I have been meaning to do some side-by-side lens comparisons for this blog. In the past I did show the differences in magnification, angle of view and image quality between the Canon EF 50mm f/1.8 II and the Opteka 500mm f/6.3. But, that was really just to see how the cheap, mirror-optics based Opteka performed. You can see the comparison here.
Yesterday I didn’t set out to do a lens comparison. My camera body was on a tripod, I had the remote trigger on it and a remote flash set up, so it just kind of happened once I started swapping lenses.
What I think is most important to demonstrate is how dramatically the angle of view changes as you move towards an 8mm focal length. There are some rare, outlier lenses but the widest lens you are likely to find is an 8mm. On a full-frame camera, 8mm will produce a 180° field of view, or something very near to it. What this means that on a full-frame you have a round image that displays everything except what is behind the camera. You can see an example here. It is an interesting effect but has limited application.
What you see here are two images, taken from the same spot, using both a Canon EF 28mm f/1.8 USM and a Zenitar 16mm f/2.8. The Canon is a pretty standard wide-angle lens. The Zenitar is a Ukrainian-made fish-eye that lacks for most of the bells and whistles we have come to expect from lenses built for a DLSR. In some ways, I like that about the Zenitar. It makes you do a little work. You have to focus and adjust the aperture manually and it gives spotty reading through your camera’s light meter when you shoot in manual mode. It can be shot in Av (Aperture Priority) mode but I generally like to tell the camera what I want it to do rather than let it make the choices for me. In manual mode you will have to experiment a little to get the right exposure, but you are only eating up space on your memory card— no great loss.
Anyway, both lenses were focused to infinity for this shot. The Zenitar was at f/8 and the Canon 28mm was at f/11. There was no real reason why I chose those f-stops. Anything above f/4 would have done the trick but the Zenitar doesn’t perform as well at wider apertures as it is prone to extreme softness at the edges and some chromatic aberration. Wide open, the 28mm could also be called soft but and also demonstrates some CA but much less so than the Zenitar.
All this is by way of explaining that both lens were set up in similar enough fashion to make for an acceptable comparison if not a perfectly controlled test. Both lenses are very sharp when stopped down this far. Although, the Zenitar did show some lens flare at the edges but that is to be expected given that the light from the windows of the monetary refectory where I was shooting was coming into the glass across the plane of the lens. Or, put another way, it was entering the lens at right angles to its orientation and thus causing some ghost-like flares.
What these two images show is the difference in angle of view and magnification between the 16 and 28mm focal lengths. A 16mm has a 97° horizontal angle of view, while the 28mm has only a 65° horizontal angle of view. The closer you get to 8mm the more rapid the change in angle of view becomes. The 12mm difference between these two lenses is almost exactly a 50% increase in angle of view. But, if you were to compare a 70mm lens to and 85mm lens you would see a change in angle of view of only 5° (the 70mm has a horizontal AoV of 29° while the 85mm has only 24°). The point is, the change in angle of view is not linear; angle of view accelerates as you approach 8mm and slows down as you recede from it.
This brings us to the second point to remember about focal lengths for the purposes of this comparison— magnification. A 50mm lens is generally considered to be a normal lens or one that has no magnification. It presents a 1:1 ratio of a subject. A wider lens shows a greater field of view while a longer lens, a 135mm for instance, shows a view of the same subject that is magnified by 2.35x. So, the 28mm does not magnify the subject in comparison to a 50mm but it does when compared to a 16mm. Thus, the two shots, from the same spot show not only a smaller angle of view but also a magnified view from the 28mm lens.
There is a common misunderstanding out there about how focal length and magnification plays out on crop-sensor cameras. Some people see the restricted angle of view that results from a crop-sensor as a magnification of the image. This misunderstanding arises from the way in which focal lengths are noted for crop-sensor cameras and the way the human eye perceives these changes.
A typical notation might indicate that the crop had a 1.6x effect on focal length. This is true in terms of angle of view. A 50mm lens on a Canon 7D, for instance, gives an angle of view equivalent to an 80mm lens on a full-frame camera (like a Canon 5D MKII). However, it does not give you the magnification of an 80mm lens which would be, on a full-frame camera, a magnification of 37.5% over the 50mm. To the human eye this reduction in angle of view is sometimes mistaken for magnification, but it isn’t.
There is no free lunch in optics and just putting a 100mm lens on a camera with a crop factor of 1.6 (i.e. a smaller sensor) does not magically give you the magnification of a 160mm lens, it just shows you the same magnification but with less of the image that would be revealed on a full-sized sensor. The photo on this page that also endeavors to explain the effect of crop-sensors demonstrates what I am talking about. When you start talking numbers, a picture truly is worth a thousand words.
Without getting too technical (or wading into territory I don’t fully grasp), if a cropped sensor were to give you a magnification gain then you would also have a different f-stop as f-stop is simply a ratio of focal length to the diameter of the iris within the lens. If this sounds too complicated then just trust me, they call them crop sensors instead of magnifying sensors for a reason.
NOTE: All How-To’s, Guides, Comparisons and such are offered as organic suggestions that will change over time and present my present state of understanding on the subject. If you have suggestions or think I got something wrong, please message me or say so in the comments. These exercises are meant to be helpful and not as the final word on the subject.