Photography Thailand Forum

[lotuseater]

I have a question about focal lengths and cropped senor format cameras. I had always thought that the focal length was marked the same regardless of the format of the lens. For example, a 24mm on a EF-S in Canon or a DX in Nikon would be the same focal length as it is on a full frame camera. The actual field of view was the thing that changed due to the sensor essentially cropping the image when using the same sensor size. A 24mm does not seem as wide on a cropped sensor camera because you are getting the 1.5 or 1.6 crop factor.

Thus a 10mm cropped format lens is a real 10mm lens but because of the crop factor it becomes a 15mm or 16mm lens.

This is what I had always understood.

A friend and I had been arguing about this and we began playing around with Nikon and Canon lenses the other day and was getting some very strange results. First we tried his 50mm f 1.8 against his 18-200mm DX lens set at 50mm. They were Almost exactly the same field of view which I expected. Then we tried his 18-200 DX against his 70-300 lens which is not a cropped sensor specific lens. We tested both of them at 200mm and I was very surprised to find that 200mm on the 18-200 was quite a bit wider than 200mm on the 70-300. A lot more than could be explained by error in our testing. I was surprised to see that 200mm was not the same between these lenses because I always though that a 200mm on a cropped format lens was the same 200mm on a non cropped format specific lens. The difference between the lenes or so I believed was that the crop lens projected an image circle that was smaller than a non crop lens and for this reason could not be used on a full frame camera, (at least not without leaving a black circle around the image, or in the case of the Nikon full frames, shutting down part of the sensor to compensate.)

I was wondering if someone could help me explain these apparently contradictory results.

Are cropped lenes labeled differently than non cropped lenes? Should a DX 200mm be the same as a non DX 200 on the same camera?
What about EF-S lenses, does the same apply? I don't have any EF-S lenes that over lap with my EF lenses so I could not run the same test for Canon.

Cheers.

[epixx]

The 18-200mm changes maximum focal length with shorter distance to the subject. This is not unusual, but more dramatic on some lenses. If you measure at infinity, they should, at least in theory, be equal.

[lotuseater]

I'm still not sure I understand. I understand that the longer the lens, the closer it is to the subject thus the longer the focal length should appear. The 70-300mm is a longer lens thus it should appear to have a longer focal length simply due to this. However the difference between the two was VERY dramatic. So much so that it took shooting the 70-300 at about 135mm to have about the same field of view as the 18-200 zoomed to 200mm. I wish I had the samples to post but it was not my camera that we were testing with.

Would the shorter distance to the subject make that much of a difference in the field of view?

[epixx]

The physical construction of the lens makes it impossible to maintain the 200mm focal length when focusing too close. It's important to remember that this kind of "superzoom" achieves its focal length using all kinds of optical tricks. If it was made the way lenses were made 20 years ago, it would have been a big, heavy behemoth that nobody would buy. But to keep the small size, compromises have to be made. So, it's only a long telephoto lens when you really need it: for subjects far away. If my memory serves me right, I think I read somewhere that it's only 160 or 170mm at minimum focusing distance,  but take that with a pinch of salt.

The phenomenon exists for many "traditional" telephoto lenses as well, but to a much smaller degree, so people don't notice. And while we're at it, there are other lenses that change with subject distance as well: many (most?) macro lenses decrease the maximum aperture when focusing at macro distances. The Nikkor 105mm f/2.8 (all versions as far as I know) is one of them. Mine, an AF D, has a maximum aperture of f/4 at minimum focusing distance. The reason is a bit different from telephoto zooms, but the concept is the same: the optical path of the lens changes.