How different is background blur between a 30mm f/1.4 and 35mm f/2 on APS-C for portraits?
Asked 1/25/2015
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I’m choosing between the Sigma 30mm f/1.4 and Canon 35mm f/2 IS for a Canon APS-C DSLR. For wider environmental portraits outdoors, typically from around 2m away, I want to know whether either lens gives noticeably stronger background blur when framing the subject similarly. I understand the focal lengths differ slightly, so distance or cropping may need to change to match composition. As a secondary question, how useful is image stabilization on the 35mm f/2 IS for low-light indoor or night shooting?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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I don't have these lenses to actually compare, but by the math, it works out to be virtually identical (presuming you accept the slightly different framing). Using an online depth-of-field calculator, it works out like this:
close limit far limit total DoF
30mm f/1.4 1.89m 2.12m 0.24m
35mm f/2.0 1.89m 2.13m 0.24m
Of course, if you change distance to match framing, a lot of things change. If, instead, you crop the 30mm image and then scale up, it's exactly as if that were a 35mm lens with circle of confusion correspondingly scaled, which works out to a tiny bit less DoF in theory, but it's so marginal it's going to be washed out in real world factors.
Basically, this isn't a big deciding factor between these lenses.
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
11y ago
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For this use, depth of field is essentially the same when framing is kept very similar. One comparison given was:
- 30mm at f/1.4: about 1.89m to 2.12m in focus
- 35mm at f/2: about 1.89m to 2.13m in focus
So total DoF is effectively identical in practice.
What changes more is background blur rendering, not the basic DoF number. Blur strength depends strongly on subject/background distances and also on entrance pupil size (roughly focal length divided by f-number). By that measure:
- 30mm f/1.4 ≈ 21.4mm entrance pupil
- 35mm f/2 ≈ 17.5mm entrance pupil
That suggests the 30mm f/1.4 can produce somewhat stronger blur potential, especially if you move closer, but the difference is not dramatic and real-world scene geometry will matter a lot more.
So background blur alone is probably not a strong reason to choose one over the other.
As for stabilization: IS helps for low-light static subjects by letting you hand-hold at slower shutter speeds, but it does not freeze subject motion. A wider aperture helps both exposure and autofocus, and can be more useful if people are moving.
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AI11y ago
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