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I am trying to increase the depth of field on my shots. Since a smaller sensor size could get me a wider DoF, I wonder if reducing image size (keeping same aspect ratio) is equivalent to reducing sensor size for this matter.

How does image size reduction actually work? By exposing a smaller area of your sensor? Less pixels? Using the whole sensor and reducing afterwards according to your image size settings?

And finally, how could this affect depth of field?

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Smaller image may mean two very different things.

If you choose Large, Medium, or Small JPG output, it is always using the full sensor size, and then may have to resample smaller (to be fewer pixels), which has less detail remaining, but it can decrease the bad effect of digital noise (simulating larger pixels).

If you resample smaller in a photo editor, you see exactly the same result. Resampling is resampling.

If you choose a crop factor, to DX or APS or a 8x10 crop, it is actually cropping the larger image, and is the same effect as actually choosing the camera with the smaller sensor in the first place. (except you will also have fewer pixels now, 24 mp cropped to DX or APS only has about 10 mp left).

If you crop later in a photo editor, you see exactly the same result. Cropping is cropping.

The same lens used at the same distance on the larger full frame sensor, or on the smaller cropped sensor, of course projects exactly the same image onto either sensor. However, the smaller cropped sensor image is smaller in two ways. 1) it has to be enlarged more later to view it the same apparent size. And 2) even so, it still shows a smaller cropped image view of the scene (it is cropped).

We perceive this crop as a telephoto effect, and we say the (Same) lens has a "Effective" focal length longer by the crop factor (because it gives the same view as a longer lens on the larger sensor). Note that zooming in a photo editor shows exactly the same telephoto effect. It is simply due to enlarging the smaller cropped image more.

To see the same (uncropped) view, with the same lens, the cropped sensor has to stand back farther (at a distance multiplied by the crop factor). Then it sees the same view (which still has to be enlarged more). But the greater distance gives more apparent depth of field (at same focal length of the same lens), distance being one factor of DOF.

But depth of field is judged at the enlarged viewed size which we see, so the greater enlargement offsets any perceived DOF increase. In all actuality, it is just a smaller cropped image viewed after enlarging it more. The only difference that exists is the smaller cropped sensor size.

  • Thank you, the very first paragraph answers my question. Reducing JPG output (no compression) uses the data from the whole sensor, so no difference at all related to DoF. Marking yours as answer due to be the first; @michael-clark 's answer also is very helpful. Thank you all very much, also for grammar correction too :) – rondador Sep 8 '15 at 8:28
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How does image size reduction actually work? By exposing a smaller area of your sensor? Less pixels? Using the whole sensor and reducing afterwards according to your image size settings?

It can work either way, depending on how your camera or editing application is told to do it.

  • When you tell a camera to create a Large, Medium, or Small JPEG, it uses the data from the entire sensor to create a file with a specific resolution of fewer total pixels by combining information from multiple pixels on the sensor. Resizing an image on your computer does the same thing.
  • When you choose higher JPEG compression, it reduces the size of the file by combining the digital description of more elements in the photo that are almost the same. Unless you need the space of the smaller files on your memory card, it is usually best to shoot with the highest quality setting (least amount of compression) in camera and then decide later just how far you can compress a specific image before the lower quality is perceivable.
  • When you choose to crop, it uses the information from only a specific part of the sensor and usually magnifies it by a higher factor to give the same display size. Some cameras can do this as you take a photo, but this is usually better done witha computer based application unless you need to save space on your memory card.

I am trying to increase the depth of field on my shots.

The only real way to do that is to use a narrower aperture (one with a higher f-number).

Be aware that when you crop, you are reducing the field of view. So you either need to use a shorter focal length lens or a longer shooting distance to get the same framing as you would have gotten without cropping. It is actually this shorter focal length and/or longer shooting distance required to get the same framing using a smaller sensor area that increases the depth of field when using the same aperture. If you crop only, without compensating for the framing, you will wind up decreasing depth of field because as you magnify the smaller image to the same viewing size, you also magnify any blur in the image. Some blur circles that looked like points are now larger and can be seen to be blurred.

In a way, depth-of-field is an illusion. There is only one plane of focus. Everything in front of or behind the point of focus is out of focus to one degree or another. What we call DoF is the area where things look, to our eyes, like they are in focus. This is based on the ability of the human eye to resolve certain minute differences at a particular distance. If the slightly out-of-focus blur is smaller than our eye's capability to resolve the detail then it appears to be in focus. When you magnify a portion of an image by making it larger or moving closer to it you allow your eye to see details that before were too close together to be seen by your eyes as separate pieces of the image.

Since things are gradually blurrier the further they are from the point of focus, as you gradually magnify the image the perceived depth of field gets narrower as the near and far points where your eyes can resolve fine details moves closer to the focal plane.

  • A very enlightening explanation. Much appreciated. I already closed the aperture all the way down (even increasing ISO), while playing with focal length and distance to the subject in order to keep the framing; just trying to figure out any other factor to increase DoF. Thank you very much. – rondador Sep 8 '15 at 8:42
  • Just remember that at smaller apertures, diffraction causes a general decrease in sharpness. – Michael C Sep 8 '15 at 11:11
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Without worrying about how the camera does it (I've seen both) just cropping the image afterwards is what you are asking.

The DOF is due to the physical pupil size of the lens, which is proportionally smaller to match the sensor. Cropping the sensor won't change that: it would be more like using a 0.5× adaptor on the lens and then cropping, to get the effect you seek.

The direct answer is to use a smaller aperture: higher f-stop numbers. But then you wish for a larger sensor to reduce the appearance of diffraction; but then you need a shorter focal length tomget the same field of view and that means the same pupil (physical) size equates to a lower f-stop and your DOF decreases.

Using a small sensor, chosen field of view, the physical blurriness of DOF is scaled up by same factor but that's trumped by the improved focusing from the smaller lens. But you have higher noise and generally poor performance from lack of light gathering.

Drawings would help. Everything is interrelated and you can't change just one effect in isolation.

  • Rather than physical pupil size, it is actually entrance pupil size. That is, the apparent size of the aperture as viewed through the front of the lens. – Michael C Sep 8 '15 at 3:40
  • I mean it's related to the actual measurement, not the relative size that f-stop denotes. I think it's more subtle than the apparent (magnified) size, since the glass must collect light from different positions, so it's further limited by the front glass size to catch it. IAC a sketch that shows a single abstract "group" doesn't make any distinction. If I get around to drawing something I'll run a draft by you first? – JDługosz Sep 8 '15 at 4:41
  • Technically, the entrance pupil is the apparent size in terms of surface area of the portion of the front of the lens upon which colimated light perpendicular to the image plane striking the front of the lens is allowed to pass through the aperture. – Michael C Sep 8 '15 at 5:36
  • So holding a caliper up to the front glass and eyeing how the hole appears (carfully sighting straight down the barrel) is the "physical" size of the funnel capturing incoming light. The leqf mechanism burried inside the guts may be a different size because it's seen through the front lenses. – JDługosz Sep 8 '15 at 5:55
  • Pretty much. To be exact you would need to sight straight down directly over one side and then straight down directly over the other. Using a caliper to measure diameter also assumes the aperture is perfectly round (it usually isn't). – Michael C Sep 8 '15 at 11:00

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