"effective" focal length of zoom lens

imamember · Feb 7, 2008

I'd like to buy a Canon DSLR but would want to use it, at least initially with my existing EOS 28 - 300mm zoom lens and am confused as to what "effective" zoom range I'd have. If I've correctly understood the posts on this issue, the smaller size of the DSLR sensor (compared to 35mm film) would essentially give my lens the magnification of a 45 - 480mm zoom ... is that correct?

Are there other issues related to using a zoom lens designed for 35mm film with a DSLR that I should know about?

Thanks,
James

mike703 · Feb 7, 2008

imamember said:
I'd like to buy a Canon DSLR but would want to use it, at least
initially with my existing EOS 28 - 300mm zoom lens and am confused
as to what "effective" zoom range I'd have. If I've correctly
understood the posts on this issue, the smaller size of the DSLR
sensor (compared to 35mm film) would essentially give my lens the
magnification of a 45 - 480mm zoom ... is that correct?

Yes, that's correct... you won't have any wide-angle with such a lens on a cropped-sensor DSLR.

Best wishes
--
Mike

GaryDeM · Feb 7, 2008

as stated you loose the the wideangle side of the zoom, while gaining on the telefoto side.

there is also an image quality gain. since lenses perform poorer at the edges than the center, you will pick up some image quality. the reason is that with a smaller image circle you no longer are using the edges, you are useing the sharper center mostly.

as focal length just multiply the crop factor(multiplier) X the lens length in mm and this equals the new focal length in mm. canon on everything but the high end dslrs use a 1.6 multiplier so a 100mm lens becomes a 160mm lens for any practical point of view.

imamember · Feb 7, 2008

Mike:

Thanks for the quick reply. On the one hand, I won't mind having that extra 180mm of "effective" focal length for photographing some wildlife but I'm disappointed to learn I'll loose my wide angle capabilities. Why the heck didn't the camera manufacturers just design the sensor to be the same size as the 35mm film it was replacing?

iMac therefore iAm · Feb 7, 2008

imamember said:
Mike:
Why the heck didn't the camera manufacturers just
design the sensor to be the same size as the 35mm film it was
replacing?

They did... the 5D, 1DS, etc... those are full frame ... and looking at the price you'll know why they went with smaller sensors for the lower end models.

imamember · Feb 7, 2008

Thanks for pointing out the "upside" of gains in image quality ... that will help soothe the pain of loosing my wide angle capability.

Telecorder · Feb 7, 2008

One additional aspect of a lens on a crop factor digital is that one will have a narrower field of view as compared to the full frame...

a 35-mm/FF camera with a 50-mm lens at f/8 and subject at 10', the field of view in focus is 6.28' near to far

a Nikon D50 1.5X crop FOV is 3.98' near to far...

http://www.dofmaster.com/dofjs.html

--
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FZee30+RD-S+OlyTC1.7X
Dee50+Nikon 35mm F2.0D-AF+Nikkor18-70DX+Tam70-300-macro-LD+BIGMA 50-500 EX HSM
(Coming soon - Nikon 70-300 VR!)
My Image Galleries --

http://www.nikonians-images.com/galleries/showgallery.php?cat=500&ppuser=121399&password=

http://Telecorder.smugmug.com/

mike703 · Feb 7, 2008

imamember said:
Thanks for pointing out the "upside" of gains in image quality ...
that will help soothe the pain of loosing my wide angle capability.

When you get the camera, why not get it with the 18-55 kit lens? It costs very little extra and will give you the wide angle. If you buy them together it's only an extra £50 (UK) for the kit lens.

Best wishes
--
Mike

rsn48 · Feb 7, 2008

When DSLR's first came out, it was understood that lets say a 100 prime lens was really a 160 lens on a Canon 1.6 partial sensor crop; that is, a 160 focal length telephoto with a "magnification" factor.

There is a difference between a crop factor and a magnification factor. If you have a software editing program, when you crop, you are removing excess picture around the final image, thus making the final image larger - appearing as though it were magnified.

But magnification is different. Lets take the argument to a bizarre level, this often helps to understand it. Lets say you are out in the wilds of North Dakota, and low and behold a wolf is on a small hill and behind him is the moon. You know to make the moon look really really large you need around a 500 lens to "magnify" the moon. So you pull out of your bag, the 500 and shot the picture - wolf on hill with moon behind him.

You decide on another picture but unfortunately you drop your lens and it smashes the front lens and it is no longer functional. You say what a minute, I have a DSLR with a crop factor of 10 (no such camera exists). You think, if I put my 50mm lens on it with a crop factor of 10, it'll be a 500 lens. So you take the remaining pictures with it. Will the moon by nice and big like in the original 500 prime lens shot? No. Because the image is cropped, framed like a 500 lens, but it is not magnified like a 500 lens. The moon will be much smaller.

The moon will look like a 50mm lens shot of the wolf and moon in background, then cropped in Picasa, to resemble the field of view a 500 lens. The moon and wolf will appear closer, but the moon won't be that wonderful full moon of the original 500 prime shot.l

Tom_N · Feb 7, 2008

rsn48 said:
Will the moon by nice and big like in the original 500 prime lens shot?

Yes, it will be, in the final print, because in the printing process, you will adjust the images from the two cameras to fit the paper size. The perspectives will also be the same.

Where the 10x crop camera may suffer is in image quality. The printouts from the two cameras' images may have the same size. But if the sensors have similar pixel sizes, the camera with the larger sensor will have much more megapixel resolution. And if they have the same megapixel count, the camera with the larger sensor will have a high-ISO / low-light noise advantage.

There also may be depth of field differences between the 50mm and 500mm lens -- and between the small and large sensors. That wouldn't affect the size of the image of the Moon, but it might affect the artistic merit of the photo.

Greg Nut · Feb 7, 2008

rsn48 said:
When DSLR's first came out, it was understood that lets say a 100
prime lens was really a 160 lens on a Canon 1.6 partial sensor crop;
that is, a 160 focal length telephoto with a "magnification" factor.

There is a difference between a crop factor and a magnification
factor. If you have a software editing program, when you crop, you
are removing excess picture around the final image, thus making the
final image larger - appearing as though it were magnified.

But magnification is different. Lets take the argument to a bizarre
level, this often helps to understand it. Lets say you are out in
the wilds of North Dakota, and low and behold a wolf is on a small
hill and behind him is the moon. You know to make the moon look
really really large you need around a 500 lens to "magnify" the moon.
So you pull out of your bag, the 500 and shot the picture - wolf on
hill with moon behind him.

You decide on another picture but unfortunately you drop your lens
and it smashes the front lens and it is no longer functional. You
say what a minute, I have a DSLR with a crop factor of 10 (no such
camera exists). You think, if I put my 50mm lens on it with a crop
factor of 10, it'll be a 500 lens. So you take the remaining
pictures with it. Will the moon by nice and big like in the original
500 prime lens shot? No. Because the image is cropped, framed like
a 500 lens, but it is not magnified like a 500 lens. The moon will
be much smaller.

The moon will look like a 50mm lens shot of the wolf and moon in
background, then cropped in Picasa, to resemble the field of view a
500 lens. The moon and wolf will appear closer, but the moon won't
be that wonderful full moon of the original 500 prime shot.l

Sorry, the image will be the same, as long as your 10x crop camera has the same resolution as the first one, or you resize to match. Caveats: DOF will be much greater with the 50mm, and image quality greater with the 500mm. The apparent size of the moon in relation to the wolf will be the same.

I do agree in part: "crop factor" is better than "magnification factor," and "equivalent focal length" is preferable to "effective focal length."

Greg Nut · Feb 7, 2008

imamember said:
I'd like to buy a Canon DSLR but would want to use it, at least
initially with my existing EOS 28 - 300mm zoom lens and am confused
as to what "effective" zoom range I'd have. If I've correctly
understood the posts on this issue, the smaller size of the DSLR
sensor (compared to 35mm film) would essentially give my lens the
magnification of a 45 - 480mm zoom ... is that correct?

Are there other issues related to using a zoom lens designed for 35mm
film with a DSLR that I should know about?

Thanks,
James

Get it with the 18-55IS to use in addition to your 28-300. The only other issue I can think of is that current DSLR's are much higher resolution than typical film, making lens defects more apparent. On the other hand, you'll only be using the center of the image, so it's probably a wash.

sherwoodpete · Feb 7, 2008

rsn48 said:
The moon and wolf will appear closer, but the moon won't
be that wonderful full moon of the original 500 prime shot.l

Assuming you are correct. Let's assume that the wolf is silhouetted against the sky and we can see the moon AND some stars. The shot with the 500mm shows the wolf, the moon and the stars.

With the 50mm and the 10x crop, we get the picture of the wolf. But hang on a minute, the moon is going to be smaller this time, and the stars too. So we get to see more stars in the sky, which were previously hidden behind the body of the wolf. Can you explain how the changed lens has allowed us to see objects hidden behind the wolf?

Well, simply it can't. light travels in straight lines and no matter which lens we use, we get the same wolf, the same moon and the same stars.
Regards,
Peter

sherwoodpete · Feb 7, 2008

I think the comparison I made previously regarding hidden stars may be somewhat back-to-front.

But to be clear about one thing, the only way that the wolf can be larger and the moon smaller is to walk towards the wolf. Changing the viewing position is the ONLY way to get the relative sizes to alter. The lens makes no difference.
Regards,
Peter

Roonal · Feb 7, 2008

sherwoodpete said:
Changing the viewing position is the ONLY way to get the relative sizes to
alter. The lens makes no difference.

The perspective changes, how "close the moon would appear to be" would change in the 50mm 10x crop image versus the true 500mm image.

http://www.luminous-landscape.com/tutorials/dof2.shtml

--
Good Day,
Roonal

'Money doesn't buy happiness, but it makes for an extravagant depression' by golf tournament sportscaster

sherwoodpete · Feb 7, 2008

Roonal said:
sherwoodpete said:

Changing the viewing position is the ONLY way to get the relative sizes to
alter. The lens makes no difference.

Click to expand...

The perspective changes, how "close the moon would appear to be"
would change in the 50mm 10x crop image versus the true 500mm image.

I'm not with you here.

Firstly perspective - this depends on only one thing, the viewing position. If the camera is moved, the perspective changes. If remaining in the same position, perspective is identical.

Secondly, how "close the moon would appear to be" - sorry, I don't know what is meant by this.

Regards,
Peter

Barrie Davis · Feb 7, 2008

Greg Nut said:
On the other hand, you'll only be using the center of the image, so it's
probably a wash.

Yes.

The idea that a lens is somehow "improved" in image quality, by cropping off the softer periphery of the image, and using only the sharper centre, is quite wrong.

The remaining image is smaller in area, will need more enlargement to fill a screen or print, and that enlargement will reduce quality by simple magnification.

In the end the effects pretty much cancel out, and the quality of the cropped lens remains very close to what it was.
--
Regards,
Baz

Roonal · Feb 7, 2008

Looked thru one of my photography books which has example photos and saw where I was mistaken.

--
Good Day,
Roonal

'Money doesn't buy happiness, but it makes for an extravagant depression' by golf tournament sportscaster

dave_s93 · Feb 7, 2008

Telecorder said:
One additional aspect of a lens on a crop factor digital is that one
will have a narrower field of view as compared to the full frame...

True. But that is what a longer "effective focal length" implies - a narrower field of view. I think you are thinking of Depth of Field because of your statement below.

Telecorder said:
a 35-mm/FF camera with a 50-mm lens at f/8 and subject at 10', the

field of view in focus is 6.28' near to far a Nikon D50 1.5X crop FOV is 3.98' near to far...

http://www.dofmaster.com/dofjs.html

Yes, you will have less DoF in this situation, but you will also get a different framing. If you keep the framing the same by using a shorter focal length or longer shooting distance, then the DoF in the FF/35mm will be thinner. See this link for a better explanation.

http://www.dofmaster.com/dof_dslr.html

Hope this helps.

bugzie · Feb 8, 2008

i think you're right, but a number of folks here have dissented from your opinion and i'm uncertain as to who's right. can we clarify who's right here?

"effective" focal length of zoom lens

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