OMD E-M1 vs Sony A-7, What's the Difference?

Started Oct 23, 2013 | Discussions thread
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larsbc Forum Pro • Posts: 12,423
Re: increased DOF

Michael Kaufman wrote:

larsbc wrote:

I think you might have things backwards. Adding an ND filter will require you to open your aperture wider, thereby narrowing your DoF.

Think of the case where you want to take a shot at F4 to have sufficient DOF for your composition. Its so bright that you need a 1/8000 shutter speed. Your camera only goes to 1/2000, so you shoot a F2 and 1/2000 instead and have a narrower DOF then you want.

You're saying that f/4 @ 1/8000 will provide the right exposure.  But your camera's max. shutter speed is 1/2000.

Here's a listing of aperture/shutter speed combinations that would equal f/4 @ 1/8000 sec:

f/2    @ 1/32,000

f/2.8 @ 1/16,000

f/4    @ 1/8,000

f/5.6 @ 1/4,000

f/8    @ 1/2,000

Basically, the larger (numerically smaller) your aperture, the faster the shutter speed you need to use.  So if f/4 requires a shutter speed beyond your camera's abilities, using f/2 isn't going to improve things at all.

You need to use f/8 in order to use the camera's fastest shutter speed of 1/2000 sec.

You could add a 2 stop ND filter and go back to F4 and 1/2000, or if you had a smaller sensor, you could shoot at F2 and have sufficient DOF.

Since you've got the relationship between change in aperture vs. change in shutter speed backwards, the above point no longer has meaning.

I think that's right.

Have a look at this DIY exposure dial to see the relationship between aperture and shutter speed:

IMO the increased DoF argument applies more for low light scenarios.

It also works in low light. Sometimes its an advantage to be able to get more DOF at a wider F-stop.

Exactly what I was saying.

The full-framers' retort, though, is that they can shoot at higher ISOs which means they can still use smaller apertures to get the same DoF as the m43 camera shooting wide open.

I wasn't arguing one way or the other, I was just explaining why someone might be happy with more DOF.

What I meant by that is this: this advantage of more DoF at a wider aperture doesn't take into consideration the fact that the 135 format sensor typically has better high ISO performance than the micro four-thirds sensor.  So, while the m43 shooter might maxed out using f/1.4 @ ISO 3200, the 135 format shooter might be able to shoot the same scene using f/2.8 @ ISO 12,800.  This would result in similar DoF and image quality.

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