Stupid Crop Factor Question

Help an idiot out.

I get that I have to multiply the focal length of EF lenses by 1.6 for my XSi. But EFS lenses are designed for crop bodies. Do I still have to do the multiplying thing for EFS lenses on the XSi, or any crop body, or is the nominal focal length the focal length?

The focal length is what is listed on the lens irrespective of the body being used. 50mm is 50mm. The field of view changes. So, to directly answer your question, yes, whether the lens is a EF or EF-S lens, if it's on a crop body you can multiple the focal length to get the effective field of view.

R1200GS wrote in post #11752009
Help an idiot out.

I get that I have to multiply the focal length of EF lenses by 1.6 for my XSi. But EFS lenses are designed for crop bodies. Do I still have to do the multiplying thing for EFS lenses on the XSi, or any crop body, or is the nominal focal length the focal length?

First off, you don't have to multiply ANY focal length used on your camera by 1.6.

Secondly, as mentioned above, all lenses are marked with their real focal length values and not some "equivalent" values. Thus, an EF-S lens set to 50mm will provide the very same field (angle) of view on your camera as an EF 50mm lens would.

I've published the following a few times, but it should help you understand the facts that so many people get badly confused.


- - - - - - - - - - - - - - - -

The "crop factor" is a reference number that relates to the difference in film or sensor size (known as the camera's "format") between two cameras like the Canon XSi and a 35mm film (or a so-called "full-frame" digital) camera. Let me list the facts:

35mm film cameras and so-called "full frame" DSLRs have a film frame or sensor size of approximately 24mm X 36mm, while the Canon XSi has an APS-C sized sensor, measuring approximately 14.8mm X 22.2mm. The other Canon APS-C format cameras - starting with the D30 in the year 2000 and progressing through all of the "digital Rebel" xxxD series, the xxD series, and today's XSi - all have sensors that are sized similar to that in the XSi.

When camera manufacturers started designing digital SLRs (DSLRs), they decided that the DSLR bodies should be about the same physical size and configuration as their 35mm film SLRs. For that reason, they concluded that they could use the line of lenses they already had for their 35mm SLRs on the new DSLRs.

All lenses designed for 35mm film cameras project an image circle onto the film that covers a 24mm X 36mm rectangle. The 35mm camera records the portion of that image circle that is defined by the opening behind the shutter for the film (24mm X 36mm in size). A digital SLR with an APS-C sized sensor only records the smaller area (approximately 14.8mm X 22.2mm) of the image circle projected by the same lens.

When you put a 100mm lens on a 35mm film camera and make a photograph, then put the same lens on a DSLR such as the Canon XSi and make a similar photograph - same subject, same position for the camera, and same focal length - and then enlarge both photographs to the same size print (4 X 6 inches, for example), it will appear as though the photo from the Canon XSi was taken with a longer lens. That is because the image recorded by the Canon XSi 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 made by Canon for the XSi (and other Canon APS-C cameras starting with the 300D - the first Digital Rebel) 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. The EF-S lenses also project deeper into the camera than the EF specification allows (the "S" referring to "Short back focus), allowing for less expensive wide-angle lens designs. However, an EF-S lens set to 40mm will produce the exact same image as an EF lens set to 40mm if both lenses are used on the same APS-C format body and both lenses are focused at "infinity". 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 the field of view of lenses used on two different format cameras.

Here's one common 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 Canon XSi. 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.

Here's another popular example: Mary Sue has been using a Canon SX120 IS point-n-shoot camera and is wanting to use a Canon 50D DSLR. She is, of course, interested in what focal lengths she would need to keep the versatility of the SX120 camera's 10X super-zoom lens. The SX120 lens is actually a 6.0mm to 60.0mm lens, but the advertising also shows the "35mm equivalent" focal length range as 36mm to 360mm. To know the focal lengths needed for the 50D, merely divide the "35mm equivalent" values by 1.6. In other words, Mary Sue would need 22.5mm on the short end and 225mm on the long end for the 50D to have the same field (angle) of view coverage as her SX120 IS camera.

The "crop factor" is NOTHING MORE than a REFERENCE between the two camera formats that lets you compare the field of view of particular focal lengths between the two formats.

The "crop factor" (as related to using lenses essentially designed for 35mm SLR cameras) is always given assuming that the 35mm format (24mm X 36mm) 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.

- - - - - - - - - - - - - - - -

gjl711 wrote in post #11752031
So, to directly answer your question, yes, whether the lens is a EF or EF-S lens, if it's on a crop body you can multiple the focal length to get the effective field of view.

I don't like that "effective field of view" phrase at all.

What you get by applying the crop factor is the focal length of a lens that would give the same field of view on a 135 format film or FF format digital body.

Unless your looking to use a full-frame digital body or a film body, what is the purpose of doing the math? Just appreciate the fact that your body is a little more "telephoto" than a FF or film body using the same lenses.

yeah why does it really matter???

Yes - exactly the same way that looking through a smaller window makes things look bigger, which is to say, not at all.

R1200GS wrote in post #11752009
Help an idiot out.

I get that I have to multiply the focal length of EF lenses by 1.6 for my XSi. But EFS lenses are designed for crop bodies. Do I still have to do the multiplying thing for EFS lenses on the XSi, or any crop body, or is the nominal focal length the focal length?

Thanks for asking this, I've wondered about the same thing myself many times.

rockygarcia wrote in post #11753799
Thanks for asking this, I've wondered about the same thing myself many times.

Glad I could help.

focus.pocus wrote in post #11753391
yeah why does it really matter???

About the only reason I asked this is because certain focal lengths have certain purposes. Like say for portraits, on a FF, focal lengths between 85 and 135 are considered "portrait" lenses. although personally I think 135 is a bit long for portraits. YMMV. Is that set in stone? No. I just wanted to know if I bought a 100mm for my crop to do portraits would i wind up with a 160mm. Could I do portraits with a 160? Sure, but I may not like the compression a 160 gives me.

gjl711 wrote in post #11752031
The focal length is what is listed on the lens irrespective of the body being used. 50mm is 50mm. The field of view changes. So, to directly answer your question, yes, whether the lens is a EF or EF-S lens, if it's on a crop body you can multiple the focal length to get the effective field of view.

I guess I forgot that I knew this from my view camera days. A 180 is a normal lens on my 4X5 but on my 8X10 it's a wide angle. If the image circle is big enough to cover an 8X10.

Thanks.

SkipD, awesome right up.

Thanks

R1200GS wrote in post #11754746
Could I do portraits with a 160? Sure, but I may not like the compression a 160 gives me.

"Compression" is not caused by a lens, and so is not affected by focal length. It is purely an aspect of perspective and is determined entirely by relative distances.

R1200GS wrote in post #11754746
About the only reason I asked this is because certain focal lengths have certain purposes. Like say for portraits, on a FF, focal lengths between 85 and 135 are considered "portrait" lenses. although personally I think 135 is a bit long for portraits. YMMV. Is that set in stone? No. I just wanted to know if I bought a 100mm for my crop to do portraits would i wind up with a 160mm. Could I do portraits with a 160? Sure, but I may not like the compression a 160 gives me.

It sounds like you may, unlike a lot of newbies who post here, understand what focal lengths are common for various purposes with 35mm film cameras. If that's true and you are thinking that you would choose 100mm on a 35mm film camera for a given purpose, you then simply divide that value by 1.6 to arrive at the focal length that would give you the same field (angle) of view with an APS-C format camera used at the same distance from the subject.

Even better, you can use a tool like this this Photography Calculator web page to determine what focal length you want to use on any DSLR based on 35mm film cameras (by selecting the "crop factor" value to arrive at a "35mm equivalent focal length") at your chosen distance to the subject to cover a certain field of view. There are other calculators out there that would let you do the same for view cameras as well.

As xarqi mentioned, perspective is controlled by the distance between the camera and various elements of the scene and not by focal lengths. For more information on perspective and how to control it, please read our "sticky" (now found in the General Photography Talk forum) tutorial titled Perspective Control in Images - Focal Length or Distance?.

Thanks for the link Skip. Yeah, I'm not really a newb. I've been a photographer since '83 and working as one since 85 and we've been digital here at work since '02. But there's still tons I don't know and am always asking questions and learning.

Thanks again everyone for the info