View Full Version : Question about lenses

16-02-2011, 12:44pm

If given the choice between 2 lenses of the same focal length but different f/x value, is it generally (not considering the build and image quality of the individual lenses) better to choose the larger aperture as this will give you more flexibility in terms of aperture size?
example: 50mm f/1.8 and 50mm f/1.2
70 - 200mm f/4 and 70 - 200mm f/2.8
If so, then why do people choose the smaller aperture lens instead? Larger aperture = higher cost?

Thank you


16-02-2011, 12:50pm
I'm not sure on all the nuts n bolts of the matter, but, the smaller the f stop a lens has, the more light it will let in . Or the less amount of light it will need in a dim situation to give a better result. Hence the greater expence.

16-02-2011, 12:56pm
true about the cost... fast lenses cost more generally.... i have a sigma 150-500 which is f6.3 at the pointy end... it cost me about $1.5k. I wouldnt even like to guess what it'd cost if it was a f2.8 or even just a f5.6!

16-02-2011, 1:16pm
with a 70-200 f/2.8, you can set the lens to perform at f/4 but with a 70-200 f/4, you CANNOT set the lens to a f/2.8.
in general, yes, higher cost. in the case of the 70-200 f/2.8 lenses are heavier than the f/4

.Cost (in most cases)
.Weight (in some cases)
.bokeh a f/2.8 photo will generally be more blur than the f/4 and the f/22 is "sharp" throughout the whole photo. -> so people pay more to get it.

16-02-2011, 2:40pm
But then as another eg, when I had minolta lenses, the f/1.7 was much sharper than the f/1.2 AT ALL APertUres. I'm basically saying that you've got to test lenses individually.

16-02-2011, 3:34pm
Larger aperture will allow more light in when wide open, and usually that means larger front element and more glass, more complex optical formula, and backgrounds more oof, which all equates to usually more weight, and always more $$.

A comparison is a 300/4 in Nikon AFS flavour costs about $1200, but the 300/2.8 variant is about $5000 for what on paper is only 1 stop of light, you pay the premium $3800 when you could just bump the ISO 1 stop for the same result. Build quality, bokeh appeal, extras like VR/IS and element coatings, focus speed etc are what usually come with the more exe versions and these are usually what pro's will buy so they come built and priced accordingly.

If you ever get the chance to see how fast a Nikon AF-S 400/2.8 ($11,000) can grab focus from one closest to infinity and the quality of the bokeh that should show where some of the $ goes in a pro grade lens.

16-02-2011, 6:50pm
So cost and weight is the factor, also the consideration of each individual build quality.
Good point on bumping the ISO 1 stop for same result, Wayne. I'll keep that in mind.
Thanks for all the feedbacks.


17-02-2011, 2:32am
Imagine you are shooting a pride of lions killing a buffalo whilst on safari in Africa in late afternoon. You dont want to miss it do you? On your trusty body of choice is a 300mm f/4.

So to keep the action sharp you are shooting Tv mode at 1/400s and wide open at f/4. You might need the ISO to be 400 for the images to be properly exposed. Then the pride drags the kill under an acacia tree to eat in the shade. Now you need to bump the ISO to 800 or 1600 to see the feast - but it is slower moving, so maybe you could keep the ISO at 400 or 800 and halve your shutter speed to 1/200s. By now your images are going to be showing some sign of strain due to noise - especially when you blow it up to a 150 x 100cm poster sized print back home.

What about if all this had been shot with a 300mm f/2.8? To start with you can keep your Tv 1/400s and drop the ISO to 200. When the action heads to the shade you only need to bump it up to 400 or 800 - or lower the shutter speed to 1/200s and keep your ISO at 200 or 400. Imagine how much better that print will look now?

For me ISO is the last thing to raise. Not to make light of it (boom, boom! :lol:), but I would rather raise the extra cash and go for a larger aperture than raise the ISO! But a smaller aperture lens at a given focal length is better than no lens at all!

18-02-2011, 11:28am
markjaffa is right:) ISO is the last resort as this begins to create noise and depending on your current set up, it could be a deal breaker. In General, a wider aperture is better but as ameerat42 mentioned, each lens is different and perform with varying degree, which dictates some testing. Quality comes at a price for 2.8 or wider, but you can get some third party gems for a fraction of the cost, albeit at the expense of auto focus etc. and of course getting a good copy.:)

18-02-2011, 12:52pm
apologies for jumping in with silly question;
If it is best not to raise the ISO, then why do cameras have ISO up to 800, 1000, 3200 etc etc. ?

18-02-2011, 12:54pm
using the two examples you have given, the reason one might choose the "slower" lens over the other is usually (but not always) because it is less, expensive, lighter, sometimes sharper or as sharp, and does virtually the same job. With higher ISO cameras with less noise these days, the absolute need for the faster lens has become less.

bottom line, horses for courses. Pick the lens that will best suit your needs. Do not be seduced by the so called flashier lens that everyone says they want. You do not need to buy the Maserati if you can comfortably use the Mazda to go to the shops and drop the kids to school. Vice versa if you are a professional speed demon.

19-02-2011, 12:26am
I've been reading about good and bad copies of the same lens and it is important to test before you buy. If in the case of a beginner, any tips on what to look out for to determine if the one you are about to buy is a good or bad copy?


19-02-2011, 12:50am
Good or Bad copy, the only way to tell is to take some photos and view them on a screen (preferably not the back of the camera). If the aint soft all good. Try some of the bigger stores as they are likely to have more than 1 or 2 of the same lenses.