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I have two canon kit lenses which came with my 1100D. While experimenting casually with my lenses, I came across something that I find peculiar. When I am focused at an object at a certain distance on my 18-55mm lens set at 55mm, I get a set of shallow depths-of-field as I move the focus ring. On the other hand, if I use the 55-250mm lens at the same distance and same focal length of 55mm (thus viewing the object at the same scale), then my depths-of-field are much wider and the amount of defocus in the foreground and background is lesser than the 18-55mm lens.

I imagine this must be because of the flexible focus of the lenses and the fact that one lens is at it's telephoto end while the other is at it's wide-angle end. I would like to understand this better though and would be glad if someone could explain the phenomenon or point me to the correct reference.

Thanks in anticipation!

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    Could you add sample photos of the same scene from both lenses? – Philip Kendall Sep 21 '15 at 11:11
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    You say you see this when shooting at the same distance and same focal length, but what apertures are you using? 55mm shot at f11 will have more DOF at f5.6, regardless of which lens you use. – Dan Wolfgang Sep 21 '15 at 15:36
  • But both lenses will be wide open during focusing. The aperture is only stopped down in the instant between when the shutter button is pressed and the shutter curtain begins to open. When seen through the viewfinder the 18-55 will be at f/5.6 at 55mm, the 55-250 will be at f/4 at 55mm. – Michael C Sep 21 '15 at 23:08
  • Are you observing this while looking through the viewfinder and moving the focus ring? Or looking at the LCD screen in Live View? Or at the resulting photos you have taken? – Michael C Sep 21 '15 at 23:16
  • Thanks for your answers. My observations were initially based on what I was seeing on live-view. I took some pictures to verify if this was indeed happening. It seems like both lenses give nearly the same defocus, except the difference based on the aperture size. I captured a focal stack using MagicLantern, so as to ensure that the sensor movement from one image to the next is fixed for both setups, and I have shown the focused and defocused images (7 steps apart) for both lenses here: dropbox.com/sh/vfq2qgwb8veiby2/AACljlQJ9O2npfSuvnz0Ei3ya?dl=0 – pkrskr Sep 22 '15 at 10:13
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With any DSLR, you are not observing the "true" aperture DOF effects through the viewfinder, due to the focusing screen in the camera. As David Kilpatrick notes in this article, the combined effects of the focusing screen and your viewfinder optics reduce the viewfinder-path max aperture to ƒ/4 – ƒ/6.3 (depending on the camera). The focusing screen in DSLRs are designed to keep the image as bright as possible by bending and slightly scattering light, but not dead-stopping light like ground glass focusing screens used to do. As such, while changing the aperture setting of the lens does affect the apparent brightness in the viewfinder, the apparent brightness is not affected as much as would be indicated by the change in aperture. That is, a 1-stop aperture change doesn't affect the viewfinder's apparent brightness by double/half.

As far as why your wide-open 55-250mm at (55mm, ƒ/4.0) appears to have deeper DOF than your 18-55mm at (55mm, ƒ/5.6), I can not answer why.

The solution recommended by Kilpatrick: use the live view. The optical path does not include the focusing screen when using live view.

Edit: See also these similar/related photo.se questions:

  • Thanks for your answer and references. After looking at captured images, and not only through the view finder and live-view, I find nearly identical defocus, except for the difference caused by slightly differing f-numbers. Please find images and more details in comments of the original question. Thanks for your time. – pkrskr Sep 22 '15 at 10:21
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I guess you have some variant of the Canon EF-S 18-55mm f/3.5-5.6 and Canon EF-S 55-250mm 4.0-5.6.

That means the widest aperture on the 18-55 is f/5.6 at 55mm (its longest zoom setting), while the tele zoom has f/4 at the same focal length (its widest setting).

So you (can) have shallower DoF with the 55-250, simply because it is faster at this length.

  • But the OP is claiming a deeper DoF with the 55-250: "the amount of defocus in the foreground and background is lesser than the 18-55mm lens." (my emphasis) – Philip Kendall Sep 21 '15 at 11:58
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    i've just seen that you describe the opposite phenomenon, that seems unexplainable to me. it would help if you looked up the respective apertures used in both cases. – ths Sep 21 '15 at 11:59
  • Apertures used in each case is irrelevant when discussing what one sees through the viewfinder. The lens is wide open regardless of aperture selected until the shutter button is pressed all the way and the mirror is moving up to allow light to strike the sensor when the shutter curtains open. The 1100D has no dedicated DoF preview button. – Michael C Sep 21 '15 at 23:14
  • they are talking about wide-open apertures... In 18-55mm case it is f/5.6, in other f/4 – Oct18 is day of silence on SE Sep 22 '15 at 6:03
  • Thanks for your answer. It seems like the defocus is nearly the same after having analyzed captured images. Please find more details in the comments of the original question. Thanks for your time. – pkrskr Sep 22 '15 at 10:19

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