Why won’t EF-S lenses work on a Full Frame body?

On Friday evening  one of my friends asked an interesting question…

“I want to upgrade to a full frame body but will my lenses work?”

By way of answering I thought I’d write this post to explain the differences but the general rule of thumb is very simple…

EF lenses can be used with both full frame bodies and crop bodies.

EF-S lenses can only be used with crop bodies

Here’s why…

Essentially there are two reasons why EF-S lenses won’t work on a full frame body.

The first is mechanical: they simply won’t fit because the mount is slightly different on a crop body. Canon have done this to prevent you from using an EF-S lens on a full frame camera body and forcing it will very likely damage the mount resulting in a hefty repair bill.

The second is because of the optics. More precisely the circle of illuminance that the lens projects on the sensor itself. The crop or APS-C sensor is quite a bit smaller than that of a full frame camera. Full frame is called “full frame” because it takes it’s name from the 35mm negative. In contrast, the sensor in a crop body is derived from the dimensions of the APS-C negative. Remember the APS or Advanced Photo System cameras from the 90’s? That’s where it comes from. I’ve drawn them to scale inthe following diagram…

 

Full Frame and APS-C DSLR Sensor Sizes
Full Frame and APS-C DSLR Sensor Sizes

(Note that these are the Canon DSLR sensor dimensions. Other manufacturers’ sensors will be different.)

As you can see, quite a difference. The more astute will also spot that the APS-C sensor size is slightly smaller than that of the old APS negative which measured 25.1mm x 16.7mm. It is also this difference that gives rise to the cropping factor or apparent mangification and is where the term “crop body” is derived. It is also how the cropping factor is actually calculated: from the ratio of the diagonal length of a full frame sensor to that of the crop sensor.

Sensor TypeWidth (mm)Height (mm)Diagonal (mm)Area mm2Aspect Ratio
Full Frame36.024.043.278641.5
APS-C22.515.027.04337.51.5
Ratio1.61.61.62.56N/A

When the lens is attached to a camera it projects a circular image onto the sensor, caled the circle of illuminance. This circular image needs to be large enough to cover the entire surface of the sensor and as you can imagine, for a full frame body it needs to be much larger than is required for a crop body. I’ve tried to illustrate it on the next sketch.

EF and EF-S Lens Circle of Illuminance
EF and EF-S Lens Circle of Illuminance

I’ve overlaid an APS-C sensor on a full frame sensor, more or less to scale. The left hand side shows the circle of illuminance from an EF lens whereas that on the right shows is for an EF-S lens. Essentially, if you could physically fit an EF-S lens onto a full frame body it wouldn’t actually do you much good as the lens isn’t designed to illuminate that size of sensor.

One thng I should point out though is they don’t actually magnify the image. A crop body will give you an apparent magnification because you are only looking at a portion of the same image that the full frame body would have given you.

 

Why do the manufacturers do this?

It allows them to design smaller, lighter and cheaper lenses that are targetted at the consumer market.  The larger the sensor, the more costly it becomes to maintain image quality at the edges of the frame. APS-C being that much smaller means that the lenses are comparitively easier to design and cheaper to manufacture than their full frame counteparts. They can be made much smaller and will consequently weigh less.

The advantage is that they’re much more affordable and easier to carry around. The apparent magnification also works with you when you use a longer lens, effectively increasing the focal length.

The disadvantage is that if you upgrade to a full frame body you’ll need to re-purchase your lenses.

What’s the best type of lens to buy?

Ultimately that really depends on your long terms strategy. If you think you’ll upgrade to a full frame body in the future it may not be advisable to buy EF-S lenses. This is certainly the stance I took several years ago when I had my EOS 20D. I wanted to buy a better lens and made the decision to only go with EF lenses. I’m glad I did. If I’d taken the advice of the sales assistant in the local camera shop it would have been a very costly mistake indeed.

 

References…

http://en.wikipedia.org/wiki/APS-C

http://en.wikipedia.org/wiki/Crop_factor

http://en.wikipedia.org/wiki/Full-frame_digital_SLR

 

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One thought on “Why won’t EF-S lenses work on a Full Frame body?

  1. Since writing this article I’d be interested to hear if anyone knows of an adaptor. e.g. a multiplier with an EF mount camera-side and an EF-S mount lens-side could theoretically allow an EF-S lens to be used with a full frame body. The mutiplier would need to be sufficient to to provide full illumination of the sensor but I’d imagine that there would be sufficient light loss to make this an impractical solution.

    I’ve no idea if such a thing exists though.

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