70-200 on a 1.6 crop

hi am i right to think that a 70-200 on a 450d would give me a focal length of 320mm and with a 1.4 con 448mm.Is this right?

you're opening a bill wall of yarn here.

nureality wrote in post #9595717
you're opening a bill wall of yarn here.

sorry you've lost me?

The focal length of this lens will always be 70-200mm, but the field-of-view when mounted on a crop body camera like a 450D will be the equivalent to a 112 to 320mm lens mounted to a full-frame or 35mm film camera.

The focal length is a built-in characteristic of the lens and will not change.

jc450d wrote in post #9595707
hi am i right to think that a 70-200 on a 450d would give me a focal length of 320mm and with a 1.4 con 448mm.Is this right?

No your focal length is 70-200 Unless you also shoot 35mm or full frame then the crop factor matters otherwise forget the math.

I'll point you to THIS thread before things get nasty

gonzogolf wrote in post #9595742
No your focal length is 70-200 Unless you also shoot 35mm or full frame then the crop factor matters otherwise forget the math.

thanks

jc450d wrote in post #9595730
sorry you've lost me?

What he's saying is that there is a lot of debate about the definition of terms on that subject.

One school of thought is that you CAN multiply by 1.6 to come up with an EQUIVALENT FL.
The other school of thought is that you CAN NOT multiply by 1.6, and that REAL focal length does not change, the image is simply cropped.
This leads to a lot of other talk about how you should operate your camera based on a REAL or EQUIVALENT focal length.

Generally, 1.4x is not disputed with the teleconverter. A 200mm lens with a 1.4x TC gets you 280mm.

My question is... where does that math get you? What are you trying to do with that number?

Well, the 1.4x teleconvertor will change the focal length...

But they are otherwise correct in that using a 200mm lens on a camera with a 1.6x crop factor gives the same field of view as 320mm would on a full frame camera but the lens still has a focal length of 200mm.

The "crop factor" is a reference number that relates to the difference in sensor size between cameras like the Rebel XTi and a 35mm film (or a so-called "full-frame" digital) camera. Let me list the facts:

35mm film cameras have a film frame size of approximately 24x36mm, while the Rebel XTi has an APS-C sized sensor, measuring approximately 15x22.5mm.

When camera manufacturers started designing digital SLR's, they decided that they should be about the same physical size of their 35mm SLR's. For that reason, they concluded that they could use the line of lenses they already had for their 35mm SLR's on the new digital SLR's.

All lenses designed for 35mm cameras project an image circle onto the film that covers the 24x36mm rectangle. The 35mm camera records the portion of that image circle that is defined by the opening behind the shutter for the film (24x36mm in size). A digital SLR with an APS-C sized sensor only records the smaller area (approximately 15x22mm) of the image circle projected by the lens.

When you put a 100mm lens on a 35mm camera and take a photograph, then put the same lens on a DSLR such as the Rebel XTi and take the same photograph - same subject, same position for the camera - with the same lens, and then enlarge both photographs to the same size print (4x6 inches, for example), it will appear as though the photo from the Rebel XTi was taken with a longer lens. That is because the image recorded by the Rebel XTi was of a SMALLER PORTION of the image circle projected by the lens - cropped, if you will - compared to the image recorded by the 35mm camera.

The special lenses for the Rebel XTi (and other Canon APS-C cameras) are called the EF-S series. These project a smaller image circle, making the lenses less expensive to design and produce in wide-angle and extreme wide-angle formats. However, an EF-S lens set to 20mm will produce the exact same image as an EF lens set to 20mm. Focal length is focal length, period.

Now to the primary point that I want to make: NOTHING about lens EVER CHANGES when you put it on different format cameras. Focal length never changes. Aperture range never changes. The only thing that would change is the apparent field of view, and that change is not a function of the lens but it is a function of the size of the sensor or film that will record the image.

The "crop factor" calculation for "35mm equivalent focal length" has only one valid use. That is for comparing lens use on two different format cameras. Here's an example: Joe took a photo of Mount Rushmore with a 35mm camera from a particular place using a 200mm lens. You want to replicate that photo with your Rebel XTi. What focal length do you need to do that from the same location that he took his photo? Divide the 200mm by 1.6 and you get the answer - 125mm. The "crop factor" is a REFERENCE between the two formats that lets you compare the field of view of particular focal lengths between the two formats.

The "crop factor" (related to using lenses essentially designed for 35mm SLR cameras) is always given assuming that the 35mm format (24x36mm) is the reference master. Something to realize, though, is that the 35mm film format is not, never has been, and never will be the "master" format against which all other camera formats are referenced. It is simply the format of the cameras that have also evolved into today's commonly used digital SLRs.


As said above, a tele-extender added to a lens will create a package with an actual focal length change as compared to the lens without the tele-extender.

I understand the point being made, but isn't this just a matter of semantics? If the original question were worded "hi am i right to think that a 70-200 on a 450d would give me the equivalent focal length of 320mm and with a 1.4 con 448mm.Is this right?" would it not be correct?

It would give you the equivalent field of view.

The focal length is a property of the lens. The field of view given by that lens depends on what kind of camera you mount it on.

Yes it will... understanding why, how or the correct terms is not important to MOST of us.

jc450d wrote in post #9595707
hi am i right to think that a 70-200 on a 450d would give me a focal length of 320mm and with a 1.4 con 448mm.Is this right?

Nope, it's 70-200mm without the teleconverter, and 98-280mm with the teleconverter.

eigga wrote in post #9596231
Yes it will... understanding why, how or the correct terms is not important to MOST of us.

I mostly agree and It bugs me when the semantics polics jump all over people, but I guess in this case it depends what you are comparing it to.

IN 35mm terms, yes, the 70-200 appears as roughly a 115-320mm lens. But if you are comparing it to other lenses you have used on your crop camera, it is better just to think in relative terms rather than converting to 35mm or FF equivalents. If the OP only has an 18-55 and is expecting the lens to look like a 320mm lens, then they might be surprised to find it only zooms in 4x closer than the 18-55 does.

So to the OP, the best way to look at it is be referencing what you currently shoot. If it is 35mm film, then yes, think of it as 200mm=320mm. But if it is other lenses on your 1.6x crop, just do the math. 200mm = 4x closer zoom than 50mm is your camera.