Crop Factor. Something to consider when buying a lens when you are shooting other than FF.

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Crop Factor. Something to consider when buying a lens when you are shooting other than FF.
1 month ago

When you shoot full frame with a prime 35mm lens at f1.4, you will get an image with a depth of field relevant to the focal length and aperture. You then bring the image into photoshop and make a crop of the image at the size and dimension of an APS sensor. The final image will have a similar angle of view of a 50mm lens. The depth of field, however, will stay the same.

Now, when you attach the same lens to an APS sensor camera, the same cropping happens, only this time it happens physically on the sensor itself, thus a 35mm lens will give somewhere around a 50mm angle of view. However, the depth of field is still based on an actual, physical 35mm lens instead of an actual, physical 50mm one.

That is why when you use an FF camera with a 50mm lens at f1.4, it will have shallower depth of field than when you use a 35mm lens at f1.4 on an APS camera although the final images will have similar angle of views, because on the full frame, the actual physical focal length is longer than on the APS camera.

To get the same depth of field on an APS camera as on FF, you need to open up more on the APS. By how much, you may ask. Well, by the same crop factor that you multiply your focal length, but this time you divide the aperture. So for the example above, to get the same depth of field, when on the FF the lens is opened up to f1.4, on the APS, it needs to be opened up to f0.93. That's by dividing the aperture (1.4) by the crop factor (1.5). What happens when you shoot at the same f1.4 on both lenses?  Well the 35mm f1.4 on APS will be equivalent to 50mm f2.1 (multiply both focal length and aperture by the crop factor, in this case 1.5X) on the FF, which means, the Dof is not as shallow as when shooting FF with a 50mm f1.4 lens.

Now the problem is, camera/lens manufacturers have failed to give this information to the general public, so many people are still clueless about this. Many people still think, well an f4 is still an f4, or an f2.8 is still an f2.8, no matter what sensor I use, right? Well yes and no. Because your f4, though still an f4, is actually based on the wider angle on the physical lens instead of the final image. Just like the example in the first paragraph. You are shooting with a wider angle lens, you just lose the width in the crop, you are not actually using a longer focal length. When camera/lens manufacturers advertise their lenses, they have the actual focal lengths and maximum f-stop, along with the equivalent focal lengths in FF, but never the equivalent f-stop on FF. Why? Because then the lens will lose its appeal.

For example, the latest Fujifilm 10-24mm f4 OIS lens. We know that the focal lengths after factoring the 1.5X crop factor becomes 15-35mm. But how many people know that the f-stop becomes equivalent to f6 (4X1.5)? Thus what they need to write is this: 10-24mm f4 (15-35mm f6), but will you buy a lens with a maximum aperture of just f6? So when a lens has a maximum aperture of f5.6 on APS, it becomes the equivalent of f8.4 (5.6X1.5) on FF. That is why many people using such lenses on APS cameras are having a hard time throwing the background out of focus, because even at 200mm (which translate to 300mm after crop factor), you won't get shallow DoF at f8.4. So now, your f2.8 lens on APS just becomes the equivalent of f4.2 on FF. And the magnification is greater on M4/3s because you have a crop factor of 2X. So at f2.8, you are actually shooting at the equivalent of f5.6.

What camera manufacturers need to do is to be honest when advertising their lenses and include the equivalent f-stops along with the equivalent focal lengths of any lenses they sell. Or, start producing and selling lenses with greater f stops, so that when being compared side by side, a 35mm lens on APS will produce images with the same angle of view and Dof as a 50mm on FF at their maximum aperture. But of course, these kinds of lenses will add bulk and price, which in turns beat the purpose of using APS sensors in the first place.

To give a clear picture, when an FF 50mm lens has a maximum aperture of f1.4, to get the similar focal length and DoF on APS, it needs to be 35mm at f0.93, and on M4/3, the lens needs to be 25mm at f0.7! much will those kinds of lenses cost, I wonder? Or if those apertures are even attainable without sacrificing size and weight. Can you imagine carrying a 12-35mm f1.4 (equivalent to 24-70mm f2.8 on FF) lens around for your M4/3 system?


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