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Variable vs constant aperture lens
Was not sure if other post went through.
Coming to the subject, I was under impression that in a constant aperture lens like Nikon 17-55mm f/2.8 we cannot change the aperture, until recently when I read this lens can have multiple f-stops. Considering the relationship between f-stop and aperture does it means this lens can have multiple aperture values? If it's true then what's different in this from a 18-55 variable aperture lens.
I'm an amateur and guess my question seems naive & stupid, but would really greatful if you could answer.
Well, there are no stupid or dumb questions. This is a questions which runs through your mind so very good you've asked it.
The thing with a variable aperture lens is, that if you zoom out, like with the 18-55 F3.5 - F5.6, is that you start @ 18 mm with F3.5. But when you zoom out to F5.6, you will have an aperture of F5.6.
With a constant aperture lens, you can use the whole range with the same aperture which is great when you are using the zoom mode on your lens. Or even if you want to make a nice bokeh.
But the most important thing is, if you zoom out with a variable aperture lens. (Lets say, a 18-200 F3.5 - F6.3) you will lose almost 3 stops of light, which you have to compensate with your ISO or your shutter speed. And sometimes you will need a faster shutter speed so you have to bump up automaticly your ISO.
And with a constant aperture lens, you will keep the same settings throughout the whole range, while using the wide angle like @ 18mm, or @ 200mm, you will have the same aperture. So you can use in theory the same settings.
Hope i answered your question.
Now -what does that mean? If a lens has a max aperture of 2.8, then that lens can allow twice as much light as one with a max aperture of 4 or eight times as much light as the 5.6 opening
Lets do a "If What, If Then".
If you have a 70 - 300 lens. You have two versions.
You have a 70 - 300mm F3.5 to F5.6
And you have a 70 - 300mm F2.8 (Which do not excist.. but well)
When you begin on your first lens. You will start with F stop F3.5 where the second lens starts at F2.8, so thats less depth of field then you are starting with the first lens at F3.5.
but when you zoom out, your F2.8 lens stays @ F2.8. But your first lens expands the aperture to F5.6 @ 300mm. So you will loose in this case 2 stops of light while zooming in. So you will have to go from shutter speed 1/500 @ 70mm, to 1/125 @ 300mm.
Even though, on all lenses you can go up to at least F20.0 if you want to. But the prices of the lenses go up, as better the F stops will be. So the 2.8 glass will always be more expensive then the expandable aperture glass.
Hope it clearified some to you.
But you considered the 200 $ more 24-70mm f/2.8?
The nikon 24-70 mm has the nano coating.. which is awsome and you got a little more zoom than with the 17-55 which is.. well.. i don't want to be harsh, but it is a kit lens...
Hope i do not offend you..
Thanks a lot for you suggestions.
I have a D7000. Initially I planned to buy 24-70 only but later realised that 17-55 on DX would give me the effective focal range of 24-70 on FX, also I was not planning to upgrade to FX very soon.
Also was bit confused whether to buy Nikon or third party 2.8 lens [and save some $s ]. But later made my mind for Nikon only. I already went Third Party [sigma] for wide angle [8-16mm].
Should I reconsider the decision again?
Well, i am not the person to say what you should buy. But in your position and with your body, i would definatly go for the 24-70 2.8. I mean, i shot with the kitlens 18-55 and it was to short for me to do stuff, if you realise that your eyes registrate around 35 to 50 mm, the 17-55 is even as much as your eyes can see.. so thats why i would go for the 24-70.
And looking at your body, lets say you go in around 2 years, maybe 3 for a new camera. Upgrading to a D800 or so would be a normal and good step from out of the D7000, and you can throw the 17-55 away in that case because it won't work on a FX camera.
And, imo the 24-70 will keep its value for if you want to resel it later on ebay.. and i think if you want to sell the 24-70 you will lose it within a day, and i don't think you get that with the 17-55.
But again, just my 2 cents.
Greetings and good luck!
Thanks again for the advice.
I can consider 24-70. The only hindrance was the effective range of 36-85mm [which felt bit strange]. But yes, I can compromise on the missed length by using my 18-55 kit lens till I upgrade .
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