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I have a Canon 7D, which has a crop factor of 1.6x.

When using a Canon EF-S lens I get the focal length stated on the lens, when using a Canon EF lens I get the focal length times 1.6 when used on the APS-C body. At least that's what I thought.

I just compared two lenses:

  • Canon EF-S 15-85mm f3.5-5.6
  • Canon EF 70-200mm f4

I took the same photo as you can see here.

The top photo is shot with the 15-85, the bottom photo with the 70-200 - both lenses are set to 70mm. Now why do they look the same?

I thought I would get 112-320mm (70mm*1.6 crop factor). What did I misunderstand about the crop factor calculation? Lightroom also displays the photos as having "70mm" and not 112mm.

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Very good comments - then I don't understand e.g. this review: the-digital-picture.com/Reviews/… (about 3/4th down the page) "It is an ideal lens for young children's soccer matches when mounted on a 1.6x FOVCF body." - why emphasize the crop factor if it doesn't change a thing concerning the focal length? –  Dennis G May 7 '13 at 18:43
    
Well, that same thing is also equally true for any telephoto lens. This Q&A goes into that in depth. –  mattdm May 7 '13 at 18:45
    
So why emphasize the 1,6x crop factor? Sure the lens is ideal for such-and-such situation. But why is it good on a "1,6x FOVCF body". There are actually more references in the prior paragraph. –  Dennis G May 7 '13 at 18:46
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LightRoom will report based on EXIF information that the lens was able to provide to the camera when taking the shot. In this case the lens reported focal length of 70mm. Usually (my observation) it has not been the case that the software would factor in the sensor size when reporting the focal length that was used to take the picture. –  Peng Tuck Kwok May 8 '13 at 2:38
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...and cropping a full-frame picture, using the same lens to achieve the same field of view (or "the same picture", if you will), means throwing away 61% of the pixels. So which is better: an 18MP image from an 18MP crop-sensor camera, or an 8MP image from a 20MP full-frame camera? That's why the emphasis on the crop sensor. –  user2719 May 8 '13 at 7:00

5 Answers 5

up vote 6 down vote accepted

Whether a lens is an EF or an EF-S lens, the actual focal length is always used. There are certain technical reasons why this is so, but the simplest is that a lens' focal length is defined as the distance from the film plane needed when the lens is focused at infinity to cast point light sources as a single point on the film plane. This doesn't change with regard to sensor size. What does change with regard to sensor size is the angle of view or field of view (FoV) that a lens of a specific focal length will include in the part of the image circle that falls on the sensor.

What this means is that if you are using a 70mm lens on a Full Frame camera, the FoV will be about 34° diagonally. The same 70mm lens on a crop sensor body will have an FoV of about 21°. That is an equivalent FoV of about a 110mm lens on the FF body, and that is why camera manufacturers say that the 70mm lens on a crop body is a 110mm equivalent. If you print photos taken of the same subject from the same distance using the same focal length lens with both cameras on the same size paper, the image from the crop sensor camera will make the subject appear larger. This is because the ratio between the crop sensor size and the print size is greater than the ratio between the larger full frame sensor and the same print size, thus the magnification factor of the photo taken with the crop sensor body is higher.

If you take both images shot at 70mm on your 7D with an EF 70-200mm f/4L and an EF-S 15-85mm f/3.5-5.6 IS they will have the same FoV because both lenses are set at 70mm. To understand the 35mm equivalent FoV they yield, both lenses need to be multiplied by the crop factor, even though one of the lenses does not project a light circle large enough to fill a full frame sensor.

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"both lenses need to be multiplied by the crop factor" - this is the best information yet! Good answer Micheal, thanks! I always assumed that the EF-S lenses have the crop factor "included" hence the -S. I guess I assumed wrong and this is very my confusion started. If both lenses are multiplied by 1,6, then of course they look the same. I would have to compare to a FF body to see a difference between the 70mm settings. –  Dennis G May 8 '13 at 8:11
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Very nicely put all around. In regards to @moontear's comment, I want to additionally stress that they only "need" to be multiplied when you want to compare between systems. If you're just working in APS-C, you can just develop as sense for how wide or narrow a certain focal length is on your camera without ever needing to use the crop factor. (24mm and down are "wide angle"; 50mm is "short telephoto" and in between is "normal".) –  mattdm May 8 '13 at 12:15
    
Since the old division for 35mm used to be 28mm and below was WA, 80mm and up was Telephoto, wouldn't the division for a crop sensor body be around 17-18mm and 50mm? –  Michael Clark May 8 '13 at 13:13
    
@MichaelClark It really comes down to where you want to draw the lines; I was using 35mm as the traditional beginning of wide angle (wikipedia agrees but without citation) but one can make an argument for 28mm too. On the other end, I think it's pretty reasonable to say that a lens stops being "normal" at somewhere around 60mm (again on 35mm full-frame), give or take a bit. –  mattdm May 9 '13 at 4:11

The crop factor applies to all lenses shot on camera with a smaller sensor. They look the same because 70mm is 70mm on both lenses, and they're both cropped in exactly the same way.

I think the answers to Is an EF 50mm f/1.4 the same as 50mm with an EF-S lens on a Canon 550D? should help.

Also check out my answer to What is Angle of View?, because the concepts are closely related, the explanation for angle of view explains crop factor as well

A smaller sensor is literally like taking a photo from a larger sensor and cropping it. This has a number of advantages in certain situations: if you crop and enlarge, as long as you still have the detail, that's more zoom "for free"; also, the center of lenses tend to have better image quality. So, in situations where you want a lot of reach and are likely to crop anyway, using a crop-sensor body is cheaper and possibly faster and has no real drawbacks. Some people argue that the smaller pixels on crop sensors even give an advantage here; I think that's open for debate, and in fact it is debated under Are full-frame cameras bad for sports photography? and Is crop-factor a bad thing?

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Thanks Matt, great summary. I'll read through all linked questions. –  Dennis G May 8 '13 at 8:12
    
+1 for the links. –  Michael Clark May 9 '13 at 3:35

Crop factor isn't a measure of a lens, it's a measure of a sensor. The lens use the same measure, which is actually the focal length of the lens, or the distance the light travels as it is brought to a single point of focus. The reason it doesn't take a lens that long is that a compound lens can move the light around within it. Crop factor is useful for comparing the effective focal length for different camera bodies with different crop factors. For example, your shot using a 70mm lens on a camera with a 1.6 crop factor would require a 112mm lens on a camera with a crop factor of 1.

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@mattdm - I think we're saying the same thing. I may have just phrased it poorly. 70mm is the same on both lenses. Even though an EF-S lens can only be used on a crop body, they measure it the same as they measure the other lenses. (ie, the Focal length it would be on a crop factor of 1.) I used the term equivalent because we're talking about a lens that can only be used on crop, but they measure it as what it would be on 35, thus 35mm equivalent. Let me know if my edit clarifies it. –  AJ Henderson May 7 '13 at 20:53
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Ah I see! The thing is, they measure focal length as it is, with no regard for crop factor whatsoever. –  mattdm May 7 '13 at 21:10
    
You say "...what the focal length will be on a crop factor of 1 or what it would be on a 35mm camera." This implies that the focal length is dependent upon the crop factor. It is not. The focal length is the distance over which initially collimated rays are brought to a focus. What would be correct when talking about FoV for a particular focal length would be to say, "...what the FoV will be on a crop factor of 1 or what it would be on a 35mm camera." –  Michael Clark May 8 '13 at 13:25
    
@MichaelClark - yeah, I was having a brain fart yesterday. Matt's last comment did it for me and I had a serious facepalm moment. I just missed that last bit of editing out the implication. Thanks for pointing it out. –  AJ Henderson May 8 '13 at 13:57

It is called a crop sensor camera, not a crop lens. The sensor is smaller, just as if you took the same image with a full frame camera and cropped it.

The lens does not know what kind of camera you are using. You could put a "crop sensor lens" on a APS-C, APS-H, FF or medium format camera, and it would still be the same. Put the OP's two lenses on each of these cameras, shoot at 70mm, and the lens still takes the same photo.

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Lenses don't take photos, they project virtual images. The size of the recording medium will affect the angle of view of the photo and photos taken from the same position with the same 70mm lens using cameras with APS-C, APS-H, FF, or MF will not be the same. –  Michael Clark May 8 '13 at 15:54
    
You are confusing things. From the same tripod location, the same 70mm lens will take the same image. Consider a photo of a person. Their head might take up all of an APS-C sensor, and only a very small part of a medium format image/sensor. On the medium format, it will probably be at least a 3/4 shot, due to the larger sensor. But the subject's head will be exactly the same size in all formats. –  Pat Farrell May 9 '13 at 2:37
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The virtual image projected will be the same size on the sensor/film, but the photo produced on prints using the same size paper made from each sensors image, or displayed on monitors with the same pixel pitch, will not be the same photo. The latter is what most people associate with the "size" of an image, and to suggest that the lens will produce the same photo regardless of the size of the recording medium is infinitely more confusing. –  Michael Clark May 9 '13 at 3:29

Easiest answer for the non technical: The lens does not provide the multiplication factor it is the sensor size of the camera. Setting both the lenses to 70mm are on the same size sensor (photo frame).

Projected onto a full size sensor both lenses would show similar results except the EF-S lens image may not give full sensor coverage (edges would be extremely poor/smaller image circle). NOTE EF-S lenses are not recommended for Full Frame incase they hit the mirror mechanism.

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