which extension tube for 150mm portrait-increase compression

[john_rogers3]

which extension tube do you recommend to add on to my 150 to achieve more compression for a head and

shoulders shot, what milimeter? what extension tube do you recommmend for my 100mm lens for using in

makro work where i am shooting something approx the size of a pack of cigarrettes??(i dont condone

smoking!!!) IL'm hoping to fill the frame. also on the 150 lens, is the proxar 1 close-up filter effective for

achieveng an increase in compression with 150 in a head and shoulders portrait?? thanks in advance John

[herb_sauer]

Increasing the extension on your lens does not alter the apparent commpression of the subject. It only changes the magnification. For more compression you need to use a longer focal length lens. The magnification, M, can be given by:

 

M = I/f - 1, where I is the image distance, ie the distance from the lens to the film (approx), and f is the focal length of the lens. To find the image distance required to get a magnification of M we re-arrange the above expression to:

 

I = f(M + 1) , so that for your 100mm lens and, lets say a 1/3rd size image of your pack of cigarettes, we need an image distance of:

 

I = 100 (1/3 + 1) =133.3mm. If your 100mm lens focusses down to 6 ft, the image distance is about 105.7mm, so you'd need 27.6mm more ie a 30mm extension tube if they make that size, or some compromises are called for.

Regards,

Herb Sauer

[john_rogers3]

sorry, compression was the wrong term..I guess what i mean is, can I

blow out the background focus wise by either using an extension tube

or perhaps a proxar filter. would these 2 things effect my infinity

focus in a way that will put the bkgd more out of focus than normally

with just the lens and no ext tube or proxar ? Is 21mm a suitable

sixe ext tube, or would you recommend another? please elaborate on

my specific questions. than

[john_rogers3]

sorry, compression was the wrong term..I guess what i mean is, can I

blow out the background focus wise by either using an extension tube

or perhaps a proxar filter. would these 2 things effect my infinity

focus in a way that will put the bkgd more out of focus than normally

with just the lens and no ext tube or proxar ? Is 21mm a suitable

sixe ext tube, or would you recommend another? please elaborate on

my specific questions. than

[oliver_sharp1]

You can do a better job of blowing out the background by using a tube, but not in quite the way that you imply. The tube lets you get much closer, so the depth of field is indeed much shallower because you are moving the camera close to the subject. I routinely use an 8mm with a 150mm lens (in fact, it's usually on the camera). That gives a very nice head/head and shoulders composition. But if you are trying to do full figure, you won't be able to move that far back with this combination (unless you are shooting my 1 year old son :-). So it only helps with the background if you take advantage of the tube to move in tight. A 21mm tube on a 150mm lens is going to have very shallow DOF but it is a bit long for most people shots; in another post Michael Heal calculated that it will let you focus on an object from 7.6 to 15 inches high. So you can do very tight headshots or abstract body-part shots, but a standard portrait won't really fit. I find that the 8 is perfect for that. For your 100mm lens, 21 is much too long for a normal portrait but the 8 is probably about right, depending on how you shoot.

[frank_ward1]

John, The inexpensive way to solve your problem is to move your subject further from the background. Increased subject to background distance equals increased lack of focus on the background. Frank