Crop factor


If you are using an α7/α9 series or equivalent full frame camera, the topic of crop factors luckily doesn’t concern you. However, if you – like me – started using adapted lenses on a NEX/α5X00/α6X00 or any other camera with a smaller sensor (i.e. Micro FourThirds, Fuji X or EOS M), the following remarks might be of interest to you.

As all Minolta MC / MD lenses are designed for 35 mm film or ‘full frame’ format, their optics produce an image in the sensor plane that just covers this image format. That also means that the projected image is larger than the sensor of APS-C or MFT cameras. Therefore, the sensor only catches a ‘crop’ from the actual image. The ratio of the image diagonals of the full frame and the sensor is expressed in the crop factor. For NEX, α5X00 and α6X00, which have an APS-C format sensor, this factor is usually given as 1.5 and more precisely calculates to 1.526. As only a fraction of the projected image is captured, the picture seems to bring you closer to your subject compared to a picture taken under identical circumstances with the same lens on a full frame camera. This results in a virtual increase in the lenses focal length equal to crop factor x original focal length. In other words: The field of view (FOV) of a 50 mm prime mounted to an APS-C camera is equivalent to the FOV of a 75 mm prime on a full frame camera.

It is often said that your f-stop (= “aperture”) changes by crop factor x original f-stop, because the equivalent focal length increases, but the aperture opening does not. That is technically not correct, because the f-stop is defined by physical focal length and physical aperture opening, both of which are fixed properties of the lens. What people mean when they say that the f-stop changes is, that your depth of field (DOF) is increased when compared to the same equivalent focal length on full frame. Coming back to the example from above, you will need a 75 mm lens on a full frame camera to get the same FOV as a 50 mm on APS-C. Everything else kept constant, the 75 mm will have a shallower depth of field.

This is an effect you can easily reproduce: Try to isolate a subject from the background with a 28 or 35 mm lens and with a 50 mm lens while keeping the same distance. You will see that you need to use a larger aperture (smaller f-stop) to achieve the same effect with the 28 or 35 mm. Ergo: The smaller your sensor, the higher your DOF at a given equivalent focal length, aperture and subject distance. There’s a short listing of DOF compared for different formats on Wikipedia. Have a look.

Using a crop format sensor also has implications on your T-stop. In contrast to the f-stop, which is a geometric quantity, the T-stop is a measure of actual light transmission and takes into account light loss inside the lens due to reflection or absorption. A higher T-stop – just like a higher f-stop – equals lower light transmission. And while the total transmission of a lens does not change when mounting it on another camera, the smaller image circle of an APS-C frame means that you do not capture typical weaknesses in the edges of the frame. One of these weaknesses is light falloff in the corners, called vignetting. Using an APS-C sensor leads to a more even exposure across the frame, as the outermost corners of the full frame image, which are most affected by vignetting, are cropped. This results in a slightly higher transmission value of the lens. As an example, a lens typical f/1.4 lens with a T-stop of T 1.7 on full frame will usually have a slightly lower T-stop of – say – 1.6 on APS-C. As vignetting usually disappears when a lens is stopped down, the T-stop difference gets smaller for higher f-numbers and mostly vanishes by f/8 or f/11.

The T-stop calculations in my reviews in some way ignore those effects, as metering is center-weighed. Therefore, vignetting does not affect the calculation and a reduction in vignetting has no effect, either. As a results, T-stops reported in my reviews tend to be slightly lower (=higher transmission) then those given in many professional reviews.

 

How does all this affect you?

The answer is: Not at all if you never used different sensor formats and just want to take photos. As Sony thankfully prints the physical (and not the equivalent) focal length on all their APS-C lenses, an adapted vintage 50 mm lens will have exactly the same FOV as the SELP1650 @ 50 mm.

If you come from a full frame system like Minolta SR or Sony A mount, though, keep in mind that all adapted lenses will give a significantly narrower FOV on the NEX/α5X00/α6X00 than on full frame. If you come from an APS-C camera and now use an α7/α9 in parallel or plan to get one, it is the other way around: The FOV of your lenses will be significantly wider on the α7/α9 than on the APS-C cam. And, as written in the beginning: If you skipped crop factor cameras altogether and went straight for an α7/α9, this isn’t of any concern as the lenses will behave identical in terms of FOV.

 


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