50mm focal length

[mikesd]

Due to the fact that I have a very small studio [12x12ft] I was wandering if the Canon 50mm f/1.8 and the f1.4 are the same as the 50mm setting on the 18-55mm kit lens. Thanks in advance for the info.

[Adam Hicks]

They're both 50mm. You could also get 50mm by using a 1.4x extender on a 17-40L.

:)

[mikesd]

Thanks Adam.

[tim]

The focal length is the same, but obvisouly the aperature isn't. That's probably stating the obvious but just in case...

[slin100]

The indicated focal length is usually only correct when the lens is focused to infinity. The field of view of a lens will usually change slightly when a lens is focused to less than infinity. The difference can sometimes be significant at close distances.

[robertwgross]

The indicated focal length is usually only correct when the lens is focused to infinity. The field of view of a lens will usually change slightly when a lens is focused to less than infinity. The difference can sometimes be significant at close distances.

Do you know why this may be the case?

We could probably answer this easily if we worked in the Canon design lab.

---Bob Gross---

[psychedelic_never]

The indicated focal length is usually only correct when the lens is focused to infinity. The field of view of a lens will usually change slightly when a lens is focused to less than infinity. The difference can sometimes be significant at close distances.

I think this is only true for lenses that use internal focusing mechanisms.

[CyberDyneSystems]

They're both 50mm. You could also get 50mm by using a 1.4x extender on a 17-40L.

:)

Not really. The Canon T-cons won't even fit on this lens....

....and even of you found a T-con that would physically fit on the 17-40mm.. it would not function correctly.

T-cons are designed to work with telephoto lenses with narrow feilds of view. A T-con on a lens this wide would cause pretty drastic vignette.

[CyberDyneSystems]

... I was wandering if the Canon 50mm f/1.8 and the f1.4 are the same as the 50mm setting on the 18-55mm kit lens. Thanks in advance for the info.

I'm not clear what you mean by "The Same"

The focal lenght is the same,. but the image quality and f/stop are dramtically different.

Both of the primes will offer superior image quality, sharpess etc. and the ability to offer functionality in much lower lighting than the zoom.

[slin100]

The indicated focal length is usually only correct when the lens is focused to infinity. The field of view of a lens will usually change slightly when a lens is focused to less than infinity. The difference can sometimes be significant at close distances.

Do you know why this may be the case?

We could probably answer this easily if we worked in the Canon design lab.

I'm not a lens designer, but if you start with the basic lens equation 1/f = 1/i + 1/o and derive image magnification i/o, you can predict that magnification will increase (and FOV will decrease) as the subject distance decreases. The lens equation only goes so far, because none of our lenses are simple, ideal lenses. There's nothing that requires the lens designer to maintain a constant focal length as the various lens groups are moved, even on a prime lens!

I just checked all of my lenses and the field of view in all but one lens varies as I move the focus ring from infinity to a closer distance:
80-200/2.8L: FOV decreases
50/1.8 Mk I: FOV decreases
28-135/3.5-5.6 IS: FOV increases
Tamron 28-75/2.8: FOV increases
17-40/4L: FOV constant
I was a bit surprised about the 17-40L. It is, however, a parfocal zoom, which means it maintains constant focus as focal length is varied. Perhaps the designers also engineered it to maintain a constant focal length as focus is varied. The 80-200L is supposed to be parfocal, too, but it doesn't maintain a constant FOV. Based on my observations, it would seem that there are a number of variables when it comes to lens design.

[FlyingPete]

The indicated focal length is usually only correct when the lens is focused to infinity. The field of view of a lens will usually change slightly when a lens is focused to less than infinity. The difference can sometimes be significant at close distances.

I have seen this when using my 75-300 at around 300mm with close up filters on it, it is quite noticeable!

[Jon]

The indicated focal length is usually only correct when the lens is focused to infinity. The field of view of a lens will usually change slightly when a lens is focused to less than infinity. The difference can sometimes be significant at close distances.

Do you know why this may be the case?

We could probably answer this easily if we worked in the Canon design lab.

I'm not a lens designer, but if you start with the basic lens equation 1/f = 1/i + 1/o and derive image magnification i/o, you can predict that magnification will increase (and FOV will decrease) as the subject distance decreases. The lens equation only goes so far, because none of our lenses are simple, ideal lenses. There's nothing that requires the lens designer to maintain a constant focal length as the various lens groups are moved, even on a prime lens!

I just checked all of my lenses and the field of view in all but one lens varies as I move the focus ring from infinity to a closer distance:
80-200/2.8L: FOV decreases
50/1.8 Mk I: FOV decreases
28-135/3.5-5.6 IS: FOV increases
Tamron 28-75/2.8: FOV increases
17-40/4L: FOV constant
I was a bit surprised about the 17-40L. It is, however, a parfocal zoom, which means it maintains constant focus as focal length is varied. Perhaps the designers also engineered it to maintain a constant focal length as focus is varied. The 80-200L is supposed to be parfocal, too, but it doesn't maintain a constant FOV. Based on my observations, it would seem that there are a number of variables when it comes to lens design.

It's important to note that what you're seeing is just the captured area of the image. The lens' area of coverage hasn't changed, just the portion of theat circle you capture. One other thing you should consider in this experiment is what focal length you're working at. Wide angle lenses (including zooms) are retro-focus designs, so it doesn't (after the fact, never really considered it before) astound me that as you close-focus a WA zoom at its widest, it would effectively become somewhat "wider". You might see different results if you were to try the same lens at full telephoto position as conditions there are quite different. And since the design requirements for a parfocal lens are signficantly different than those for variable-focus zooms, I'd expect its' behaviour to more closely approach that of a prime or tele-zoom.

I have seen this when using my 75-300 at around 300mm with close up filters on it, it is quite noticeable!
That's because using a close-up lens physically alters the focal length of the compound lens. That's how close-up lenses work. If you change the focal length of the lens, of course you'll change the field of view.