Extension tube + Telephoto ZOOM = ??

[felixdd]

OK, so I found an old Sigma 75-200mm Pentax K mount for $15 at my local thrift store. At the same thrift store, I found a SMC Pentax 50mm f1.7 attached to a sigma 2x telemacro adapter...the whole thing selling for $10. I don't shoot Pentax, but my brother-in-law does, and he's looking for some legacy glass so I thought I'd pick it up for him.

So all things aside, the telemacro adapter is pretty cool in that you can remove the elements from the adapter and it'll become an extension tube.

Now I know extension tubes are purely for reducing the distance between yourself and the subject and boost magnification. At 75mm, I can achieve focus at ~10cm vs. 2m without the tube. I get great magnification as well.

As I go towards the tele-end of the lens, what I find is that I don't get any further image magnification; the image gets fuzzy instead. I find that I need to step back, such that, at 250mm, I need to be almost 2-3m away; subjectively, I get less magnification.

Am I right in thinking it as the following: as I get more tele, my field-of-view, measured in degrees, gets more and more acute (which is how a telephoto lens works), but my minimum focal distance gets pushed back such that my subject occupies the same dimensions within the frame. Therefore, I'm not getting any magnification?

If the above is right, then question 2 = how do I get more magnification? Do I use a tele-converter instead?

(Pics to follow)

 

[480sparky]

Odd thing about using a tube on a zoom. You start to loose the zoom function of the lens. It turns it more into a fine-focus capacity.

This is normal. Just compose and focus, or rather, focus and compose, as necessary. And focus becomes more of 'move the camera/lens back-n-forth' than it does 'turn the focus ring'.


I had one of those 2-in-1 devices back in my film days. I wish someone would smarten up and make them again for todays' DSLRs. They saved a lot of bulk & weight being two tools in one.

 

[felixdd]

Image 1: taken at ~10cm from subject, lens set at 75mm, closest focus possible, with extension tube

IMG_8796 by .felixdd, on Flickr


Image 2, taken at ~150cm from subject, lens set at 250mm, closest focus possible, with extension tube

IMG_8797 by .felixdd, on Flickr

 

[felixdd]

Ha! Someone beat me to the punch in replying.

OK So I'm no longer as confused I guess. Thanks for the clarification!

 

[apaflo]

With a "simple lens", the distance from the sensor to the lens has to be equal to the focal length to focus at infinity, and farther than that to focus closer. So consider a 50mm lens vs a 200mm lens. 50mm extension for one does the same as 200mm extension for the other!

Adding extension tubes to a zoom lens lets you see that in practice. Any given extra extension at the shortest focal length has far more effect than it does at the longest focal length. (And conversely, a diopter lens in front of a zoom will have less effect at shorter focal lengths than it does a longer focal lengths.)

But lenses are not a "simple lens" design anymore. Older zooms will change the length of the lens tube as they are focused or zoomed, and many modern lenses won't. That is due to what is called "Internal Focus" (IF) design. The lens is optimized to be mounted a fixed distance from the sensor, and works best only at that distance. The use of extension tubes and bellows has declined since IF designs have become popular. Another effect is that IF zoom lenses change their focal length as the are focused closer.

The benefits of IF lens designs are many. Today most macro lenses are just as sharp at infinity as they are close up, where older macro lenses were well corrected for astimatism only at closer distances. Zooms that don't change their length when focused or zoomed are another, but the big issues really are the compromises are matched much tigher across the focus and zoom ranges to avoid various abberations. An added benefit from that is the use of teleconverters, which magnify such defects, will cause less degradation. The overall effect is that high end zoom lenses, while quite expensive, are now just as sharp as fixed focal length lenses for practical purposes.

And all of these characteristics we take advantage of every day are directly related to the effects you noticed in using an extenstion tube at different focal lengths!