Newbie question about zooming

[Panda]

I know that when I zoom, say from 25mm to 300mm, a very steady hand is need, otherwise the picture will blur. The same thing happens to a lesser degree when I zoom from 25mm to 38mm.

Question: Assuming everything else is the same (i.e., same anti-shake feature, same mechanism, same shutter speed, same sensitivity level, etc.), will I ALWAYS get the same amount of "shakiness" when the focal length is the same?

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Examples

For Camera A with 25-250 mm lens, and Camera B with 40-400 mm lens:

Case1: Camera A is zoomed to 40 mm, while Camera B is at its widest focal length of 40 mm.

Case2: Camera A is zoomed up from 25 mm to 50 mm, while Camera B is zoomed a little from it widest view of 40 mm to 50 mm.

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Thank you in advance for your help.

[xeliex]

Hey Panda,

When referring to absolute focal lengths numbers, we have to factor in the device the glass in mounted on. For example, on a full frame SLR, 300mm is 300mm in 35mm standards (like a golden rule). However or a Canon 40D the 300mm is effectively 480mm and 450mm on Nikon bodys (except the full frame Nikons).

Let's take a look at your examples.

Case 1: Camera A and camera B both are at 40mm. Assuming that their digital sensors (or films) are of the same size, yes, you will need the same shutter speed to avoid camera shakes. It doesn't matter what the "zoom" is, it's the effective focal length that matters.

A rule of thumb photographers use is 1/x shutter speed where x is the focal length. In your example you can go (theoretically) down to 1/40 shutter speed and still be considered safe. However, if you are shooting crop cameras, your minimun safe speed will be more like 1/60.

Case 2: Same. It's the effective focal range that matters. Keep in mind though that anti-shake throws a big donkey bone in what was said above. Different cameras and different lenses have various stabilization mechanisms that can varry greatly from manufcaturer / model to another.

Image stabilization on some lenses claim to give you 3-4 extra stops, so in your example here if you are shooting a crop canon body, your 50mm needs 1/80 or faster shutter speed without stabilizer, but possibly can go down to 1/15 (theoretically).

[sebasj]

Well, I'm not sure if I'd agree in all those statements, Xeliex There's the appropiate magnification with regards of the focal length, and then the apparent magnification which is product of the sensor crop.

Now, in 35mm, the rule of thumb of shutter reciprocity to focal length applies specifically to the focal length magnification. The other magnification, the one that comes from sensor crop, is apparent, not real. Hence, I think that it still applies the rule of thumb that you described, despite the apparent behaviour of the focal length.

However this observation, the slowest speed usable for a given focal length depends on a lot of things.

To answer Panda's question, for a given focal length, theoretically the same amount of shakiness would come despite the lens. However, what can you hold more steadily, a 40-300 or a 20-40? Here is when other factors come in: your pulse, balance between camera and lens, etc. This is why is pretty hard to state anything in absolute terms of veracity.

Best,
Sebas.

[Panda]

Thanks, guys.

Panda