35mm on DX is equivalent to 35mm x 1.5 = 52.5mm on FX. Likewise, 50mm on DX is equivalent to 50mm x 1.5 = 75mm. So, which focal length works best depends on why you want a prime. If you are getting the prime for "classic" or head-and-shoulder portraits, 50mm will likely work better.
The reason is price to performance ratio. While the Canon 35mm f1.4 L II USM is a fantastic lens, the Canon 35mm f2 is $1,100 cheaper, offers outstanding image quality, and has image stabilization
Here too, let's look at the 50mm lens vs 35mm lens with obvious samples. To avoid interfering in a particularly quiet, intimate moment, take care to have a small f/1.8 or f/2 prime on your camera. In other situations, when you're not afraid to disturb someone and can easily get in on the action, it's the 35mm that will make viewers feel closer
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