Fast lens needed

[nikthegeek]

HI guys,

the focus is allright when i use my sigma 18- 80 (ithink) 2.8 but on these longer lenses they are rubbish.

I will be at focus on Sunday shall investigate further

Thanks Guys

any other points are most welcome

nik

[miketoll]

Autofocus needs light to focus and the more the better. A fast lens helps autofocus in low light for any given body as for every stop that the lens is faster twice the light enters the body and hits the detector. Lenses focus wide open then close down to the taking aperture after focus. The focusing motor for Canon systems is in the lens itself so better lenses will focus quicker. The 70-200 L zoom is a top lens so no doubt focuses quickly.

[nikthegeek]

Excellent thanks for that, i couldn't understand the relationship between the aperture and the auto focus



NIK

[nirofo]

Originally Posted by Alex1994

The aperture makes no difference to the speed of the autofocus. The AF is on the camera, so if it's going in and out it's the camera's fault. If that happens override it manually, at small apertures you will get a good enough DoF for the subject to be in focus.

I'm afraid you've got that one wrong, if you want the lens to focus as fast as the camera is able you need the largest aperture you can get. A 70-200 f2.8 wil leave a 70-210 f3.5 to f5.6 standing when it comes to low light photography, in fact it will probably leave it standing in all lighting conditions.

nirofo.

[miketoll]

Ah yes the f2.8 L 70-200 would be better still , only snag is it costs a tad more! £1,400 new according to camera price buster.

[Alex1994]

Originally Posted by nirofo

I'm afraid you've got that one wrong, if you want the lens to focus as fast as the camera is able you need the largest aperture you can get. A 70-200 f2.8 wil leave a 70-210 f3.5 to f5.6 standing when it comes to low light photography, in fact it will probably leave it standing in all lighting conditions.

nirofo.

Bummer forgot that the camera focusses through the lens! Thanks for putting me right.

[robski]

Basically your looking at the interaction of a number of factors.

First of all you have the camera body. The 5D range design was never intended to be used for "action" photography. From user reviews it strength is image quality especially in low light but a common gripe is about focus. I understand that the focus system is basically that used on the x0D range. So not exactly the bees knees in the Canon line up for ultra critically focusing applications. But for your application I suspect it will be good enough.

The auto focus motor speed. Canon developed a Ring Ultrasonic Motor (USM) which is virtually silent and extremely fast. They are fitted to their more expensive lens. A micromotor USM is fitted to their cheaper range which are noisier and somewhat slower. The penalty of going for a cheaper Sigma equivalent was to suffer a very slow auto focus motor. Some recent models of Sigma are now fitted with a fast HSM focus motor.

The distance the motor has to travel to cover the entire focus range of the lens. This is dependent on the lens focal length and its focus range. This can be a considerable distance on close focusing telephoto lens and very short for wide angle lens. Some lens are fitted with a focus range limit switch to restrict the lens working range so that the motor is not travelling from one end to the other all the time.

Contrast. Basically in non technical terms an auto focus sensor is looking for the point where the image contrast has peaked. It needs to see a difference in light levels from the subject before it can get to work. This is why it is almost impossible to focus on blank scene such a sheet of white paper. In general the more light on the scene and passed through the lens will give the sensor a better start position due to the improved contrast. With wide aperture lens a factor to be taken into account is its optical performance wide open. A common issue seen on some 1.4f and 2.8f lens is reduced contrast wide open and reported unreliable focusing issues in low light. Google can be your friend to track down and avoid such problem lens.

[wolfie]

The two best lenses for your camera/situation would be without doubt the Canon 24-70 f/2.8 and the 70-200 f/2.8.
If you want IS lens, substitute the 24-70 for the Canon EF 24-105mm f4 L IS USM Lens.

If you're shooting "tethered" from a tripod, then go for the cheaper non stabalised version
Expensive, very, but they're the best you can get.

A good place to get them is "Flash Cameras" not what I would call an ideal trading name, but this is the company name of the most respected Ian Kerso. [email protected].

Don't let the fact that he's an Ebay trader put you off, I've bought several items for my 7D of him this year.



Canon 70-200MM 2.8 IS LENS £1,239.00 (Ebay), but less if you deal direct.

(£1500 at Warehouse express)

Type Kerso in your search bar for reviews of this company.

Harry

[nikthegeek]

Thanks again guys

NIK

[nikthegeek]

I have also noticed another odd thing about the auto focus.

When i use the changeable autofocus points they are no where near as fast as the centre point especially the extreme points.

Any thoughts