Deeper f/stop Q's

Van Isle

I understand what the numbers mean and what they do. I can control depth of field, I love my nifty fifty, my slow 18-200mm tweaks my melon in lower light action sometimes...but why, please explain...

WHY CANNOT EVERY LENS BE OPENED RIGHT UP TO F/1? :dunno Why not? If your lens had poor light transmission, wouldn't you want it to get more light by opening the aperature? :dunno

pls 'splain. Thanks,

VI

pyroPrints.com

I'll second that question, I'm assuming there is oem sorta optical threshold, but i'd be nice to know how it works

LiquidAir

For simple lenses, the widest aperture is the focal length of the lens divided by the diameter of the objective, so a 200mm f/1 lens would have minimum objective diameter of 200mm to open to f/1. 200mm diamater is a big hunk of glass. Real world SLR lenses are complicated, particularly retro-focus designs (read wide angle) and zooms, so often the actual limiting aperture is not the objective.

Here is a sample of a relatively simple lens: my 200/2.8 has a filter diameter of 72mm. 200mm/2.8 = 71.4mm.

Large objectives are hard to manufacture. For telephoto lenses, you'll find that the objective diameter is a fairly good predictor of the price of the lens. In particular, lenses objectives larger than around 77mm tend to be frighteningly expensive.

DavidTO

LiquidAir wrote:

In particular, lenses objectives larger than around 77mm tend to be frighteningly expensive.

And heavy!

pathfinder

There is no theoretical reason that you cannot make an f0.95 lens at 100, 200 or 400mm ( that's just slightly faster than f1.0) - but the practical reality is that large front objective lenses get very large, very fast, and get very expensive. Also, as lenses get larger in diameter they get quite a bit thicker also, which increases the number of optical corrections required due to increasing lens thickness. So the lenses may be faster, but they may be giving up sharpenss and contrast at the same time. Larger lenses require heavier lens barrels, larger iris aperture diaphragms, etc etc.

The real answer is that manufacturers already make most lenses in as fast an aperture as most folks are willing to pay. Check out the price difference between the Canon EOS 85mm f1.8 and the 85mm f1.2 L - The price quadruples, and in many ways the cheaper lens is the better lens in terms of size, weight, and focusing speed.

The market for lenses that cost more than $2K or $3K apiece is pretty small unless you are spending OPM (other people's money) - that is to say, only governments are willing to spend unlimited sums to shoot pictures in very low light.:D It is cheaper to buy a good Speedlite.

Van Isle

Duh! When someone points it out to you (thanks EVERYONE!) it becomes frighteningly obvious. i.e. an objective lens of 72mm cannot be much more open than, say, 72mm...despite the 200mm "focal length" achieved by complex optics (in a zoom). Wow. This place rocks

VI: less confused, and now loaded with more (to my friends) UFI

TylerW

Simpple anser: My 50mm prime can't open up to f/1.0 because my wallet can't open up to the cost of $5000

claudermilk

It is a pretty simple answer: cost & size. For a good example of the size issue, look at Sigma's new monster--the lens is enormous & has to be in order to get the f2.8 at 500mm. http://www.pma-show.com/0176/sigma/slrlens/sigma_apo_lens/
Imagine how truly huge that thing would have been at f1.0? You'd need a truck to move it, and a second mortgage to buy it (not like this one doesn't).