View Full Version : Smallest aperture for good focus ? ? ?

04-30-2005, 05:08 PM
Hey guys I am doing lens research here I have read many places that in low light circumstances you need a fast lens. I understand that to mean that the F value needs to be low, big aperature (ie F/1.4 is faster than F/2.8) Correct me if I am wrong I may be getting it all backa$$wards. However, I am curious to learn where the term "fast" stops being an accurate discritpion. for instance, I thought I read somewhere that the camera needs an aperature of at least 5.6 to focus in low light without the help of an AF illuminator? Does anyone know if this is true or can anyone out there offer up some comments aimed at helping me figure this out. Like for instance is there a rule of thumb out there that suggests a certain F value for a certain mm of focal lenght for focusing in low light circumstances???
Thanks in advance!

The main motivation behind this question is that I am looking to get a telephoto lens for the XT and I was wondering if at 300mm [add crop factor of 1.6 = 480mm 35mm equiv.) is F/5.6 going to be too slow if I am shooting in a low light situation... say outside shooting at wildlife in a shaded area??? I may very well be using the term low light incorrectly, I won't be shooting a bar at night or in a dark old church with these focal lenghts... Im just primarily concerned with lower light outdoors.. not full sun.. partial shade maybe... that sorta thing. Thank you all!

04-30-2005, 05:38 PM
I'm not a wildlife photography expert, but I think that lens is fine for outdoor use. If I remember correctly, what one might consider "poor" light outdoors is actually better than "good" light indoors.

Here's a page with a chart that gives relative light levels for various scenes:

04-30-2005, 05:43 PM
I would think that is true but I've been wrong before. With a larger aperture ie: f/2.8 will be brighter than say a f/4.0 aperture, meaning better performance in low light situations. Some lens perform better stepped down from their largest opening, at least my Sigma does.

To get good focused images without a tripod a 300mm lens is basically gonna needed a 1/300 sec. shutter speed. So that could create a problem if handholding in shaded lighting. However with the ability to adjust the ISO speed higher will help too.

04-30-2005, 05:49 PM
Also, regarding your question about needing f/5.6 for autofocusing... according to Bob Atkins, a lens (for Canon) will not even attempt to autofocus unless the aperture can open up to f/5.6 or wider. Here's a quote from him talking about lenses with extenders attached (which is when this issue typically comes into play):
autofocus is possible as long as the maximum aperture of the prime lens/extender combination is at least f/5.6

Check it out here (http://bobatkins.com/photography/eosfaq/eosfaq24/0toc.html#4lenses16)

04-30-2005, 05:56 PM
Thank you guys for your input. I do have a tripod that can be used if lighting/shutterspeed requires.

04-30-2005, 06:56 PM
aparmley ... a good investment if using a tripod is the Canon RS-60E3 Remote Switch. Plugs into the XT and works the shutter same as the button release does. Sells for around $25.00.

04-30-2005, 08:19 PM

The remote shutter switch you are talking about [25 bucks, not bad I was expecting it to be more.] does it function exactly like the shutter button, half press focus, full press shutter release?

04-30-2005, 09:49 PM
Yes just like the camera button and its very portable, not big at all.

05-01-2005, 09:16 AM
You mean it isn't the size of a suit case... LOL :D

05-01-2005, 11:34 AM
Regarding your question: "when does a lens get fast?" Here's a thread I found on another forum that answers this question:


05-01-2005, 12:29 PM
Hey Nate thanks for the link. That was an informative post. Thank you