Crop sensors and depth of field

So a crop camera has wider depth of field?

Depends how you look at it. The way I look at it: “only because you will use it differently. If you use it to take the same picture, you’ll get the same depth of field”. But of course for that “same picture” you’ll need a different lens.

OK, this is complicated. So let me just show you by example.

Picture one: full frame 1Ds MkIII camera, 50mm lens at f/1.4.

MVWS0030-50mm-1.4

(You can click to see the large version).

Picture two: crop camera, a Canon 7D, with a 35mm lens also set to f/1.4. That 35mm lens on a crop camera is about the same as a 50mm lens on a full frame camera; i.e. enables me to be at the same point and yet get roughly the same picture. Now I see this:

IMG_1270-35mm-1.4-adjust+15

As you see, this gives you roughly the same depth of field. If there is any difference, it is minor. SO:

A 35mm lens on a crop camera gives you roughly the same picture as a 50mm lens on a full frame camera, and roughly the same depth of field.

A few notes, by the way:

  1. These results are consistent; not just “one randomly picked picture”.
  2. The DOF (depth of field, i.e. how much is sharp) is extremely narrow. Great care is needed when shooting at f/1.4!.
  3. I was aiming a single focus spot at the dot (near the 10) in all pictures (On the 7D, I was using the extra small focus spot).
  4. I have noticed that the Canon cameras (or is it the lenses?) focus too closely (they “front-focus”) when open at f/1.4. By f/2.8, this effect is either gone or too small to see clearly. In the last picture, I added an adjustment of +15.  This does not change the depth of field; it is just to make both pictures about the same.

Point 2 explains a few things to me, by the way. That’s part of the Canon-effect, more on which in the next few days; it’s why I have to adjust extra when wide open.

0 thoughts on “Crop sensors and depth of field

  1. Try this test:

    Take your 35mm lens and take a picture of a scenic. Then, standing in the same spot, take a longer lens say a 100mm and shoot various parts of the same scene at the same aperture[make it easy in yourself and shoot between f/2 – f/4] . Then go back to the image shot with the 35 and enlarge the same areas as shot with the 100 and compare DoF – should be quite similar [depending on your focusing accuracy with the 100].

  2. Won’t the image compression still be different between 35mm @ 1.6 crop factor vs. 50mm at FF? Isn’t that a matter of optics and not crop factor?

    IOW, wouldn’t a stand of trees shot at 300mm on 5DMk2 appear to be closer together than the same shot at 187.5mm on a 7D? Likewise, wouldn’t a portrait shot at 10mm on a 1.6 crop factor show more exaggerated facial features than at 16mm on a full frame?

  3. Well, John, if it is different, that difference is very minor: look at the images. If you can spot a difference in depth of field, you’re better at it than I am. DOF is determined majorly by distance to subject and by aperture, and of course those are identical in the two shots.

    Acytaully, it is the distance that causes the distortion in facial features in your example. widen angle means you are closer. It’s not the lens. So if I were taking that shot from the same spot, as I was in my example, then I would see the same things in the shot.

  4. Hi Michael. Your assessment that “DOF is determined majorly by distance to subject and by aperture” identifies just two of four factors involved. And “majorly”? Something is either a contributory factor or it isn’t.

    Depth of field is determined by a combination of four factors: focal length, circle of confusion, aperture, and camera-subject distance. If all four are identical then there is no difference in depth of field between full-frame and crop-sensor cameras fitted with identical lenses. Any claim to the contrary is not comparing like with like. It is no use comparing identical camera views because one of the determining factors must be changed in order to achieve the same view.

    Regards,
    Andy S

  5. True – but of course there is a good case that we should compare effective lens length since that is what we “work with”. A 1Ds with a 50mm lens is like a 7D with a 35mm lens. To the photographer, they are identical.

    And those two, from the same viewpoint, give the same picture with the same depth of field.

  6. Hey Michael!

    Thanks for clearing up depth of field… but I was wondering if the crop factor, or more importantly, size of the sensor, affects the sharpness of the picture??? I.e. is there much difference between a Canon 1.6 factor and a Nikon 1.5???

    Thanks.

  7. Hi!

    Nope – no real difference.Sharpness is affected by number of pixels, and by quality of lens. And by the resolving power of the lens, but that does not make much difference here I get very sharp pictures out of my 7D (1.6x) – as sharp as those out of my 1Ds MkIII (full frame).

  8. Thanks!

    I have one more question and it might be a bit off topic for this particular conversation… (and forgive me if this is a very basic type of question, I’m still very new at photography) but, what is the purpose of having a larger sensor if it doesn’t affect picture quality???

  9. Ah! No problem at all. A larger sensor results in a lower noise image. Also, larger sensor means wide angle lenses work like wide angle lenses. Those are the two huge reasons!

    Ad you’re doing fine, I would have thought. Isn’t photography fun?

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