Photo Tips #7 : Crop Factor Revisited

Posted on Monday 12 September 2011

I talked about crop factor in the previous installment, it’s the reason why an identical lens mounted on a camera with a smaller sensor will appear to have a local focal length. I thought that a diagram might help clarify things further, so let’s try this. Imagine you’re looking straight down the lens onto the sensor, mounted at the back of your camera.

Crop Factor Diagram
Crop Factor (Large Sensor)

On the left you see the sensor, represented by the yellow box. You can see that the image projected onto it by the lens is a little larger than the sensor – this is quite normal. Whatever does hit the sensor becomes your final image, shown on the right. Now imagine the sensor is smaller – the lens is exactly the same, so the projected image is the same, but less of it will hit the sensor. The final image is also the same size (we’re assuming that both sensors have the same resolution – the same megapixel count). End result? The final image looks like it’s taken with a longer lens, it’s been magnified. The crop factor is a measure of how much smaller the sensor is, and thus how much longer the lens appears to be. It’s called the crop factor because it has the effect of cropping (trimming) the image.

Crop Factor Diagram
Crop Factor (Smaller Sensor)

Hopefully that makes things a little clearer, if you have any questions feel free to leave them in the comments!

    September 14, 2011 | 5:04 am

    Ah, didn’t realise this. Is it metaphorical or literal?
    If literal, it seems that the lens doesn’t project all of its ‘incoming captured image’ on to the sensor. A consequence of that would be that it is only using the central portion of the lens – so less abberation + lens edge distortion, but the downside would be the sensor only gets 1/(1.6*1.6) = 40% of the incoming light.

    September 14, 2011 | 9:25 am

    As far as I know it’s entirely literal. The distance between the mount and the sensor is the same (has to be, or it’d be out of focus), and the sensor is smaller, hence the crop. You’re right about less distortion, most lenses are also sharpest in their center portion which is another advantage. Finally, much less chance of vignetting from filters or hoods. In terms of light capture, it is getting less on the smaller sensor, but the light incident per area is the same so the brightness of the image shouldn’t be affected (as I understand it). That would show itself as needing a higher aperture on an APS-C for the same situation, and that’s not something I’ve ever heard of. Smaller sensors do have their own problems of course, due to the smaller pixels and smaller distance between them – more noise, more clipping and less sharpness.

    September 14, 2011 | 10:06 am

    So after a little research I think I was wrong about the perspective thing, so I deleted it.
    It’s worth noting that the various lens types made specifically for APS-C sensors (Canon’s EF-S mount for example) specifically have a smaller image circle, so this effect doesn’t happen. It does mean, however, that they either work differently on a full frame (Nikon’s actually crop the image digitally in a FF camera to remove the black borders!) or don’t even fit (you can’t put an EF-S lens on a Canon FF body). That said, the published length of these lenses is still at the 35mm standard, so you still have to apply the crop factor to get the effective length. I think they did this to avoid even more confusion 🙂

    October 11, 2011 | 7:01 pm

    Re your last comment – Yes, I just noticed that the lens for the canon digital had a smaller ‘hole’ in the body end of the lens than Cora’s old Minolta 35mm film SLR which has a Tamron zoom lens. – So they dont ‘waste’ all that light after all 😉 It pays to look rather than theorise Trants!

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