silly question, does lowest f affect a lens at other f settings?
 

Does an f2.8 (say) lens collect up more light than an f.5.6 lens through its design geometry or is correct exposure independent of lens?

This prompted by the fact that my Dunnock shot about this time last year
http://www.worldphotographyforum.com...4173&nocache=1
worked at f13 using sigma 50-150 2.8 lens giving adequate DOF at 150 about 1.2M away (its a crop of approx 50% width of the frame taken) in what I thought was terrible light, admittedly ISO800. Perhaps more light than I thought bounced around by snow??

Struggling now with ISO200 f9 1/250 400mm in very good light and not getting enough DOF for similar sized bird using Nikkor 80-400 at closest focus ie 2.3M

Related question if not already answered - what circumstances would f.2.8 or lower be useful for other than pankakes flat on?

Chris,

Short answer.

f2.8 allows for use of wider aperture than say a f5.6 lens in low light or to limit dof. Once it reaches f5.6 then any advantage of that wider aperture is lost, other than a brighter viewing screen and more responsive A/F.

Dof on a 400mm lens is very shallow particularly at wide apertures close up to the subject.

Don

Depth of field increases with the distance from the subject so, at the 50mm end of your zoom, a landscape shot at F2.8 should show reasonable d o f.

As I understand it, the actual size of the aperture when viewed from the front of the lens controls depth of field. Thus, at a given distance your lens at 150mm, F 2.8, will have the same d o f as a 300mm at F5.6 or a 600mm at F11.

Taking this a bit further, if you put yourself in the subject's position, then from a great distance even a big fast telephoto lens will appear to have just a pinhole-sized aperture - hence those 'stacked-up' effects of distant detail are all pretty sharp even though they may be some distance apart in reality. It's the distance that provides the effect, the lens just selects how much of the scene is recorded.

Please feel free to shoot me down in flames if I've got this wrong ;)

Another difference is that, most of the time, fast lenses (say f/2.8 instead of f/5.6) are better quality lenses. There is no law of optics making this so, it's economics. Manufacturers rarely spend whatever it takes to make a really high-quality lens in (e.g) 50mm f/3.5 (which is slow for a 50mm) or 400mm f/6.3, and when they pull out all the stops to make a fast lens (e.g., 50mm f/1.2 or 400mm f/2.8) they also take the trouble to get just about everything else as good as possible.

The result, in practice, is that a fast lens is usually a good quality one, even though you may not be actually using the maximum aperture. It is also nearly always quite a bit bigger, heavier, and much more expensive.

Finally, only a handful of the very best lenses perform at their best wide open. Most lenses are sharper a stop or two down from their maximum aperture. So, if you want to shoot at f/1.8 with a 50mm lens, you could use a typical cheap plastic f/1.8 lens wide open (which will return a so-so result) or an expensive f/1.2 stopped down to f/1.8, which will not only provide you with better quality glass in the first place, it will also be performing close to its best because you have stopped it down a little.

With all that said, don't worry about it too much. The real difference between top-quality glass and cheaper lenses isn't really image quality in good, normal conditions. Let's face it, depending on the type of photography you do, most of the time you will be shooting at f/8 or f/11 anyway (for depth of field) and at f/8 or f/11, nearly all lenses perform pretty well.

For example, in good light at f/8 or f/11, there is practically no quality difference between my $130 plastic Canon 50mm f/1.8 II and my $800 Canon EF-S 60mm macro. Similarly, given good light and a modest aperture, at 35mm my old $100 EF-S 18-55 delivered nearly the same quality as my vastly bigger and heavier $1700 24-105.

So why spend the extra? Obviously, a cheap, slow lens can't do the low-light or shallow depth of field shots that an expensive fast one can do. Sometimes this matters, sometimes it doesn't: depends on the sort of photography you do. More importantly, in my view, top-quality lenses (which are usually fast lenses) seem to have a way of performing well even when the light is bad. I'm not sure why this is, but I suspect that it is probably nothing more than the accumulation of a lot of small differences, the sum total of all those small, expensive, attention-to-detail tweaks the manufacturer made when putting the expensive lens together.

f2.8 or faster lets in more light = faster shutter speed = no more milky waterfalls!!! To make the most of the larger aperture, you will need to understand hyperfocal distances. Try Cambridgeincolour for a good explanation and hyperfocal distrance charts (you'll also understand why I don't bother with fast lens).

Sorry Mike but I do not understand this. Using a fast lens at wide apertures can isolate the subject from the background as in the attatched pic. Hyperfocal distance immaterial.

No more milky waterfalls ??? Just stop the lens down.

Chris,

I will look for my 400mm dof tables and draw up a chart so we can all understand your situation as it relates to your post 1.

Don

PS : Decent engraved dof scales are something I miss on modern lenses. My old AIs stuff is good for that.

Thank you all. I am also a fan of Cambridge in colour and as you know Mike, it also has links to more erudite though usually more messy sites, but hyperfocal distance is a stage too far for me at present and the drift of it seemed to end up a matter of what the eye can really see or not.

I agree with Tannin about buying the best lens(es) one can afford and is prepared to carry.

I did use the CinC DOF on-line calculator (chart provided with Nikkor 80-400 VR a bit small to read at 400mm end) and was rather shocked by the results for 400mm at minimum (2.3M) range, but this is merely confirmation of what I was finding with flowers and dozy birds. I need to think a bit more about the combination of aperture size and speed advisable at relevant distance and focal length. I have no regrets on choosing this as my main lens, just a matter of learning how to get the best out of it.

My lens fund is a bit empty and I was finding your ruminations in the other thread interesting, Tannin, but not necessarily going to slipstream you! I am also a bit frustrated over getting the sensor clean on D80, so less keen on dozens of lens changes. Ken Rockwell suggests converting the lens for quasi-macro using Canon D500 close up ring and am inclined to try that in due course.

Does the D80 not have a sensor cleaner built-in? I thought all the recent releases had a sensor shaker these days. Or have you got some stubborn dirt, Chris?

Like you, I got frustrated with dirt on the sensor, a bit fanatical in fact, as I have bought extra bodies over the last year or so and don't need to change lenses nearly as often now. My last remaining non-self-cleaning body, a 20D, essentially never has the 10-22 unplugged from it, and the 40Ds I am most likely to need different lenses on largely look after themselves.

"Ruminations". That's a nice word for it. I'd have called it a "dummy-spit" myself, but them I'm not really a gentleman. :)

Hyperfocal distance is actually pretty simple if it's explained right, and so long as you don't want to go into too much detail, but I'll have a crack at that in another post.

No need to bother then Chris ?

As for dust on the sensor then it may be down to the speed of operation of the zoom ring giving a pump action to disturb any dust in the mirror box.
I have only one zoom 12-24, all other lenses being fixed focal. Necessarily I do lots of lens changes, and dust has not been a significant problem. Only ever used a Rocket Blower.

Relative to Tannins dust experience I do not live in a hostile environment. Avoid lens changing on a dusty steam rally field.

Don

Hi Chris, Delkin Sensor Scope was a good investment for me. This link takes you to Scope & Cleaning Kit:

http://www.fotosense.co.uk/shop/Delk...D=5157&tabID=2

But the best cleaner is not to change lens in poor environment. If you have to - try changing lens with camera and lens in an old film changing bag - Your not after a good light seal but a reasonable environment around the change area - so you don't have to get them all inside - just the gaskets/flanges.

As for your 80-400 Nikkor - I have that lens and find it great in all that I use it for, but I mainly use Shutter priority at Min 1/500 s whatever the focal length and always tripod/monopod or beanbag support. If going Close up I switch to a 105 micro Nikkor or 70-200 f2.8 VR with short extension tube and Monopod.