Canon 100mm USM macro... zooms when focussing?

OK, I may be going crazy, but when I focus my 100mm macro (or rather swing it from one extreme to the other, instead of focus on anything in particular) it seems to zoom slightly. When focussed far away, objects seem further away; when focussed close in, objects seem closer.

Is this common when focussing any lens (only other lens I have is the kit and I can't bear to put it on my camera any more), specific to a macro lens, specific to fixed-length-when-focussing lenses, specific to this lens in particular, any combination of the above, or merely a product of my fevered mind?

Cheers

(on another note, I was initially disappointed with some of the shots I got from this lens, but on taking it out into the sun I've decided that I'm the problem )

Rawling wrote in post #3441250
Cheers

Have you been drinking?

Drinking?!

It's 10 A.M. in the morning here!

... no, that's not an outright denial. But no, I haven't been drinking

Working at hig magnification just makes this effect very noticeable.
Try focusing on something further away and the effect reduces.

You shouldve said you were drinking cause now I have no explanation for you.

I was gonna say, maybe you were stumbling back and forth while looking through the view finder and you really thought you were staying still. Zooming with your feet.

There will always be a small amount of change when focusing due to the optics moving in the lens, but as Choderboy has pointed out, in a macro lens at close focus distances this will be more pronounced.
A similar thing will happen with a zoom, when you zoom in or out the focus will shift slightly.

I was trying this on something ~1m away, which I guess is relatively high magnification. Cheers for the feedback, I'll keep an eye out for this in other lenses I use.

(and no, I was seated - no stumblin anywhere, thank you very much)

I have some notes around somewhere on various combinations of tubes / lenses.
I wanted to record the frame size so I could work out what tube length would be required for various size subjects.

With (I can't remeber how many ) tubes on my 200 2.8 , the frame width varied between approx 7cm and 10cm from min focus to infinity focus range.

So in practice I do use the focus ring as a zoom ring.

Greetings,
Shooting at those cery close distance, ie macro distance, it is the magnification effect. Thats why it looks like zooming. Thats the character of a macro lens. Enjoy...

The 100mm macro USM is an internal focus lens. The way these work is to shorten the lens focal length to focus closer, this is much more noticeable in a macro lens than an ordinary one. If you do the sums it probably shortens to about 70mm focal length at life size.

Alternatively there are lenses that focus by "Overall Linear Extension", these move the whole optical group away from the sensor to focus closer. In the case of a macro lens this means the lens extends out by the focal length at life size. An example is the Tamon 90mm.

Generally the internal focus design is better as it gives the designer more ways of optimizing the design to reduce aberrations over the magnification range and the lens does not get longer when you use it. Also AF is faster as less weight needs to be moved about.

Interestingly the working distance (between the end of the lens and the subject) is not worse than with an OLE lens, this is because as the focal length reduces as the lens is focused closer for more magnification, the front principle point is moved forward compensating for the reduction in focal length.

Lester Wareham wrote in post #3442613
...If you do the sums it probably shortens to about 70mm focal length at life size...

So you mean it is actually a 70-100 zoom lens

I have never used the lens, but I find this a bit extreme to believe

Best regards,
Andy

Andy_T wrote in post #3442714
So you mean it is actually a 70-100 zoom lens

I have never used the lens, but I find this a bit extreme to believe

Best regards,
Andy

No it is a prime lens in that it has one focal length, that focal length changes as a function of distance. You can see the effect as the change of pupillary magnification (the ratio of the apparent size of the lens stop seen from the front and rear.

What the lens is doing is moving the rear principle point away from the sensor to focus closer without adding extension; a by product of this is the change of focal length.

This is true for all the internal focus lenses, with a normal lens the focal length change is a few percent, but because of the large focus range on a macro lens it becomes very significant.

In a way this technology does blur the difference between zoom and prime technology.

To add to Lester's reply, it is not only IF primes that do this, but also quite a few of the newer zoom lenses. These zooms may well have their indicated focal length at infinity, but when focusing nearby, the focal length also changes. They also get shorter, in focal length that is.

BTW, this is one of the reasons why modern macro lenses have so many lens elements, as compared to older designs, and have larger maximum apertures. Although it allows better aberration corrections, larger apertures, and IF, hence (relatively) fast AF, in macro lenses, this is the price you pay. A very small price, IMO.

Kind regards, Wim

wimg wrote in post #3443142
These zooms may well have their indicated focal length at infinity, but when focusing nearby, the focal length also changes. They also get shorter, in focal length that is.

I understand that, but what amazed me was the amount (70 mm instead of 100) mentioned by Lester.

Best regards,
Andy

Hi Andy,

Andy_T wrote in post #3443161
I understand that, but what amazed me was the amount (70 mm instead of 100) mentioned by Lester.

Best regards,
Andy

Yes, but then the macro focuses really nearby . Even with zooms at middle distance you may find 15-20% shorter lengths than indicated on the barrel.

Another reason to use a zoom as a variable prime .

Kind regards, Wim