Crop Factor: Why you multiply the aperture by the crop factor when comparing lenses
Why Photography Stack Exchange recommends it
Photography Stack Exchange commenters and answerers link it when explaining that crop sensors and full frame cameras give equivalent images once aperture is scaled by crop factor, not just focal length.
Recommended by 2 people across 3 different months, 2015 to 2019.
What it is
A short explainer from Tony and Chelsea Northrup on crop factor, showing why you multiply both focal length and aperture by the crop factor when comparing lenses across different sensor sizes. It demonstrates the effect on depth of field and noise with side-by-side examples.
What people said
Recommends Goat · 2016-11 · Photography Stack Exchange · on “Large Sensor vs Large Aperture”
The fact is, it makes no difference as far as DoF, images noise etc. goes provide you use crop factor correctly. Small and big sensors are made of the same waffle, the only thing that changes is the camera settings. Read this: [LINK] Or see this: [this video]
Recommends confetti · 2018-10 · Photography Stack Exchange · on “Why don't my Canon EOS 600D vs 5D Mark II comparisons meet my expectations?”
When using the same lens on full-frame vs crop sensor, you have to take a virtual change of the aperture into account too. This [this video] video explains and demonstrates it really well.
Recommends confetti · 2019-02 · Photography Stack Exchange · on “Would a 50mm lens on a Canon APS-C crop produce the exact same image as an 80mm lens on a full frame camera?”
I was looking into FF vs. Crop 2 days ago a LOT, and these two YouTube Videos were the very best out of like 20 I've watched and actually helped me fully understand it: [LINK] [LINK] - Also a nice one: [this video]
Cites OldGeeksGuide · 2015-04 · Photography Stack Exchange · on “Which current Canon APS-C cameras can use older EOS EF lenses?”
As others have pointed out, any EF lens should work on modern EOS digital SLR's. One thing to keep in mind, though, is that they will only look about the same on Full Frame digital SLR's, e.g. 1D, 5D or 6D series, because the sensor in those cameras is similar in size to the size of film exposed in a 35mm film camera. On other cameras, "APS-C" or "crop sensor" cameras, e.g. all the rebels, the sensor is smaller, which is modeled by a "crop factor" of 1.6. This means that in terms of field of view, your 100-300mm lens will appear more like a 160-480mm lens, and your 28-80mm will look more like a 45-130mm. The subject of crop factors is a little more complex than that, but that's the gist. In
Cites Goat · 2016-03 · Photography Stack Exchange · on “Do I use the crop factor in calculating aperture size and area?”
Right, the focal length remains the same no matter what size the sensor is. The effective focal length however is based on sensor size, it's called crop factor, but that was implied in the OP's question initially, so I didn't find any reason to repeat it. F-stop is based on the effective focal length, as I said, the aperture diameter is fixed, however effective focal length is not, even though the physical focal length is always fixed. But crop factor is an abstraction, it's a way to calculate how to get a certain result. So yes, crop factor includes changes to F-stop, for-instance: Sigma 18-35mm F/1.8 is the physical dimensions and obviously can't be changed. However it's an APS-C sensor on
Cites Nathan Samsoedien · 2019-03 · Photography Stack Exchange · on “Does it make sense to buy a Sigma Art f1.4 for a crop sensor body?”
@mattdm Why does that make no sense? The depth of field will be more shallow on a FF when you shoot wide open, therefore there should be less bokeh on crop sensor. See example at [this video]