Technical focal length EXIF question.

[TiCoyote]

I'm shooting with a Digital Rebel. I have the 28-135 USM lens. I was trying to explore the possibilities of using a wide angle close to the subject to highlight depth and give a sense of elongation.

Here's the anomaly: When I set the zoom on the lens to 135 mm, the EXIF data on the computer shows " focal length: 135 mm." However, when I set the lens to 28 mm, the EXIF shows 47mm. Why does this happen? Is is because the camera is APS-C? If so, wouldn't it affect the 135mm side as well?

Or is it just a problem with outdated firmware?

Whenever I shoot with my 50 mm, the EXIF displays 50mm.

 

[Plato]

Are you sure that you're zoomed in to 28mm?

I have an old discontinued Nikon 28-105 lens with macro capability, which is nothing more than the ability to focus really really close. However, macro capability exists only from 50-105. If you want to protect yourself from accidentally zooming to outside the macro range, you set a "macro/normal" switch to "macro," which locks the lens so that it will not zoom to less than 50mm. You can take regular pics all day long with the switch set to "macro" and, as long as you don't need to zoom to less than 50mm, there is no problem. HOWEVER, if you rotate the zoom ring without looking at it, you might THINK that you are zooming to 28mm.

 

[Garbz]

The ExIF always should record the actual focal length. Software sometimes calculates the 35mm equivalent focal length based on the crop factor, but none of this has anything to do with the downright weird inconsistencies since 28*1.6 = 45mm and not 47mm.

Something may be screwy with the firmware or your process.

 

[Derrel]

It seems very possible that the 28m m lens focal length is really NOT 28mm.

Okay....thanks to google, I see that the Canon 28-135 f/3.5~5.6 IS USM is "actually" a 29.63 to 137.90mm lens under critical bench testing in Pop Photo's one million dollar testing facility.

Canon Canon 28-135mm f/3.5-5.6 IS USM AF - Lens Review from Experts at Popular Photography- Photo Tips

Mystery solved? 29.63 x 1.6 = 47.408mm

 

[TiCoyote]

Derrel, that gets me halfway there, but 137*1.6=220ish. Besides, 50*1.6=80.

 

[Dwig]

...

Or is it just a problem with outdated firmware? ...

Its vastly more likely that its an issue with the lens and not the body.

One of two things are likely happening:

1. When focusing extremely close, the lens is actually shifting focal length as part of the focusing process and this altered focal length is being reported to the body. It would seem a big odd that the shift is to a longer focal length, but it may be something Canon chose to do to balance image quality at the close distances.

2. The lens is faulty and isn't signaling the FL correctly. There are electrical contacts in the lens barrel that transfer the mechanical zoom system positioning into electrical signals. These can fail.

You'd have to compare your lens to another sample of the same lens to determine whether its a characteristic of the lens version in general or just your sample.