Earlier today, Frederick Van Johnson and I recorded a pilot for a new series from This Week in Photo. The series is tentatively called All About the Gear and the pilot episode is The Olympus OM-D vs Sony NEX 7 – Is Mirrorless More, or Less?
In the show I made the statement (at about the 8:00 mark) to the effect that the depth-of-field on a micro four-thirds (MFT) camera at a given aperture (say f/4) would be the equivalent of f/8 on a crop-sensor camera like the Sony NEX-7. What I should have said was that it would have been the equivalent of f/8 on a full-frame (FF) body. That’s what happens when you try to talk faster than your brain can operate.
In any case, in the YouTube comments, Jamie MacDonald challenged that statement, saying “f/1.8 is f/1.8 It is ‘sensor agnostic’ if you will.” Jamie is technically correct and I was taking a shortcut for the benefit of simplicity, for which I apologize. However this continues to be a much misunderstood issue, which I think deserves a more complete explanation.
Imagine you’re taking a picture of a tree with two cameras, both equipped with zoom lenses, from the same location. One is an MFT and the other is FF. Let’s assume both are set to an aperture of f/4. You compose your shot on the MFT camera and it turns out to require a focal length of 45mm. Now you compose the exact same shot on the FF camera. The tree is the same size (in terms of percentage of frame height) in both viewfinders. If you check the focal length on the FF camera, you’ll see it’s set to about 90mm. Same shot. Same aperture. Different focal lengths.
Now go home and look at both images. You’ll see that the image from the MFT camera has a much greater depth-of-field, while the one from the FF has much more boken, or blurring as you get away from the in-focus area of the image. Why, because of the difference in real focal length as opposed to equivalent focal length. (We say that 45mm on the MFT camera is the full-frame equivalent of 90mm because of the size of object in the frame from the same distance.)
As Jamie correctly points out, this is not actually due to the aperture, since it’s the same in both cases. But here’s the thing: If instead of shooting the FF image at f/4, you use f/8, now you would find the depth-of-field in the resulting images was nearly identical.
The superzoom on my big Nikons is 28-300mm. An equivalent superzoom on an MFT camera is 14-150mm. Those lens/camera combinations give me the same image-area range. I most-often shoot in aperture-priority mode, so with my FF camera I might decide to use f/11 to give me the desired depth-of-field for a particular shot. In order to get the same depth-of-field using the MFT camera, I need to set it to f/5.6.
So what does this mean in practice? When using lenses of equivalent focal lengths such as the kit zooms on each, to get the same depth-of-field as a full-frame camera, you need to use an aperture about two stops wider on an MFT camera and about one stop wider on an APS-C crop-sensor camera.
Hope that helps.
And please: give us feedback on the new show. Here, on the TWiP blog or on the YouTube channel.
Blogarithms 
Doug,
Thank you for the follow up! On the Youtube video I assumed this is what you were alluding to but didn’t want anyone else who may not be familiar w/ the µ4/3 system to be confused.
I’ve always been a fan of Mr. Van Jonson, and now I believe you have gained a new fan as well. I hope to see you on more Hangouts and podcasts.
Good day sir.
Jamie A. MacDonald
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Doug,
Just found your blog linked from your website and will subscribe as it has just the kind of information I find useful to my own work. Re; this post, I thank you for a clearly articulated explanation of how depth of field is relative to aperture AND focal length. Like so many things in photography we need a standard reference so we don’t end up comparing apples to oranges.
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Doug,
I read your “Sensor Size Affect Depth-of-Field” post. I understand the depth-of-field relationship. Could you clarify for me, would a micro four-thirds camera with a 50 mm equivalent (25 mm) set at f2.8 have the same ASA and speed settings of the same scene as a full frame camera at f2.8 50 mm lens? In other words an f2.8 lens is an f2.8 lens regardless of camera?
Allan
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You’re correct, Allan. f/2.8 will always be f/2.8. The amount of light transmitted (re exposure) and the depth of field don’t change as you switch sensor size. What changes is the field of view. So when you go from a full-frame sensor to one that has a crop factor of 2x, you get a cropped image. Still the same exposure and depth-of-field. (Note that you probably have, however, cropped off the worst of any distortion and any vignetting or other problems in the corners, so you might be able to get away with a lens of lower quality and still have a good image. But if you now want to get the same field of view as you did on the full-frame sensor, you need to switch to a lens with 1/2 the focal length. If you shoot at the same f/2.8 aperture as before, your depth of field will be greater than with the original lens. And because you’re now using a wider-angle lens,yYou have to open up to f/1.4 (if you can) to retain the depth-of-field you had with the longer lens on the full-frame body.
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