Cannot get lens to it's lowest f stop
Nov 20, 2011 21:30:51 #
georgebs
Loc: Georgia
Hi all, I'm new here. Hope I am not violating any rule itch this post. Anyway, I just bought a Canon t3i. It's amazing.......at least to me. However, there is one question I have. It came with a lens that is supposed to have an f stop of 3.5. No matter what I do, I cannot get an f stop of lower than 4.5. I'm new to the DSLR scene and have been using "automatics". Is there somethingwrong with lens or am I just not aware of something?
Thanks for any responses. George
Thanks for any responses. George
Nov 20, 2011 21:38:10 #
If your lens is a ZOOM lens, it may be zoomed out too far. Many low-price (not low-quality) consumer zooms have a variable aperture - for example, the 18-55 f/3.5-4.5 IS. At 18mm, you can open the aperture to f/3.5. But zoomed out to 55mm, the widest you can go is f/4.5. The kick-over to get to f/3.5 probably comes down around 20-22mm
Even many high quality lenses do this - my $1600 100-400mm L lens is f/4.5 at the wide end, but out at 400mm I can only open it to f/5.6
Hope this helps.
Even many high quality lenses do this - my $1600 100-400mm L lens is f/4.5 at the wide end, but out at 400mm I can only open it to f/5.6
Hope this helps.
Nov 20, 2011 23:31:43 #
georgebs
Loc: Georgia
Wow! Right on the money. Glad you are on this forum. Thanks very much for the quick (and correct) response. Thought I was going to have to take it back. Thanks again.
Nov 20, 2011 23:40:44 #
Actually, the aperture does not change, but the relationship between aperture width to lens length changes as a zoom lens extends. The "f" stands for fraction, f/4 means that the aperture width is 1/4 the length of the lens. Also, as the aperture closes down (becomes smaller, letting in less light), the fraction also becomes smaller. f/8 is smaller than f/4.
Nov 21, 2011 01:36:56 #
georgebs
Loc: Georgia
Ah so. This is also very good information. Thank you very much. When I was checking the f stop based on focal length, I noticed that the f/3.5 came while the Lens was not at its visible shortest. For instance, when it was totally zoomed out (it is an 18-55) I started bringing it back to 18. It went all the way in and then started to go out. It went out a short distance before the f/3.5 could be seen. I wou have suspected that the f/3.5 would be achieved when the lens was at its shortest. Does that sound right to you?
Again, thanks very much for the info.
Again, thanks very much for the info.
Nov 21, 2011 01:49:01 #
georgebs wrote:
When I was checking the f stop based on focal length, I noticed that the f/3.5 came while the Lens was not at its visible shortest.
I also own a Nikkor 18-55 zoom lens. The "node" point that you noted is about 37-mm, which is the "normal" lens length for a DX-size sensor (similar to a 55-mm normal lens for a 35-mm film camera). Lens groups inside the zoom move in different directions for both wide angle and short telephoto, so the lens lengthens for both wide & tele. It is the "virtual" lens length that is used to calculate f-stop.
Nov 21, 2011 10:01:12 #
you learn something new everyday.
I never knew that f stands for fraction.
I have always told people to think in fraction, but never put the two togeather.
thanks
I never knew that f stands for fraction.
I have always told people to think in fraction, but never put the two togeather.
thanks
Nov 21, 2011 12:18:06 #
georgebs
Loc: Georgia
Understand now. I "played" with the various positions of the lens and mapped f stops. Thank you very much for sharing your knowledge.
Nov 24, 2011 06:23:26 #
Dun1
Loc: Atlanta, GA
If you have say an 18-55 lens set it on 18 take the camera off green or auto and use manual, or AV to dial in the f stop you desire. If you move the lens away from 18 the lens opening will vary depending on the length you set. You may only achieve f 3.5 at 18 or so depending on the light, you may only achieve f/6 or so at 55mm. As you zoom the lens out of extend the lens the f stop will more than likely increase, as several have mentioned. If you have a 70-200 2.8 lens and you are using the auto settings you may not be able to get the lens to open to 2.8. If you set the camera on manual, or AV you can dial in the f stop you wish
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