Depth of field question- Crop / Full Frame

Started 8 months ago | Discussions thread
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As easy as 1, 2, 3!

Pk8887 wrote:

I'm looking for a crop set camera for sports to lose some weight and gain and gain some length but am confused about depth of field and would like some help figuring out the following.

I like the background separation look I can get at 100mm f2.8 on my full frame 5dmk iii.

If I were to buy a crop camera and the lens was f2.8 from what I understood the background separation wouldnt be as obvious and the apparent depth of field wouldn't be as shallow (similar to f4 or f4.5 as i have read) due to the smaller sensor.

BUT as the crop would be a longer focal length in reality (1.5 x 100mm) and as focal length impacts the depth of field would it actually be similar?

In practical sense if I'm using a 2.8 lens on crop camera will I be able to achieve a similar shallow depth of field/backgroudn separation compared to a full frame equivalent at the same 'looking' focal lengths.

I may have just confused myself!

There are three ways to look at the situation:

  1. Shooting a scene from the same position with 100mm f/2.8 on both FF and 1.6x.
  2. Backing up with APS-C or getting closer with FF using 100mm f/2.8 on both to achieve the same framing (but necessarily different perspective).
  3. Shooting the same scene from the same position with the same framing (meaning that we use a 1.6x the focal length on FF as we use on 1.6x).

OK, for scenario #1: 100mm f/2.8 on 1.6x will give a 1.6x tighter framing and a 1.6x more shallow DOF than 100mm f/2.8 on FF.

For scenario #2: FF will have a 1.6x more shallow DOF whether you back up with crop or move closer with FF when using 100mm f/2.8 on both.

For scenario #3: To get the same framing from the same position as 100mm on FF (which will also result in the same perspective), you'd use 62.5mm on 1.6x.  In this situation, FF will have a 1.6x more shallow DOF than 1.6x. Alternatively, 100mm f/4.5 on FF will have the same DOF as 62.5mm f/2.8 on 1.6x or 100mm f/2.8 on FF will have the same DOF as 62.5mm f/1.8 on 1.6x.

Hope that helps!

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