It's important to know exactly what you'll be photographing. Now that might sound strange but if all you want to do is astronomical photos then you'll likely be buying a T2 adaptor for your camera and putting it onto a telescope - no need to buy any lenses, so why buy any? If all you do is sport action photos then all you need is a 70 - 200 zoom or 70 - 300 zoom or perhaps a couple of fixed focal length lenses (also known as prime lenses) of 200, 300 or 400mm focal length.
A lot of amateurs will fret that they don't have the "correct" focal length for every conceivable occasion. This fretting is what costs them a lot of money, pointlessly. If you want to cover every focal length then get a 28 - 1000 zoom compact and use that. The lens on a zoom compact will reach every focal length though the quality of the photos obtained may leave room for improvement.
I am a staunch advocate of prime lenses. Zooms are good but switching cameras with different focal length primes fitted is a lot faster than zooming and a lot kinder to the lens. Zooms are not designed to be anything more than a compromise. They're far better now than they ever used to be but they're still not all that great. At the wide end they often display barrel distortion (where straight lines bend toward the edges of the picture) and at the narrow end they often display pincushion distortion (where straight lines bend toward the centre of the picture). At the long end they can lose several stops of light too. An f4.5 - 5.6 zoom lens will be f4.5 at its wide end and f5.6 at its long end. Immediately the light has been reduced by 50%. This is no good whatsoever for most things. So, zooms are slow, heavy and cumbersome - their only redeeming feature is they're cheap. I have a Canon 17-85IS lens. It was about $500 a few years ago, brand new. It's acceptable but not a great lens. At the wide end there is distortion and at the long end it's quite a dark lens. The killer is - it only replaces three prime lenses. Were I to replace this with prime lenses then I would go for the following: wide, standard and telephoto lenses. For my use because I use a camera with a smaller sensor, I'd go for possibly 16/17/18mm wide angle, 28/35 standard lens and maybe an 85mm telephoto.
The interesting thing about lenses is that the longer the lens is, the less difference there is per millimeter. Thus a 14mm lens is a lot wider than a 17mm lens but a 200mm lens is not that much different from a 135mm lens. Between a 28mm and a 35mm the difference is barely noticeable in use. I made the mistake of buying both when I used 35mm.
When buying lenses, it's usually best to go for the best lens possible. A cheap independent lens is only going to give your photographs the quality of a cheap independent lens. Even if you buy the lens new, it's not going to be anywhere near as good as a lens produced by the camera manufacturer. As an example, I have a Tamron 17-35 lens that was inexpensive. I barely use it because the photo quality - even at the optimum aperture isn't that great. Now we're talking about cost. As an example, the 14mm Canon prime lens is about $2,500 at the time of writing. It's serious money. The Canon 10 - 22mm zoom is $860 by comparison. It's easy to see why people say "Hmm... Serious money for one lens or not-so-serious money for a zoom". Then they look at the independent lenses and say "Hmm but I can get a 12 - 24 from Tokina for only $400". This is where things unravel very rapidly. When you start to let your wallet dictate your lens choice is when you start on the spiral of regret. It is better to decide what you want and then to get then to save for it than to buy things that are never going to be as good. As the proverb goes: I'm too poor to buy cheap things. At best you will buy a cheap lens, be disappointed and sell it for a fraction of what you paid then buy a more expensive one and repeat the cycle many times over until by the time you get the lens you should have bought in the first place you'll have paid its price many times over on buying rubbish and losing money selling it to the next sucker.
Cheap lenses will suffer from distortion, chromatic aberration (or funky colored lines around areas of high-contrast), soft images (where lines aren't quite as sharp as they should be) and a whole host of other issues.
At this point I'd better say don't buy stuff with loans. If you can't afford it, save. If you can't save then you either don't want it that much or you don't have enough money to take up photography. Loans are just a way the fools among us pay somebody else to do our saving for us. Putting the money that would be spent on loan repayments into the bank would earn interest and then at the end you'd get the lens that much sooner. Paying for it with a loan means you're paying interest on the loan plus other charges and it'll take a lot longer before it's paid for and should the unexpected happen, you won't have the money to fall back on instead of getting the lens but you'll have a loan to repay and an unexpected event to cope with. Double jeopardy.
As far as camera bodies are concerned, these days they're all pretty good. I'm still using 8 megapixel cameras from 5 years ago. I have no problems with them. Incidentally, the current average is about 18 megapixel. That sounds a lot but the reality is that it's really not that much different. 8 megapixels yields an image 3456 pixels on the long side. 18 megapixels yields an image 5184 pixels on the long side. At 200 dpi the difference in print size is 17 inches to 25 inches. It's about 50% bigger and that's all. Your printer will still only pump out at most an 11 x 19 print and dropping the 8mp print to 180dpi will give that extra couple of inches with no noticeable drop in quality. I'm not going to spend any effort on telling you what body to get. I simply don't care. I'll use any camera body that comes my way.
Now onto the main thrust. We've discussed why prime lenses are better than zooms and why cheap junk lenses are a pathway to regret. Now let's look at focal lengths. This is what the question is really asking though the above is how we get to the answer.
Using 35mm as the example (some digital SLRs have an effective 1.5 multiplication factor on 35mm equivalent lenses such that a 50mm lens on a 35mm camera might have an effective 85mm focal length on some digital SLRs), the standard lenses would be:
85mm - 100mm for portraits and some would even go as far as 135mm
24mm - 28mm for wide-angle
50mm - the "standard" lens that most cameras used to come with. It's about equivalent to the focal length of the human eye.
300mm - 400mm for sport
500mm - 1000mm for bird/aviation/wildlife photography
With the 1.5 and 1.6 multiplication factor on many lenses, this would mean wider lenses would be needed than above. Thus:
24mm would need to be 1.5 - 1.6 times wider - thus 16mm etc
50mm would need to be 35mm etc
You can probably see how that all works out. Thus, for a digital SLR with a multiplication factor, a 14mm lens would be $2,500 from Canon. The equivalent in full-frame format would be 21mm - this could be either interpreted as 20mm or 24mm at $540 or $850 respectively. Thus, getting a 20mm lens and a more expensive full-frame body could well be more economical than buying a body with a smaller sensor and a 14mm lens. Indeed, the savings would be $2,000 approx on the lens alone.
At the longer end, a body with a multiplication factor does extend a long lens at no extra cost. A 1,000mm lens becomes a 1,500mm lens at no extra cost. Long focal lengths are where the cost racks up tremendously though. Canon's 800mm lens comes in at over $13,000 - that's right - thirteen thousand dollars. That's enough to buy a brand new car or a mobile home. Who said photography is cheap? Canon does have a longer lens that's 5,200mm long and weighs about 220lbs. That lens costs over $100,000 and is made to order.
The irrevocable conclusion has to be therefore that for longer focal lengths a body with a multiplication factor is an asset but a penalty at wider focal lengths.
In defense of smaller zooms and multiplication factor cameras, this is a photograph I took in Key West last week at an hotel I was staying in. The resolution of the lens isn't bad and the various aberrations aren't bad either. It is not a top quality lens. It's a Tamron 17-35 zoom on a Canon XT body. The same scene taken with a prime lens would most likely have been a lot better. Having said that I took one body and one lens and stuck with them the whole holiday. The images are acceptable though not the best quality obtainable.
Now, it's up to you to decide where to go with this information. The same camera/lens took the following photograph of the night sky. Not a bad pairing. They could be improved on though not without spending a lot more money.
Speaking of starfields, I'm planning a trip sometime to photograph the Milky Way from one of the more remote hilltops. That should be interesting.
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