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06-02-2009, 02:02 AM   #1
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What does the DSLR do with the focal length info when using MF lenses?

When I mount an old MF lens (e.g., Pentax-A 50mm) on my K200D it asks me to dial in the lens' focal length. What does the camera do with this info? I tried entering different focal lengths for the same lens and it didn't seem to make any difference to photo taking.

06-02-2009, 02:09 AM   #2
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It's for the Shake Reduction.
You should enter the real focal length (50mm in this case) or the closest value, if not available.
06-02-2009, 05:04 AM   #3
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im with Ari, more info needed...
06-02-2009, 06:28 AM   #4
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It's not just for manual focus lenses, it's for any lens that doesn't transmit the focal length information to the camera. (I'm pretty sure such lenses exist in autofocus too, though all my AF lenses are relatively new so they do give the camera the focal length info.)

It uses the info for Shake Reduction - a larger focal length will need more SR than a short focal length. It also stores this information in the exif focal length field, which can be handy after the fact to identify which lens you were using. Unfortunately there are "holes" in the available numbers - no 90mm, no 105mm (and I'm up to three distinct 105mm lenses now!)...

If you put in a higher value for the SR than the lens actually is, you may see excess blurring as the sensor will try to do more SR than it should.

06-02-2009, 07:06 AM   #5
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QuoteOriginally posted by Groucho Quote

If you put in a higher value for the SR than the lens actually is, you may see excess blurring as the sensor will try to do more SR than it should.
Actually if you put in a smaller number you are essentially reducing the shake
reduction compensation which will also blur your images more than inputting the correct focal length.
06-02-2009, 07:08 AM   #6
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it should also show up in the EXIF
06-02-2009, 07:10 AM   #7
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Psychedelic Dinosaur - Focal Length Converter
06-02-2009, 07:46 AM   #8
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You should not have to do any conversion when using a manual lens. If you put a 50mm lens on the camera input 50mm. For manual zooms it was said that best practice is to input the median focal length (or closest approximation thereof) again with NO FOV conversions.

06-02-2009, 08:31 AM   #9
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Sorry Cosmo, as nice as that web page is it will confuse the issue for this thread.

As has been pointed out by more than one poster you enter the focal length of the lens, no maths or conversion needed.
06-02-2009, 12:49 PM   #10
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QuoteOriginally posted by jco Quote
Actually if you put in a smaller number you are essentially reducing the shake
reduction compensation which will also blur your images more than inputting the correct focal length.
True, but by entering too low a focal length, you won't make it worse than *no* SR. Entering too high a focal *can* make it worse than no SR at all - but only if it's *way* too high (like entering 200 for a 28mm lens).

How it works is pretty simple. The SR systems know much the camera is shaking, but if it doesn't know the focal length of the lens, it doens't know how big and effect the shake has on the picture. Consider that with a wide angle lens looking at an object, you can move the camera quite a lot and the object stays completely within the frame; put on a telephoto lens, and the same amount of motion might move the subject completely out of the frame. So the SR system has to work harder to counteract the same amount of shake when used with telephoto lenses.
06-05-2009, 12:00 PM   #11
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So when fitting a MF zoom lens I should use the maximum zoom value? An 80-200 lens should be entered as 200 instead of the midpoint, around 135?
I have an old Sears branded 80-200 which is very smooth to MF and zoom, and fun to play around with around the neighborhood.
06-05-2009, 12:34 PM   #12
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The "conventional wisdom" is to use the lower focal length because too little SR is supposedly better than too much. That is if you think you'll be using the full range of the zoom. If you know you'll be at the long end mostly then set it to the max, conversely if you'll mostly be at the short end then use that.
06-05-2009, 12:49 PM   #13
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QuoteOriginally posted by TER-OR Quote
So when fitting a MF zoom lens I should use the maximum zoom value? An 80-200 lens should be entered as 200 instead of the midpoint, around 135?
I have an old Sears branded 80-200 which is very smooth to MF and zoom, and fun to play around with around the neighborhood.
I think ideally, I would run a lot of tests at various zoom with the camera set for a constant focal length for shake reduction. See what the quality differences are for a range of possible settings. You may in fact find that you are satisfied with the results by setting the focal length to the max end, the min end, or even an average.

Everyone is giving sound advice, but you won't be totally satisfied until you know what works.
06-05-2009, 04:23 PM   #14
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QuoteOriginally posted by TER-OR Quote
So when fitting a MF zoom lens I should use the maximum zoom value? An 80-200 lens should be entered as 200 instead of the midpoint, around 135?
No. If you follow my explanation above, you'll see that in you're really at 70mm, telling SR you are at 200mm will result in it moving the sensor *too much* - it will be moving the sensor by the much larger amount that would have been necessary for a 200mm lens. With the result you could get a picture that is shakier than no SR at all. Whereas entering too low a value might not move the sensor as much as it should, but it won't be worse than nothing - just not as good as it could be.

So if can't anticipate where you'll actually be using the lens when you mount and and don't wish to change the setting while shooting, best results would be to set it to something toward the bottom of the range - maybe 100-ish. But really, it would depend on whether you'd "mostly" be using it at the long end, the sort end, or right in the middle.
02-24-2010, 03:16 PM   #15
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QuoteOriginally posted by Cosmo Quote
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