akashyap
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grahamgibson wrote:
The 24GM cropped 1.5x will render like a 36/2.1.
I think most of the difference you are referring to is coming down to degree of focus (DOF). Crop mode and/or sensor size doesn't do anything to change the DOF. It's a matter of the focal length, f-stop and distance to subject. You can figure out the "blur potential" or compare effective DOF equivalency by dividing the focal length by the aperture of any lens. At a given distance from a subject, two lenses with the same number will have the same DOF. Here are some examples for various lenses:
24/1.4 = 17.1
24/1.8 = 13.3
28/2 = 14
35/1.4 = 25
35/2 = 17.5
35/2.8 = 12.5
40/2 = 20
The higher the number, the narrower DOF you get (and more background blur). Crop mode doesn't change that at all. You multiply both the focal length and the aperture by the crop factor (APS-C being 1.5x), but the blur potential stays the same. So the Zony 24/1.8 will be the same as a 36mm f2.7 lens.
A couple of caveats to this when you're looking at actual images:
- Lens manufacturers will round focal lengths sometimes, so it's not always exact.
- Lenses do have differing out-of-focus/bokeh renderings, but it doesn't affect the amount of blur, just the look.
- Field curvature in the focal plane can cause two like lenses to render the background a bit differently towards the edges.
Some examples. The APS-C 24/1.8, FE 28/2, and FE 35/2.8 all look pretty similar cropped to the same framing. Here's a shot I took with the Samyang 35/2.8 and Sony 28/2 (cropped):
https://farm1.staticflickr.com/894/41291620692_e53150e6b2_h.jpg
Also, another comparison you might find interesting:
https://www.talkemount.com/threads/sony-zeiss-alpha-e-mount-35mm-equivalent-shootout-distagon-biogon-and-two-sonnars-compared.13393/
Fun fact--it works the other way too. This is the math behind the look of bokeh panoramas (aka brenizer). Stitching together shots from a 135/1.8 (=75) can create the look of an imaginary 35mm f0.47 lens, for example....Show more →
Thanks for this post! It seems to give a quantitative way of understanding the effect I was noticing: 35/1.4 and 35/2 renders differently than a cropped 24/1.8 and 35/2.8, and cropped a 24/1.4 might give me the 35/2 effect I was looking for. The examples are also very good.
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