Telephoto lens f-stops

Alan Sandham

Alan Sandham

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Hi all,
I am still trying to get to grips with my Canon 500D and 28-135mm lens. I am sometimes getting softer results than I used to get with my old 400D with the 750-300mm lens.
Would you say that because the new lens is advertised as f3-5 to f5.6 that I should keep the AV mode f-stop within this range. I have seen a lot of good landscapes with AV of f8 or f10, or am I misunderstanding something here??

Any advice very welcome:bonk:
 
The f3.5 is the widest aperture you can get an at 28mm
At 135 it will be 5.6 as the aperture is not constant through the zoom range.
 
Thanks for the quick response. So if f5-6 is the widest the lens can get am i spoiling the effect by selecting, say f10 on the camera?
 
5.6 is the widest the lens can get at 135mm.

Of course you can set f10 subject to iso and shutter speed and the amount of light all the way down to f22 or f32 if the lens stops down that much (sorry Nikon bloke here )to obtain the depth of field you need in a particular shot
 
Alan Sandham said:
Thanks for the quick response. So if f5-6 is the widest the lens can get am i spoiling the effect by selecting, say f10 on the camera?
Click to expand...

F5.6 is the widest at maximum zoom, F3.5 wider and available at the shorter end (28mm range). A lot of lenses can generally perform are better not at their maximum appature, so no your not spoiling anything with setting F10. :thumbs:
 
You are misunderstanding something.

The f/3.5-5.6 rating of your lens is telling you the maximum (i.e. largest) aperture that the lens offers. It varies according to the focal length. At 28mm it will be f/3.5 and at 135mm it will be f/5.6. The largest available aperture lets in the most light, which is why it is useful to know. However, it also offers the least depth of field and rarely offers the best image quality. Your lens, from memory, offers a minimum (i.e. smallest) aperture of f/22 to f/36. This means that at 28mm you can choose any aperture from f/3.5 to f/22, and at 135mm your choice ranges from f/5.6 to f/36.

More expensive lenses typically offer a fixed aperture that does not vary as the lens zooms, e.g. the Canon 24-105mm f/4 IS, which is F/4 all the way from 24mm to 105mm.

For landscape photography, depth of field (meaning how much of the image from front to back is in focus) is often very important, It is thus common to "stop down" the lens to f/8, f/11, f/16 etc to ensure that all the important elements from the foreground to the background are in focus.

As far as image quality goes, most low-ish cost lenses are at their best in the middle of their aperture range. The review of your lens here shows that effect, at the wide end, with best performance at f/5.6. At other focal lengths, performance of this lens is relatively constant with aperture.

It is generally best not to close the aperture down too far unless you really need all of that depth of field. An effect known as diffraction starts hitting image quality from around f/11 onwards on crop-frame cameras such as yours.

Hope some of that helps.

It doesn't really explain why you are getting soft images, however! You're best off posting a soft image with as many details of how it was taken as possible for others to look at and help you diagnose.
 
the maximum f-stop is what you should use if the light is low and you want to keep the shutter speed as short as possible.

however, it's not a target. the lens will be sharper away from this limit, so f8 might be best for landscapes or f11, f16 perhaps.

you need to experiment with the lens.

people harp on about having to have f2.8 glass when they don't shoot low light, or need that depth of field effect. In decent light most people don't shoot at the maximum aperture if they can help it, unless aiming for a specific effect as mentioned
 
The numbers indicate the biggest aperture (smaller the f number the bigger the aperture ) that a lens can work at, as your lens is not a constant aperture lens the aperture will change as you zoom from 75-300mm, the bigger the aperture the more light is let into the camera allowing you to use higher shutter speeds which is important with a longer zoom as you need to use higher shutter speeds to reduce camera shake, normally you should aim to have least 1.6 X focal length as a shutter speed to eliminate camera shake so for 200mm you need at least 1/320, by reducing the aperture (bigger f number) you are reducing the amount of light entering the camera so the camera compensates by reducing the shutter speed and you may get blurred images due to camera shake.
 
Re softness between the 500d and 400d, I had a similar problem when I upgraded to the 50d from the 400d. I think it's down to the higher pixel count giving a much more detailed image. I'm ok with it now Ive got used to it.
 
Thanks for the very useful comments. I will continue to experiment. Perhaps get the tripod out to confirm it's camera shake.:thumbs:
 
Make sure you have IS turned off with this lens if you are using a tripod. Mine is very very soft when I forget.
 
Thanks for all your comments. I think Garynlea's reply was useful also, in that it could be just me getting used to the digic 4 processor with the extra 5m pixels.

http://www.talkphotography.co.uk/gallery/showphoto.php?photo=65337

This example above was up in Cheshire a couple of weeks ago. It sharpens up in photoshop ok. No tripod on this one but I will try one with it next time I am out.

Many thanks once again.:thumbs:
 
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