EF-S lenses: Does the focal length indicate the "crop"?

[Stewx]

Is an 18-55mm EF-S lens on a body with a "cropped" sensor like the Rebel XT actually still 18-55mm, or is that still multiplied by 1.6?

[Wilt]

Stew, 18-55 on the lens *is* 18-55mm, period.

Mounted on an APS-C body, due to the smaller sensor size 'cropping off' part of the image, your APS-C camera captures an image which is similar in content to using 29-88mm lens on a 35mm camera (the frame of reference that so many photographers are accustomed to using prior to digital SLR) or a FF dSLR.

[Stewx]

So then why is there vignetting when you use it [by force] on an full frame camera?

[dontblink]

There is vignetting because the image circle produced by the lens is smaller than the sensor on a full frame camera.

The focal length of the lens is still 18-55.

[Wilt]

Because Focal Length has NOTHING to do with Image Circle size!
I can show you a 75mm lens for 35mm photography, a 75mm lens for medium format photography, and a 75mm lens for 4x5 sheet film photography. ALL have the same focal length, but the image circles start small (35mm) and go to large (4x5)!

They behave differently due to format size: 75mm lens for 35mm photography is a short telephoto, a 75mm lens for medium format photography is a normal lens, and a 75mm lens for 4x5 sheet film photography is a very wide angle lens.

A hypothetical 75mm EFS lens has a smaller image circle than a 35mm format lens, but it is still a 75mm lens!

[Mark_Cohran]

Because Focal Length has NOTHING to do with Image Circle size!
I can show you a 75mm lens for 35mm photography, a 75mm lens for medium format photography, and a 75mm lens for 4x5 sheet film photography. ALL have the same focal length, but the image circles start small (35mm) and go to large (4x5)!

They behave differently due to format size: 75mm lens for 35mm photography is a short telephoto, a 75mm lens for medium format photography is a normal lens, and a 75mm lens for 4x5 sheet film photography is a very wide angle lens.

A hypothetical 75mm EFS lens has a smaller image circle than a 35mm format lens, but it is still a 75mm lens!

Somebody should make this a sticky - not that it would stop the quesiton from being asked about a dozen times a week. :)

Mark

[SkipD]

Stewx - read this thread (from the stickies) (http://photography-on-the.net/forum/showthread.php?t=45388) and you should get a little better idea about the stupid "crop factor" and you should then understand the only type of situation where it matters.

[Mark_Cohran]

Stewx - read this thread (from the stickies) (http://photography-on-the.net/forum/showthread.php?t=45388) and you should get a little better idea about the stupid "crop factor" and you should then understand the only type of situation where it matters.

So it is a sticky, after all. :) (I wasn't trying to be sarcastic in my previous post, I promise). I did look first, though. :)

Mark

[crn3371]

when you use it [by force] on an full frame camera?

That's just downright scary. Forcing a 18-55 on a full frame body.

[Mum2J&M]

You gotta admit, this all does get just a LITTLE confusing!

[ScottE]

You gotta admit, this all does get just a LITTLE confusing!

That is why you should never bother thinking about conversion factors. Learn what your lens does on your camera and don't worry about what it would do on someone else's camera.

You need to learn to previsualize the picture you want to take and mount the lens on your camera that will allow you to take that picture. The actual numbers on the lens (or the numbers that would be on the lens if you were using a 35 mm film camera) are not as useful as just becoming familiar with the performance of your own equipment.

[Wilt]

That is why you should never bother thinking about conversion factors. Learn what your lens does on your camera and don't worry about what it would do on someone else's camera.

You need to learn to previsualize the picture you want to take and mount the lens on your camera that will allow you to take that picture. The actual numbers on the lens (or the numbers that would be on the lens if you were using a 35 mm film camera) are not as useful as just becoming familiar with the performance of your own equipment.

Precisely!!! Generations of photographers have moved between 35mm and medium format and 4x5 cameras interchangeably, for different assignments, and they NEVER have to resort to 'conversion factors' or 'crop factors'. They merely learn the behavior of a lens of a given focal length, on a format.

The ONLY time they MIGHT resort (initially) to a conversion factor is if they love the look of 24mm lens on 35mm, and they want a similar effect on 4x5 camera, they would 'calculate' what focal length gives that same framing. But it is NOT necessary to use any conversion factor!!!...

One mere compares the format height in millimeters vs. the lens focal length. So, [15mm lens in APS-C] is like using a [24mm lens on 35mm] is like using [55mm lens on a 6x7(cm)] camera is like using a [90mm lens on a 4x5(in)] camera! So the photographer knows to go shopping for a 90mm lens for his large format camera, to get similar framing at the same shooting distance.
In tele lengths...150mm on APS-C is like 240mm on 35mm is like 500mm on 6x7 is like 900mm(!) on 4x5. Simple. No conversion factors, it is simply based upon FRAME SIZE!


Confusion has arisen only because the press and the community have turned it into a confusing mess! :(

[angryhampster]

You gotta admit, this all does get just a LITTLE confusing!



..Not really actually.

[Eagle]

This shot of Belmondo's shows it all.
http://photography-on-the.net/forum/showpost.php?p=101495&postcount=1
It's from the thread that Skip linked to.
No matter what sensor size or film format the focal length stays the same. The object you are shotting will be the same size from any camera, the rest of the frame surrounding your subject changes. And it doesn't really change you just see different amounts of it depending on sensor or film size.