Talk to me about lenses?

[sjt269]

Hey all,

I'm considering getting back into photography after a few-year gap while I've been at uni. As a teenager, I was pretty into photography and I had a Fuji bridge camera (which I still have) that got a lot of use, but now that I'm earning money, I'm considering going for an entry-level DSLR and learning photography properly. I'm pretty much set on the Nikon D3100, though I'm in no rush and I'm waiting for the price to come back down to the ~£250 it was a few weeks before Christmas. I can't afford the D5100 at the moment, though I'll keep my eye out for any not-to-be-missed offers.

The thing that's throwing me though is the step after buying the camera and the kit lens - buying additional lenses. I know this is a while away for me, from a purely financial standpoint at least, but I'd like to know what I'm buying into. I've got a vague working understanding of shutter speeds, apertures and focal lengths, and I've been doing a lot of reading lately, but never having used a DSLR or a specialised aftermarket lens I'm still left puzzled by a few things, so I hope you good folks here can help me out with a few things.


i) Why have an 18-55mm and a 55-200mm lens when you could have one lens that does 18-200mm? I picked those numbers purely as an illustration, but I don't see why you'd want 2 lenses to cover a range of focal lengths when one could do it. Is it price, or is it a case of jack of all trades, master of none?

ii) Carrying on from that, I don't understand primes. They seem to be fixed focal length lenses (or is there more to them?), which I take to mean they have no zoom capability. Again, why would you want a 50mm f1.8 prime (which I think is one that's mentioned a lot) over a lens that covers 18-55mm and on the face of it seems more versatile? I can only guess that the performance of the specialised lens at 50mm must be a lot better than the non-prime lens, but I don't quite understand how. Does anyone have any examples of pictures taken at the same focal length on a prime and non-prime to highlight the advantages of a prime lens?

iii) Again, fixed focal length lenses. The lenses say things like '50mm f1.8' or 40mm f2.8G', which seems to imply fixed focal length and aperture, but I see people on the forum here talking about using a variety of apertures with these lenses. Am I misunderstanding forum posts? Is the aperture stated in the spec a limiting value, rather than a fixed value? Why then do some lenses state a particular range of apertures?

iv) What's meant by 'fast' prime?


I know these are all noob questions, but I'm at the stage where every new thing I read confuses me further! Any replies will be much appreciated.

Thanks,
Steven

 

[sjt269]

Fantastic, thank you both! I think that may take a few read-throughs, but that's exactly what I was after.

Bokeh, this is what is know as how nice is the blurry bits in the photo. General population don't really pay attention to this, not consciously. They just see blur, but subconsciously, I believe that when someone see a photo with a nicer bokeh, it makes a nicer photo. And a nicer lens generally gives a nicer bokeh.

I've seen a few comparisons of this recently while searching for what 'bokeh' meant, and you're right that I've never consciously looked at the blur before, but looking at two almost identical photographs taken with different lenses, the difference is quite striking. I didn't realise primes were particularly nice for this though.

Light is the key to a good photograph, understand light will go a long way improving your images.

I like to think I have a pretty good understanding of light (I'm a theoretical physicist), though it's on a rather more microscopic and mathematical level than is useful for photography!

So am I right in saying that the same f-stop value at two different focal lengths corresponds to a different physical aperture size? So a 50mm at f4, say, isn't really directly comparable to a 100mm at f4?

And on the subject of zooms, is it right that longer focal length = more zoomed in, and if so, is this always true or just a rule of thumb? I'm more used to telescopes, where the magnification is given by the ratio of focal lengths of the lens (eyepiece) and the telescope itself, but I've never seen camera zoom described in quite the same way. In the case of the telescope, I can see that the magnification has a simple relation to the lens focal length, so is this simplicity also true for zoom lenses in cameras?

hbuzz, you've mentioned full frame and crop sensors there, so I'll take the opportunity to ask this question too - what's the difference, and what's the effect on the focal length? I keep seeing qualifications when talking about lenses on full frame cameras as 'equivalent to...' some other focal length on a crop sensor camera. I'm also under the impression that crop sensor cameras are the 'cheaper' normal cameras, and full frames are the elite top few. Is this right?

 

[sjt269]

So you need to know the sensor size of your camera to be able to work out exactly how the lens will act? And do different non-full-frame DSLRs tend to have wildly different sensor sizes, or is there some standard they all adhere to?