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I stepped into macro photography with a 50 mm f1.8 lens and a passive extension tube. Since the tube has no electrical features and lens has no on-lens aperture control, I am left with extremely shallow depth of field, annoyingly shallow even for macro shots.

Using a passive extension tube, I am trying to put the subject in focus but objects appear to be out of focus on the resulting image.

Since the definition of an object being in focus changes with respect to the distance of the observer, the observer being my eye through the viewfinder or the CMOS sensor of the camera, I am wondering if it is possible for these two observers to be experiencing two differently focused images or is it just my inexperience with the tube that ruins my images? Might there be a focus difference between these two observers that can be ignored with normal lenses but becomes significant with such shallow depth of field?

And for the record my eyes are fine, I had them checked to a doctor :)

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First off, even at it's close focusing distance of 1.5', that 50mm is only going to have a quarter inch (6mm) depth of field. At anything under 1', you will have nothing in focus.

Now, to explain why some things are in focus: It sounds like you are having issues focusing because of the focusing screen in your DSLR. Modern DSLRs have a focusing screen intended to brighten the image you are viewing with slower lenses most commonly used in conjunction with AF - nearly everyone uses a kit lens and AF never venturing into MF.

The downside of the focusing screen is that when you get a wide aperture and resulting shallow depth of field, you are actually seeing far more in focus through the viewfinder than is actually in focus on the sensor. This is far more problematic for you because you are left with f/1.8 at close distances where the depth of field is, for all intents and purposes, non-existent.

Also, it sounds like you are misunderstanding depth of field a little bit. You mention that the depth of field is too shallow, even for macro. As you get closer to your subject, depth of field decreases. For macro work, you actually want to stop down the aperture so you increase depth of field as much as you possibly can to keep your entire subject in focus.

Even if you did change the focusing screen to a split prism screen, which would accurately show you what is in focus at shallower depths of field, you are still going to have a massively frustrating time getting anything in focus with that lens stuck at f/1.8 at close distances.

Again -- Even at it's close focusing distance of 1.5', that 50mm is only going to have a quarter inch (6mm) depth of field. At anything under 1', you will have nothing in focus.

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Do you have a source for your assertion that more will be in focus in the viewfinder? I have never heard that before. Seeing as the image coming through the lens is what hits the sensor when the mirror flips up, I can't see how it can be the case, but I will stand ready to be corrected. –  ElendilTheTall Feb 17 '13 at 16:48
4  
@ElendilTheTall See the answers to photo.stackexchange.com/questions/4175/… –  mattdm Feb 17 '13 at 18:01
    
Today's viewfinders bring in light from only an f/8 or so fraction of the lens rear element view. Get an old lens that can manually stop down and see what you get at all aperture settings. Surprise, fast (wide aperture) lens don't come through the viewfinder in the expected way, as mattdm's link describes. –  Skaperen Feb 18 '13 at 1:19

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