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Let me start with the general question:

Consider the following situation. I want to take a picture of a particular object (that is, not a scenic shot). I can alter my distance from the subject and I can use the zoom on my camera. By altering both at the same time, I can keep the subject the same size in the picture but take different photos. My question is: what changes as I do this?

The obvious answer is "depth of field". So I'm really asking about what other effects I should be aware of.

Background: I've recently upgraded from a (decent) compact camera to one of the micro 4/3rds range and been experimenting with the new-found control over my photographs. Fairly early on, I learnt about "depth of field" and enjoyed taking some photos exploiting variations in that. The photograph that I'm currently trying to take is of a water drop splashing in a puddle (I've a fairly constant source of this due to a leaky gutter!). I've gotten a fairly decent photograph just by taking it on standard settings, but I'm curious as to whether or not I can do better. Obviously, taking the shot with as fast shutter speed as possible is the main thing to do, so I'm particularly interested in whether varying the focal length affects the range of shutter speeds, but I'd also be interested in other factors that are affected by focal length.

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If you adjust both zoom and distance to keep the subject the same size, then the subject magnification is a constant. If you also keep the f-stop the same, then your DOF is in fact unchanged. So your obvious answer is not correct. –  philw Sep 18 '10 at 17:45
    
Depth of field is only approximately constant with respect to magnification if you get close enough the depth of field will change. –  Matt Grum Sep 18 '10 at 21:38

5 Answers 5

up vote 5 down vote accepted

With regards to compression, as has already been stated if you move closer and zoom out you'll get more of the background in as you compress the perspective less. If you're photographing a single object against a plain background the effect is more subtle but you will get the same compression. For this reason longer focal lengths are considered more flattering for portrait photography as they make 'sticking out' features, such as the nose smaller.

Going back to the original question, zoom affects the usable range of shutter speeds, in that camera movement is exaggerated by a longer focal length, thus limiting the range of shutter speeds you can use without a tripod.

An often quoted rule is that you can hand hold shots for 1/f seconds, where f is the focal length. So you can expect to hand hold a shot at 1/50s with a 50mm lens but will need to up the shutter speed to 1/100s for 1 100mm lens. This rule is just an approximation, and you should take into account both crop factor and number of megapixels as this will influence the degree to which camera motion blur is visible in images!

Zoom can also change the aperture f number, since the term f/4.0 means the width of the aperture is the focal length divided by 4.0, so increase the focal length and the f-ratio gets bigger too (some more expensive lenses are designed so the apparent aperture increases with focal length to keep the f-ratio constant).

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The stuff about the shutter speed is exactly the sort of thing I wanted to know! Thanks. –  Loop Space Sep 28 '10 at 19:47

The most obvious change that images shot with different focal length while keeping the main subject size intact will show is perspective. By using a longer focal length (and, by necessity, increasing the distance between the camera and the main subject), you will get a more "compact" perspective, with the background appearing to be closer to the subject than when shooting with a shorter focal length. Here is a very clear demonstration of that.

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2  
That is a very clear demonstration! Thanks. (This particular effect isn't relevant for my specific case, but is clearly very important in the general case.) –  Loop Space Sep 18 '10 at 12:51
    
Another demonstration is the standard video shot, where the camera moves in as the operator zooms out, keeping subject size constant but appearing to "expand" the background through the shot. This is the main utility of zooms other than cropping: getting the perspective you want. –  philw Sep 18 '10 at 17:36
    
This effect makes for interesting video when watching the Tour de France on TV. Often the background will grow while the cyclist shrinks. –  Bart van Heukelom Sep 18 '10 at 20:19
    
Is this the same as background compression? –  jfklein13 Sep 18 '10 at 21:37
    
@philw Cropping has the same effect as zooming on perspective. –  Evan Krall Sep 19 '10 at 20:12

Apart from the perspective and depth of field there are some rather minor (or technical, if you like) aspects:

Changing focal length alters optical configuration of the lens. Diaphragm has to open or close to keep the same aperture (f in f/4 means focal length, so for f = 24 mm, f/4 means opening it to 6 mm, on 105 mm end of the zoom you need approximately 26 mm). Apart from the effect on the depth of field, this also means that diaphragm blades might become more visible. So zooming may change the look of out of focus parts of the picture.

Additionally, different grouping of lens element may alter look of flares when you shoot into the sun, or cause the image to be soft on certain zoom settings.

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It depends on the design of the lens, for some more expensive lenses the diaphragm doesn't need move when you zoom to maintain the same f ratio. The quoted f stop actually refers to the apparent size of the aperture which rarely corresponds exactly to the physical size (nowhere in a 600mm f/4 lens is there a 15cm hole!). It's possible for the apparent size of the aperture to change without the physical size changing thus the blades don't have to move as you zoom to maintain f/2.8 on the 70-200 f/2.8, for example. –  Matt Grum Sep 18 '10 at 22:00

If you are taking a picture of subject X, and subject Y is in the background 10 feet further, changing the zoom will change the relative sizes of X and Y in the photo. If you are closeup with a wide angle shot of X, then subject Y will be much smaller. If you are far away with a telephoto shot of X, then subject Y will be about the same size.

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Not often a problem, but the fact that the front of the lens rotates may be an issue when using a linear polarizing filter.

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Front of the lens rarely rotates when zooming. All Canon lenses I've seen (such as kit EF-S 18-55 mm) rotate the front only when focusing, when you zoom it stays the same way. –  che Sep 19 '10 at 9:55

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