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I would like to buy a wide-aperture lens for an APS-C DSLR (Canon EOS 500D, crop factor 1.6x) with equivalent standard focal length. However, I was unable to decide between Sigma 30mm f/1.4 and Canon EF 35mm f/2 IS. The Canon is surely optically better, even has an image stabilizer and can be used on full-frame cameras if I decide to buy one in the future.

But I am mainly concerned with the background blur in terms of a rather wider portrait in exterior (forest, street), shot from 2 meters or a little less. The background blur is very similar in close-up images but I was unable to find a good comparison shot over longer distance (with deep background).

Would someone please be able to take two photos (at f/1.4 and f/2) similar to the portraits described with respect to the focal difference (so that the object in focus has the same magnification, what should be e.g. at 2 meters from 30mm and 2 1/3 for 35mm - potentially with a cropped photo from a 30mm lens)? I assume that with the 30mm lens, I could always step forwards and focus a bit closer to make the background more blurred. Unfortunately, I only own the EF-S 17-85mm f/4-5.6 whith which I can't simulate even crops from such wide-aperture lenses. I am totally aware that portraits are not a domain of such short focal length lenses but the reason I am buying one of there is that I am often limited by space. The possibility to shot nice portraits with it would come as an extra value to me.

As a side note for anyone knowing, how much necessary is the image stabilization in low-light conditions like indoor or even night photography?

Thank you.

Kyselejsyreček
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2 Answers2

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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.

mattdm
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    Because they're so similar, I doubt anyone will have both lenses. However, if someone does, I'll happily delete my theoretical answer in favor of a practical example. :) – mattdm Jan 26 '15 at 03:28
  • When you match the framing by moving the 35mm further away to compensate for the narrower FOV total DoF changes to 0.32m – Matt Grum Jan 26 '15 at 11:46
  • Yes, I've tried these computations myself. I didn't think the lenses were so similar since I couldn't find shallow-DOF photos from 35mm on pixelpeeper.com similar to those from 30mm. I was assuming that I could always step forwards and focus closer with the 30mm to create a shallower DOF, instead of cropping the image and losing the wider range. Nor did I expect someone would have exactly these lenses but with a 30mm or a similar lens, one could simulate the comparison saying that some of the peripherals in the farther photo would be cropped. Anyway, thank you for your answer. – Kyselejsyreček Jan 26 '15 at 12:09
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This is a perfect example of the term "depth of field" being misused. You don't actually want shallow depth of field for portrait shots - it's annoying and can make things difficult if you have more than one subject.

What you want is subject isolation with a nice blurred background. Mattdm has demonstrated the depth of field is neigh on identical with these two lenses (though when you alter the distance for equal framing you do get a difference).

However the amount of background blur is different. Background blur is dependent on distance but is also proportional to the size of the entrance pupil (the image of the aperture stop viewed through the front of the lens).

The entrance pupil diamater can be determined by dividing the focal length by the f-number. So for the 30mm f/1.4 the entrance pupil is 21.4mm and the 35mm f/2.0 entrance pupil is 17.5mm. The 30mm lens will produce more blur, but there's not a lot in it.

Unless you're limited by space, you can actually get much more background blur using a longer focal length, such as a 50mm or 85mm lens.

Matt Grum
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  • Thank you. I was considering lenses with longer focal lengths too, but since being limited by space is often my case, they'll probably be one of my future choices. – Kyselejsyreček Jan 26 '15 at 12:24
  • The wide aperture just comes as an extra property to the 30mm and 35mm lenses and I only wanted to see how they are likely to perform with wide portrait photography since this was the reason why I originally wanted to buy the cheaper non-stabilized 30mm Sigma. – Kyselejsyreček Jan 26 '15 at 12:28