lenses

Hi guys, im trying to workout advantages of using fast lenses and disadvantges.

Also im a little confused about constant aperture/lenses and variable lenses

can anyone help clarify

ta

Fast lenses -

Advantages
* Work in darker surroundings
* Able to use faster shutter speeds
* Lower f-stops can isolate the subject more effectively

Disadvantages
* Heavier
* Expensive
* Can be soft (ie not as sharp)
* Focus becomes more critical at lower f-stops

It depends what you want to do. Fast lenses are not always better. I've had fast (non-Pentax) lenses that had problems with flare and sharpness. I now have a couple of fast Pentax lenses (eg 50mm/f1.4) that are just amazing.

Regarding constant aperture zooms - if you use automatic exposure modes, and/or TTL flash then there's not really a problem at all. Constant aperture zooms are perhaps more difficult to make, and are therefore often more expensive.

If you need fast glass - then there's no alternative.
If you're hoping to get good glass, then fast is not necessarily the answer. You have to consider the whole characteristics: bokeh, flare, colour, contrast, resolution - and not just speed.

Hope that helps
Matt

PS: I moved the topic as it suited the section "Pentax Lenses" more

Thanks Matt.

Matt is correct when describing fast prime lenses, however when it comes to fast zooms lenses (e.g. F2.8 ones) then it's my experience that they are usually sharper than slow zooms so one of the disadvantages mentioned "Can be soft (ie not as sharp)" doesn't usually apply. The weight and cost disadvantages still exists however, and to a greater extent than with primes.

If we compare the popular 80-200mm range then the F2.8 zooms (including the 3rd party ones) perform at a significantly higher level than the F4-F5.6 consumer zooms, however they are a lot heavier and a lot more expensive. Despite that it's my opinion that a 80-200mm F2.8 is one of the most useful lenses available.