why?The question arose of "I have a high quality prime such as a 20mm f1.7, so why would I bother buying a telephoto?", so I thought I'd have a whack at answering it. The asker professed to have some years experience with 35mm and seemed happy with just cropping. I covered this from another angle back in 2010 here, but as my purpose was different I thought I'd redo it entirely.
To some extent you can "increase" the focal length of your lens by cropping - which is exactly why cameras with smaller sensors than the 35mm standard format are called "crop cameras" and exactly why there is the idea of a focal length multiplier (or effective focal length). This is often a bewildering topic for beginners (and even seasoned 35mm photographers who never dealt with 645, 6x7 and large format).
The confusion often comes from the misunderstanding of f-numbers. These (often called f-stops, but that's also confusing) were designed to allow photographers to determine exposure back when a hand held light meter was used to determine exposure and the photographer wanted to change lens (and keep the exposure the same).
Commonly the confusion comes from discussions involving Depth of Field (DoF)
So if all things were equal then a standard lens could work as a telephoto by just cropping. However, if we use cropping (essentially maths) to make our 20mm into a virtual 50mm then we would just set our apertures to be the same and all will work out the same. Of course all things aren't equal, so I thought I'd explain this with pictures.
Firstly, lets take a shot of my mate with a 20mm f1.7 lens on my GH1
This was taken with the camera on a tripod, using the Panasonic 20mm f1.7 @ f1.7 and focused with face recognition.
Now, lets put on a 50mm lens (which is telephoto for 4/3) and have a look at that.
A quick glance at the image seems to show that DoF is quite similar to the central portion of the image taken with the 20mm ... so, lets crop the guts out of it and have a look.
Well actually its close isn't it, perhaps if I'd stopped the 50mm down to 5 it would be even closer. But you can see the point here (I hope).
Whats different?Firstly I encourage you to "right click" on the above images and open them up in separate tabs (or windows). I think you'll see quickly that even at the modest sizes we have that there is much more information (look at hair strands) in the image from the 50mm and not much at all in the image from the 20mm (cropped image). This is because we are essentially concentrating the image to be clearer with the scaled down 50mm image, but dilluting the 20mm image with magnification (and pushing its limits). Essentially we are pixel peeping with the 20mm but giving the 50mm room to breath.
Next the 50mm is able to stop down to further ... so lets look at that
Which I'm sure you can see is even more shallow DoF with the focus being drawn to the eyes and the background is just a diffuse background. Personally I prefer the look of this lens at f1.8 or f2, which is still more shallow DoF than f4 yet has a higher contrast.
Then there is the point that the image from the 20mm started out at 4000x3000 pixels and just by cropping alone wound up at 1512 x 1181 pixels .. which is a paltry 1.8 MPixels and really only good for a 4x5 postcard print (or the web).
In a tight corner that may be enough ... but should you shoot for that? Well that's up to you.
ClarityLastly there is the amount of clarity available. The 20mm is a sharp lens, but it has a maximum of about 7Perceptual Megapixels, while the Olympus 45mm has more like 9 ... by cropping you end up with 2 and given that the Panasonic had less to start with you may end up with less Perceptual Megapixels.
I'm really pleased with my legacy FD 50mm f1.4 lens(well and for that matter even more pleased with my legacy OM 50mm f1.8 which is just as sharp (see post here) and cost so little (like $18) its ridiculous. Indeed for folks now (such wasn't available to me when I got into micro43) the Olympus 45mm f1.8 is available in good condition used from around $300 or the Sigma 60mm f2.8 even less ... great optics well within grasp of many and if not then the legacy option is IMO fantastic value. Hell this portrait was taken with my Pentax 110 50mm f2.8 in another mates garage.
and that lens is ultra compact and light (and cheap too).