Questions about Extension tubes (not found searching)

[NateS]

I have a couple of questions about extension tubes. First off....I have a Sigma 70-300 APO DG Macro that has 1:2 magnification at 300mm. I usually shoot at betwenn f12-f18 with a shutterspeed at between 1/250 to 1/500 to darken the background and always use an off camera diffused flash. Using this setup, I get suprisingly sharp and clear photos (considering the lens).

So...question 1. Do extension tubes change the quality/sharpness of the lens? I understand that there is no glass and it's only moving my rear glass farther from the body.......but does that affect IQ and sharpness?

Question 2 - I've searched and searched but haven't found it. I'd like to get as close to 1:1 as possible and only plan to buy 1 extension tube (this only to hold me over until I can afford a true Macro lens). How can I figure up what each mm of extension tube will change my magnification to? One place said you take the focal length (300mm) divided by the extension (say 25mm) which would make it 1:12 obviously isn't right since I'm already at 1:2.

Any help is hugely appreciated.

 

[Mike_E]

#1--no there is no change.

#2-- I never bothered with that as they usually come in sets of three. Just start with the small one and go from there until you get what you want..

(you will find that you can use the zoom to focus- which is a nice thing. Enjoy them.

 

[NateS]

Thanks for your answer on number 1.

I really need an answer to number 2 though if anyone can. I am not buying a set of 3....only plan to buy one considering it's about 90 bucks difference. If I'm gonna spend $160 on extension tubes, I'd throw another $100 and buy a used 3rd party macro.

 

[icassell]

Thanks for your answer on number 1.

I really need an answer to number 2 though if anyone can. I am not buying a set of 3....only plan to buy one considering it's about 90 bucks difference. If I'm gonna spend $160 on extension tubes, I'd throw another $100 and buy a used 3rd party macro.

You might consider getting the ones that are not electronically coupled. They are much much much cheaper and you may not really need the AF when you're doing macro anyway. Remember, they have NO optics, so you don't have to worry about that. You can get a set of 3 manual ones for under $30 on ebay.

 

[NateS]

You might consider getting the ones that are not electronically coupled. They are much much much cheaper and you may not really need the AF when you're doing macro anyway. Remember, they have NO optics, so you don't have to worry about that. You can get a set of 3 manual ones for under $30 on ebay.

Seriously? I had no idea you could pick them up for that cheap. I usually autofocus at 1:2 but getting toward 1:1 I'd probably be forced to manually focus anyway so I suppose that doesn't matter. I'm not sure that I would have metering either, but that wouldn't even matter much the way that I shoot my macros. I'll check that out.

Still would like to know if there's any kind of computation to make to find the mag. ratio.

 

[icassell]

An example (I haven't dealt with this particular vendor, but price is similar all over):

http://cgi.ebay.com/3-Ring-Macro-Ex...ryZ79000QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

 

[icassell]

Still would like to know if there's any kind of computation to make to find the mag. ratio.

I found this:


http://scubageek.com/articles/tubes.pdf

These are the same guys that sell the V2 wireless things:

http://www.gadgetinfinity.com/product.php?productid=16874

 

[DeadEye]

My experance with extension is using a bellows ( just think of it as an adjustable length extension tube ) The longer the tube the more magnification you will get . The focal length of the lens used will not effect the magnification only the focal distance. In other words if I wanted to fill the frame with a penny. I would find the proper extension length on the bellows or tube to fill the frame. If I were using a 50mm lens the penny would be very close to touching the lens. If I were using a 135mm it would be several inches or feet away. Best results for me were allways shooting wide open not stopped down , Yes I know it sounds counter intutive but diffraction at that magnification plays a major part.

 

[Bifurcator]

Imperfections in the lens (it's CA qualities) are also magnified so using pro or high grade glass (lenses) helps ALLOT!!

I actually have a MUCH better time (more fun and as good or better results!) using a $40 high grade AC close up lens. IMHO it's the second best way to work with macro. The #1 best being a high quality dedicated Macro lens alone. Here's a little chart I put together on close-up lenses: http://thephotoforum.com/forum/showpost.php?p=1268864&postcount=30