APSC Crop Factor and lens Aperture...

[FowlersFreeTime]

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:

Understanding Crop Factor

There is a great deal of confusion surrounding crop factor, and it is particularly difficult to explain, but let’s give it a try, shall we?Before we dive in, let me dispel two vicious rumors related to crop factor that are circulating through the photography (Internet) world today: Crop factor...

www.bhphotovideo.com

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.

 

[JDinTulsa]

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.

 

[FowlersFreeTime]

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:

• 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!

 

[JDinTulsa]

Thanks Guys! That helps! Now, I just need the $'s! Hope Santa comes to our house this year!