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10-21-2010, 10:52 PM - 1 Like   #1
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What focal length for a manual zoom?

Hi guys
I just picked up a 35 - 105 A zoom.......lordy its sharp! but what focal length should I enter? I tried 100 and 70...... does it make any difference? or switch the s/r off?
cheers
Jan


Last edited by janstew; 10-21-2010 at 10:53 PM. Reason: I have a miss spelling in the title but I cant edit it!
10-21-2010, 11:22 PM   #2
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It's always a tricky one - what focal length to enter when the camera doesn't have the information supplied by the lens.

The reason you need to supply the info is that SR uses the focal length in its calculations. If the wrong info is supplied, then SR will be compromised, or worse!

If you were to enter 35mm and shoot at 105mm, then SR effectiveness ought to be reduced to 35/105 or a mere 33.3% of what it should be. At least you'd get some SR!

If you were to enter 105mm and shoot at 35mm, the SR mechanism will respond more vigorously than is required, by a factor of 105/35 or 3 times. The net result would be (as I see it) shake being overcompensated for, and therefore magnified by 2x (and in the direction opposite to the causing movement).

From this, the best compromise setting would seem to be midway, i.e. 70mm. This should result in SR effectiveness from 50% (at the extremes of the zoom range) to 100% (at 70mm).
10-21-2010, 11:25 PM   #3
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I generally choose one focal length and leave the lens there if I want to use SR on a manual zoom. If I'm zooming all over the place I'll turn it off. Under some conditions the shake reduction system will blur the image if the setting is too far from the focal length you have the lens set at - I've swapped from a 400mm prime to a 28mm prime while forgetting to change settings and the resulting images were unusable due to blur from the camera trying to compensate for a much longer lens.

Try playing around with different settings and see how far you can zoom from the focal length the shake reduction is set at before you can spot the difference in the image.
10-21-2010, 11:35 PM   #4
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Ok I get that......and its pretty much what I thought but its great to be able to confirm it! I will play with it over the weekend and see what I can come up with.
Many thanks
Jan

10-22-2010, 05:21 AM   #5
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You're very welcome, BUT I've realised I haven't done my sums right:

I reckon that the calculation you need to make for a zoom lens is as follows:

x = H*L/(H-L)

where x is the focal length input to the camera, H is the max zoom FL and L is the min FL.

Also, the min SR effectiveness will be (x/H) * 100%

So, for your 35-105 zoom, x works out at 52.5mm, and min SR effectiveness is about 50%.

Since you can't input 52.5 to the camera, it's preferable to go to the next value LOWER than the exact value (so, 50mm here). (This principle of course should be extended to primes with "unusual" FLs.)

Many apologies for misleading you (and everyone else)!
10-22-2010, 05:29 AM   #6
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QuoteOriginally posted by m42man Quote
Since you can't input 52.5 to the camera, it's preferable to go to the next value LOWER than the exact value (so, 50mm here). (This principle of course should be extended to primes with "unusual" FLs.)
Why that? I usually put the closest value (for instance for my 125mm I put 135mm not 105mm).
10-22-2010, 06:15 AM   #7
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It is good to underestimate.

Your zoom is a push-pull correct?

I used to think my takumar bayonet was really blurry at 200mm, but then I realized that I was often actually shooting at 180 - 200, because when I was busy focusing, I would pull the zoom a tiny bit. Now I set SR to 150 or 180, and things are ok.


Basically I ask, what length am I trying to shoot? I set SR to be 20 ish mm below that. If I go below that value (say, try to shoot at 100), I just shoot without SR. It's really not that graceful but I'm sure the price was right .
10-22-2010, 10:22 AM   #8
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QuoteOriginally posted by Zav Quote
Why that? I usually put the closest value (for instance for my 125mm I put 135mm not 105mm).
Yes, you're quite right - in most situations it will be better to choose the "closer" of the 2 options. The situation only gets out of hand in gross situations, where you can actually get shake multiplication if you input too large a focal length (whereas too small merely reduces the SR to total ineffectiveness). Actually, for small mismatches, the differences will be minimal.

(I really must try inputting something like 200mm when I'm using a 28mm lens. It should be interesting to see the results!)

10-22-2010, 10:55 AM   #9
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QuoteOriginally posted by m42man Quote
Yes, you're quite right - in most situations it will be better to choose the "closer" of the 2 options. The situation only gets out of hand in gross situations, where you can actually get shake multiplication if you input too large a focal length (whereas too small merely reduces the SR to total ineffectiveness). Actually, for small mismatches, the differences will be minimal.

(I really must try inputting something like 200mm when I'm using a 28mm lens. It should be interesting to see the results!)
I once tried my 180 with a 58mm value. It was all blurred..
10-22-2010, 11:13 AM   #10
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Helpful thread!

And welcome to the 35-105mm cult, ricstew--it's a weird & wonderful lens isn't it?
10-22-2010, 12:08 PM   #11
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Its certainly a different lens! I only recieved it yesterday but will be able to give it a good try this weekend. The macro function is somewhat mystifying.......and its a heavy little thing! I havent quite worked out how to hold it yet!
cheers
Jan
10-22-2010, 12:24 PM   #12
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Yeah, it certainly has its quirks but gives great results.
10-22-2010, 03:03 PM   #13
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QuoteOriginally posted by ricstew Quote
Its certainly a different lens! I only recieved it yesterday but will be able to give it a good try this weekend. The macro function is somewhat mystifying.......and its a heavy little thing! I havent quite worked out how to hold it yet!
cheers
Jan
"Heavy"!!?.....Naaaah...try the A28/135......now that's a HEAVY lens....a good one too, but never much mention of it on here.
Cheers, Pickles.
10-22-2010, 08:16 PM   #14
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HEAVY? Naw, try the Vivitar Series 1 Version 1 70-210/3.5 (880g) or the Lil'Bigma 170-500/5-6.3 (1270g). But I digress. I'm slowly getting rid of my old manual zooms because I *do* zoom a lot and I *don't* want to twiddle with SR. (I'll put another batch on eBay this weekend -- my ID is r*m.)

Yes, I'm keeping that brilliant Vivitar and a few others, like a Tokina RMC 35-135/3.5-4.5, and a Promaster-Tamron 60-300/4-5.6. My practice with them was mentioned above: pick a single FL and feed that to the SR-bot, or zoom around with SR turned off. Longer heavier manual zooms are best used on tripod anyway, where SR is irrelevant. Or so I'm told.
10-22-2010, 10:17 PM   #15
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QuoteOriginally posted by m42man Quote
You're very welcome, BUT I've realised I haven't done my sums right:

I reckon that the calculation you need to make for a zoom lens is as follows:

x = H*L/(H-L)

where x is the focal length input to the camera, H is the max zoom FL and L is the min FL.

Also, the min SR effectiveness will be (x/H) * 100%

So, for your 35-105 zoom, x works out at 52.5mm, and min SR effectiveness is about 50%.

Since you can't input 52.5 to the camera, it's preferable to go to the next value LOWER than the exact value (so, 50mm here). (This principle of course should be extended to primes with "unusual" FLs.)

Many apologies for misleading you (and everyone else)!
I'll give this a try. So far I've always used the shortest focal length for the SR value.
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