The relationship between sensor size and lenses is often a source of confusion. So here is a quick guide that doesn’t use any complex geometry or advanced maths, just some methodical common sense.

Lens optics do not change on a crop-sensor camera body

There is an enduring myth that a lens fitted to a camera body with a different sensor size has different optical properties. No matter what you may have heard to the contrary, a lens provides the same unchanged optics on any camera to which it is attached. 

Just to push that point home, let’s do a little thought experiment (or you can do this for-real if you are so inclined). First we’ll take a photo with a lens at its maximum aperture on a full-frame camera body. Then we’ll put the same lens on an APS-C camera body and take the same shot at the same maximum f-stop. 

Now we’re going to compare the two photos side-by-side. Just before we do that, since the APS-C sensor is a third smaller in width and height, let’s even-up the score by cropping around our full-frame photo, getting rid of a third of its width and height.

Now for the grand reveal… Well I’m shocked. Both photos are pretty much identical. Same field of view, same depth of field, same exposure. This is because the attributes of a lens do not change, no matter what camera they are on.

So what’s this about crop-factors and how the lens behaves differently? To find out let’s understand what a crop-factor is, because it will give us a very handy metric that we will refer to frequently.

Crop factor – explained

If a lens is designed to provide an image for a full-frame sensor, and that lens is used on a camera body with a smaller sensor, it will still be transmitting the same size image. The smaller sensor will only see the part of the image that falls onto it. The rest will be cropped away. Hence the term crop-factor.

A sensor’s crop-factor is its width compared to another sensor – almost invariably a full-frame sensor. A full-frame sensor is (give or take)  one and a half times as wide in any direction as an APS-C sensor, giving the APS-C sensor a crop-factor of 1.5. I could just as easily have used a Micro Four Thirds sensor for this discussion with a crop factor of 2, which is to say that a full-frame sensor is twice as wide. 

If you want to be more exacting you can calculate the relative crop factor for any sensor (dribbling into keyboard warning) by measuring the diagonal length of each sensor, then divide the diagonal of the full-frame sensor by the diagonal of the other one. This will work for any two sensors, even if they are different shapes. 

What this means in practical terms is that if you put a 50mm full-frame lens onto a camera body with an APS-C sensor, there will be approximately a 1.5 times crop factor. The image produced looks as though it has been put through some photo editing software and the outer portion of the image cropped away.

Does this affect the lens aperture?

No. And, well, yes. A lens, as has been pointed out already, does not change its properties, no matter what camera it is attached to. But we can infer that, since a cropped sensor is smaller, it will collect less light.

This reduced light capture results in a similar effect as if there were a smaller aperture. The effective aperture can be calculated by multiplying the aperture of the lens by the crop-factor. Just to be clear on this point; this calculation is being made against the lens aperture as a convenient way for us to understand its effect, but the lowered light transference is nothing to do with the lens and everything to do with the sensor size.

In practice, the camera manufacturer will almost certainly make compromises elsewhere in order to keep the lens operating as expected at its intended f-stop. For example, the sensitivity of the sensor may be tweaked up.

Whatever compromise is made on a crop-sensor camera, it will result in reduced low-light performance. In practice, as sensor performance has improved, this has become much less of an issue and the latest crop-sensor cameras can be expected to put in pretty good low-light performance. See ISO-invariance-explained.

What about depth of field?

What? Wasn’t it clear? The attributes of a lens do not change. If you put a 50mm lens from a full-frame camera onto an APS-C camera, we know that it will have a narrower field of view like a 75mm lens (focal length x crop-factor) because of the cropped off area. The depth of field will remain unchanged. It’s still a 50mm lens and it will have the depth of field of a 50mm lens. 

However, now is a good time to mention that depth of field also increases with subject distance. This is where things get a little tricky. If you use a full-frame lens on a crop-sensor camera, you need to stand further away from your subject to get the same composition. And if you are further from the subject, as I just pointed out, the depth of field is greater, so the effective bokeh is reduced since more of the image is within the field of focus.

You may, in a moment of brilliance, decided to use a 35mm lens on your 1.5 times crop-sensor camera because it will give roughly the same composition as the 50mm lens (50 ÷ 1.5). But you can’t beat the laws of physics. A 35mm lens can only produce the bokeh of a 35mm lens. It hasn’t magically turned into a 50mm lens. That’s because the shorter the focal length of a lens, the deeper the depth of field at any given aperture. A lens with a focal length of 135mm will undoubtedly be able to resolve a shallower depth of field than a 28mm lens at the same aperture so the shorter lens cannot produce as much bokeh.

So are crop-sensor cameras all that they are cracked up to be?

A crop-sensor camera doesn’t collect as much light as a full-frame camera. For this reason, some compromises must be made. In practice, the compromise turns out to be marginal. The full-frame sensor size, as a standard, is somewhat arbitrary. It is based on the original 35mm film size and that was based purely on the convenience of available movie film stock.

As sensor technology becomes better at what it does, the loss of image quality is falling below what the human eye can detect, and so it is becoming increasingly less relevant for most practical purposes, unless there is a need to blow the image up to a large size. In that case, sensor size is still very much a factor.

The single area where a larger sensor reigns supreme is in producing bokeh as a result of attaining a shallow depth of field. While this is inevitable, there are plenty of lenses for crop sensor cameras that still give satisfying enough bokeh that you may reasonably decide that you can live without a full-frame.

My APS-C Fujifilm camera may at times have a little less bokeh than I want. I occasionally compensate (cheat) by adding a little blur to the background of my photos in post processing. It has one huge advantage over a behemoth full-frame DSLR. It’s smaller, lighter, enjoyably unobtrusive, and so I am much more likely to have it with me, in my hand, when it’s needed.  On the basis that the best camera is the one you have with you, my Fujifilm X-series wins hands down.

And to take this whole argument to its logical conclusion, the sensor on your smartphone camera is positively tiny, and yet there is no arguing how good they have become. Not to mention that some of them now fake in some pretty convincing bokeh. Having said that, there is nothing like the real thing. Is there?

Quick summary

• To calculate crop factor: divide the diagonal length of the full-frame sensor by diagonal length of the crop-sensor.
• The effective focal length of any lens used on a crop sensor camera is the stated focal length times the crop factor. 
• The maximum aperture of an interchangeable lens is whatever the lens says it is.
• The reduced light collecting capability in a crop-sensor camera is caused solely as a result of less light falling on the smaller sensor’s surface area. This is compensated by design within the camera.
•  Depth of field increases with distance. So if your subject needs to be further away to get the same composition on a crop-sensor camera, the depth of field will be greater.
• If you use, as an example, a 25mm lens on an Micro Four Thirds camera, the field of view will approximate to a 50mm lens on a full-frame camera. The depth of field will remain that of a 25mm lens. The lens attributes never change.