Originally posted by bxf ..
You are in a situation where you want to take a photo, and you've determined that with your FF gear, you will use a 50mm lens at f1.8, ISO 100. Due to the nature of the subject, DOF is not an issue.
What lens and setting do I need in order to obtain the "same" (in practical terms) image with my APS-C camera. Do I need your theoretical f1.1 lens, or will a 31mm/35mm f1.8 do? If the latter, does it not mean that this constant obsession with equivalence is exaggerated, in the sense that it is not always applicable?
To get the same photo, you would need about a 33mm f/1.1 lens on aps-c. Note that the image would in most cases be just as 'good' if you shot it with a 35mm f/1.8 wide-open. Think of it this way: FF in a lot of cases just expands your options with DOF control.
Why would you want to shoot something like a 33mm f/1.1? One reason is it lets you completely obliterate a messy kitchen in the background while shooting from 'normal' distance
50mm @ f/1.8 == 33mm f/1.1 on aps-c
Or a messy neighbor garage area in the background of a short telephoto shot...
180mm @ f/2.8 == 120mm f/1.8 on aps-c
Or just allows you to very subtly remove a subject from it's environment
just a tad more, pull it out, highlight it, if there's a quiet moment to be seen amidst chaos:
180mm f/2.8 == 120mm f/1.8 on aps-c:
When it comes to aesthetics, sometimes a small difference can make an outsized impression. Artists will be the first to tell you that
It also allows you to get sharper at the plane of focus while maintaining an option for DOF control. For instance, you could shoot your 50mm at f/2.8, nearing the lens's probable sweet spot for sharpness, and get the same subject isolation you'd need to shoot a 35mm f/1.8 wide-open to get - and a 35mm is probably not going to be as sharp wide-open and may show more CA.
To your subsequent angry comments about equivalence ; 'equivalence' just describes a relationship between formats, so you can know what to expect if you're shooting a 50mm lens on micro 4/3, or aps-c, or FF, or Medium format. It's still a 50mm lens, but will behave much differently across formats, in FOV and DOF. 'equivalency' provides you a method to determine how it will perform, what it will show you on other formats. Getting mad at equivalence is like getting mad at a mathematical equation.
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