Why are some sigma lenses "crop only"?

Like the sugma 30mm 1.4...its crop only. So do I get 30mm instead of 48mm on my 1.6x crop 60D if I use it?

It's quite apparent that you don't understand what "crop factor" issues are all about.

The focal length of any SLR lens (or range, in the case of a zoom lens) is marked on the lens. The focal length of a lens DOES NOT CHANGE when you mount it on different format cameras. It's purely a function of the lens.

Regardless of the lens design, all lenses of the same focal length will provide the same field (or angle) of view on the same format camera. "Format" refers to the size of the film frame or digital sensor in a camera.

Please read the following article (which I've published here a few times) very carefully. I think you may be able to learn a lot from it.


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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 7D 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 7D has an APS-C sized sensor, measuring approximately 14.9mm X 22.3mm. The whole line of 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 7D - all have sensors that are approximately the same size (± 0.2mm).

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 14.9mm X 22.3mm) 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 7D 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 7D was taken with a longer lens. That is because the image recorded by the Canon 7D 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 7D (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. In addition, all lenses designed for SLR cameras (and most other "real" cameras) are marked with their actual focal length. No lenses for SLRs are marked with a "35mm Equivalent Focal Length" value, though you may find this on some point-n-shoot camera lenses.

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" is NOTHING MORE than a REFERENCE between two camera formats that lets you compare the field of view of particular focal lengths between the two formats. For the photographer who started with an APS-C format DSLR and has never used a 35mm format camera (at least enough to have developed a feel for what certain focal lengths provide him/her), the "crop factor" calculations can be completely forgotten for day-to-day lens selections. Only when comparing two camera formats is the "crop factor" useful.

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 7D. 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" (as related to using lenses essentially designed for 35mm SLR cameras) is always given assuming that the 35mm film 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.

Beginning photographers are often first confronted with the crop factor puzzle when choosing their first DSLR camera. Intuitively, "Full Frame" sounds better than "Cropped", as if one is getting a complete camera instead of a partial camera. There are very few really significant differences (other than features) between similar-generation cameras of different formats. The fact is that both format cameras can be used to make essentially identical images, though different focal lengths will be needed on them to keep the framing the same.

Beginning photographers are also confronted with "crop factor" issues when buying lenses. Focal length (translated to how big or small of a field of view you want) is the first factor to consider when asking the “which lens?” question. The beginner doesn't have to convert every focal length to its "35mm equivalent focal length" value but they should know that, on their APS-C camera, a 28mm lens isn't going to be wide angle but instead is a "normal" focal length and that a 250mm lens is going to be a rather long telephoto.

Many photographers who are new to DSLRs have acquired some very wrong ideas about "crop factor" issues. For example, they sometimes expect the focal lengths of EF-S lenses to be different (as in the focal lengths being pre-converted for the "crop factor") than the focal lengths of EF lens. This is completely false, as all SLR lenses are marked with their actual focal lengths. In addition, many new photographers who use APS-C format DSLRs seem to have been fed with the idea that they need to employ the "crop factor" calculations whenever thinking about using lenses on their cameras. This is generally not necessary at all as I have outlined above.

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Now to your main question - A "crop only" third-party (Sigma, Tamron, etc.) lens is one that is designed to project an image onto the smaller sensors in many of today's consumer-grade digital cameras rather than filling the film frame or digital sensor in a so-called "full-frame" camera. That's really the only difference. Canon's EF-S mount lenses have an additional design difference in that they can project further into the mirror box than their EF mount lenses. The purpose of that was to make it less expensive to design and produce shorter focal length lenses for the APS-C format cameras.

Sometimes Skip gets overenthusiastic!

Here is an illustration depicting the first sentence in the last paragraph...the inner rectangle is the APS-C frame, the full image is the FF frame, the red circle is (conceptually) the Image Circle of a lens designed 'for crop only'...if you tried to use it on a FF camera, the area outside the red circle would be only partially visible.

thedcmule2 wrote in post #14417593
Like the sugma 30mm 1.4...its crop only. So do I get 30mm instead of 48mm on my 1.6x crop 60D if I use it?

Lenses designed for "crop" bodies are simply engineered to provide a smaller "image circle" or "field of view" that will "fit" the smaller sensor.

As a result, it can be more cost-effective to produce a variety of lenses at the "wider" end of the scale, although these lenses won't work "well" with full-frame bodies -- they will tend to "vignette" at wider focal lengths.

But the newer design allows lenses such as the Canon EF-S 10-22 as well as some third party lenses to be procuced and sold at a very nice price point!

thedcmule2 wrote in post #14417593
Like the sugma 30mm 1.4...its crop only. So do I get 30mm instead of 48mm on my 1.6x crop 60D if I use it?

You hit the lottery with the first answer to your post. I doubt you could have been any luckier to have him be the first to answer your question.

Thanks to everyone who posted. I read the article and it does clear up a lot of misconceptions I had.

Since its just a sensor size reference between cameras, would I be able to use a 50mm lens on a 1.6 body and step back far enough to record a wider view like if I had used the 50mm on a full frame?

thedcmule2 wrote in post #14419302
Thanks to everyone who posted. I read the article and it does clear up a lot of misconceptions I had.

Since its just a sensor size reference between cameras, would I be able to use a 50mm lens on a 1.6 body and step back far enough to record a wider view like if I had used the 50mm on a full frame?

You'd get the field of view but a difference in "Depth of Field", meaning a woder range of the shot that would be in "acceptable focus" and a change in "perspective".

For more on perspective, read this thread:

https://photography-on-the.net/forum/showthre​ad.php?t=672913

tonylong wrote in post #14419346
You'd get the field of view but a difference in "Depth of Field", meaning a woder range of the shot that would be in "acceptable focus" and a change in "perspective".

1. FF with 50mm f/4 at 10' sees 4.7'x7.1' FOV, DOF zone is 2.43' deep
   
2. APS-C with 50mm f/4 at 16' sees 4.7'x7.1' FOV with different perspective than #1, DOF zone is 3.92' deep
   
3. APS-C with 30mm f/4 at 10' sees 4.7'x7.1' FOV with same perspective as #1, DOF zone is 4.02' deep

The images below were taken with a Tamron 17-50 at 17mm. It's one of those "crop factor only" lenses.

The 5D image is sized 1.6 times larger than the 20D image, to properly illustrate the real difference between camera formats. With a smaller format, you don't get more "reach," you don't get "the equivalent of a longer focal length." You just get less picture.

From a Canon 20D (1.6x sensor)

You guys are doing a great job with answering these questions. The images and illustrations really help someone grasp these concept. For someone to really retain data when studying, they need equal parts significance (the written) and equal parts mass (the actual thing). Of course, nothing would be better than to have two cameras with different sensor sizes and some lenses and a free afternoon to do some testing.

frugivore wrote in post #14420679
You guys are doing a great job with answering these questions. The images and illustrations really help someone grasp these concept. For someone to really retain data when studying, they need equal parts significance (the written) and equal parts mass (the actual thing). Of course, nothing would be better than to have two cameras with different sensor sizes and some lenses and a free afternoon to do some testing.

Heh! The concepts are basic enough that it really doesn't require much testing, but it can be fun to play!

In my day=to-day photography, I'm almost always shooting with my ff camera, but when I want "more pixels on target" and don't have the lens to fully frame a subject with my ff camera, I'll go for the crop! An example was "shooting the moon" last week where I used my 100-400 lens with a 1.4x TC, and grabbed my trusty ol' 30D rather than my ff 5DC to get the better resolution in the cropped result:

Actually, as to the "crop only" language, I think the more important interpretation of that is that the lens physically projects farther back into the camera. If it is used on a full frame DSLR the mirror would hit it when it swings up to expose the sensor. Less important, I believe is whether you get a full sensor image or a circular image. You could always crop in the circle, but you wouldn't want the mirror to hit the back of the lens.

BobOh wrote in post #14426214
Actually, as to the "crop only" language, I think the more important interpretation of that is that the lens physically projects farther back into the camera. If it is used on a full frame DSLR the mirror would hit it when it swings up to expose the sensor. Less important, I believe is whether you get a full sensor image or a circular image. You could always crop in the circle, but you wouldn't want the mirror to hit the back of the lens.

This only applies to Canon EF-S mount lenses. None of the third-party lenses for APS-C Canon cameras (such as Sigma, Tamron, and the rest) that I have ever heard of use Canon's EF-S standards. Thus, none of them extend into the mirror box any more than the Canon EF standard allows.

OK, good to know.