The viewfinder is a lie. There, I said it. Unpopular statement, I know. Especially since all a viewfinder does at one level is bounces the picture the sensor will see up into a little tunnel, but despite that, the viewfinder is a lie. Or, at least, its depth-of-field is.
There’s a lot of humbug these days about lenses with focus issues (front focus / back focus, what have you). This is the first half of a set of posts I have about things to check before you pull a chicken little and run to avoid the falling sky.
Here’s the deal: To better aid in manual focusing, most DSLRs use a micro-matte etched focusing screen. These are very good at enhancing micro-contrast and such things that we humans use to determine whether something is in focus.
The problem is, they often muck a bit with depth of field. I can’t speak for every system or every model, but, those of you with live view that allows for off-the-sensor magnification can test this yourselves. I first found it when playing with my old 50mm f1.8 OM lens, which has a shallower depth of field then my E-system was optimized for. I was looking through the viewfinder, as I prefer to do, and was a bit disappointed there wasn’t more bokeh happening. Snapped the shot and–woah!–the picture had a completely different bokeh than my viewfinder had showed me. I was confused about this for a while, and was mucking about with things, so I flipped to LV and zoomed in to 7x for some super-precise manual focusing and I noticed the picture in LV looked just like the ones the camera was actually taking. Then I remembered the matte focus screen and had an “aha!” moment.
The moral here is, sometimes the matte screen shows a greater depth of field than the camera is actually going to expose. So, while you’re looking through your viewfinder, something might look in great focus but you fire the shot and your focus was off. There’re chance, if you’re experiencing this wide-open only, that your camera picked a point for focus (or even your eye, if you manual focus) that fell within the matte screen’s depth of field, but was in a different spot than you had expected in the actual depth of field. Trust me, I’ve done it.
Next time: Auto-focus and the imaginary plane