Let There Be Light – The Truth About Crop Sensors and Lenses

I consider myself a photography geek. I love the technical side of photography. Learning about how lenses work, the reasons why aperture, shutter speed and ISO contrbute what they do to image quality, different sensor designs, the technical differences between full-frame vs. crop sensor, etc. etc.

And more than seven years into this full-time journey, I thought I had heard most of the explanations about why cameras work they way they do. I get it. I can explain it. Even as new technology is released, I revel in doing deep dives into that too.

Of course, I should state that none of this helps the artistic expression in my photography nor will reading this article help your artistic expression. But it does lay the groundwork for quick decisions about how to possibly achieve a specific artistic effect. For example, to get that creamy bokeh, I know I need to do x, y, z. So, to be clear, knowing how your camera works isn’t the be all and end all of being a good photographer. But it will get you part way down that road.

That said, I find it fascinating when I run across a technical fact that I didn’t know. That’s what this post is today.

Camera manufacturers have led us down the garden path on many things. It’s understandable – they want to sell equipment. But I think this particular fact is almost a bit of malpractice on their part. I have never heard it referenced in any brochure or ad.

What is that fact?

When you buy a lens for a camera, the lens is typically described by its focal length(s), range of aperture settings and whether it can be used on both full-frame cameras and on cameras with smaller sensors.

In the case of camera brands with smaller sensors – Fuji for example – their dedicated lenses are often described by two focal length numbers. The actual focal length inscribed on the barrel, and the full-frame equivalent. It is well accepted that the crop factor of the camera has to be applied to the published focal length of the lens to determine the actual image circle or field of view of the image that will be captured by a particular crop sensor camera, at least in relation to full-frame cameras. This is because those of us who have shot with film at some point in our lives have come to know the field of view of a full-frame camera and may use that as our standard. So with a Fuji lens, and a 1.6 crop factor camera, a lens advertised as 35mm will actually capture the same field of view as a 56mm lens on a full-frame camera.

With me so far? So here’s where I really get geeky.

To be clear, if the same lens is capturing the same scene on two different camera bodies – one full-frame and one crop sensor – the image projected by the lens (also known as the image circle) will be exactly the same, but the amount of that image recorded by the sensor will be different, by the crop factor of each camera. In the case of that same Fuji, the field of view recorded will be 1.6 times smaller. Here’s what’s important: the lens, if it can be used on both bodies, itself does EXACTLY the same thing on both bodies.

And yet, the lens manufacturer will provide the two numbers mentioned earlier – the inscribed focal length and the focal length as a full-frame equivalent. This isn’t associated with the body, where it properly belongs, but with the lens. So the proper way to interpret the second number is as follows: on a crop-sensor body, the resulting image is the same as one that would be captured by a 56mm lens on a full-frame body. The lens is “behaving” as though it was 56mm.

Ready for more?

Imagine this. You have one setup with one lens and one subject. The camera will be on a tripod at a fixed distance from the subject. But you have two camera bodies. One full-frame and one crop sensor. For ease of comparison, let’s imagine the crop sensor has a crop factor of 2x.

You shoot the subject at 100mm on the full-frame body. You want to replicate that same field of view with a crop sensor body. And the answer is: shoot at 50mm! Exactly right. Only then will the field of view be the same. In other words, The lens behaves as though it has twice the focal length.

But did you know that there are other “behaviours” that change in the switch from full-frame to crop body? This is what the lens manufacturers don’t tell you – at least not in an obvious way. One behaviour is around depth of field, the other is around image noise. Both are related to the fact (at least I call it a fact) that the lens, when attached to the crop sensor body, also behaves as though its aperture was two stops slower.

Let’s deal with light first. As mentioned, the image circle created by the lens is the same regardless of the body used. But, the sensor gathers less of that light, because it is smaller. How much less? Less by the same crop factor as already mentioned. So, a lens set to f/2.8 on a full-frame body which is then switched to a crop-sensor body with 2x crop will behave as though it has been set to f/5.6 (two stops slower) on the crop-sensor camera.

What’s the practical implication of this? When purchasing a new lens advertised at f/2.8 for a crop-sensor camera with a 2x crop factor, it will actually behave, at its widest aperture, as an f/5.6 lens on a full-frame camera. I find it very strange that camera and lens manufacturers somehow don’t mention this. And I can also attest to having experienced it.

In terms of the results when shooting, the implications of f/2.8 vs. f/5.6 relate to the ability to separate background from foreground (depth of field) and to the amount of noise in the image. I always wondered why my Fuji crop sensor camera images never seemed to have nice background blur as compared to my Canon 5D Mark III full-frame images. And always a lot more noise. It was very frustrating.

I had never heard of these aperture-specific implications before, not in any class I’ve taken, not in any book I’ve read, certainly not in any advertisments I’ve seen. It was only when trolling YouTube that I discovered any mention of the issue. And this was in a post from 2014. So it’s been known for a while.

Think about it – a camera company wants you to pay 4-figures for a high quality piece of glass, but because you are using a crop sensor camera, no creamy bokeh for you. But in those situations where extending the depth of field is useful, such as in macro photography, the use of a crop sensor camera may actually be preferred.

I also should mention that not everyone agrees with the suggestion that crop factors need to be applied to aperture too. Unless you have actually experienced the difference and were puzzled by it, you might be one of those who think this is nonsense. But I think the disagreement stems from several other things as well:

1. The amount of light gathered by the lens at a particular aperture opening is the same regardless of the camera, so by definition it can’t be different just because you use the lens on different bodies. But neither is the image circle different. Just as we have found it helpful to quote full-frame equivalents for focal length in order to understand the resulting image captured, it is equally meaningful to quote full-frame equivalents for aperture, for the same purpose.
2. The newest camera systems often have dedicated lens lines that no longer cross between full-frame and crop sensor bodies. That is true with the Fuji line for sure. So the aperture settings are likely more true in these situations and don’t result in unexpected behaviours. Having said that, my experience with the Fuji crop sensor cameras up to two years ago is exactly as I have described for both depth of field and noise and I would have appreciated knowing that sooner.
3. Where the same lenses can be used on both body types, the latest technology and computational photography algorithms can perhaps compensate for any unexpected behaviours arising from the optical and mechanical components and provide a more expected experience. We already have full-frame cameras that can instantly apply a crop (and its associated behaviours) or provide a dial-selected level of computational blur directly to the background in camera.

And one last point. There will be fewer and fewer photographers over time who have any memory of what a full-frame film experience used to be. So it may be less and less important over time to recognize that standard and identify the features of equipment in comparison to it.

So how’s that for your geek fix for today? At the very least, if you are a user of crop sensor camera bodies, you’ll be in a better position to understand the potential behaviour of that next new lens you might purchase in the future.