APSC Crop Factor and lens Aperture...

FowlersFreeTime

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It is fairly common to come across comments online about the crop factor and using full frame lenses on our APSC cameras. I get it, APSC cameras are aimed at the novice, and we often have a lot to learn, but that doesn't mean we should learn incorrect facts. One of the fallacies that needs to stop is the belief that the aperture value of the lens is affected by the crop factor: it is not.

I came across this article while offering supporting evidence to a fellow a6400 owner over on the Facebook group and thought I would share here as well:

From the article:
In photography, aperture is expressed as a ratio of the focal length to the diameter of the aperture opening. The ratio is commonly referred to as an f/number, f/stop, focal ratio, f/ratio, or relative aperture... This ratio is based on physical measurements and is completely independent of the size of the camera’s sensor or the size of the film you are shooting. Sensor size has an effect on depth of field, but not because it changes aperture. Aperture is independent of film frame or sensor size.
 
Excellent article that helps explain the differences in sensor sizes! I do still have a question that I didn't see answered. (although I did kind of scan thru it): Does a Sony "APS-C" lens (ie., E 50 mm F1.8 OSS) have the same field-of-view as the equivalent FF lens (ie., FE 50mm F2.5 G Lens)? Or, do you still need to do the math to figure out the "normal" FOV? And, do third-party manufacturers (Sigma, Tamron, ect.) follow the same format/rules?

I hope that makes sense. This is something I've been struggling with while researching primes for my A6600. In the end, I suppose it doesn't really matter. FOV is what it is. I just don't have the funds to buy a lens then find out it won't work the way I need, and have to wait and get what I actually need/want.
 
Excellent article that helps explain the differences in sensor sizes! I do still have a question that I didn't see answered. (although I did kind of scan thru it): Does a Sony "APS-C" lens (ie., E 50 mm F1.8 OSS) have the same field-of-view as the equivalent FF lens (ie., FE 50mm F2.5 G Lens)? Or, do you still need to do the math to figure out the "normal" FOV? And, do third-party manufacturers (Sigma, Tamron, ect.) follow the same format/rules?

I hope that makes sense. This is something I've been struggling with while researching primes for my A6600. In the end, I suppose it doesn't really matter. FOV is what it is. I just don't have the funds to buy a lens then find out it won't work the way I need, and have to wait and get what I actually need/want.
Oh no I get it man, I went through the same thing.
So as the article explains, the measurements such as "50mm" and the f-stop are mathematically derived based on the physical construction of the lens. The effective field of view of a 50mm lens on an APSC sensor is the focal length * crop factor.
But on a full frame, the same lens only has a 50mm field of view.

Now, if you reverse that order and put an "APSC" 50mm lens on a Full frame camera, you get the correct 50mm field of view in the center of the picture, but because a good portion of the sensor is blocked you get intense vignette-ing like this:
TS560x560_2440978.jpg
  • NIKON D800
  • 10.0 mm
  • ƒ/8
  • 1/125 sec
  • ISO 100

To allow full frame users to use APSC lenses, or to give them a bit of extra reach at the expense of pixels, Full frame cameras can operate in "crop mode" which only uses the center of their larger sensor to mimic an APSC sensor.

Hope that helps!
 
Keep it simple. As someone who shoots M-4/3 and APS-C, I have had to wade through the nonsense to gain my understanding. Why this needs to be so difficult is beyond me. Here it is straight-up:

An f/XX lens is always going to be an F/XX lens regardless of what platform it's on. An f/2.8 lens in M-4/3 is the same as an f/2.8 FF lens, speaking relatively. If I take a FF lens and mount it on a smaller sensor camera, does it change the aperture? Nope.

What DOES change is the amount of light hitting the sensor, which affects only the way the f-stop acts, not it's physical characteristics. Because the sensor is smaller the light is cut by roughly an amount equal to the difference in area of the two sensors which causes the f stop to act smaller. And because it's acting smaller, it also affects depth of field.

How much? Well, that depends on a lot of factors. The rule of thumb is 1-1/2 stops from FF to APS-C, and 2 stops from FF to M-4/3. However, with the continuous advances in sensors dynamic range and computers that gap has narrowed significantly. I try to use 1 stop to make my guesstimates. So, if I know that I have a 1 stop advantage in light on a FF sensor, I can reason that an f/4 lens on a FF would act like an f/3.3 (3.5) lens on APS-C, or F/2.8 on M-4/3, with regard to the amount of light hitting the sensor, crop, and DOF.

As for the crop factor, it's simpler yet. The sensor is smaller in area, it's like enlarging the center of the image. All you have to do to see this effect is crop an image on your computer. Crop the outside of the image equally if you're in the same aspect ratio to see exactly what the sensor sees. Crop it by 25% to see an APS-C representation, or 50% to see M-43.
 
Excellent article that helps explain the differences in sensor sizes! I do still have a question that I didn't see answered. (although I did kind of scan thru it): Does a Sony "APS-C" lens (ie., E 50 mm F1.8 OSS) have the same field-of-view as the equivalent FF lens (ie., FE 50mm F2.5 G Lens)?
Yes. A 50mm lens is going to have roughly the same FOV as another 50mm lens. In order to get an equal FOV you need to adjust the lens based on the sensor. This is why kit lenses differ from format to format.

In the world of photography, a 50mm lens was always considered the standard for 35mm cameras because it represents roughly the same image size that the human eye sees. So, everything is based off of that.

These lenses all represent the same FOV based on their respective sensors:

FF: 50mm
APS-C: 37.5mm
M-4/3: 25mm

So even a 50mm APS-C lens will have a 75mm FOV or FL equivalence.
 
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Thanks Guys! That helps! Now, I just need the $'s! Hope Santa comes to our house this year! :)
 
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