Fast Lenses

[Tywais]

I've been researching lenses to try and determine what's best for me (shades of Neverdie ). The price differences between a fast lens with the same focal ranges and slower ones are significant. Let's say for example: 70-200mm f3.5-f5.6 versus 70-200mm at a fixed f2.8. So, what are the mechanics that make a fast lens fast and so much more expensive?

I've been reading this site, what's a fast lens and it is quite informative but doesn't quite explain the mechanics of fast lenses.

Another naive question, if one has a prime 50mm f1.4, are you restricted to only f1.4 or can this be adjusted? Reason is that an f1.4 DOF will be pretty shallow and would like to know if the f stop can be adjusted to control DOF.

Really rusty on my technical knowledge as you can see.

[MarkBKK]

I've been researching lenses to try and determine what's best for me (shades of Neverdie ). The price differences between a fast lens with the same focal ranges and slower ones are significant. Let's say for example: 70-200mm f3.5-f5.6 versus 70-200mm at a fixed f2.8. So, what are the mechanics that make a fast lens fast and so much more expensive?

I've been reading this site, what's a fast lens and it is quite informative but doesn't quite explain the mechanics of fast lenses.

Another naive question, if one has a prime 50mm f1.4, are you restricted to only f1.4 or can this be adjusted? Reason is that an f1.4 DOF will be pretty shallow and would like to know if the f stop can be adjusted to control DOF.

Really rusty on my technical knowledge as you can see.

I'm sure someone will disagree, but the term ''fast lens'' generally seems to be applied to lenses with widest apertures of f/2.8 or lower (but it's all relative). I think the mechanics are: 1) better glass, and 2) allowing more light in. An f/1.4 lens can typically be stopped down to f/16, allowing a fairly wide range of apertures and DoF. One advantage of fast glass is that most lenses are at their sharpest a couple of stops up from wide open, so an f/1.4 lens may be at its sharpest at f/1.8 or f/2, say, at which apertures the lens still has a shallow DoF and still lets plenty of light in for taking pics in low light levels, whereas an f/3.5 lens may be at its sharpest at f/4 or f/5.6.

[The Vulcan]

Agree with "MarkBKK" as to a lens being F2.8 or lower. However, disagree with statement that +2 stops is sharpest.

Most fast lenses are sharpest at f8/f11

Fast lenses are optimum for low light (brighter image in VF), lower iso usage due to aperture range, stability (if no IS) because you can extend the shutter speed by a wider aperture, DOF and (usually) IQ.

I mean, who wants a 35-70mm zoom that is F4.5-F5.6 which Contax do for their G series RF's. Absolutely no DOF control and a waste of space. Unusable at (say) iso 100 inpoor light. Pretty much the same as your 70-200 where the 70mm is f3.5!

If you can't "open up" you can't throw the background OOF, and you'll also need a higher iso.

Irrespective of the lens rating (i.e. F1.4) you can use the full range from (say) F1.4 - F22

My 50mm F1.4 is at it's sharpest (and tested) at F8 BUT - I almost always shoot it at F2 for OOF and it's still VERY sharp at this aperture.

Edited August 21, 2009 by The Vulcan

[MarkBKK]

Agree with "MarkBKK" as to a lens being F2.8 or lower. However, disagree with statement that +2 stops is sharpest.

Most fast lenses are sharpest at f8/f11

Fast lenses are optimum for low light (brighter image in VF), lower iso usage due to aperture range, stability (if no IS) because you can extend the shutter speed by a wider aperture, DOF and (usually) IQ.

I mean, who wants a 35-70mm zoom that is F4.5-F5.6 which Contax do for their G series RF's. Absolutely no DOF control and a waste of space. Unusable at (say) iso 100 inpoor light. Pretty much the same as your 70-200 where the 70mm is f3.5!

If you can't "open up" you can't throw the background OOF, and you'll also need a higher iso.

Irrespective of the lens rating (i.e. F1.4) you can use the full range from (say) F1.4 - F22

My 50mm F1.4 is at it's sharpest (and tested) at F8 BUT - I almost always shoot it at F2 for OOF and it's still VERY sharp at this aperture.

And I agree with Vulcan. I should have said a few stops :-)

[Tywais]

Thanks for all the input. I noticed that the fast lenses, given same focal length between them and slower lens, are heavier. So, that would suggest bigger glass all around getting more light collection and probably higher quality thus explaining the higher costs.

[The Vulcan]

Thanks for all the input. I noticed that the fast lenses, given same focal length between them and slower lens, are heavier. So, that would suggest bigger glass all around getting more light collection and probably higher quality thus explaining the higher costs.

Dead right - usually attributable to the additional elements and the increase in glass size viz, cheapo lens has 55mm element, fast lens 77mm.

[Nordlys]

Most fast lenses are sharpest at f8/f11

Aren't most lenses sharpest at F8 fast or not, with fast lenses yielding wider range of sharpness than slower lenses?

Dead right - usually attributable to the additional elements and the increase in glass size viz, cheapo lens has 55mm element, fast lens 77mm.

Also the solid metal casing as compared to plastic body for cheapo lenses that makes fast lenses heavier. All my Canon L lenses seem to come in metal body (magnesium alloy?), completely different build quality from non-L lenses. Also rubber O ring for weather sealing that usually doesn't come with cheaper lenses (at least for Canon).

So, what are the mechanics that make a fast lens fast and so much more expensive?

Aspheric lens, extra-low dispersion glass, fluorite often used for optical element for Canon L series expensive (read fast) lenses.

Edited August 21, 2009 by Nordlys

[The Vulcan]

Most fast lenses are sharpest at f8/f11

Aren't most lenses sharpest at F8 fast or not, with fast lenses yielding wider range of sharpness than slower lenses?

Dead right - usually attributable to the additional elements and the increase in glass size viz, cheapo lens has 55mm element, fast lens 77mm.

Also the solid metal casing as compared to plastic body for cheapo lenses that makes fast lenses heavier. All my Canon L lenses seem to come with very solid build body whereas non-L lenses I used to own all came in plastic body.

Some of the cheapo lenses never get sharp!

[Pez]

I get the feeling you have been looking at Nikon lenses. The FX F2.8 are the premium lenses amid at the pro or the serious amateur with deep pockets , the rest are a compromise leading to affordability. Optics that vary in aperture with focal length are much cheaper to make, with the added advantage of reduced weight and lower cost. It gives you a choice super fast+maximum Q or a compromise - up to you. The Nikon F2.8 Zoom lenses are built of metal high durability but they also have a lot of dust sealing built in. The DX versions have bodies containing high grade plastic and catering for a smaller sensor - horses for courses.

You did alarm me that your question was amid high but when you asked if a 50mm F1.4 could be used at other apertures. There is only one type of camera lens that had a fixed aperture - mirror lens but they now seem to not be made any more. What the specs state is the MAXIMUM aperture and if you look further they will state the minimum ie f16.

Previous postings state the best performance is 2 stops down etc - it all depends on the individual lens, very often the poorest performance is at minimum aperture. Nowadays they are good at most stops!!! Just look at the optical performance charts.

[Nordlys]

Another naive question, if one has a prime 50mm f1.4, are you restricted to only f1.4 or can this be adjusted? Reason is that an f1.4 DOF will be pretty shallow and would like to know if the f stop can be adjusted to control DOF.

They indicate the maximum aperture. And don't forget it's not just the maximum aperture but also the focal length that determines the DOF. My 135mm F2 lens's DOF is probably shallower at F2.8 than 50mm at F1.4.

[Tywais]

And don't forget it's not just the maximum aperture but also the focal length that determines the DOF.

I did manage to forget that even though I posted in the pinned topics a DOF Calculator.

Not sure what I was thinking when I asked that question on fixed f-stops, it's obvious to me now when I think about the specs being maximum.

Pez, when you say mirror lens I assume you mean a catadioptric lens. I had a 500 mm catadioptric for my Nikon FE years ago. I also stumbled across one recently for sale on a site and appears they are still being made though how well they would work on an APS-C sensor may be questionable.

500mm f/6.3 catadioptric lens

Actually this one looks like the one I had years ago and believe it was Vivatar also > http://www.barthworks.com/nikon/vivitar800mm.htm

[astral]

Back when I only had prime lenses, the slowest was F2.8

and the fastest was f1.2.

I did not carry a flash or tripod, but managed some good shots in quite dark places,

mainly interior shots on my travels.

I would steady myself against a wall or pillar,

put the camera on the floor, pointing up for ceilings.

With modern zooms f3.5 or 4 seems to be fast............

I did look at the Canon 70-200 F2.8. An awesome lens, but way to heavy for me

to lug around. I settled for the F4 IS

[Nordlys]

Pez, when you say mirror lens I assume you mean a catadioptric lens. I had a 500 mm catadioptric for my Nikon FE years ago. I also stumbled across one recently for sale on a site and appears they are still being made though how well they would work on an APS-C sensor may be questionable.

I also had a Canon's old new FD mount 500mm F8. It's not a lens serious or professional photographers favored, but it was sharp enough for me and I loved the circular bokeh it creates. The aperture is fixed at slow F8 but you rarely needed it smaller aperture than that so it was not a problem. And the focal length makes the lens only practical for outdoor use so you'd almost always use it under daylight.

[Tywais]

I thought I would tag this onto my topic rather then creating a new one. I'm close to making my final decision for my dSLR and lenses. The lenses are a tough decision but this is my choice now.

A Nikon D90 with a Nikon AF-S DX VR 18-200MM. F/3.5-5.6G IF-ED and a Nikon AF 50mm f/1.4D prime. This site has the combination D90 & 18-200at a price of around 51k > Zoom Camera. What is confusing is that is significantly less then at many other on-line stores I've checked where they show the D90 body at 35k and the 18-200VR at 31k for a total of 66k.

Possibly that online store is selling gray market products? What do you think of the lens choice? The 18-200 would be the general purpose carry everywhere lens, and the fast 50mm for indoor and low light situations. Also the 18-200 VR is a G series (gelded, meaning reduced functionality, specifically no aperture ring) so not sure if that is an issue with the D90.

AF-S DX VR 18-200mm

Nikon AF 50mm f/1.4D

A useful page showing Nikon lens compatibility charts and what all the designators mean such as DX, VR etc. Nikonians.org

[Nordlys]

Nikon AF-S DX VR 18-200MM. F/3.5-5.6G IF-ED.

So all that tips you got above was not enough to convince you into buying fast (or premium) lenses?

Edited August 23, 2009 by Nordlys

[Tywais]

Nikon AF-S DX VR 18-200MM. F/3.5-5.6G IF-ED.

So all that tips you got above was not enough to convince you into buying fast (or premium) lenses?

I'm convinced but my bank account isn't.