Focal length change on a prime??

Janice

Janice

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Janice
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Yes
Absolutely stupid question... sorry!! :)

But Im never too proud to ask a stupid question.. that's how I learn!

OK.. Using a prime lens the focal length is always the same. I was taking shots of flowers using a sigma 105 macro.

While getting it into focus manually, I noticed that when it is out of focus I can see the whole flower but when it is in focus I can see less of the flower.

How can that be if the focal length is always the same?
 
IIRC, this is called 'breathing'. It happens on all optics that I know of, and is simply due to parts moving which will create a slight difference in focal length.

My Sigma 10-20 has practically no breathing, but that's probably because SWA lenses have a very broad focussing scale - i.e., anything from around 3 feet onwards is just infinity focus. My fifty (both of them in fact) however changes by at least two or three MM during breathing.

http://en.wikipedia.org/wiki/Breathing_(lens)
 
The focal length of all lenses will change slightly due to focussing. The focal length of a prime lens will only be accurate when focussed to infinity.
 
The light is refracted as it passed through the lens, only when it comes into sharp focus at the focal plane in the camera is it exactly the right 'size' related to the lens's focal length...
As it passes through the sharp plane of focus, the viewed image will appear 'larger' depending on which side of the focal plane the point of focus occurs...
4627-004-E3B847C3.gif


More easily understood in this simple diagram: as the light rays pass through the lens they converge at a point - this is the point of focus for that lens - by placing the camera's focal plane at that point (or designing all lenses for that camera so that all of their focal points converge at that point), images will be in focus...
Unfortunatley this 'simple' lens is fixed-focus, to enable a variable-focus lens, one that the user can select different subjects at different distances from the camera, that point of focus becomes floating as the light rays passing through the lens are 'bent' more or less acutely in order to focus the image at the camera's focal plane.
As you turn the focus ring, the point of focus shifts slightly depending on the camera-to-subject distance...so out of focus images appear larger than the properly focussed image.
 
Yv said:
breathing - so in other words, whilst you might have a 36inch chest, it becomes 37 when you breath in? :D
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Wouldn't that be when you breathe out ? :thinking:
 
Was thinking backwards...

... it's your stomach that gets bigger when you breathe out!
 
trencheel303 said:
Was thinking backwards...

... it's your stomach that gets bigger when you breathe out!
Click to expand...

I'm beginning to worry about the model & type of diaphragm you have in your body :p


Back on topic, that's a very good question raised Janice, thank you for having the guts to post this.
 
hmmm, just worked out my diaphragm has around 3,000,000 actuations so far, I can only be thankful I am not made by Canon!!

Sorry I digress, Arkady's diagram and explanation is spot on

Paul
 
When you alter the focus on a macro lens you effectively change the magnification, at the closest focus point you will get maximum magnification 1:1 and if i focus 39cm away i only get 1:2 on a Canon 100mm. You can see the magnification / focus distance relationship on the distance scale on the lens.
 
Taztastic said:
hmmm, just worked out my diaphragm has around 3,000,000 actuations so far, I can only be thankful I am not made by Canon!!

Sorry I digress, Arkady's diagram and explanation is spot on

Paul
Click to expand...

I don't know how fast you breathe, but with no exertion (sat on my fat one) I breathe about 1 breath every 5 seconds. Apparently the average is between 10 and 20 bpm with exertion between 20 and 60 bpm.

12 per minute
720 per hour
17,280 per day
6,307,200 per year
321,667,200 in my 51 years, I would suspect you could add a further 60% for exercise, exertion and sex

I would think I have taken more than 500,000,000 breaths in my 51 years!
 
Thanks for that, Rob... I understand it now. It's quite a curse having a nature that cannot rest until I know exactly how and why of everything!!! :D

As for the breathing part... we discovered the other day that when I breath in and out, its my stomach that moves and that is all.
But when my husband breaths in and out his chest and shoulders move but his stomach is completely still.

I guess we all fall into one of those categories, with YV being in the chest movers one!! :) :)
 
EdBray said:
I don't know how fast you breathe, but with no exertion (sat on my fat one) I breathe about 1 breath every 5 seconds. Apparently the average is between 10 and 20 bpm with exertion between 20 and 60 bpm.

12 per minute
720 per hour
17,280 per day
6,307,200 per year
321,667,200 in my 51 years, I would suspect you could add a further 60% for exercise, exertion and sex

I would think I have taken more than 500,000,000 breaths in my 51 years!
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Does that mean you have spent 60% of your life exerting yourself having sex!!! you need to get a camera...you might live longer:bonk:
 
EdBray said:
The focal length of all lenses will change slightly due to focussing. The focal length of a prime lens will only be accurate when focussed to infinity.
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That's true. All lenses change focal length very slightly on focusing closer, but it's much more apparent with some zooms and macros.

Some macro lenses are a bit of a cheat TBH and they deliberately change focal length at closer distances to reduce the amount of extension needed (or internal focusing movement) to get down to the required magnification. There are upsides and downsides.

The main upside of simplified construction is price. I guess you could also argue a slight benefit in that with some lenses, the increase in exposure needed at 1:1 is not two stops as demanded by the laws of physics, but perhaps 1.7 stops or a little less. That's another cheat though, as it's the result of the f/number becoming effectively lower as the focal length decreases. In other words, it's still two stops, but from a lower base, ie it's not a 105mm f/2.8 lens at 1:1, but maybe an 85mm f/2.4 (just a rough guess).

The downside is the working distance is reduced, which isn't good news considering that's usually the reason you buy a 100mm macro in the first place. I just looked up some figures and at 1:1 the Sigma 105 macro focuses 122mm from the front element. By comparison, the more expensive Canon 100mm macro gives you 150mm, a useful bit more, when you would expect that being slightly shorter nominal focal length it would be less than the Sigma... :thinking:
 
HoppyUK said:
. I just looked up some figures and at 1:1 the Sigma 105 macro focuses 122mm from the front element. By comparison, the more expensive Canon 100mm macro gives you 150mm, a useful bit more, when you would expect that being slightly shorter nominal focal length it would be less than the Sigma... :thinking:
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The Canon lens has internal focus whereas the sigmas hasn't, so the sigma's lens barrel is at full extension at 1:1.
 
Dogfish_magnet said:
The Canon lens has internal focus whereas the sigmas hasn't, so the sigma's lens barrel is at full extension at 1:1.
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For sure, they are optically quite different. The point is though that in terms of working distance, ie distance from the front element to the subject, there is less room with the Sigma than the Canon when from the nominal focal length alone you would expect there to be more.
 
HoppyUK said:
For sure, they are optically quite different. The point is though that in terms of working distance, ie distance from the front element to the subject, there is less room with the Sigma than the Canon when from the nominal focal length alone you would expect there to be more.
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Yes but forget MWD, as the front element moves on the sigma and look at MFD, which is the distance from the focal plane which is fixed , the sigma is 310mm and the canon is only 300mm, as i said the reason the MWD is less is because the front element moves closer to the subject as you increase magnification on the sigma.
 
Dogfish_magnet said:
Yes but forget MWD, as the front element moves on the sigma and look at MFD, which is the distance from the focal plane which is fixed , the sigma is 310mm and the canon is only 300mm, as i said the reason the MWD is less is because the front element moves closer to the subject as you increase magnification on the sigma.
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Or maybe it is less because the focal length reduces more? Which fits with the OP's observation.

But whatever the reason, the fact remains that with the 105mm Sigma you get less working distance than with the 100mm Canon, whereas from the nominal focal length you would expect it to be slightly more.

If you're looking at TheDigitalPicture.com, there are a couple of other interesting examples quoted on there. The Tamron 90mm, which also extends like the Sigma, has minimum working distance of only 99mm, whereas the Canon 60mm, which doesn't extend like it's bigger brother, with a nominal focal length that is 33% shorter has a MWD that is only 9% less at 90mm.
 
HoppyUK said:
Or maybe it is less because the focal length reduces more? Which fits with the OP's observation.

But whatever the reason, the fact remains that with the 105mm Sigma you get less working distance than with the 100mm Canon, whereas from the nominal focal length you would expect it to be slightly more.
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I calculated the focal length of the EF100/2.8 at MFD to be 78mm. Alas, I didn't go through this exercise when I had the Siggy 105.

Bob
 
dont breathe on the lens though, unless you want a fog filter result...:)
 
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