quality at all, you just lose light and have a much smaller range of focus, since you lose infinity by using extension tubes.

 

[louist]

Definitely the Kenko set, performance wise, there's no difference, the Canon ones are just build a little nicer. I have the EF25 only because I got it cheap off Ebay, else I would have gone with the Kenko in a heartbeat.

 

[wetWolger]

Thanks for the replies. I knew you could stack them together the reason I asked how much diffrence 11mm makes was just to get an idea of if a little change makes a big diffrence. Or what.

Also can anyone comment on my friends statment that if your lens has few apature fins it will distort the image when using an extension tube? Again my lens can stop down to 3.5f. Thanks

 

[reefrunner69]

Also can anyone comment on my friends statment that if your lens has few apature fins it will distort the image when using an extension tube? Again my lens can stop down to 3.5f. Thanks

Using extension tubes will not distort the image, but what may be possible is that the increased magnification will make the flaws the lens has more apparent. Any distortion noticed would also be there without the ext tube, it is simply more apparent due to the magnicifation.

 

[louist]

Also can anyone comment on my friends statment that if your lens has few apature fins it will distort the image when using an extension tube? Again my lens can stop down to 3.5f. Thanks

Fewer aperture blades means that out-of-focus (OOF) highlights will not be a smooth circular shape. Say if your lens has 5 aperture blades, OOF highlights will have a pentagonal shape, while if you lens has 8 aperture blades, then it will have a octogonal shape. Now, given a lens with n blades, as n approaches infinity, the aperture opening begins to resemble a circle. When does it look like a circle? I believe that will have to do with the property of the light (sorry won't comment on the hardcore physics of light without embarassing myself!).

So to put it simply: your friend is wrong.

However, as aperture size decreases (larger f-number) there will be increasing distortion due to increased diffraction as light passes through the smaller opening. Thus, the cure to shallow DoF isn't just to shoot at f32 all the time. Distortion begins to appear at around f22. There is little increase in DoF from f22 to f32, thus decreaseing your aperture size by going from f22 to f32 is a bad trade-off for light loss, since the gain isn't great.

Again my lens can stop down to 3.5f

That should say "open up to f3.5" not "stop down". Stopping down refers to going from say f3.5 to f8. The verbs "stopping" and "opening" is referring to the physical size of the aperture opening (actually it's the surface area of the opening) and not the value of the aperture. Aperture values are written as f3.5, f8 etc, but in fact they are actually 1/f3.5 and 1/f8. Now it's apparent why large f-number = smaller aperture opening since the demoniator is larger!

Lastly, how much would 11mm of extension gain you in magnification? Well that depends on the focal length of your lens. It goes by this formula:

Magnification (M) = total extension / focal length, where focal length and total extension are measured in mm

If you are using a zoom lens, then just keep the focal length constant, then do your calc.If your lens is a macro lens capable of x magnification without accessories, then you simply add M to x to get the final magnification. How about examples?

Assuming these lensees are not macro lenses.
50mm lens with 25mm extension tube = 0.5x magnification
100mm lens with 25mm extension tube = 0.25x
180 mm lens with 25mm extension tube = 0.14x

Thus you can see to increase the magnification of the 100mm lens to 1x, you will in fact need 100mm worth of extension! Considering each 25mm of ext. cost you 2-stops of light, your viewfinder would be pitch dark.

Anyhow, I am rambling, the story is actually quite a bit longer than this, but ta ta!