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06-07-2013, 04:43 PM   #1
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EXIF / Focal Length / DA 18-135 WR

I've been trying to shoot with my zoom at specific focal lengths to understand those FLs and consider whether primes in those lengths would be useful or not. But, but I've noticed that the focal length in my EXIF data never matches where I had it set exactly. If I'm going 55m, for example, I'm not surprised if it's off, but I just shot some at 24, which is clearly marked on the zoom and it shows as 21mm in the EXIF. I've done that with other lengths as well with similar results.

Which is more likely to be accurate? The markings on the lens or the EXIF data?

06-07-2013, 05:16 PM   #2
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It occurred to me that I have one prime in a focal length that is printed on the register of my zoom, so set up my tripod, focused on a birdfeeder and took a shot with my M100/f4 Macro and one with my Zoom @ 100mm. They are both below.


DA 18-135 WR @ 100mm



M100/f4 Macro

Interestingly enough, the EXIF data does indicate 100mm, I didn't think it would, but I'm not sure that they look the same in terms of magnification. What do you think?
<EDIT> the M100 is definitely a tighter crop. Look at the windows on the house and the branch on the tree. It's not much, but it's as different, probably as the 24mm photo that I tried to take that showed as a 21mm photo in the EXIF data.

So, that would suggest....wait, what does this mean?
06-07-2013, 05:23 PM   #3
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To me both look same in terms of magnification.
06-07-2013, 05:55 PM   #4
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Camera moved to the left on the M 100mm shot. More tree is showing on the left and less window on the right.

06-07-2013, 06:44 PM   #5
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QuoteOriginally posted by jatrax Quote
Camera moved to the left on the M 100mm shot. More tree is showing on the left and less window on the right.
Yeah, I think you're right. I looked more critically after the first response.

So, the consensus is that 100mm is 100mm and if my EXIF data suggests that the focal length is not where I thought I was setting it, it's probably the imprecision of the register on the zoom, the EXIF data is correct?
06-07-2013, 07:16 PM   #6
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Also, focal length is not an absolute measure of field of view.
06-07-2013, 07:25 PM   #7
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QuoteOriginally posted by troika Quote
So, the consensus is that 100mm is 100mm and if my EXIF data suggests that the focal length is not where I thought I was setting it, it's probably the imprecision of the register on the zoom, the EXIF data is correct?
Well, it looks correct at 100mm. I'm not sure how the lens tells the camera what the focal length is, but I can think that it might have a little slop in it or might not be 100% accurate, or it might only report in 'steps'. Not saying it is or isn't just that all this test seems to indicate is that at 100mm it is fine.

A definitive test would involve a prime lens of each of the marked focal lengths and using a target that is easier to measure with. And a totally locked down tripod.
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