Beginner’s Corner – Crop Sensors

By Charles MacPherson– 

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So what in the world is a “crop sensor” anyway?

There are two kinds of sensors in DSLRs – full frame and crop.

Full-frame sensors are exactly the size of 35mm film (24mm X 36mm) Crop sensors are smaller and come in different sizes.

The reason that the sensor size is important is that it changes the way a lens behaves.  To explain that, we first have to define a couple of things about lenses, specifically their focal length.

The focal length of DSLR lenses are spoken of in millimeters.  You’ll hear about a 50mm lens, a 600mm lens, a 24mm lens and many others.  That “mm” specification tells us how much the lens magnifies the scene we’re looking at.

50-55mm is considered “normal”.  That means that looking at the scene through the camera gives you the same view as you see with your eye.

Anything above that gets into the telephoto category.  So a 200mm lens is like looking through a telescope.  a 600mm lens is like looking through a bigger telescope.

Anything less than 50mm is like looking through the wrong end of the binoculars – everything looks further away, and shows you a wider angle of view.  So looking through a 35mm lens makes things look farther away than it is.  A 24mm lens, even more so.

Now with that out of the way, let’s talk about how the size of the sensor figures into all of this.

Crop sensor cameras have a “crop factor”.  Most Nikons are 1.5.  Most Canons are 1.6, except for some of the 1D series, which are 1.3.

These sensors have the effect of multiplying the focal length by the crop factor, thereby increasing their effective focal length.

Here’s what the difference looks like:

Full Frame - Canon 5D MkII
Full Frame.  Canon 5D MkII.
600mm lens, 600mm effective focal length

1.3 Crop Factor -Canon 1D MkIV
1.3 Crop factor.  Canon 1D MkIV.
600mm lens, 780mm effective focal length.

1.6 Crop factor - Canon 7D
1.6 Crop factor.  Canon 7D
600mm lens, 960mm effective focal length

The crop factor works in your favor when you want a longer focal length.  For example, in wildlife and sports photography.

But it works against you when you want the widest possible focal length, as you might for landscapes, by making your nice wide-angle into less of one.  For example, my Canon 16-35mm L II zoom acts like a 25.6mm-56mm on my Canon 7D camera (1.6 crop factor).

In general, full-frame sensors exhibit lower digital noise and will help you to create a better shallow depth-of-field.  Cameras with crop sensors cost less.  Those – and the effect on focal length are the facts to keep in mind when you’re buying or using a DSLR.

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9 Comments

  1. The article regarding a cropped sensor is misleading. Although many people believe that multiplying the sensor crop factor does what your article suggests, it simply is not true. In fact the crop sensor factor is a multiplier used to multiply the diagonal of the sensor to achieve a full-frame sensor. Another words, if you have a 1.3 sensor, then when you multiply the length of the diagonal of that sensor by 1.3, the number you get would the diagonal of a full-frame sensor. This is why as the crop sensor value increases, the size of the sensor decreases. It has nothing whatsoever to do with multiplying the telescopic value of the lens to see a new telescopic value. In fact, it really means that your lens, focused on the same spot in an image, cannot possibly obtain as much image at the telescopic value of the lens as it would on a full-frame camera. If you think about the logic here, you will realize how facetious it is to use the value to correct for the telescopic value of the lens.
    I have both a full-frame camera and a 1.3x sensor camera (5D Mark III and 1D Mark IV, respectively) and compared images with the same lens on both cameras, back-to-back. You just get less image, since the sensor is smaller on the 1D, than you do on the full-frame camera, 5D.
    This concept of multiplying telescopic lens values has been around for some time. Unfortunately, it is absurd.

  2. I enjoyed reading your description of full frame vs cropped. I somewhat grasped it but understand it much better now. Glad you used a 7D as an example as that is what I use.

  3. Robert,

    Thank you for your feedback. I’ve shared them with the author of the article, and will post his thoughts when he replies.

  4. I tend to agree with Robert. The lens itself doesn’t change its focal length, but the potential picture area is reduced. Further, the actual image is not the same when using, for example, a full frame sensor with a 200mm lens and a 133mm lens on a 1.5x sensor camera, creating a seemingly 200mm lens image. Distant objects appear more compressed with higher telephoto magnifications than with fewer millimeters, so the inherent lens characteristics of a 200mm vs. a 133mm lens is different. A very long time ago, one of the photography magazines explained this in great detail, before there ever was an issue of sensor size. The author was explaining the difference between an 8×10 print using a full frame negative enlarging from a telephoto picture and enlarging the same area from the negative made from a wide angle lens.

  5. Technicalities aside, the statement in the article that: “In general, full-frame sensors exhibit lower digital noise and will help you to create a better shallow depth-of-field” should probably have read …”and will help you create better depth of field” not “a better shallow depth of field.” Using a 50mm lens on a crop sensor with a 1.5 multiplication factor will give you less apparent depth of field than if it were used on a camera with a full-frame sensor. Used full-frame, its 50mm depth of field will be better (greater) than the same lens which becomes the equivalent of 75mm on the smaller crop sensor. The longer the focal length, the less depth of field. The shorter the focal length, the more depth of field.

  6. Robert is correct, and the article is wrong, or at least misleading. The crop factor affects the field of view. A larger crop factor means a narrower field of view, not increased power of the lens.

  7. Robert,

    Interesting comments, not sure I follow your logic. It seems that you are forgetting the pixel size of the cameras in your argument. If I shoot a 600mm with a Canon 5D MII with approximately 20mp and a 600mm with a Canon 7D with approximately 18mp, I get “an apparent increase in magnification” by virtue of the number of megapixels that provide me a good level of detail on my 7D. In essence, if I cropped my image from my Canon 5D MII to equal the field of view (which the author hinted at with the discussion of the affect of lens on a crop camera) of my Canon 7D, I would be left with an image of approximately 12.5mp vice the 18mp of the 7D. I simply would not have the same pixel density as I do on the 7D. Granted, it is not true magnification, but rather is a difference in FOV. But the end result is essentially the same. Not to mention that on a crop sensor camera, I don’t have to worry about corner resolution as much as I do on a full frame camera such as my 5D MII. Your argument would be more correct if the number of pixels on the 7D were more than a third less than that of a full frame camera. Of course, all bets are off if you are using a Nikon D800 with 36mp. ;-) I guess this is why most of the wildlife, bird, and sports guys I know use a crop sensor, it gets them “closer” to the action than does a full frame camera, even though a full frame typically has more dynamic range than a crop sensor.

    Peace,

    Dave Cote

  8. Thank you for opening the discussion of Sensor size and lens focal length. I have two camera, one is 7360 x 4912 pixels, the other 6000 x 4000. These are the number of pixels in my images. When I use the same lens on either camera, the size of the projection on the sensor is the same. So which camera gives me the better image? Let’s assume that the two sensors are the same size. No brainer, the more pixels, the better resolution. What happens when the sensors are different sizes? If the larger sensor has the smaller number of pixels, again a no brainer, the smaller sensor with more pixels will again give a better resolution, assuming that the image can fit on the smaller sensor. Now what happens when you have more pixels on the larger sensor? You have to look at pixel density. Dividing the number of pixels by the length, gives 255 pixels per millimeter on one camera and 205 pixels per millimeter on the other.
    With the same lens, you will get a more pixels per millimeter with the smaller sensor. So why do I have the two cameras? Quite often I shoot birds at a distance. I can not fill the screen with the image and I shall be cropping the image later. I want the highest pixel density,(the smaller sensor in tis case.) Sometimes I can fill the sensor with the image and I want the most pixels I can get of the image, (the larger, less dense sensor.)

    Nikon D800 – great for close up. Nikon D7100 great for telephoto work.

  9. As the article’s author, I’m surprised that there is so much technical nit-picking over such a simple concept.

    First, please notice that the title of the article starts with “Beginner’s Corner”.

    These are a series of articles I’ve written to help those who are just starting out in photography, with the intent to de-mystify some of the technical terms and concepts they might like to have explained.

    Many, like this one, are answers to questions folks submit through my website.

    This purpose of this article is simply to explain to beginners the difference in sensor sizes and the effect is has on the image, which I believe it did quite clearly.

    It’s not meant to be an exhaustive thesis on the electromechanical design of digital sensors.

    To clarify, no, a crop does not multiply the focal length. It has the EFFECT of multiplying the focal length, which is precisely what I wrote in the article.

    I think it’s more useful to concentrate less on physics, math and one-upsmanship and more on creating great images.

    Sound good?

    - Charlie MacPherson
    http://www.TheAmazingImage.com

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